[chimera]

It is important to remember that the SFP is not in primary containment for a reason. The purpose of primary containment is to contain the energy and materials likely to be released in the design-basis accident. Typically, for LWRs, this involves sudden loss of coolant (like a double guillotine-break in the primary coolant piping) while the reactor is at power and near the end of a fuel cycle, so the fission product inventory is maximum. The primary containment must withstand the initial energy release from steam flashing and coolant blowdown from depressurization of the primary coolant system, and the release of any core material from large-scale pressure vessel failure.

Obviously, the accident scenarios for the SFP are different. There is no operating core. The fission product inventory is lower. So there is less material available, and less stored energy to drive those materials out into the environment. It all comes down to decay heat management, which as we have seen with Fukushima is exactly the cause of the problems they face.

So I expect here we'll see added emphasis on assuring availability of emergency AC power and systems to supplement the cooling of both reactor cores and the SFP. Fukushima is a classic example of a common-mode failure. One event (earthquake-tsunami) took out multiple redundant systems (offsite power and emergency generators). That is the real lesson to be learned here. I hope cooler heads prevail and we learn the right lesson.

[bvw]

It all comes down to decay heat management, which as we have seen with Fukushima is exactly the cause of the problems they face.

Decay heat management, containing the residual radioactivity, and since 9/11 it should have been clear -- but evidently was not -- preventing the breach of the pit by a single missile or explosive. I would not call the pit a "secondary containment" area, given how easy it is to breach. Did all the air in that outer shell of a steel building at the Tepco plant go through filters, go was it open to atmosphere?

True, a pit at the top of vessel is needed during fuel cycling, but leaving much fuel for any length of time there is bad, and the reason -- the ENGINEERING reason, for good engineers learn from mistakes -- because when that pit is breached the whole of the plant -- the primary containment and the major plumbing, controls and power supplies are made very difficult to impossible to access.

In general on-site spent fuel has to be better secured.