Author Archives: Dr. Zoomie

What is Alpha and Gamma Spectroscopy?

Dear Dr. Zoomie – we’ve got to do some environmental sampling and the lab asked if we wanted to have alpha or gamma spectroscopy done. It sounds important, but I’m not sure what they mean or what they’re used for. Can you help me out?

Spectroscopy can be incredibly valuable, but only if you need to use it. We’ll get into that in a minute – first, let’s talk about what it is. So let’s start with the science part and then go to when you might need to use it.

First – radionuclides give off alpha, beta, or gamma radiation. For reasons that are too complicated to get into here, beta radiation isn’t amenable to spectroscopy – but alpha and gamma radiation are.

Second – we can analyze the alpha and gamma radiation to see how much energy the radiation has. Every nuclide gives off radiation with very specific energy. Cesium-137, for example, gives off a gamma ray that has exactly 662 thousand electron volts (662 keV) and cobalt-60 gives off two gamma rays with energies of 1170 and 1330 keV. These are as specific as fingerprints – anytime you see a gamma with an energy of 662 keV you can bet that it comes from Cs-137. One way to think about it is that every alpha- and gamma-emitting nuclide has a distinctive “fingerprint” of radiation where the identifying features are the energies of the emitted radiation. So if we can measure the radiation energies – alpha or gamma – then we can identify exactly what radionuclides are there.

Third – it’s relatively easy (and not terribly expensive – no more than a few tens of thousands of dollars) to perform gamma spectroscopy, but the cheap and easy way of doing it isn’t tremendously precise. To do gamma spectroscopy well costs over $100 K and is best done by a radiochemistry laboratory. Alpha spectroscopy is another kettle of fish entirely – there is (as of now) no quick, easy, or cheap way to do alpha spec – you pretty much have to have a trained chemist and a full-blown laboratory with a few hundred thousand dollars’ worth of equipment.

The graphics here are from some gamma spectra I made in early 2016 – the top one is Cs-137 and the bottom one is Co-60. Looking at these, it’s easy to see how the gamma spectrum can be used for radionuclide ID – you can do the same thing with alpha radiation; I just don’t have the equipment (or the training) to make these measurements in my lab.

Gamma Spectra - Cs-137

Gamma Spectra – Cs-137

Gamma Spectra - Co-60

Gamma Spectra – Co-60

OK – so this is how spectroscopy works; the next question is when you might need to use it. Here are some examples.

Several years ago a rail car filled with scrap metal set off a radiation detector at a steel mill. The gondola car was sent back to its point of origin, which happened to be just a few miles from where I was working at the time. The scrap metal dealer didn’t have the ability to figure out what was causing the alarm – it might have been natural radioactivity or it might have been something manmade; nobody knew how to handle the material until the radioactivity could be identified. Using gamma spectroscopy we were able to identify the source of radioactivity as radium (specifically Ra-226), which made it possible to find a disposal site.

Another example was with one of my consulting clients – they measured elevated levels of radioactivity at a site where they had used thorium in decades past. We had to find out if the elevated radiation levels were due to their operations with thorium or something else. By identifying the specific isotopes present (using both alpha and gamma spectroscopy) we were able to show that my client hadn’t contaminated the site with thorium; the extra radioactivity was naturally occurring and came from some coal-processing that had taken place on the adjacent site nearly a century earlier. This meant that it wasn’t due to any actions of my client and, not only that, wasn’t anything that required remedial action.

There are tons of other examples I could give but they all come down to pretty much the same thing – there are times that you need to know exactly what types of radioactivity you might (or might not) have and the best way to make this ID is through alpha or gamma spectroscopy. If you have to do a lot of it then you might want to do the work yourself, otherwise it makes more sense to send samples off to a commercial lab. One caution – gamma spectroscopy is fairly inexpensive, but alpha spec can cost a few hundred dollars per analysis and a thousand dollars or so to completely analyze a sample. But if the alternative could call for spending hundreds of thousands – or millions – of dollars in cleanup and/or waste disposal, this is money well-spent.

The New Radioactive Source Security Regulations in 10 CFR Part 37

Dear Dr. Z – I’ve been hearing about new source security regulations in 10 CFR part 37 but I’m not quite sure what they mean or how they might affect me – can you fill me in?

Without knowing exactly what sorts of sources you have I can’t give a precise answer to this. But let me tell what I’ve noticed in these regs and how they’ve affected me – hopefully this will help you out with your program.

I was Radiation Safety Officer at a major university and hospital from the late 1990s through the first few years of the oughts and one of my responsibilities was to help assure the safety and security of our radioactive materials. When I took the job (pre-September 11) my major concern was that a disgruntled grad student would try to dump low levels of radioactive materials into a colleagues lunch – this had happened at Brown University, NIH, and a few other places in the previous decades. But in the aftermath of the 9/11 attacks and the subsequent arrest of putative “dirty bomber” Jose Padilla in May, 2002 my worries changed dramatically – instead of theft of relatively small amounts of radioactivity by an amateur, I had to worry about trying to thwart an attack by terrorists or professional thieves; an entirely different kettle of fish.

Most of the time I am in favor of less-prescriptive regulatory guidance – as an experienced radiation safety professional I’d rather come up with my own solutions to, say, keeping radiation exposures as low as reasonably achievable (ALARA, the guiding philosophy of radiation safety in most of the world’s nations). But faced with so sudden and so dramatic a change in paradigm – and when faced with a problem that was outside my expertise in radiation safety – I found myself wishing for more guidance from my regulators. Nobody will be injured by swallowing minor amounts of radioactivity – the worst that had happened before 9/11 – but even if the radioactivity from a dirty bomb attack is not dangerous, the explosion and panic can be deadly (not to mention the cost of a cleanup). I’m a health physicist, not an expert in security or counter-terrorism – what I really wanted from my regulators was advice on how to secure my potentially dangerous sources from a threat that was outside all of my experience.

While the regulators didn’t exactly spring into prompt action, the Nuclear Regulatory Commission did issue a series of orders beginning in 2005 that helped clarify measures that could be taken to help safeguard high-risk radioactive materials. These are about to be collected into a single new regulation, 10 CFR 37, that will put much of this information in one place.

As a former Radiation Safety Officer (RSO) I think it’s great to collect all of the disparate NRC orders into a single new regulation, but I find myself wishing the NRC had gone further, keeping in mind that most radiation safety professionals aren’t security experts. For example, the upcoming rule has tons of verbiage devoted to telling licensees how to establish a program to ensure the trustworthiness and reliability of workers granted access to radioactive materials, but it doesn’t provide any guidance on how to actually secure specific radioactive materials (For example, are normal locks OK, or is keycard access needed to control access to a 1 Ci Cs-137 source?).

One of the keystones of these rules is the requirement that people must be deemed “trustworthy and reliable” before they are permitted to have unescorted access to high-activity radioactive sources, and this is one of the parts that I’m not sure I’d feel comfortable with as an RSO. Part of the T&R program is specified in the rules – requiring fingerprinting and a background check. But part of it is left to the judgment of the T&R officer. The T&R officer is required to have a background check and to be fingerprinted, and the NRC verifies their trustworthiness and reliability. But not much more is written – the T&R officer can be the RSO, the head of security, a manager in Human Resources, or virtually anyone else proposed by the licensee. The reason this gives me pause is that, as an RSO I was our organization’s most knowledgeable radiation safety professional, but security is not my game – I know how to select technically competent staff and how to find a technician who will give me a full day’s work, but I’m not trained in how to evaluate a person as a possible saboteur or terrorist. I’d rather have Security handle this task, but under the new regulation there’s no requirement that security evaluations be performed by security professionals.

The NRC will undoubtedly be developing guidance on how to implement the new regulations – there is a draft out (dated 2010) that I hope will be updated and issued to help licensees figure out how to comply with both the letter and the intent of the new regulations. Absent such guidance we are likely to end up with a hodge-podge of approaches to radioactive source security by RSOs who are professional health physicists, physicians or medical technologists, industrial radiographers, and so forth. It would have been helpful for the NRC to have required participation by a security professional – the head of institutional security for large organizations or a security consultant for smaller ones (there are a number of relatively small businesses that possess dangerously high-activity sources).

