Monthly Archives: January 2016


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.

If time permits – and if the estimated dose is fairly high (more than 10% of a dose limit is my general rule of thumb, although others will do this for any calculated dose greater than 100 mrem) – you should do more than one technique and compare the results. Say the worker’s typical dose is 50-100 mrem monthly for the last several years, that his co-workers received doses of 50-80 mrem for the same month, and your measurements suggest a dose of 120 mrem for that month. These are all in the general ballpark – close enough that you could justify assigning a dose of anywhere between 80-120 mrem. If you want to use the average you’d pick 100 mrem, but I’d be a little more conservative and would likely go with 120 mrem. Where you have to really think, however, is if there’s a wide disparity – say your calculations, instead of showing 120 mrem, showed he might have received 3000 mrem during that month? You can always go with the measurements and assign a dose of 3000 mrem, but I’d want to do some more checking first. In this case, I’d re-run the dose measurements and calculations to see if I made a mistake. If my measurements and math seem to work out, I might even call a consultant to do an independent dose assessment as well as to review the work I’d done to see if these numbers could somehow be reconciled. You might have to go through this process a few times until you’ve got an answer you can feel comfortable about – once you reach this point you’ll need to contact your dosimetry vendor to ask them to assign this dose to the worker for that month. Oh – something to remember – if you base any part of your dose assignment on radiation surveys you performed, you need to file your survey results away with your dosimetry records and then hang onto them for as long as your company has a radioactive materials license.

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 - H-Bomb

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.