Monthly Archives: August 2016

Mixed Oxide (MOX) Explained

Dear Dr. Zoomie – every now and again there’s something in the news about something called MOX. Some people seem to like it and it seems to make others unhappy. Can you tell me what the story is?

This is one of those issues that, like you, most people don’t know much about – in fact, most of the population probably doesn’t know anything about it at all. Those who do follow the controversy tend to come down heavily in favor or vehemently against – not many are on the fence. With that as a bit of a prelude, here’s what the facts are – I’ll let you decide how you feel about the issue.

To start, MOX stands for Mixed Oxide. What this refers to is the fact that MOX fuel contains both uranium and plutonium mixed together with each metal in a chemical form called an oxide (rust, for example, is iron oxide). Since the uranium and plutonium are both fissionable, the reactor generates power by fissioning both of these elements instead of just the uranium.


Mixed oxide, or MOX fuel, is a mix of plutonium and natural or depleted uranium which behaves similarly (though not identically) to the enriched uranium feed for which most nuclear reactors were designed. MOX is an alternative to low enriched uranium (LEU) fuel used in the light water reactors which predominate nuclear power generation.

This plutonium can come from one of two sources. One possibility is that plutonium from former nuclear weapons – or from plutonium stockpiles – can be mixed in with standard uranium fuel. Alternately, plutonium can be extracted from spent reactor fuel (more on this in a moment) and mixed in with fissionable uranium to form reactor fuel. In either case, this is sort of like blending ethanol in with gasoline – they both burn so you can mix them together and still fuel your car.


As for where plutonium is created…well, this gets interesting. Whether the plutonium in the MOX fuel comes from weapons or weapons stockpiles, or from spent reactor fuel it was produced in the same way – inside a nuclear reactor. In fact, every operating reactor produces plutonium during its operation. Every nuclear reactor generates power by nuclear fission and the atoms most likely to cause these fissions are the lighter isotope of uranium, U-235. But if reactor fuel is, say, 6% U-235 (at the upper range for commercial reactor fuel) then the other 94% is U-238. The core of a reactor has a lot of neutrons flying around (the neutrons come from fission) and many of those neutrons will be captured by U-238 atoms to form U-239. U-239, in turn, is radioactive and it emits beta radiation to become fissionable plutonium (Pu-239). So normal fission results in the formation of Pu-239 in every single nuclear reactor on the planet. The question is what happens to the Pu-239 after it’s formed.

Some of the Pu-239 fissions as the fuel continues to produce energy. Some of it will capture another neutron to become Pu-240 (which also fissions) or can even capture more neutrons to form heavier forms of plutonium. But not all of the Pu-239 will fission or capture neutrons – a lot of it simply remains in the fuel until it’s removed from the reactor as spent fuel. And this is where some of the controversy starts to arise – since all spent reactor fuel contains Pu-239, a nation (or a sophisticated terrorist group) can chemically process the spent fuel to extract the plutonium. In truth, much of this will be marginally useable for nuclear weapons because of the Pu-240 and heavier isotopes. But the fact remains that it’s still there and it can be extracted by anyone with access to the right chemistry, handling equipment, and the rest of the necessary technology. This is, in fact, one reason that the US gave up reprocessing spent reactor fuel – since the plutonium presents a possible proliferation risk if the fuel is reprocessed, deciding not to reprocess the fuel means that the Pu-239 remains locked up in the fuel rods alongside the highly radioactive fission products, rather than being extracted for possible theft or diversion into nuclear weapons.

Another approach – no matter where the Pu-239 comes from (reactor fuel or nuclear weapons stockpiles) – is to deliberately mix the Pu-239 in with uranium fuel and to use it to produce energy. The thinking here is that, if the Pu-239 is sequestered away inside of spent fuel, it still exists and can be extracted at some time in the future; or if it’s locked in a secure bunker it still exists and can be stolen or formed into weapons by a nation that has decided it’s time to make new weapons again. On the other hand, according to this line of thought, if the Pu-239 is used to make reactor fuel, the actual atoms of Pu-239 are split and can never be fissioned again – if the plutonium is fissioned then it no longer exists; no more than carbon dioxide and water vapor can be reconstituted into the gasoline that they once were. When the Pu-239 is fissioned it is forever gone from this world and it will never again be a proliferation risk.

On the other hand, groups that oppose MOX fuel point out that, until the fuel is inserted into a reactor and fissioned, it poses even more of a security risk. The thinking here is that, normally, Pu-239 is either locked up inside of secure storage vaults or is locked up within dangerously radioactive spent fuel rods (which are, themselves, locked up behind multiple barriers). Either way, the plutonium is fairly secure.

By comparison, those who are opposed to the use of MOX fuel point out that it makes the plutonium much easier to steal. First, it’s already been separated from the dangerous fission products (if the source is spent reactor fuel). Second – and even more important – the fuel itself can be stolen when it’s in transit to the reactor in which it will be used. Typically, the most vulnerable time for any dangerous material is during shipping, when it is outside the normal security barriers and safety systems. No matter how much security is in place, materials in transit are never as secure as when they’re locked up behind multiple barriers in a hardened facility.

So this is the situation – we have (quite literally) tons of plutonium in various places around the world, and every nuclear reactor is producing more all the time. It doesn’t matter whether you are pro- or anti-nuclear power – the fact is that the world we live in has this material in it. That being the case, the question – and the controversy – is what to do with this material. Do we lock it up to achieve maximum security (but the plutonium continues to exist)? Or do we add it to uranium to form MOX so that it can be destroyed forever (but introducing potential vulnerabilities during transit)? There is logic to both positions – the question is whether the ability to gain useful energy, as well as removing the plutonium from the face of the Earth, is worth the added risk of theft.