OK – so what? We’ve got a new law on the way that should help to consolidate a lot of the existing rules and orders on radioactive materials security, and (hopefully) some guidance on how to implement these new laws. It doesn’t seem to do any harm, and by putting so much in one place it can certainly make things easier for the licensee. What’s not to love?

The biggest thing is that this seems to be a rule written by administrators, for administrators. Don’t get me wrong – the administrative stuff is important! It’s nice to know the standards I should meet in order to decide (and demonstrate) that a person should be allowed to have unescorted access to high-activity radioactive sources. It’s also good to force licensees to have a security plan, to know when to notify law enforcement agencies that something has gone wrong, and so forth. But there’s more to security than getting the paperwork right, and that’s where licensees could use some more help – what would be great would be specific practical guidance.

Say I’m applying for a new radioactive materials license and I am to be RSO at a small facility with just enough radioactive materials to fall under this new rule, but we’re not large enough to have its own security force. I know that I need to secure the sources, but what constitutes adequate security? Is a padlock sufficient, or should I have a full-blown safe or vault? Should I put in motion detectors and, if so, what specifications should they meet? What about cameras (and if so, what kind and how many)? Do I need to have an alarm that automatically sounds at the local police precinct? And so forth and so on…. I need more than a bunch of file folders filled with plans and lists – what I need is a list of vendors and model numbers, or at least a list of minimum acceptable specifications for my cameras and locks. In short, I want a radioactive materials security program that will keep secure my sources against theft in addition to meeting paperwork requirements.

All in all, the new rule is a good start – at the least it makes it easier to figure out where all of the requirements are located. But from a practical standpoint it’s no more than a start – let’s hope that more practical assistance is on its way.

Why Do We Use Highly Enriched Uranium Research Reactors (HEUs)?

Hi, Dr. Zoomie – I’ve read some stories about research reactors that are fueled with weapons-grade uranium, which we now worry about as a proliferation risk. Why in the world would anybody do something like that? Didn’t they think this could be a problem?

Every so often we hear something in the news about nuclear reactors fueled with highly enriched uranium (HEU); usually with regards to nuclear weapons proliferation. Back in the good old days both the US and USSR constructed over a hundred small HEU-fueled reactors and shipped them all over the world – Uruguay had one, there were some in the Balkans, the Ohio State University reactor was fueled with weapons-grade uranium, and there were plenty more. Given today’s concerns about locking up and accounting for every gram of weapons-grade uranium it’s only natural to wonder “What were they thinking?”
Even today there are a number of reactors fueled with uranium that could be turned into nuclear weapons. One category is military reactors – the nuclear reactor on my submarine was fueled with HEU. But outside of the military, the biggest reason to use high enrichments is for research and to produce radioactive materials for research and medicine. Here’s why.

First a little bit of background. Two kinds of radionuclides are produced in nuclear reactors – in one, stable atoms that capture a neutron can become radioactive by a process called neutron activation and the products are called neutron activation products (activation products for short). Cobalt-60 is a neutron activation product, formed when stable cobalt-59 captures a neutron to become radioactive cobalt-60. In the other process, a uranium atoms splits (fissions) and the fission products are radioactive; these include the nuclides we saw in Fukushima (radioactive isotopes of iodine and cesium mainly) as well as molydebenum-99 (the parent nuclide of technetium-99 that is the workhorse of nuclear medicine) and others.

So – to create a neutron activation product you need two things – target atoms (such as cobalt-59) and neutrons; the neutrons come from uranium fission. All things being equal, a larger number of neutrons means a larger amount of radioactive product; a higher flux of neutrons means more rapid production. The way to get a lot of neutrons is to have a lot of fissions – the more fissionable atoms that are crammed into a volume, the more neutrons. A higher uranium enrichment is the best way to cram the highest number of fissionable atoms into a volume and, thus, to increase the neutron flux.

With fission products this rule works double – a larger number of fissionable atoms not only boosts the neutron flux but it also gives a larger number of target atoms to fission. So, again, using more highly enriched uranium produces a higher yield of the desired nuclides. And with nuclides that have a low probability of being produced the only way to make useful quantities is to use more-enriched uranium.

In both cases we end up with the same choice – do we choose the greater profits from HEU or the lower risks of LEU? An LEU-fueled reactor can do everything that an HEU-fueled one can – just more slowly.

At the moment it looks as though the nation is moving towards security rather than production. Unfortunately for the nuclear medicine industry this coincides with the shutdown of some Canadian reactors that produced medical isotopes, causing some shortages in our supplies of medical nuclides.

With medical science using radionuclides in ever-increasing amounts, this places a strain on our nuclear medicine system (with the exception of PET nuclides, which are produced on-site in a type of particle accelerator called a cyclotron). Our only real options are to cut back on nuclear medicine procedures or to build more isotope production reactors.
There is more to HEU-fueled reactors than producing medical nuclides – they’re also used to produce nuclides for industry, for basic research (bombarding rocks with neutrons, for example, can tell us what the rocks are made of), developing and testing nuclear instruments, and more. All of these things go more quickly with a higher neutron flux, but they can also be done in a less neutron-rich environment. When we put it all together we pretty much have to conclude that HEU-fueled reactors are nice, but they’re not essential. If our priority is to make the largest amounts of nuclides possible then we need the HEU-fueled reactors; if security is more important then we have to shut them all down and replace them with the slower (but more proliferation-resistant) LEU-fueled devices.

How Do You Do An Inventory And Leak Test Sources?

Hi, Dr. Zoomie – I just took over as RSO and found out that I have to do an inventory and leak testing of my sources. Can you tell me how often I have to do these and the best way to go about it? Thanks!

Wow – I can sense your excitement with the topic! And, I have to say, I can sympathize – inventory is one of those things that we all have to do, but that nobody really gets excited about. But it’s got to be done – and done correctly – even if only because they’re regulatory requirements. Not to mention the fact that leak testing your sources will let you know if one might be starting to give you problems. Let’s take them one at a time.


Most of the time you’re going to have to inventory your sources every six months – this is the default periodicity written into the regs. But you need to check this in your license; sometimes regulators will require a quarterly inventory. Your inventory has got to account for every source and it has got to be documented in writing, including your initials or signature confirming that you performed and vouch for the inventory.

There’s more to an inventory, though, than just accounting for the sources – you also have to account for the activity of radioactive materials in your possession. This means that, when you bring in new radioactive sources, you need to add them to your inventory and, as radioactive materials leave your facility, you again need to update your inventory accordingly.

Remember, too, that there are a number of ways that radioactive materials can leave your inventory, and you should account for all of these. For example:

  • Your inventory should include corrections due to radioactive decay so that it shows (at least every 6 months) the decayed activity rather than the strength of the source as it was when you first bought it.
  • If you’re using radioactive materials in experiments, medical procedures, industrial processes, and so forth remember to reflect what is used up and leaves your facility (for example, radiopharmaecuticals in your patients).
  • As you dispose of radioactive waste, your inventory also has to be updated.
  • Devices with radioactive sources (electron capture devices inside of gas chromatographs are one example) should not only be on your inventory, but should be updated if you sell or dispose of the devices.

These are only a few example – again, make sure that you’re updating your inventory so that it reflects not only the total number of sources (and tracks each individual source), but also the decay-corrected activity of each one.

Something else to keep in mind is that there are a number of ways you can perform an inventory.

