I suspect the reason plant designs don't attempt to harness the decay heat is that in one key accident scenario (massive LOCA) you aren't going to be able to generate any steam pressure from core heat. Being able to address this scenario is essential to getting licensed. So a secondary power system that doesn't rely on the plant at all (batteries, diesel generators, etc.) is mandatory.
>From the standpoint of the plant designers, the above reasoning means the decay heat subsystem looks like a completely unnecessary extra cost. They already have the mandatory secondary and there's no licensing requirement for a tertiary power system that may not work in some failure scenarios. Jeff On Fri, Nov 23, 2012 at 3:18 PM, Jed Rothwell <[email protected]> wrote: > Abd ul-Rahman Lomax <[email protected]> wrote: > > I'll admit I don't get this. The reactor stays hot because of residual >> radioactivity. And if it isn't cooled, it gets *hotter* than normal >> operation under power. So there should be enough power there to the >> turbines to keep it -- and maybe the fuel storage ponds -- cool. >> > > There is not enough power to drive the main turbines. I think it takes at > least 600 MW of heat from the reactors to drive the turbines at 200 MWe > (20% of normal capacity). After a SCRAM the power is reduced to around 5%, > and it falls rapidly after that. > > I suppose you could have smaller auxiliary steam turbines. I think at some > plants, some of the initial response is powered by main reactor steam. But > the overhead for the pumps and other equipment operating is something like > 15% so they would not be enough to keep the clockwork going. Whereas if all > you want to do is keep cooling water flowing through the reactor into the > cooling towers, a much smaller set of pumps will suffice. > > As I said, the aux systems have never been destroyed in any previous > accident. They would not have been destroyed in this one if anyone had > imagined a tsunami this large might strike. They could have located the > equipment where the tsunami did not reach, or they could have built a > higher seawall. The accident could have been prevented easily if they had > known it was coming. You cannot anticipate everything . . . > > Someone did, in fact, anticipate this. He wrote a report pointing to > historic evidence for a tsunami at this location a thousand years ago. As > someone else pointed out, they think of everything in cases like this. > After a major accident at a nuclear plant, or with a large modern airplane, > you can always find an engineering report worrying about that problem. But > you cannot fix every possible problem. If you tried, the power plant would > always be under repair being retrofitted; the airplane would never leave > the ground. > > - Jed > >