How Do You Respond to a Radioactive Spill?

Dear Dr. Zoomie – I run a radiation safety program at a small laboratory and I’m working on our emergency response procedures. I was wondering if you can tell me what goes into responding to a radioactive spill.

How Do You Respond to a Radioactive Spill?

How Do You Respond to a Radioactive Spill?

Good question – and a very good topic! I ran an academic radiation safety program for several years and spills were most common type of incident we had; on average, about one spill a week. Most of these were fairly minor – a researcher with a leaky pipette for example, or a little spray from around the cap of a stock vial when you open it up. Of course you can have larger spills too – we had a researcher once drop 100 ml of radioactive solution on the floor, and (in our hospital) had an incontinent patient who urinated on the floor – nearly a liter of radioactive urine. Anyhow, what it means is that you are likely to have spills ranging from nearly invisible to fairly significant, and you have to know how to respond.

The acronym we learned in the Navy was SWIMS – it stands for:

•    Stop the spill
•    Warn other people
•    Isolate the spill area
•    Minimize radiation exposure
•    Stop ventilation if possible and if it will help

Here’s what that means….

Stop the spill doesn’t mean cleaning it up so much as stopping it from getting worse. So you want to pick up (wearing gloves so your hands don’t get contaminated) the bottle or vial or whatever you might have dropped or knocked over (if that’s what happened) and put some absorbent materials over whatever spilled to keep it from spreading further.

Warn other people about the spill. Let the RSO know about it so he or she can send help. Let people in the vicinity know about the spill so that they don’t walk into your spill area (also so that people who might be contaminated – anybody closer than about a meter away – stay put so they don’t spread contamination around). Even if it’s a minor spill you have nothing to lose by warning others – especially Radiation Safety.

Isolate the spill area to keep people from wandering in and getting contaminated. This means putting up physical barriers – rope, tape, even a table across a doorway – something that people have to physically move in order to cross. What you should do is to give yourself enough room to work – at least a meter past the furthest droplet you can see. Once a spill boundary is put up, you shouldn’t let anybody enter the spill area unless they have proper protective equipment (PPE) – at the least, gloves, shoe covers, and a lab coat. And once somebody is inside the spill area, only Radiation Safety should be permitted to survey them out of the area.

Minimizing radiation exposure is not so much a procedural step as a way to approach the incident. Remember – there is nothing life-endangering about a spill and you don’t have to rush in, unthinking, to save the day. Take a moment – give yourself the luxury of thinking about what’s happened and the best way to deal with it. Do you need respiratory protection? Do you have your gloves on? Do you have the right materials on-hand to clean it up? Or do you need to wait until you can get the right materials to clean it up safely? By doing this you’ll be minimizing everybody’s exposure.

Stop ventilation if possible and if desirable – but this isn’t something that needs to be done every time. First – running the ventilation can spread radioactivity through the ventilation system, which can cause problems. In addition, air blowing on a spill – especially if it’s volatile – can cause the activity to go into the air, turning it into an inhalation concern. So if you can turn off the ventilation then it will keep these things from happening. On the other hand, if you don’t know how to turn off the ventilation or if you have to stand in the middle of the spill to do so then you might want to leave it be for the moment. Oh – also, don’t forget to stop ALL the sources of ventilation if you make this decision. This includes, say, refrigerators and freezers (the compressor blows out air), pumps (the motors usually have vents), and even computers or projectors that have vents that might blow onto your spill.

Another quick comment about these actions – SWIMS is a mnemonic to help you remember these steps – they do not have to be done in that order. Just make sure you remember to do them!

Once you’ve got through these actions you’ve earned the right to take a short break – at this point things aren’t getting better, but they’re also not getting any worse. So take a minute to think about what you’ve got and the best way to clean it up and restore the area. Cleaning up is part of this – here’s a little on that.

•    First, work from the outside of the spill area towards the inside, and to work from the top to the bottom (if the spill is, say, on a table or countertop and has dripped onto the floor).

•    Most of the time, commercial cleaners will work just fine; although you might want to use a specialty cleanup product if you have radioactive metals (cobalt, cesium, etc.) in the spill – this is mostly at nuclear power plants, though, and not so much at universities.

•    As you clean, you should put the cleaning materials (paper towels, bench pads, or whatever you’re using) into a plastic bag as you use them. If someone is holding the bag they should be wearing gloves to keep from being contaminated. And every now and again, survey the cleaning materials to make sure your cleanup is having an effect – if you start finding that no contamination is coming off on the paper towels (or whatever you’re using) then either the area is fully decontaminated OR the contamination is fixed to the surface and you should move on to another location.

•    Finally, you’ll have to survey the entire spill area when you think cleanup is completed in order to show that contamination levels are acceptable. Usually this means less than 1000 dpm per 100 square cm, but these vary depending on the radionuclide and the type of radiation it emits. A good reference is a document called RegGuide 18.6 – it’s a Nuclear Regulatory Commission regulatory guidance document that, even after nearly 40 years, is still the standard reference on the issue.

There’s a lot more on this topic than what I’ve got here, but what I’ve got here will get you off to a good start. Hopefully you won’t have to use this often but, if you do, I hope this helps out.

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.