  • Best, of course, is to visually sight each source – to lay eyes on them so you can say “I saw source #12345 on this date in this location.”
  • You might not be able to do this for sources that are installed inside of devices (the Ni-63 source inside of an electron capture device for example). One option in this case is to confirm that the device operates. If an electron capture device only operates when the source is installed, then showing that it operates properly will also show that the source is present.
  • Alternately, you can check radiation levels on the outside of a device to confirm the source is present. Say (for example) you have an irradiator that normally reads 2 mR/hr with the source inside. As long as you get a reading of about 2 mR/hr then you have a high degree of confidence that the source is still there. But if readings drop to, say, 20 µR/hr (0.02 mR/hr) then you’ve got to wonder what happened to the source.
  • If you have a source (or several sources) in storage then you’re allowed to inventory the sources by confirming that the storage location is sealed and has remained sealed since your last inventory.
  • And if you can’t figure out how to inventory your source(s), call the manufacturer and ask for a copy of their inventory procedure – they should be happy to send you a copy that you can use.

Finally, if you find out that you can’t account for a source, you have to tell your regulators. According to 10 CFR 20.2201, you have to make an immediate telephone report to your regulators if the source is more than 1000 times as active as the amount requiring licensing – but only if the circumstances were such that a member of the public might be exposed to radiation from the source. The next paragraph of the regs also notes that you have to make a written report to your regulators within 30 days of losing any source that’s more than 10 times the licensable quantity.

As an example – according to 10 CFR 30.71, Schedule B the limit for exempt sources of Cs-137 is 10 µCi (you’ll also find this in Appendix C to 10 CFR 20). This means that any Cs-137 source that’s smaller than 10 µCi is exempt from regulations. If you lose a source that’s less than 100 µCi (10 times the exemption limit) you don’t have to report anything to your regulators. If you lose a source that’s more than 100 µCi you need to make a written report within a month and, if you find that a source greater than 10 mCi has been lost, stolen, or gone missing then you need to call your regulators to inform them of this as soon as you realize what’s happened.

Leak testing

Something else you’ll have to do periodically is to check your sealed sources for leakage (you don’t have to check unsealed sources such as vials of stock solution used for experiments, or syringes filled with radiopharmaceuticals). In addition to unsealed sources there are other sources you don’t have to leak test:

  • Beta- and gamma-emitting radionuclides don’t need to be leak tested as long as the activity is less than 100 µCi
  • Alpha-emitting sources don’t need to be leak tested as long as the activity is less than 10 µCi
  • Sources filled with radioactive gases (such as Kr-85) need not be leak-tested

Specific leak testing requirements are found in a number of places in the regulations:

  • 10 CFR 32.59 (for companies that manufacture and sell items that contain sources)
  • 10 CFR 34.27 (industrial radiography) and 10 CFR 34.67 (for depleted uranium shielding)
  • 10 CFR 35.67 (medical sources)
  • 10 CFR 39.35 (for well logging sources)

There may be others; these are the ones that showed up in a search of the NRC website.

The purpose of leak testing is fairly obvious – to check to see if your radioactive sources are still sealed tightly or if they’re leaking. A leaking source can be a big deal – a single leaking source at a university I used to work at was responsible for contaminating an entire laboratory and adjacent rooms and a different leaking source contaminated an entire warehouse.

Performing leak testing on lower-activity sources can be fairly straight-forward; simply wipe the entire source with a piece of filter paper or a cotton-tipped swab. But you might not be able to do this – the source might be locked away inside a piece of equipment, for example. If this is the case, what you need to do is to wipe the outside of the source holder or an access panel. You’re not going to be permitted to disassemble a device to get at the source, so you should try to get as close as you can without doing so. One specific example would be a self-shielded irradiator (such as a blood bank irradiator) – this is a device with a drawer or a sample holder that’s exposed to the radiation, even though it’s heavily shielded to prevent exposing you to radiation. In this case, wiping the sample holder will suffice for a leak test since, if the source is leaking, the sample holder will be contaminated.

You’ll want to be careful when you’re leak testing high-activity sources. First, you should make a habit of doing all of your leak testing with a cotton-tipped applicator and not with a wipe you’re holding in your hands. And for any source that’s more than about 1 mCi in activity you should also be holding the source (if at all) with tweezers or tongs to avoid a high dose to your fingers. And with higher-activity sources even this might not be enough to ensure your safety. For example, industrial radiography sources often contain several tens – even up to a couple of hundred – curies of activity; forget about dose to your hands, this much activity could push you over an annual dose limit fairly quickly. For sources like this you want to survey the guide tube that the source travels through, or the shutter or exit port that the source is extended through – the thinking is to survey something that the source passes through or comes in contact with under the assumption that, if the source is leaking, these are the things that will become contaminated.

There are still more ways to check for leakage. I once had to perform leak tests on a 50,000 Ci source of Cs-137 that was stored in a large pool of water. First, the only way to wipe the source directly would have required scuba gear, which I’m not qualified to use. But more importantly, there’s no way in the world I’m going to go anywhere close to a source of that activity – such a source is an immediate danger to health and safety. On the other hand, cesium is soluble and any leakage would quickly dissolve into the water of the pool. Our approach was to take a liter of the water, evaporate it away, and count the residue in our gamma counter; if the source were leaking then this would show in the sample.

As with various methods for inventorying your sources, if you can’t figure out the best way to safely perform a leak test, ask the vendor. They’re certain to have developed a procedure as a part of their radiation safety program and they should be happy to share it with you.

Taking the sample is a start, but you also have to count the sample to show whether or not the source is leaking. A lot of licensees will send their wipes to a consultant or a laboratory for counting, but this isn’t always necessary as long as you can demonstrate that your counting methodology is capable of detecting leakage. Here’s how you can do that. And keep in mind – this is an example only – you’ll have to do this for the equipment you actually use at your facility!

  1. To start – the standard you have to be able to detect is 185 Bq (0.005 µCi). This is 185 radioactive decays per second, or 11,100 decays per minute.
  2. So – say you’re using a 1”x1” sodium iodide detector for the counting and the source contains Cs-137. The counting efficiency of this detector for Cs-137 is about 10% (you can look this up, but it’s better to have this determined when you send your meter out for calibration).
  3. In addition, normal background count rates with this detector will be about 500-1000 counts per minute (cpm).
  4. If you’re using a 1”x1” sodium iodide detector to count a wipe that holds 11,100 dpm of activity, you’ll get a count rate of 1110 cpm.
  5. For the final step, you can point out that the count rate you’d see from 185 Bq of contamination with your detector is at least double the normal background count rate you see with this same detector.
  6. This is “visible” to your detector; thus, your analysis methodology is capable of detecting a leaking source.

You might still want to have a consultant do your leak test counting for you – that’s your choice. But if you’d rather count your own leak testing, this is how you can get started.

Radioactive Decay: Half-life, Mass, & Activity Per Gram

Dear Dr. Zoomie – I am trying to brush up on some of my radiation knowledge and am having some trouble figuring out some of the calculations and concepts about radioactivity. For example, I know there’s some sort of relationship between a nuclide’s half-life, its mass, and the amount of activity per gram but I’m not quite sure how these all go together. Can you help explain how it all works?

You’re right – these factors are all tied together, and the relationship is fairly straightforward. That’s the good news – but it will take going through a bit of math to see how these all fit in with one another. Luckily it’s not too involved, so let’s walk through it a step at a time. First, let’s start with some equations!

Decay constant (λ)

The first concept is something called a decay constant, which is represented mathematically with the Greek letter lambda (λ). The decay constant is simply the probability that given atom will have a radioactive decay in a particular amount of time. Or, if you have a bunch of radioactive atoms, the decay constant tells us how many of those (what percentage of them) will decay away in a given amount of time. The equation is  . The natural logarithm of 2 (ln 2) is roughly equal to 0.693 and t1/2 is the half-life of the nuclide you’re calculating activity for. So for Co-60 (which has a half-life of 5.27 years) the decay constant is equal to 0.693/5.27 years = 0.1315 yr-1, which you would read as 0.1315 per year. What this tells us is that any particular Co-60 atom has a 13.15% chance of decaying during the course of a year, or that 13.15% of the atoms in a bunch of Co-60 will decay during a year.

