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NUCENG is offline
Aug5-11, 03:58 PM
Sci Advisor
P: 916
Quote Quote by westfield View Post
I don't think its that black and white - I'm also speaking as a layman so someone please correct me if I'm mistaken, early depressurization to me would mean the first line of defence on a loss of power is gone, namely HPCI and RCIC both of which require high pressure steam to work as I understand it. Obvioulsy the operators would have wanted to try and keep the ability to use those systems so initially depressurization would be the last thing they want to do. The only other systems they had to control heat in the reactors were the LP ones which all require electricity (again, as I understand it).

Of course Unit#1 with its Isolation Condenser instead of RCIC is a different case. That should have worked fine without power, something else went wrong there, perhaps as simple as running out of water.

I guess my point is that the operators are trying to remove the heat from the core to prevent a meltdown, if the heat is under control there will be no pressure issues and therefore no need to vent AT ALL - From what I understand of the systems early depressurization and venting prematurely will not help the heat problem and will remove several critical systems from the picture.

From the sparse reports I've read the operators at Fukushima 1 could not keep the IC and RCIC systems running for some reason and this is one of the most important questions in my mind. If they had functioning IC and RCIC then we might not even be here on this forum now.

Perhaps someone here in the business could clear these aspects of ECCS up for us?
Is it a bad thing to lose those steam driven HP systems and try "fight" the fight with a depressureized reactor?
Let me try. I will address the IC first as it is a very simple system. I will post a follow up on the more complicated possibilities with RCIC later.

The IC basically is a sytem to take reactor steam and condense it in a heat exchanger then route the condensate back to the vessel, removing heat in the process. The system runs on natural circulation. The steam rises to the condenser and is condensed. The condensate is cooler than the water being heated in the vessel so it flows back into the vessel. That is the theory. In operation all that is necessary is to open valves to allow the condensate to flow back into the vessel. The standpipe of condensate is kept filled by steam which is continuously available to the condenser. The condenser is basically a water tank that boils off and is vented to atmosphere. To keep it running all that is needed is to continue to add water to the tank. Since the tank is vented this can be done by a portable pump or fire truck.

Failure modes are azlso relatively straight forward. If the valve can't be opened the system won't work. At unit 1 the system was started, but apparently was stopped over concern about exceeding a design limit on cooldown rate. Later they tried to restart the IC, but it is not clear whether it worked. Power to the valve may have failed. The valve itself may have failed or the high temperatures in containment could have caused boiling in the condensate standpipe. This would have broken the driving force for natural circulation. Other possibilities are that the tank was damaged and leaked or boiled dry removing the coolant from the heat exchanger.

I have looked at the data dump from TEPCO from the first hour after the erathquake. It is clear that the IC was initiated and stopped after about 15 minutes. Following the tsunami there was no active instrumentation readings released so it is not clear what prevented reinitiation. The concern about cooldown rate was probably a mistake since the vessel was probably already on the way to core damage due to the extended SBO. I do think the mode of failure will be easy to identify when conditions permit examining the piping and valves.

Hope this helps. I will try to post on RCIC later tonight.