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:

Ci/gram

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?

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.

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.

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

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 they cause a meltdown or a release of radioactivity to the environment?

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.

Conclusions

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.

TRANSCRIPT:

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.

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.

Dose Reconstruction – What Happens if a Rad Workers Loses A Dosimeter?

Dear Dr. Zoomie – one of my rad workers lost his dosimeter. He normally gets some radiation exposure from working with our equipment – some gives off x-rays and some uses radioactive sources. What should I do?

From the information that you’ve provided I think this is something that has to be taken care of sooner rather than later – having said that I’m guessing that this is important rather than urgent. In other words, you have to make sure you meet regulatory requirements, but the worker’s health is probably not at risk (at least, based on the fact that you said the worker normally gets exposure, but you didn’t say that he normally is close to dose limits – and there’s a huge gap between reaching a dose limit and facing potential health effects). So – if my assumptions are correct – what you need to do is to come up with a reasonable dose estimate that you can provide to your dosimetry vendor so you can ask them to assign that dose to the worker in his dosimetry record. Here’s how you can go performing a dose estimate. And remember – make sure you document everything!

What I’m going to do is to lay out a bunch of techniques you can use – in some cases, a single one might do the trick; in other cases you might have to try several or even all of these.

One thing you can do is to look at the worker’s past dosimetry reports to see what level of exposure he’s received in the past. If, for example, his exposure has typically been between 50-100 mrem monthly AND if his workload for the month with missing dosimetry was typical then you can make a safe guess that, during the month in question, he probably received no less than 50 mrem and no more than 100 mrem. In this case, as the RSO, I could justify assigning a dose of 100 mrem to the worker for the month. But it would be nice (if possible) to come up with a second estimate to justify the first. So what else can you do?

You didn’t say anything about co-workers, but if you have more than just this one person who does this sort of work, you can check to see what dose his colleagues received during the moth with missing dosimetry. If their doses are normally comparable to the dose of the man with the missing dosimetry then you can see how much exposure they received and use that as the basis for a dose assignment. Say you have three other rad workers who received 50, 80, and 60 mrem for the month in question – being conservative, you can assign a dose of 80 mrem to the worker.

You have yet another option – and this one will require a little more work, but it’s likely to be most accurate. First, take a look at what the worker did during the month in question. For example, if he’s a radiographer you’re going to have to see how many jobs he went out on, which source he used for each job, and how many shots each job entailed. Or if he’s a nuclear medicine technologist, you need to see how many patients he saw and how much dose each received. Once you’ve figured out what the worker did you can start making radiation measurements. For example, if the worker did three radiography jobs with a 50 Ci source of Co-60, you can set up a mock radiography shot that mimics the jobsites he would have worked (in a safe location). Then you can perform a dummy shot in which you run the source out and retract it while you measure the radiation exposure – this will tell you how much dose your worker received during a single shot; this can be used to calculate his total dose for ALL of the short he performed during that month. For example, if you measure a dose of 5 mrem for one shot and he performed a total of 15 shots during the month you can assign a dose of 75 mrem for his radiography duties for that month. Alternately, you can perform this same measurement on a real shot and use the results to make the same calculation.

There’s another way to accomplish this, say, for someone who works in radiation areas or around radioactive materials. In this case, you need to try to estimate how much time the worker spent in each radiation area and then go to these areas yourself to measure the radiation levels. Say (for example) he spent a half hour performing sealed source leak tests – you’d need to go to your source storage and measure radiation levels. If you measure 20 mR/hr then you multiply this by a half hour and determine that his source leak tests gave him a dose of 10 mrem. Repeat this for every other task he performed and add these numbers together – this is the worker’s assigned dose for that month.

