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Fukushima update
PermaLinkPart of the fun of this reading-between-the-lines and informed speculation business is seeing how right or wrong one is as hard facts start coming out. One of my speculations, that the HPSI pumps didn’t start was wrong.
The utility has supplied the IAEA with official INES notices of the events and these contain some hard data describing the first few hours of the evolution. As reported here, the HPSI pumps DID start and did operate properly until the water in the torus became too hot.
When the water in the torus approaches the boiling point, a couple of things happen.
- The steam discharge from the HPSI pump is no longer condensed and simply builds up pressure in the torus and primary containment.
- The HPSI pump, which has to have a certain net positive suction head (NPSH) starts to cavitate and lose pumping efficiency. At some point when the water temperature reaches nearly 100 deg C, it simply can’t pump. It’s a multi-thousand HP pump no larger than a compact car and so the specific energy transfer to the water is very high.
A long term total station blackout accompanied by flooding is beyond the design basis of the plant. The design basis assumes a functioning RHR system that removes heat from the torus water. Without the RHR, the torus water simply heated to the boiling point at which point the HPSI and LPSI pumps became ineffective.
Speculation alert: I imagine that if RHR circulation can be restored to the torus, probably by external pumps, the LPSI pumps should restart and resume their feeds to the reactors. This would be MUCH better than actually pumping seawater directly into the reactor vessels. I suspect that this is what they’re doing.
More updates as information becomes available
Posted by neonjohn on March 18th, 2011 under Energy, Nuclear
March 18th, 2011 at 9:53 pm
Thanks again for all the good info
March 19th, 2011 at 8:12 pm
That makes sense, the HPSI and LPSI turbines are heat engines and heat engines require a significant difference in temperature between two points to operate.
Now that we know this I think the question becomes why did the water in the torus heat up so quickly? I forget exactly how long it was between the Tsunami hitting and the hydrogen gas explosions but from memory I think it was about a day.
That means that even with the HPSI operating and coolant flowing between the torus and core, the core heated up enough in about 24 hours to expose the fuel cladding to steam, suggesting that the water in the torus is at best only a short-term heat sink for the core.
It would be interesting to find out how long the system was designed to operate in this state and compare it to how long it actually lasted.
Perhaps for future reactors it will be necessary to have more steam turbines that run directly off the reactor but which instead generate electricity that can then be used to run electric pumps for the RHR circulation.
Does that make sense?