The next part of this is to turn this into a measure of radioactivity. Radioactivity measures the number of radioactive decays an amount of radioactivity will undergo in a second. One curie of radioactive material will undergo 37 billion decays every second; one Becquerel of radioactivity will undergo 1 decay every second. The way to find out how many atoms will decay in a given amount of time is to count (or calculate) the number of radioactive atoms and to multiply this by the fraction of those atoms that will decay in a second. Mathematically, this is written as A= λN where A is the amount of radioactivity (decays per second) and N is the number of radioactive atoms.

OK – so say you have a billion atoms of Co-60 and you want to figure out how much radioactivity this represents. Using this equation, over the course of a year 13.15% of these atoms will decay, so one billion Co-60 atoms will undergo 131.5 million radioactive decays, giving a decay rate of 131.5 million decays per year. This is easier to calculate with if we use scientific notation – written this way we’ve got 1.315×108 decays per year. Unfortunately, our instruments don’t read out in counts per year so we have to convert this to a more useful measure. There are roughly 31.4 million (3.14×107 seconds in a year, so we divide 1.315×108 decays per year by 3.14×107 seconds per year to get a decay rate of about 4.2 decays per second (dps), which is about 4.2 Bq, measuring radioactivity in SI units. To convert to the US units of curies we have to remember that one Ci is 37 billion Bq, so 4.2 Bq = 1.13×10-7 mCi, or about 113 pCi (a pCi, or pico-curie, is a trillionth of a curie. So a billion atoms of Co-60 gives us 113 pCi of radioactivity.

Now, let’s see what happens when you have a nuclide with a different half-life. What if, instead of Co-60, you have a billion atoms of Cs-137 (which has a half-life of about 30 years)?

First, the decay constant for Cs-137 is going to be different;  so the same billion atoms will produce only 23.1 million decays per year, compared to the 131.5 million Co-60 decays. So Cs-137, which has a longer half-life, decays more slowly than the shorter-lived Co-60. And this is a good rule of thumb – for the same number of radioactive atoms, the ones with the longer half-lives will decay more slowly (will have lower levels of radioactivity). And as a corollary, nuclides with shorter half-lives will be more intensely radioactive for the same number of radioactive atoms.

Specific activity

OK – so this tells us how half-life and radioactivity go together. But we don’t count atoms (at least, not normally) – weight is how we normally measure things. So a more useful measure – a more useful calculation to perform – will tell us how much radioactivity we have per gram of material; the name for this is “specific activity.” And when we look at specific activity we have to take into account not only a nuclide’s half-life, but also how massive the atoms are. This gets a little more complicated, but there’s a bit of a shortcut at the end that makes things a bit easier.

Say we have one gram of Co-60. If we want to use the equation we already know then we have to figure out how many atoms of Co-60 there are in a gram. This is the somewhat complex part. Going back to high school chemistry, remember that the mass of one mole of anything (the number of grams that it weighs) is equal to the molecular (or atomic) mass. The mass part is easy – it’s just the numerical part of the nuclide. So one mole of Co-60 weighs 60 grams (just as one mole of Cs-137 will weigh 137 grams, and one mole of Ra-226 will weigh 226 grams). And one mole of Co-60 (or anything else, for that matter) has 6.022×1023 atoms, a number called Avogadro’s Number, after the 19th century scientist who first calculated it). So the number of atoms in one gram of Co-60 is equal to one sixtieth of Avogadro’s number, or  atoms of Co-60. And from here, we just multiply by the decay constant, which we know from earlier to be 0.1315 per year. Doing this tells us that one gram of Co-60 will undergo about 1.315×1021 decays per year, or about 4.2×1013 decays every second. Remember that one curie will undergo 37 billion (3.7×1010) decays per second:  So one gram of Co-60 gives us 1132 Ci – this is the specific activity of Co-60.

Using the same reasoning as before, we can guess that a nuclide with a longer half-life should have a lower specific activity since fewer atoms are decaying every second. And adding to that, a heavier radionuclide should also have a lower specific activity (fewer Ci or Bq per gram) because there are fewer atoms in a gram when each atom is heavier.

So here’s how it all goes together. All else being equal:

  • A shorter half-life means a more radioactive nuclide (more Ci or Bq for the same number of atoms)
  • A longer half-life means lower activity (fewer Ci or Bq per gram for the same number of atoms)
  • A heavier nuclide (the number part of the nuclide is larger) means fewer atoms in a gram, so there’s less radioactivity per gram
  • A lighter nuclide means more atoms per gram, so a higher specific activity
  • And if both half-life AND weight change then you have to figure out which is more significant before you can tell.

An easier way to do the math

OK – that’s the “pure” way to figure this out, but as I promised earlier, there’s an easier way to do it – just compare to a radionuclide with a known specific activity. And the one I compare against is the nuclide that gave us the definition of 1 Ci – Ra-226.

One gram of Ra-226 has an activity of 1 Ci (not exactly, but close enough for our purposes) and Ra-226 has a half-life of 1600 years. So a radionuclide with a similar mass and a shorter half-life will have a higher specific activity. Consider Am-241, with a half-life of about 432 years and a mass that’s fairly close to that of Ra-226. Roughly speaking, Am-241 has a half-life that’s about a quarter as long as Ra-226 so we’d expect to see four times as many atoms decay in the same amount of time. This means that 1 gram of Am-241 should have a little less than 4 Ci of activity. And if we look it up, we find that Am-241 has a specific activity of 3.4 Ci/gm – right in the ballpark of what we predicted.

Another one we can try is Co-60. In this case we have to correct for both the mass and the half-life:


Remember – a longer half-life means a lower decay constant (fewer atoms decay in the same amount of time) and a heavier atom means that there are fewer atoms in a gram of the pure nuclide. This is very close to what we calculated earlier, close enough to not worry about the relatively minor discrepancy that comes from the rounding off we’ve been doing.

Wrapping up

After having gone through all of that I have to admit that there aren’t times you’ll need to go through this, and there aren’t many radiation safety officers who need to make these calculations. Those who manufacture radioactive sources need to know this of course. If you’re working in a nuclear pharmacy or in radiation oncology then these calculations will also be useful. And if you’re a scientist using radionuclides for your research then these are calculations you need to be able to perform. And, I guess, if you hang out at geek bars, this particular skill set might win you a free drink (or, then again, maybe not…). But if you’re not in one of these groups (and if you’re not studying up for an exam of some sort) then you can probably get by with using a spreadsheet for all of this.

Decommissioning – Meeting “Free Release” Criteria

Dear Dr. Z – I have a room and some equipment that was used for isotopic work that I want to close out. My regulators said something about decommissioning and meeting “free release” criteria. What in the world does this mean? And can you tell me how I can reach this exalted state? Thanks!

Boy – you’re about to have a lot of fun. Or maybe not…but let’s be optimistic! The big picture is that you have to be able to demonstrate to your regulators that you’ve moved all of the radioactive materials out of the room you’re decommissioning – including any radioactive contamination that might be present – and that you’ve cleaned up all of the equipment that was used to work with radioactive materials. That’s the “decommissioning” side of things; “free release” is another way of saying “release for unrestricted use,” which means that it’s been cleaned up to the point where you can do whatever you want with it. “Free release” means that you can turn the room into a lunch room without worrying that they’ll be harmed from the radioactivity, or that you can throw the equipment into the trash or give to your dog to play with – again, without risk to Fido. So that’s the big picture – now let’s get a little into the nitty-gritty.

Let’s start with the decommissioning process – what do your regulators expect you to do to show that you’ve properly closed out a room?