There’s one related topic I’d like to mention before signing off – what to do if the dosimetry report comes back showing a dose that seems way too high. Say a worker’s badge reads 45 rem (45,000 mrem) one month. This is a high dose – not dangerously high, but far higher than the worker’s annual dose limit. Unless the worker was responding to a radiological emergency there’s no acceptable reason to have so high a dose – you need to try to figure out if the dose is real. With some dosimeters, you can ask for an assessment to see if the badge was attached to the worker during the exposure (this is called “static/dynamic imaging” by one vendor). But this should be verified by a dose reconstruction – you should do everything described above: compare his dose with previous months, compare his dose with co-workers, look at his work schedule, AND make dose measurements in all the areas where he worked (and under the same conditions under which he was exposed); maybe consider calling in a consultant as well.

But with a dose this high you have an option that a lower dose doesn’t give you – you can send a blood sample off for biodosimetry; most likely a procedure called chromosome aberration analysis. This looks at the chromosomes to see if they’ve been damaged by the radiation; if so, the amount of damage can be used to estimate the dose. The body is the ultimate arbiter of dose – no matter how high a dose the dosimeter shows, if the body shows it received a small dose then the dosimeter must be wrong.

OK – having said all of this, I have to acknowledge that you can get even deeper into dose reconstruction than this, but if what I’ve described above doesn’t solve your problem then you really need to bring in a consultant to help you out. In addition, if your worker had any sort of an uptake (inhalation or ingestion) then you should really consider bringing a consultant in as soon as possible. But barring one of these possibilities, this should stand you in good stead. Good luck!

North Korea – What Did They Detonate?

Dear Dr. Zoomie – I just heard that North Korea claims to have developed a hydrogen bomb, but our experts say it’s probably just a regular fission bomb – or maybe a boosted device. This is all Greek to me – what’s the difference between these?

Good question! And I know the terminology can be a bit confusing, so let me see if I can help shed some light. But first, some basics.

First, where the energy comes from. Conventional explosives get their energy from breaking chemical bonds – breaking or rearranging chemical bonds releases a few electron volts each (the electron volt is a unit of energy that makes sense on an atomic or molecular level). By comparison, nuclear reactions involve rearranging the nuclear structure of an atom and nuclear reactions release millions of electron volts. So a single nuclear reaction releases as much energy as at least a million chemical reactions.

Next – where the energy comes from in a nuclear reaction. Some atoms are so big that they barely hold themselves together; hit them with a neutron and they’ll split apart, releasing all that energy. They also release additional neutrons, and if those neutrons are absorbed (and cause fissions in) additional nuclei then the reaction will grow, as will the energy release. And since all of this happens in the merest fraction of a second (timescales are on the order of nanoseconds), the power output grows…well…explosively. Fission weapons (using uranium-235 or plutonium-239) make use of this process exclusively. But fission weapons can be horribly inefficient – it’s not uncommon for over 90% of the fissionable material to be blown apart before it can be fissioned. We’ll get back to that in a moment.

Another way of producing nuclear energy is from slamming light atoms together hard enough that they stick, forming a larger atom. This is how the sun makes energy – hydrogen atoms stick together to form helium; three helium atoms can fuse to form carbon, and so forth. But fusion can only happen under extraordinary conditions – specifically the conditions that we see in the center of the sun. In a weapon, these conditions are generated using a fission explosion – a fission bomb goes off, igniting the fusion reaction. Now the question becomes how much fusion takes place and how much energy does it produces.

Most importantly, this fusion also generates neutrons, and these neutrons are vitally important in a boosted weapon. In a boosted weapon, enough fusion fuel is put in the center of the bomb to produce copious numbers of neutrons, but not enough to produce a lot of energy. But these neutrons are crucial because they can be captured by some of that 90% of the fuel that normally is untouched – if you can simply double the number of U-235 or Pu-239 atoms that fission you’ll double the weapon’s yield. So this is a boosted weapon – a “typical” fission bomb with a smidgeon of fusion fuel in the center – but the fusion fuel is there to produce neutrons. You can think of this as the nuclear equivalent of blowing on a fire – you’re not directly adding significant energy to the fire, but you’re helping the existing fuel to burn more efficiently.

