GURPS effects of exposure to a nuclear fuel source

[Michael Thayne]

GURPS Disasters: Meltdown and Fallout goes into some detail about dramatic ways nuclear technology can do bad stuff to you. But what if there's no meltdown, no fallout, and instead you're just directly exposed to a nuclear fuel source—say because some idiot decided to dissect a nuclear bomb or radioisotope thermoelectric generator? I've tried to research this and it turns out to be a somewhat confusing subject, because AFAICT the main types of nuclear fuel are all alpha emitters. Since alpha particles don't penetrate skin very well, alpha emitters are only dangerous if ingested or inhaled. Except alpha emitters often have much more dangerous isotopes in their decay chains. So how do you figure out the rads accumulated for, say, handling a fuel rod / pellet / etc. with no better protection than a pair of iron tongs?

[Anthony]

An unused nuclear fuel pellet really isn't a short term radiation hazard at all (long term cancer risk, but not radiation sickness), unless you manage to turn bomb parts supercritical. A fuel pellet that's actually been used inside of a reactor can be extremely hot because of fission byproducts but that effect will be extremely variable with time.

[thrash]

Have you looked at OSHA's Radiation Emergency Preparedness and Response page? There's a lot about the more dramatic stuff, but they also cover (e.g.) "Lost, found, or orphan (i.e., no longer under proper control, abandoned) radioactive material sources." This includes links and cross-references (as in the quote).

[Anthony]

For those who are curious: if you have purified U-235, something like 92% of total decay energy will be alpha decay from U-235 to Th-231, another 8% will be Th-231 to Pa-231, and unless you leave it for thousands of years, everything else is negligible (Th-231 has a half-life of 25.5 hours so it reaches equilibrium quickly, but Pa-231 has a half-life of 32,000 years). Decay rate is 4.5e-16/sec with a decay energy of 2e+9J/g for an energy output of 9e-7W/g. 1J/kg is 100 rads, so for a 70 kg person that works out to 1.3e-6 rad/sec (about 1 rad per 9 days) if you absorb it all, which pretty much means swallowing powdered uranium.

Now, for nuclear fuel that has been used in a reactor, radiation output starts at around 6.5% of the total thermal power output of the reactor (so probably more like 20% of electrical power output), which will kill people impressively fast.

[Michael Thayne]

Originally Posted by Anthony

For those who are curious: if you have purified U-235, something like 92% of total decay energy will be alpha decay from U-235 to Th-231, another 8% will be Th-231 to Pa-231, and unless you leave it for thousands of years, everything else is negligible (Th-231 has a half-life of 25.5 hours so it reaches equilibrium quickly, but Pa-231 has a half-life of 32,000 years). Decay rate is 4.5e-16/sec with a decay energy of 2e+9J/g for an energy output of 9e-7W/g. 1J/kg is 100 rads, so for a 70 kg person that works out to 1.3e-6 rad/sec (about 1 rad per 9 days) if you absorb it all, which pretty much means swallowing powdered uranium.

Now, for nuclear fuel that has been used in a reactor, radiation output starts at around 6.5% of the total thermal power output of the reactor (so probably more like 20% of electrical power output), which will kill people impressively fast.

This is really helpful. What's the equivalent math look like for the plutonium-238 in an RTG?

[Anthony]

Originally Posted by Michael Thayne

This is really helpful. What's the equivalent math look like for the plutonium-238 in an RTG?

The main decay path there is Pu-238 -> U-234, which is an alpha reaction, so also mostly an internal risk, though the total amount of radioactivity is much larger.

[whswhs]

Of course, if you swallow powdered uranium or plutonium, there are going to be unpleasant chemical effects that may well be much faster.

[Anthony]

Originally Posted by whswhs

Of course, if you swallow powdered uranium or plutonium, there are going to be unpleasant chemical effects that may well be much faster.

Maybe for U-235 (I believe the chemical toxicity of U-238 is greater than the radiation risk, but I don't think the same is true for U-235). Not for Pu-238 (which is around ten million times as radioactive), a milligram will give you a fatal dose in something like a month.

[dcarson]

For swimming in a radioactive storage pool. https://what-if.xkcd.com/29/

[Michael Thayne]

For the kind of plot that's bouncing around in my head, maybe I should use a Strontium-90 RTG. I'm guessing decay rate as it shows up in Anthony's math is 1 - 0.5^(1/[half-life in seconds])? And when I Google decay energies I get a value in MeV, is the thing I need to do convert from MeV to Joules, multiply by Avogadro's number, and divide by the atomic mass to get decay energy in J/g? Not a scientist, mostly just trying to do my best to remember what I learned in high school.