First, you’ve got to be able to show that you’ve moved all of your radioactive materials out of the room. If this is the only room where you use or store radioactive materials then you have to be able to show that you’ve got rid of the stuff – the only acceptable ways to do this are by transferring it to a licensed radioactive waste disposal facility or by transferring it to another radioactive materials licensee. Whichever of these you choose, you’ve got to be able to document the transfer or disposal – you will have copies of shipping papers and the manifest if you ship materials for disposal, or a letter (and possibly shipping papers) from whomever you transfer your radioactive materials to. But be careful! First, you can only transfer your radioactive materials to someone whose radioactive materials license will permit them to possess the source(s) you’re transferring to them – and you need to see a copy of the license before you ship the materials to them (whether the recipient is a waste disposal site or another licensed facility). And second, you have to be able to document the transfer – this is what the shipping papers and (if possible) a letter or email from the recipient confirming what they received from you.

Second, you’ll also need to show that you’ve cleaned up any and all contamination in the room so that it can be released for unrestricted use – this gets into the same ground as releasing equipment for unrestricted use. How to do the decontamination is beyond the scope of this posting – although soap and water or most commercial cleaning products will normally do the trick unless contamination has soaked into a porous surface or has chemically attached itself to whatever it is that you’re trying to clean up. But what we’ll go over here are the cleanup limits, regardless as to what it is that you’re cleaning up. Here, the fundamental document you’ll be referring to (unless your regulators have other requirements) is the Nuclear Regulatory Commission’s Regulatory Guide 1.86 (Termination of Operating Licenses for Nuclear Reactors). And the key part of this document is the table on Page 5. Here’s the table, along with a discussion of how to use it.

Nuclide (a) Average

(b, c)

Maximum (b, d) Removable (b, e)
DPM/100 cm2
Uranium (natural), U-235, U-238, and associated decay products 5000 (α) 15,000 (α) 1000 (α)
Transuranics, Ra-226, Ra-228, Th-230, Th-228, Pa-231, Ac-227, I-125, I-129 100 300 20
Thorium (natural), Th-232, Sr-90, Ra-223, Ra-224, U-232, I-126, I-131, I-133 1000 3000 200
Beta-gamma emitters (nuclides with decay modes other than alpha emission or spontaneous fission) other than Sr-90 and others noted above) 5000 15,000 1000
  • (a) Where surface contamination by both alpha- and beta-gamma-emitting nuclides exists, the limits established for alpha- and beta-gamma-emitting nuclides should apply independently
  • (b) As used in this table, dpm (disintegrations per minute) means the rate of emission by radioactive material as determined by correcting the counts per minute observed by an appropriate detector for background, efficiency, and geometric factors associated with the instrumentation
  • (c) Measurements of average contaminant should not be averaged over more than 1 square meter. For objects of less surface area, the average should be derived for each such object.
  • (d) The maximum contamination level applies to an area of not more than 100 cm2

  • (e) The amount of removable radioactive material per 100 cm2 of surface area should be determined by wiping that area with dry filter or soft absorbent paper, applying moderate pressure, and assessing the amount of radioactive material on the wipe with an appropriate instrument of known efficiency. When removable contamination on objects of less surface area is determined, the pertinent levels should be reduced proportionally and the entire surface should be wiped.

Here’s how to interpret Reg Guide 1.86:

  • Notice that there are two types of contamination – removable and fixed. And with the fixed contamination, there is both the maximum and average levels. Removable contamination (according to Note e) is contamination that will come off when you wipe the surface with a piece of dry filter paper; fixed contamination is whatever can’t be wiped off.
  • We look at contamination in terms of disintegrations per 100 square centimeters. So what we do is to survey (or wipe) an area of 100 square cm (about 4”x4”) to see how much contamination we is there.
  • What you’re looking for is the amount of contamination, but you have to remember that there’s background radiation also and you don’t want to find yourself cleaning up contamination that isn’t actually there. Before you start to survey (or count a wipe) you’ll want to turn on your meter when you’re a distance away from any contamination to see what it reads. With a 1”x1” sodium detector, for example, background is likely to be several hundred cpm. So if you’re reading, say, 1900 cpm with your detector and the background count rate is 500 cpm then the net counts (the amount of contamination) is 1900-500=1400 cpm.
  • Here, we have to convert from the counts per minute (cpm) shown on your instrument to disintegrations per minute (dpm) required by regulations. The way to do this is by knowing the counting efficiency of your meter for the radionuclide you’re surveying for. For example, say you’re surveying for Cs-137 with a 1”x1” sodium iodide detector. When you send it for calibration you can ask the calibration laboratory determine your counting efficiency, which will probably be in the neighborhood of 5-10%. A 10% counting efficiency means that, for every 10 gammas that are given off by the Cs-137, only one is counted. So to turn cpm into dpm, you have to divide the meter reading (cpm) by the counting efficiency. In other words, dpm = cpm/efficiency. So if you’re reading, say, 1400 cpm with your detector, you have 1400/10% = 1400/0.1 = 14,000 dpm.
  • OK – now that all of that’s out of the way, here’s how you use the table.
    1. There are different limits for a variety of nuclides. If you have, for example, I-131 then you’re only allowed to have 200 dpm/100 square cm of removable contamination, average contamination levels of up to 1000 dpm/100 square cm (averaged over 1 square meter), and no single location can be higher than 3000 dpm/100 square cm.
    2. The first three rows list the limits for a number of specific radionuclides. And if what you’ve got isn’t one of the ones that’s specifically listed then you go to the final row. This is the most commonly used set of cleanup standards and you’ll frequently hear health physicists talk about 1000 dpm/100 square cm or removable (or smearable) contamination.
    3. So what you have to do is to first know what radionuclide(s) you’ve got, then determine the appropriate cleanup limits for each. After this, you survey with a survey meter to measure total contamination over each square meter and, finally, take a number of smear wipes to see how much of this contamination is removable versus fixed. Finally, if you exceed these limits, you’ll need to decontaminate until the limits are met.

Third, there will need to be a formal closeout survey. This is where your regulators or (more likely) a contractor will come in to check to make sure that you’ve done everything properly, including your decontamination. This will include a contamination (and likely a radiation) survey to confirm your own readings. Once the survey confirms that your lab is cleared up (and cleared out) then you can go on to the next step.

Finally, we need to talk about documentation. I already mentioned the need to show what you did with your radioactive materials – the shipping papers and whatever transfer documents you might have. You’ll also need to keep copies of your own survey maps, showing the radiation and contamination levels you measured while you were closing out and cleaning up. Or if you have a consultant or contractor do this work, make sure that they document it properly. Incidentally, there’s also a requirement for you to keep records of any spills or areas of contamination with long-lived nuclides – your regulators might want to check these locations to make sure that you got them properly cleaned up.

When you’ve done all of your work, you’ll need to write to your regulators, requesting to terminate your license. You’ll have to include all of your documentation so that your regulators will know that you’ve properly disposed of your radioactive materials and cleaned up your lab, then they might want to pay you a visit to verify your work. When they’re satisfied that everything is OK they’ll go ahead and let you close out your lab and terminate your license.


Should I Be Concerned About Japan Dumping Tritium from the Fukushima Plant?

Dear Dr. Zoomie – What is it about the tritium they’ve got stored up at Fukushima? I heard this is something that they’re going to just dump into the ocean – is this something I should worry about? Do I have to give up on sushi for the next ten years? And where does the tritium come from, anyways?

There are a couple of questions here – let’s start with the most fundamental one first: where does tritium come from.

Tritium is a radioactive form of hydrogen. Normal hydrogen is the most common element in the universe – over 70% of the atoms in the universe are hydrogen atoms. This hydrogen is mostly the simplest atom of all – a single proton mated with a single electron. But elements come in different “flavors” – you can add a neutron to an atom of any element without changing it into a different element. So we can add a neutron to hydrogen to form a new atom – one with a proton, an electron, and a neutron – that’s a heavier form of hydrogen that’s called deuterium.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

The Fukushima I Nuclear Power Plant after the 2011 Tōhoku earthquake and tsunami. Reactor 1 to 4 from right to left.

You can take this a step further and add yet another neutron to our deuterium atom to produce something that still has a single proton and electron, but that now has two neutrons – this is called tritium, and tritium is also slightly radioactive. Of course, to do this we have to find a source of neutrons – in a nuclear reactor those are produced in copious quantities by the nuclear fission that’s taking place. So this is where the tritium comes from – hydrogen atoms in the water absorb neutrons from nuclear fission to form radioactive tritium in the reactor coolant.