Of course, if you put more fusion fuel (this can be a mixture of hydrogen isotopes deuterium and tritium, often combined with lithium to form lithium deuteride or lithium tritide) then you get more energy from fusion – at some point the fusion is not only producing a ton of neutrons, but a significant amount of energy as well. This is where we transition from a boosted fission weapon to an out-and-out thermonuclear (or hydrogen) bomb. And this, too, is where the weapons designers have to decide what to do with all of the fusion neutrons – they can use them to cause still more fission, to produce a great deal of radioactivity (for example, adding stable cobalt to the weapon can produce radioactive cobalt-60), or they can let them escape to make a “neutron bomb” that produces high levels of radiation while sparing the infrastructure. Since fusion doesn’t result in radioactive fission products it’s considered to be fairly clean – especially compared to fission weapons.

Ivy Mike was the codename given to the first test of a full-scale thermonuclear device, in which part of the explosive yield comes from nuclear fusion. It was detonated on November 1, 1952 by the United States on Enewetak, an atoll in the Pacific Ocean, as part of Operation Ivy. The device was the first full test of the Teller-Ulam design, a staged fusion bomb, and was the first successful test of a hydrogen bomb.

Finally, there’s one more point I’d like to address – North Korea’s claims that their weapons have been miniaturized. First, this is potentially important because unless a device can be delivered, it can’t really be considered to be a weapon. The smaller (physically) a weapon is, the more easily it can be used. And to be used on a missile, where every cubic inch and every ounce matters, the smaller a weapon can be made, the better. The problem is that it’s not easy to make a compact nuclear weapon – physics itself places some constraints (you have to have a critical mass of fuel, in addition to the explosives to set it off plus the electronics plus the casing and so forth. You can trim a lot of this somewhat – for example, a boosted weapon will require less uranium to achieve the same yield – but there are limits. It takes a lot of science and engineering to press up against these limits, as well as a lot of testing to make sure that, when you’re pushing the limits of the science, that your weapon will actually work the way you intend. Think of electronics – it’s easy to make something that’s large, but making something tiny can be hard. So North Korea’s claims to have developed miniaturized nuclear weapons is potentially alarming – but also somewhat dubious.

There’s a LOT more discussion we could have here, but to go beyond this level would take up the better part of a book (rather than a blog posting). But if you’re interested in how nuclear weapons work (at an unclassified level) you can go online to the Nuclear Weapons Archive; if you’re interested in the effects, try reading The Effects of Nuclear Weapons – these are all unclassified documents. In addition, Richard Rhodes’ books The Making of the Atomic Bomb and Dark Sun: The Making of the Hydrogen Bomb are outstanding histories of these two projects. In addition, the magazine The Progressive published a piece titled The H-Bomb Secret in their November, 1979 issue; this can still be downloaded at no charge.

How Do I Protect Myself from Background Radiation?

Dear Dr. Zoomie – I just heard that we’re always exposed to radiation and it sort of scares me. And on top of that, there’s the radiation from Fukushima. Can I just line my walls with lead? And how much lead do I need to cut down this radiation to something safe?

Well…I think you might be overly concerned here. First, you’ve got to remember that radiation from natural sources has been a part of life on Earth since it first formed – it’s safe to say that every single organism that has ever lived on Earth has been exposed to radiation. Not only that, but there’s ample evidence that these natural background radiation levels today are the lowest they have ever been. This means that today we are being exposed to less radiation (from natural sources) than were our distant ancestors.

It’s also important to realize that natural radiation varies considerably from place to place on Earth. Some places – the American Gulf Coast, for example, most of Hawaii, Japan, and so forth – have unusually low levels of natural radiation (more on this in a moment) while other places (Ramsar Iran, the American Colorado Plateau, Guapari Brazil, and others) have elevated levels of natural radiation. Interestingly, cancer rates don’t seem to be at all correlated with natural radiation levels. For example, the parts of the US with the highest natural radiation levels also have among the lowest cancer rates in the country. Some people have actually used this fact to suggest that low levels of radiation might make us healthier – I’m not sure I can agree with that, though, because there are so many other things that can affect cancer rates. For example, the population along the Gulf Coast is generally older than in the Rocky Mountains, there are more people who smoke, and the area has a significant petrochemical industry – all of these could serve to increase cancer rates compared to the Rocky Mountain states. The most we can say is that the effects of these different radiation levels is fairly low – low enough to be swamped by the other factors.

