RF posted a news exerpt in comments, here’s a bit:
Meanwhile, conditions at the plant appeared to worsen Wednesday, with white smoke pouring from the reactor complex and a dangerous surge in radiation levels…
Let’s look at where all this crap is coming from in the first place. This will be old-hat to many of you, so I’ll try to whiz through. Then I’ll list some things that can cause these rad levels to change. You guys can provide the ones I miss.
If we could run nuke plants without making radioactive materials, we would. Well, except for the ones designed to make useful isotopes, like the HFIR.
The sole purpose of commercial nukes is to produce electricity. If we could, we’d skip building the facility entirely and just say “Electromellius!” or something. Say, that might work for coal plants too.
What’s mostly sought from fission is the kinetic (heat) energy of the fission fragments. That heat either flashes primary water to steam (BWR) or heats pressurized water that flashes secondary water to steam (PWR). The decay heat of radioisotopes adds to the heat production, but we would happily forego that if we could. Even better would be only extremely short-lived isotopes, but that’s just being greedy.
So, steam, turbine/generator, electricity.
The radioactive materials are produced in two ways: the fragments of what was uranium are extremely radioactive. More details on those later, but Cesium-137 is a prime example of an undesirable side effect.
And lots of free neutrons are produced, which is necessary for sustained fission. All elements can absorb neutrons, producing isotopes both stable and unstable. The second sort add to the problem.
So, these increasing radiation levels. The worst is continued neutron production, because that means the radioactive inventory may still be increasing. With no neutrons, the inventory is (way too slowly) decreasing.
(Yes, I know there’s an exception, but it probably doesn’t apply here.)
So, say the inventory is slowly decreasing but the dose rate somewhere is still increasing. There’s an old, old Health Physics exposure principle called “time, distance, and shielding”. It’s really just formalized common sense. To minimize your dose you minimize your time and maximize your distance and shielding.
Since time is working for us (again, not at scales to do us much good) then distance and/or shielding are working against us. Rad material is moving, shortening distance and/or angling around shielding. That means dose rates are decreasing elsewhere.
This is a long enough post. It didn’t really whiz, did it?
How are dose rates changing? Basically, we’re producing more materials or what’s there is drawing nearer.
If the problem is that shielding and/or fuel is moving, is it also possible that different types of material are being exposed to and absorbing neutrons from the slowly decreasing inventory in such a way as to favor the production of different decay chains and they are responsible for some of the increased radiation count? Or is that a second order effect that can be ignored?
You’re very close to that exception I mentioned, involving decay chains. But neutrons aren’t necessary. The difference in half-lives between parent and daughter nuclides increases total activity at certain times. There’s a good page on it here, and the graphics let you grasp the concept at a glance.
Yeah, I wasn’t thinking about alpha and beta capture at all (I should have) and not thinking clearly at all about the decay chain as a whole (visions of dlambda/dT daisy chains were scaring me). The link was nice — I was having trouble visualizing what the preconditions were for the ensemble activity rate to increase w/ time.
There was a quote once to the effect of “Education is what remains after you’ve forgotten everything you ever learned.” Sometimes I feel that way about my nuc/high energy past.
Thanks for your great posts!
Thank you.
Could it be a degradation of the shielding? There are reports one of the pools (#4?) is now empty of water. That would allow temperatures to increase and certainly it means the shielding from the water is gone.
Oh, absolutely. Good catch, and my omission. Rad materials can move around shielding. And some of them can remove the shielding. The fuel pools all have a “makeup” water system used routinely and specifically for evaporation.