Tritium is also produced in the atmosphere, by the way, in a natural process that’s been happening since the Earth first formed. When cosmic radiation strikes atoms in the atmosphere it can break up some of these atoms; some of the time these fragments will include tritium atoms. This has been measured for decades and the rate of tritium formation is well-known. In fact, at any time, there’s just a tad less than 1019 Bq of tritium from natural sources in the environment. Oh – the Bq (Becquerel) is a measure of radioactivity (one Bq of activity will give you 1 radioactive decay every second). We’ll get back to this shortly.

Tritium atoms are still atoms of hydrogen and they behave the same as any hydrogen atom, so the tritium remains part of the water molecules. From a radiological standpoint, tritium has a half-life of about 12.27 years, which means it doesn’t decay away very quickly at all and will be with us for awhile. On the other hand, it only gives off a low-energy beta particle – so low-energy that it’s almost impossible to detect without specialized radiation equipment. This means that tritium is hard to detect – it also means that it’s not very dangerous (more on this shortly).

So – reactor coolant is water that circulates through the reactor where it’s exposed to neutrons. Some of these neutrons are absorbed by hydrogen – and then by deuterium – atoms to form tritium. This means that the water that comes out of an operating nuclear reactor is going to contain tritium – it also means that tritium formation stops when the reactor shuts down since the neutrons are produced from fission, which stops when the reactor shuts down. Thus, the only tritium present is whatever was in the reactor coolant of the three operating reactors at the time the reactors shut down. As we all know, there have been leaks of this coolant into the sea and into the local groundwater which means that tritium has been entering the environment in both of these places. The tritium that’s already entered the ocean can’t be recovered; the tritium in the groundwater is another story – the Japanese have been recovering contaminated groundwater for a few years, including the tritium it contains, and they have to do something with it.

It’s important to remember that tritium is chemically identical to regular hydrogen. This means that tritiated water can’t be chemically processed to remove the tritium. And, since the tritium is part of the water, it also means that the tritium can’t be filtered out. In fact, there’s really only one effective way to remove tritium from the normal hydrogen in water – distillation. Since tritium is heavier than normal hydrogen (because of those two extra neutrons), tritiated water is about 10% heavier than regular water; this means that it evaporates a little more slowly, so repeated distillations will gradually strip off the lighter water, concentrating the tritiated water in what remains. The problem is that this process is slow and expensive – it doesn’t always make sense to go this route.

OK – so the Japanese have tons of water that holds – according to the news article you referenced – about 3.4×1015 Bq (over 90,000 curies) of tritium. This is a lot of tritium to have in one place – but it’s not even close to the nearly 1019 Bq of tritium that’s already in the oceans from natural sources. In fact, there’s about 3000 times as much tritium in the environment from natural sources as there is contained in all of the water tanks at the Fukushima site. This is important for a couple of reasons – first, because it means that tritium is a part of the natural environment and, second, because it means that the tritium held at the Fukushima site is (almost literally) a drop in the bucket compared to natural tritium. So it seems reasonable to conclude that, even if this water is dumped into the ocean, it won’t be enough to cause massive health or environmental problems.

The problem is a little more complex than that of course. For example, natural tritium is distributed throughout all of the oceans, the atmosphere, and even within the bodies of every organism on Earth. On the other hand, the tritium from the Fukushima water will be discharged into a small part of the Pacific Ocean – it’s likely that the local concentrations will be higher than natural tritium concentrations. This is true – but we have to remember that it’s a BIG ocean and the tritium will diffuse into it and will mix fairly quickly. And the organisms that are exposed to it are going to change out the tritium the same as we do (more on that in a moment) – over the space of a few weeks or (at most) months the tritium will pass out of their bodies.

When tritium is in the body it pretty much circulates with all the other hydrogen – it sticks to the body’s fluids. This means that, instead of targeting a specific organ (iodine, for example, concentrates in the thyroid gland) tritium affects the whole body. This sounds bad but, in fact, it means that the entire body gets a low dose of radiation rather than one particular organ receiving a high radiation dose. The other factor is that we are constantly changing out the water in our bodies as we drink, breathe, sweat, and urinate – this means that the tritium in the body tends to leave fairly quickly; and the more we drink, the faster it leaves. So if you do have an uptake of tritium, the best thing to do is to drink plenty of fluids so you urinate out the tritium in your body and to dilute what remains. Incidentally, there’s an urban legend that the best way to do this is to drink a lot of beer…I found out recently that this originated with an offhand quip made by one of my colleagues. When I asked him about it he acknowledged inspiring the comment and said it can’t hurt – but pretty much any liquid intake will have the same effect. If you like tea, drink tea; if you like water, drink water; if you like beer, drink beer. The important thing is to drink (and urinate) – what you drink is almost incidental.

So – let’s recap a little bit.

First, tritium is a form of hydrogen that is chemically identical to any other form of hydrogen, it just has a few extra neutrons added to it.

Second, tritium is produced in nuclear reactors, and also in nature. Our planet always has tritium in the environment – the tritium in the Fukushima water is about 0.03% of the tritium found in nature.

Third, if the Japanese government does dump all of this into the environment it will probably increase tritium concentrations in the water – and probably in the organisms living in that water. But this will pass as the water mixes in with the ocean water and it will almost certainly have no long-lasting environmental impact.

Finally, tritiated water mixes in with all of our body fluids and it passes out of our bodies in the same way as the water we might have drunk after working out. To speed up this normal changeover, simply drink more fluids.

Terrorists in Belgium – What Can Terrorists Do to a Nuclear Reactor?

Plant Doel; seen from the north. Doel, Beveren, East Flanders, Belgium

Plant Doel; seen from the north. Doel, Beveren, East Flanders, Belgium

Dear Dr. Zoomie – I read in the news that those terrorists in Belgium were trying to get into a nuclear reactor. What can terrorists do to a nuclear reactor? Can they melt it down? Or make a dirty bomb? How worried should we be?

This is a fairly complicated question and there are a few things to think about:

  • Can terrorists gain access to a nuclear reactor?
  • Can they cause a meltdown or a release of radioactivity to the environment?
  • Can they gain access to radioactive materials to make a dirty bomb?

Let’s take these one at a time.

First – can terrorists gain access to a nuclear reactor? Well – the best answer to this is maybe. Nuclear reactors are guarded by some pretty well-trained security forces. In addition, every nuclear reactor is located within some law enforcement jurisdiction – a sheriff’s office, police department, state police, and so forth – in addition to whatever federal assets (FBI, military, etc.) might be available. A terrorist group can certainly attack a nuclear reactor plant – either head-on or by cutting through the security fence – but there will be a fight that will slow them down. I have to admit that I’m not a security expert (and the exact security arrangements are classified) so I can’t predict which side would prevail. But even if the reactor security staff are overcome, the other assets will be on their way as soon as they’re notified – even just delaying the terrorists a bit makes it more likely that law enforcement or other forces can arrive in time to lend a hand.

Let’s assume that a terrorist group is able to get past the security force and/or the security perimeter – the next question is whether or not they can gain access to the reactor control room or to areas where radioactive materials are stored.

Both of these are certainly possible but, unless they know exactly how the power station is laid out, it could take them awhile to figure out where to head. Most of these sites are fairly large and they have a dozen or more buildings, some of which are fairly large. Eventually it might be possible for a terrorist group to gain access to the control room or to radioactive materials storage areas, but it will likely take some time.