Did you know Bananas contain naturally occurring radioactive material in the form of Potassium-40?

As far as where this natural radiation comes from, there are four main sources, and these all change from place to place. They are:

• Radon in our homes and buildings. Radon comes from the decay of uranium in the rocks and soils. Radon is a gas; once it forms it will seep through the soil and into our homes – since it’s heavier than air it collects in basements, subway tunnels, caves, and so forth. Places with higher levels of uranium and thorium in the rocks and soils will have higher concentrations of radon in the air. On the other hand, since radon is such a heavy gas it’s not a concern above the first floor of any building. We receive about 150-250 mrem annually from radon, depending on the local geology and the amount of time we spend in areas where the radon might collect.
• Radiation from our own bodies. As mentioned earlier, a tiny fraction of potassium is naturally radioactive, including the potassium in our own bodies. In addition, we’re always exposed to radioactive carbon (carbon-14) and hydrogen (H-3, also called tritium) that’s formed in the atmosphere from interactions with cosmic rays. Finally, we’re always ingesting or inhaling trace amounts of dust that can contain uranium or thorium. All told, we receive about 40 mrem each year from all of these sources of radiation in our own bodies.
• Cosmic radiation from the sun and stars. The sun gives off high-energy particles as the solar wind. In addition, distant stars give off their own stellar winds and some of those particles escape into space as well – some of these stars, by the way, are far more active and energetic than our sun and the particles they give off have far more energy than what our own sun emits. As if that weren’t enough, every few decades a massive star explodes in our galaxy in one of the most energetic events in the universe – a supernova. The particles from these explosions also permeate space. When any of these particles – whether from our sun or from elsewhere in the galaxy – slam into our atmosphere they can generate what are called cosmic ray air showers; cascades of particles and gamma rays that percolate down to the ground to expose us to radiation. In general we receive from 20-40 mrem each year from cosmic radiation.

Putting all of this together, we receive about 250-300 mrem each year from natural radiation, with some variability depending on where we live and what we eat. And remember – first, there’s no way to get away from this and second, we’ve been exposed to this radiation for as long as there has been life on Earth.

Another mistake that people make is taking too small a sample and counting it for too short a period of time. If you have elevated readings after counting, say, a 1 liter sample of rainwater for an hour, it’s a lot more compelling than having an elevated reading following a 1-minute count of a one-milliliter sample. This is another problem that people run into – they’ll take a small sample, count it for a minute, and if the count rate is a little higher than background then they start worrying about contamination. Most of the time, though, counting a larger sample for a longer period of time (many environmental samples are counted for over 12 hours) will show us that there’s really no contamination. OK – so let me put all of this together!

Did you know we receive anywhere from 20 to 40 mrem each year from cosmic radiation? – Graphic by W. Kent Tobiska

What Should Be Included in a Training Program for Radiation Workers?

Hello, Dr. Z – I’m a Radiation Safety Officer (RSO). I am putting together a training program for my radiation workers. What do I need to include to make sure I cover all the bases? Thanks!

Wow – great question! And very near and dear to my heart – I’ve been teaching in one way or another since the early 1980s. So let’s see how much I can tackle here.

First, let’s start off with who is considered to be a radiation worker. And the fact is that there is no regulatory definition of what a radiation workers is – this is something that you will have to define for yourself based on your understanding of your facility. The bottom line is that anybody who is NOT a radiation worker is limited to 100 mrem of exposure annually, so if you have anybody who you can reasonably expect to reach or exceed this level of exposure then you need to designate them as a rad worker and give them appropriate training. You might do this based on radiation dose rate measurements (if a person works for 2000 hours a year in a radiation field of 0.05 mrem/hr they’ll receive 100 mrem in a year).

