narcoleptish wrote:Question time:
How/why do spent fuel rods begin to heat up. "Spent" makes me think they aren't functional anymore.
OK, first of all, I'm not an expert on this -- my engineering background is civil, not nuclear. But I'll do my best.
Nuclear fuel isn't like a tank of gasoline, where it starts at "full", you drain it at a constant rate until it runs out, at which point it's "empty". It's an exponential decay process whereby the fuel's power gets smaller and smaller but never completely stops. At some point the primary fission fuel drops below what can be economically used in the reactor, at which point they remove the fuel.
But it's still experiencing fission decay, both from the low levels of U or Pt that remain, and from continued decay of the fission products that accumulated while the fuel was being consumed in the core (strontium, cesium, iodine, etc.) So ... the stuff is still quite dangerous (at first) and also still capable of producing heat. To deal with both these problems, they generally put the spent fuel in a "pool" (literally, it's filled with water) on-site, for some long period of time (months???) until the various radioisotopes have decayed still further. The water in the pool serves two purposes -- it keeps the spent fuel cool, and it blocks emission from the fuel rods.
The water needs to be actively circulated through the pool to dump off the waste heat. If this circulation stops, or the water is somehow removed, the spent fuel rods will begin to overheat. Their protective casing (cladding) can then be damaged. The worst-case scenario is that they get so hot that the cladding catches fire and bits of the still-radioactive fission decay products from the fuel rods are dispersed into the environment.
In the current situation, they're worried that the water in the spent fuel pools is boiling. If too much evaporates, the rods start to get exposed, and they can't lose heat efficiently, so they heat up further. Hopefully they can get cold water circulating through there ASAP. It's a bad sign that multiple SPFs are apparently heating up and they're talking about jury-rigged methods to get more water in -- dropping it from helicopters, sprayed from firehoses, etc. That's not going to work very long.
At least, that's my understanding. If someone here knows more about this, I'll happily defer to her/him.
However, the heated water and steam also reacts with the zirconium in the cladding around the fuel rods, damaging the cladding and splitting the H2O, producing hydrogen gas, which accumulates at the top of the reactor building and eventually explodes in a dramatic PUFF!
Is a certain amount of hydrogen needed before it is combustible? Would some kind of a periodic ignition system be possible that would ignite the hydrogen in smaller, less damaging "puffs" in times of emergency such as this?
Yes, that's a very sensible idea. I recall reading some discussion of this and can't remember whether they have such a system, and if so why it's not working, or if they don't use something like this, what the reasoning is. Recall, though that the fundamental problem here was the loss of power from the diesel backup generators -- if there was an ignition system, maybe it doesn't function when there isn't external power to the plant (yes, believe it or not, these reactors that normally produce electricity do need to be receiving external power from the grid, generators, or batteries to function during an emergency).