Second – can terrorists cause a reactor meltdown? The answer to this one is “probably not,” and almost certainly not without some sort of insider help. First, every reactor plant has a number of automatic safety systems that are designed to keep it from melting down. It’s not necessarily hard to make a reactor plant shut down – they’re designed to do so automatically in order to keep the reactor safe – but it IS hard to damage the plant; causing damage requires not only a fairly detailed understanding of how nuclear reactors work, but also some very detailed knowledge of how a particular reactor plant works. For example, a terrorist would have to know the reactor plant well enough to know how to override or thwart all of the various safety systems because it only takes a single operating one to keep a meltdown from occurring. And to put this in perspective, I spent 8 years operating Naval nuclear reactors and I’d be hard-pressed to cause a meltdown in one of the reactors that I actually operated – I can’t picture walking into the control room of an unknown reactor plant and being able to do much of anything, let alone finding the right controls to operate in the correct sequence to override a dozen or so safety features to cause a meltdown. I’m not saying it’s impossible – but it certainly isn’t a simple matter.

And forget entering the reactor compartment itself and, say, setting explosive charges to damage the reactor directly – there are locked doors, entry controls, and other security features to defeat, not to mention knowing enough to understand exactly where to set the charges to cause damage that would lead to a reactor meltdown. The reactor compartment itself will be have very high radiation levels, but not high enough to really have an impact – the biggest obstacle would be not knowing which components are vitally important. Even someone with schematics of the reactor compartment might not be able to figure out what to do – I can tell you from personal experience that there’s a huge difference between looking at a schematic and the actual welter of pipes and cables, just as there’s a huge difference between looking at a schematic of your car’s engine and the engine itself. Again – I’m not saying that it’s impossible, but it’s highly unlikely.

What about a release of radioactivity to the environment? OK – so let’s say that a terrorist group manages to cause a meltdown. Can they turn this into a Chernobyl- or Fukushima-style environmental disaster? Believe it or not, this is also more difficult than it seems, mainly because the reactor plant is surrounded by a robust containment (that’s the dome or cylinder that we associate with reactor plants). This is a shell of reinforced concrete that’s about three to six feet thick and even the reactor melts down, the radioactivity has to find a way into the environment. This means finding a way to breach the containment. There are openings in the containment as well as pipes, drains, ducts, and so forth to let air and fluids in and out. But all of these openings are designed to slam shut (and stay shut) in the event of any emergency. The whole plant is designed to keep radioactivity from reaching the environment and it does it quite well. At Three Mile Island, for example, the reactor melted down but the containment worked as designed. Some radioactivity was released – but it was released deliberately when the operators vented hydrogen (and some of the gaseous radionuclides) to the environment to prevent a hydrogen explosion. And as we all know, the Fukushima accident released a lot of radioactivity, but the containment here was damaged by the earthquake as well as by the hydrogen explosions. As above – it’s not impossible to conceive of a terrorist group first causing a meltdown and then finding a way to cause an environmental release – but it’s very unlikely.

OK – so what about the last concern? Can terrorists get their hands on the makings of a dirty bomb? Here, the odds might be a bit higher since making a dirty bomb only requires radioactive material, and nuclear reactor sites are full of these – there’s radioactive waste, the reactor itself, the spent fuel, and maybe some radioactive sources as well. Let’s think about each of these.

The greatest amount of radioactivity is in the reactor core, but this is pretty much impossible to remove since it’s locked up inside the reactor. So we can rule this out. Spent fuel is also pretty hard to make off with – the spent fuel rods are fairly long and unwieldy (a few tens of feet in length) and they’re stored in either a huge spent fuel pool or they’ll be inside a spent fuel casks that are at least ten feet tall, several feet in diameter, and weigh many tons. Either way, it’s not plausible that a terrorist group is going to make off with spent reactor fuel. And even if they do, they’d still have to find a way to break into the cask (or the fuel rods) to release the radioactivity – another difficult task.

There is a bunch of radioactive waste at a reactor site – mostly low-level stuff like contaminated paper towels, gloves worn by radiation workers, and so forth. This low-level stuff has to be controlled as radioactive materials, but it’s hardly the sort of thing to inspire fear – I know, because I ran a radioactive waste program at a few universities in addition to keeping track of the waste on the submarine I was assigned to. There are some higher-level wastes – reactor components removed for repair or replacement, the filters used to purify the water, and so forth. But high-level waste have to be kept in secure locations – at the very least, finding and getting at them will cause further delay. They also have to be kept heavily shielded for safety purposes – this makes them hard to sneak off with or to breach…even more delay for potential thieves.

Finally, every reactor plant will have radioactive sources that it uses for a number of purposes. Most of these sources are going to be fairly low-activity but a few will be stronger – these might be able to cause a fair amount of contamination if strapped to an explosive. These will also be secured according to regulation and the highest-activity ones will also be shielded. Again – none of this will prevent a theft, but it all adds to the difficulty of making off with these sources and using them for nefarious purposes.


So let’s put this all together. First, a terrorist group can always attack a nuclear reactor plant but they’ll be going up against a trained security force as well as the local, state, and maybe even federal response that’s sure to follow. If they manage to get into the reactor site, they’ve got to figure out which building contains their target, not to mention figuring out which room (or rooms) they need to enter. If they’re trying to cause a meltdown they have to understand the reactor and its controls well enough to override all of the safety systems that are designed to prevent exactly that. And if they’re trying to cause a release of radioactivity to the environment, they have to go even further and create a hole of some sort in a thick shell of reinforced concrete. Finally, if they’re after stealing radioactive materials to make a dirty bomb, they have to find some of the high-activity radioactive waste or high-activity sources – and they then have to spirit them away.

Is all of this possible? Well…it’s not impossible, but it’s pretty unlikely unless there’s an insider involved. As I mentioned earlier – I’m not sure I could do it, and I think it’s safe to say that my level of knowledge is higher than that of most.


Radiation Safety Training for Graduate Students (A Little Comic Relief)


I’m your neighborhood University Radiation Safety Officer.  My staff and I have three primary goals: don’t let anyone get hurt, don’t let anyone break any laws, and provide as much support as we can to the people who bring in grant money.  Unless, of course, they are trying to do things that are dangerous or illegal.  Professionally, I’m a health physicist.  Health physics is the profession that works with radiation safety.  It started off as a code term during World War II, to keep our enemies from knowing that we were doing research into radiation biology.  Its success is best measured by the fact that, after nearly 60 years, virtually nobody has yet figured out what it means.

Radiation may well be the second most innocuous thing in your laboratories, just ahead of your telephone.  It won’t cut you, electrocute you, fall on your foot, burn you, poison you, infect you, or do many of the other things that your normal lab equipment and reagents can do.  It’s less dangerous than your drive to work, and is easier to detect than most of your chemicals.  You won’t receive enough radiation at work to make you ill no matter which hypothesis you believe in to describe our response to radiation exposure, and if you have funny-looking children it’s probably the fault of your in-laws and not the radiation in your lab.  If you really want to reduce your risks, consider giving up eating pickles.  It’s a fact that everyone who ate a pickle in 1838 is now dead, strongly suggesting that pickles are deadly and should be avoided at all costs.

We are all exposed to radiation from background sources daily.  Exposure to radon is the most significant, primarily because we prefer to work and sleep indoors instead of under the stars.  Of course, some believe that this condemns us to an early death from lung cancer, but sheltering ourselves from the elements may have some positive aspects, too, and you should consider continuing this habit if at all possible.  We can’t do much about our exposure to internal radionuclides because potassium deficiency has been shown to have adverse health effects that slightly outweigh the potential ill effects from exposure to K-40 radiation.  Other sources of natural radiation are radionuclides in rocks and soil and cosmic radiation.  If these concern you, you may wish to consider moving to a coral island near the equator, if you can find grant support to do so.  Otherwise, you may be relieved to note that background radiation is something that’s been with us for billions of years, and it’s now thought possible that we have adapted to live with it quite safely.

Moving to the realm of working safely with radiation and radioactivity, there are a few principles you should keep in mind.  First, you should take reasonable precautions to reduce your radiation exposure.  The three tenants of health physicists are time, distance, and shielding.  You want to reduce the amount of time you spend in a radiation field, but don’t rush your experiments to the point that you make mistakes or cause spills.  You should work at the greatest distance possible from your stock vials, but working at distances greater than arms’ length has been shown to negatively affect data quality.  You should also try to interpose shielding between yourself and the radioactive materials you are working with.  This can be plastic, leaded glass, or slender co-workers.