Alternately, you might decide to designate everyone who works in a particular area or who performs specific tasks as a rad worker – everyone who operates an x-ray cabinet for QA/QC measurements for example, or everyone who works in a veterinary x-ray suite. Something else to consider is that some types of workers are likely never going to receive 100 mrem in a year – but they should be considered rad workers nevertheless. Think, for example, of laboratory technicians who handle radionuclides every day – even though these workers almost never receive any measureable dose at all (at most universities, anyhow) they’re working directly with radioactive materials in a form that can easily cause contamination.

Pictured here, a Radiation Safety Officer (RSO) gives a short demonstration on how to properly use a survey meter.

The founder of Nevada Technical Associates, Inc., the late Dr. Robert Holloway, frequently conducted Radiation Safety Officer Training courses, and other short courses on Radiation Safety.

How the training (both initial and refresher) is given is not specified in the regs – I will always give the initial training in person, but I’ve found a number of ways to give the refresher training. I’ve required rad workers to watch instructional DVDs, to read the refresher training materials, I’ve had on-line refresher training, and also given annual classes. I’ll also accept (only for refresher training) certificates from related radiation training – for example, if a person attends a class on responding to radiation emergencies. Whenever possible I prefer to give all training in person, but sometimes this just isn’t possible. The bottom line is to make sure that you can document that all of your rad workers have received the training and that the training meets the regulatory requirements. As to what those requirement are…keep reading!

The same regulation mentioned earlier also tells us what has to be included in the training:

• Kept informed of the storage, transfer, or use of radiation and/or radioactive material;
• Instructed in the health protection problems associated with exposure to radiation and/or radioactive material, in precautions or procedures to minimize exposure, and in the purposes and functions of protective devices employed;
• Instructed in, and required to observe, to the extent within the workers control, the applicable provisions of Commission regulations and licenses for the protection of personnel from exposure to radiation and/or radioactive material;
• Instructed of their responsibility to report promptly to the licensee any condition which may lead to or cause a violation of Commission regulations and licenses or unnecessary exposure to radiation and/or radioactive material;
• Instructed in the appropriate response to warnings made in the event of any unusual occurrence or malfunction that may involve exposure to radiation and/or radioactive material; and
• Advised as to the radiation exposure reports which workers may request

In addition to these, you should also consider discussing program-specific topics; what these are is up to you. For example, I would include a discussion of the most common problems we found when we inspected our radiation laboratories, I’d go over a few instructive case studies, and would also show the rad workers (most of them were laboratory technicians or graduate students) how to work safely with radioactivity in their labs. Oh – and at the end of the rad worker training you should give an exam to the students to confirm that they’ve been paying attention. Typically your regulators will be looking to make sure that they receive a score of 70% or higher – anyone who can’t get this score should be required to re-take the training, hoping the information will stick a little better the second time around.

Now a few odds and ends….

• You need to keep records of everything. Students should sign in for the training (keep the sign-in sheets) and their exam grades should be recorded; these records should be kept for at least a year, until they take their refresher training (and pass the exam for that).
• You, as Radiation Safety Officer (RSO), also need to have periodic refresher training, presumably to keep you from just talking to yourself for a half hour or so every year. And since you have to have a far higher level of knowledge than any of your rad workers, you should seriously consider taking a 2-3 day class for your refresher training, but this class need not be called “RSO refresher training.” Many regulators will accept any radiation-related class you find – I’ve had RSO refresher students in classes I’ve taught on radiological terrorism, radiation instruments, Naturally-Occurring Radioactive Materials, and more.
• You might also have to receive more specialized training. For example, if you’re transporting or shipping radioactive materials then you have to receive training in this every three years. If you have (for example) an irradiator then you’ll probably have to teach people how to safely use the irradiator before they’re allowed to do so. And so forth….
• And there’s no specific requirement as to how long the training should last – I’ve been to rad worker training that lasted as little as a half hour and to training that took three full days – you’re going to have to balance what the regulations require, the site-specific materials you’d like to go over, as well as the amount of time you’re able to pry your workers away from their normal duties.

That’s about it on training – I can go into more details but, really, from here it gets to be very program-specific. I hope it helps – and good luck!

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