MartinFocazio-A couple of questions about BWR systems in this crisis mode.

M-So, basically, what you're implying here is that the formerly closed loop of a BWR system, which is now lacking a condenser system, is operating as an open loop system (sea water in and steam out) which, while bad, is far less bad than an exposed core and the possibility of a 4,000 degree core temp that would drop through the bottom until it stalls X feet (50? 70?) below the reactor building, and likely making a big mess of things as it goes. The boric acid they are also dumping in is to deliver boron to help damp the reaction, correct?

U- Right. I don't know where the "steam out" is going for sure. It may be going to atmosphere thru an overpressure relief valve. Or it's going to an overpressure relief catch tank with only gasses going to atmosphere. Hopefully that's intact. Boron is a neutron absorber they are using to ensure the reaction is really shut down. Control rods may not supply sufficient negative reactivity to be d*mn sure or the design required boron injection for shutdown maintenance activities.
BTW- since it's a boiling water reactor, boiling the water in the core isn't automatically a big problem. Better a BWR having this problem than a PWR. The temperature of the fuel pellets is the important factor. Boiling in a PWR is automatically a Bad Thing. Then again, the boiling occurring now is perhaps not removing sufficient heat to keep the fuel pellets intact because there's no circulation under pressure like for normal ops.

M-I know that the decay heat output will diminish, but I was of the opinion that the lifetime of the fission products is quite long - but are you implying that the highly uncontrolled fission reaction is, by simple fact that it's highly uncontrolled, consuming fissile materials so that there will be less material to react, and thus less time spent reacting?

U- There are the prompt decay products which have half-lives of minutes to hours and the long term decay products which make spent fuel rods radioactive for years to millenia. The initial decay products decay to other radioactive elements which themselves decay further some of which are radioactive and they decay further and so on. At reactor scram the prompt decay products are numerous and produce nearly 10% of the reactor power that was steady-state just before the scram. This 10% drops off quickly (exponential decay) to something like 1% after a week (I probably have this figure wrong) Handling this overshoot reaction heating is what has to be done to prevent melting. It's a significant factor in a normal shutdown plan because it takes time and attention to deal with it.

M-I know a core breach would be exceedingly bad, not so much for the fact that the core itself is melting, but from the influx of water into the hole that would result and the explosive reaction that would happen as cooling water, metals and so on mixed with the ultra-high temperature slag. That's the Bad Thing that I think we're talking about here, right?

M-And even a meltdown with a large "eruption" of steam and other particulates would eventually stabilize, right?

U- "core breach" might mean different things to different people. Influx of water into the hole.... doesn't exactly match here.
The potential problem here is- loss of coolant sufficient to allow rods to uncover, rods uncover then the zirc fuel cladding "corrodes" (at a high rate btw creating H2 gas as the zirc oxidizes), exposing uranium pellets. This alone will release radioactive gasses to the area above the reactor which I think is venting to atmosphere. Erosion/corrosion of the pellets will release stuff into the liquid/steam AND if the fuel pellets melt, then their geometry wants to change. If the zirc cladding ruptures from the heat (probably will) the u-slag can escape the cladding and drain downwards.

U-At this point it's still only airborne gas (albeit radioactive) release from pellet, thru breached cladding, to relief valve venting.

U- But once the u-slag collects in a pool, there is the chance that the new geometry (of a puddle of hot fissionable slag) can go to criticality then supercriticality creating uncontrolled heat and radiation and fission products. Of course eventually the active badness has to stop as fuel is consumed or splattered about to where it no longer has the geometry to remain critical. Even Chernobyl stopped thrashing about.



I may not have these things quite right. Just thinking thru what I know to a new situation.