In addition to these factors, we require you to wear lab coats and gloves to minimize the potential for contaminating yourself or your clothing.  An added precaution is that you should minimize the amount of exposed skin that you can spill or splash onto.  While you need not wear a wetsuit, you should not wear skirts, shorts, kilts, flip-flops, or similar items of clothing while doing “hot” work.  Needless to say, “Naked co-ed gene sequencing” is frowned upon.  You are also advised that eating, drinking, smoking, chewing tobacco, applying cosmetics, using dental floss, watching cooking shows, storing food, and having watermelon seed-spitting contests in posted laboratories is prohibited.

In spite of your best efforts, it’s likely that, at some point, you’ll end up causing a radioactive spill or contaminating yourself.  If that happens, don’t panic, and take some immediate actions to try to make the situation better.

In case of a spill, you need to let other people know there’s a problem and keep it from getting worse.  Although you might be embarrassed at your little faux pas, you don’t want to have people unwittingly walking through the spill.  So you should let other people know that you have caused a spill and where it is.  Also, don’t be shy about contacting Radiation Safety.  We’ll help out if we need to, and we promise not to make you feel like an idiot for having done something that we’ve all done, too.  After hours, call Security and they’ll page me.  I am sometimes a bit gruff when I’m under-caffeinated, but I’ll apologize for any unkind things I might have said after my second cup of coffee.  Nobody should be held responsible for anything they say within ten minutes of waking up.  If you don’t call, I’ll probably say worse things and I won’t apologize afterwards.

The next thing you want to do is to try to isolate the spill.  You have probably noticed that this is a university and that many academics are not fully aware of their surroundings at all times.  This means that you should make an effort to keep people out of the spill area by putting up physical barriers that will at least cause people to slow down a little bit before they wander into your spill area.  Your spill kit should include a dart gun – anyone trying to enter the spill area without wearing a lab coat, gloves, and shoe covers should be darted and the bodies can be stacked out of the way.

If you manage to contaminate yourself, don’t panic.  Give us a call to let us know, and try to dial the phone with an uncontaminated hand so we don’t have to take your phone, too.  Then, you should try to decontaminate yourself using mild soap and warm to cool water.  Don’t use a wire brush, cleanser, scrub pads, steel wool, lye, needle guns or any other substances that might damage the skin.  I have only seen a few cases in which this didn’t work.  In one case, we simply taped a plastic bag over the person’s hand and the activity was sweated out in less than a day.  We typically don’t recommend this approach for facial contamination, but we are willing to consider all options to help out some of our more valued researchers.

That’s about all I have for this class.  I’d like to thank you for showing up and staying awake, because I know that most people would rather have a root canal without anesthesia than attend yet another mandatory training program.  Go out, have fun, and call us if you have any questions or problems.

How Was Alexander Litvinenko Killed?

Hello Dr. Zoomie! There was something in the news again about a Russian guy, Alexander Litvinenko and how he was killed with radiation. I remember something about this from about ten years ago and didn’t quite understand what was happening. Can you tell me what’s going on? And was there any risk from this to anyone else?

Good question – and an interesting story! The short version is that Alexander Litvinenko was a Russian who was killed after drinking tea laced with polonium (specifically, Po-210). Extensive radiation surveys by the British found a fairly extensive trail of contamination around the city, and even on a number of commercial aircraft. But in spite of extensive contamination, nobody except for Litvinenko was harmed by the radioactivity. OK – that’s the short version; now let’s dig a little deeper.

First – Litvinenko was indeed given some polonium in his tea. Polonium-210 is a naturally-occurring radionuclide that comes from the decay of natural uranium (U-238) in the rocks and soils. Po-210 can also be manufactured in nuclear reactors, and this is where the material used to kill Litvinenko probably came from. This particular radionuclide emits only alpha radiation – alphas are highly damaging to living cells, but are too weak to penetrate the dead layer of skin cells that comprise our epidermis so as long as they stay on the outside of our bodies they can’t hurt us. But once inside, it doesn’t take much for an alpha-emitter to be dangerous – the amount of radioactivity administered to Litvinenkowas less than the weight of a single grain of salt, but it was enough to be fatal.

Once administered, the radioactivity has to get into the blood – in Litvinenko’s case it was absorbed from the digestive tract. In actuality, only about 10% of the polonium was absorbed and the rest passed out of his body, so that grain of salt amount of Po-210 was ten times as much as was needed to kill him. After he started feeling ill, Litvinenko went to the hospital, where they initially suspected radiation but then went on to look at other types of toxins. In fact, Litvinenko was hospitalized for a few weeks before the doctors realized he was poisoned with radioactivity; by that time it was too late to help him – we just don’t know how to keep people alive when they’ve received a high enough dose of radiation.

According to research performed by the Russians in the 1960s, polonium in the body tends to concentrate in the hair follicles and to come out in perspiration when we sweat. It’s also very mobile – in the laboratory polonium was known to move around readily, spreading contamination all over the place. This means that, as Litvinenko moved around London after he was poisoned and before he was admitted to the hospital, he was shedding contaminated hairs and perspiration and, once on the ground, the polonium was able to spread from the hairs to the ground. So when British radiation specialists started performing surveys, they were able to find traces of polonium wherever Litvinenko had traveled. Not only that, but they also found polonium on a number of commercial aircraft – presumably contamination from whoever it was that brought the polonium from Russia to London.

With all that contamination, there was a lot of worry about the effects of the contamination on the health of Londoners or travelers flying on the jets that were contaminated. Luckily, there simply wasn’t enough polonium to cause problems – in spite of the low levels needed to cause harm. The reason is that we are very good at detecting very low levels of radioactivity – the fact that scientists are able to detect radioactivity doesn’t necessarily mean that it’s present at harmful levels. For example, you might be able to see a single speck of dust on a table – but just because you can see it doesn’t mean that it’s harmful to you. One radiation scientist was asked by a reporter if simply flying on contaminated airplanes posed a health risk to passengers – he replied “Not unless you lick the seats,” which not only sums up the radiological hazards, but is probably good advice on any plane! When all was said and done, not only was polonium found in a number of locations in London, but over 50 other nations were involved as well – traces of polonium were found in some, and others had citizens who were exposed to the polonium in London and who had to be checked when they returned home.

One really important thing to mention here is the effect of all of this on the hospital staff. Remember – Litvinenko was in the hospital for a few weeks before anybody knew that he had radioactivity in his body, let alone that he was shedding it with every hair that fell out of his head or from his body. In spite of that, not a single hospital worker had a significant uptake of polonium when they were tested – at the very least, this is a testament to the typical precautions taken by hospital workers to keep themselves safe.

Interestingly, when Litvinenko first checked into the hospital blood samples were sent off to be analyzed for radioactivity. But they were checked for gamma radiation, and Po-210 is very nearly a pure alpha emitter – because the detector being used was only sensitive to gamma radiation, the polonium contamination was missed. It wasn’t until several weeks later that the samples were re-analyzed for alpha contamination that the polonium was recognized.

There’s a lot more to this story, but to go into more detail would take much more space than we have here. But out of all of this there are a few interesting points that are worth mentioning:

  • Alpha radioactivity is not only dangerous in very small quantities, but is also hard to detect without the correct instrument,
  • Radiation poisoning is not always easy for doctors to recognize,
  • Routine medical precautions offer good protection from low levels of radioactive contamination,
  • We can detect radioactivity at levels that are much lower than what can cause harm.

To the best of my knowledge, Litvinenko is the only person to be assassinated this way, although there are those who believe that Yasser Arafat might also have been killed in the same manner (although this is more conjecture than fact at this time). As far as we know, this was a unique event – a tragedy for Litvinenko and his family – that will hopefully not be repeated.