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TMI 2 Question

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nspunx4:
Hello,

First I would like to thank everyone for all of the knowledge I have received from this forum. I am just an interested "civilian" who enjoys learning about nuclear power.

My question is this... Can anyone explain why all of the official reports on the TMI 2 incident state that the emergency feed water block valves being closed for the first 8 minutes into the transient had no direct effect on the outcome other than to possibly distract the operators? From my very rudimentary and uneducated understanding of a PWR I thought the introduction of cold emergency feed water at the start of the loss of main feed water would have caused the steam generators to accept more heat from the RCS and kept the pressure from increasing thereby causing the pilot operated relief valve on the pressurizer to remain closed?

I am sure I am missing something here possibly the effect of the once through steam generator design B&W used?

Thank you in advance!

jams723:
First, due to the transient, the PORV open on the initial overpressure and failed to fully reseat.  Because the design had other high pressure pumps that were injecting cool water into the reactor vessel the EFW would have been beneficial but not having it at first did not have any effect on the transient.  The B&W design at that time was to allow the PORV valve to cycle.  You also have to avoid having too much heat transfer which is overcooling and has it's own issues.  EFW is very effective on the once through steam generator.

Bottom line is they had a pressurizer steam space Leak (the PORV), did not recognize it and turned off the high pressure injection pumps... That is the point when the EFW Block valves being closed affected the transient.

nspunx4:
I think I understand now. The initial over pressure condition would have been present with or without immediate introduction of EFW. Thank You!

thenuttyneutron:
Assuming the PORV reclosed after it lifted the first time and they had EFW, this disaster would not have occurred.  B&W plants are very sensitive to a loss of feedwater event.  There are only about 40 gallons per inch in the steam generator and you need several hundreds of gallons per minute to remove the decay heat of the reactor (~250 gpm 10 min post trip).

The loss of subcooling margin is very serious for a PWR.  As long as the Reactor Coolant system is subcooled, adequate core cooling is assured.  The PORV being stuck open did not cause the core melt.  The fact that they overrode safety systems that had automatically started as designed by stopping them is where they went bad.  Even after all of this, the core was probably ok.  The final hit was when they tripped the Reactor Coolant Pumps and removed all heat removal from the core.


--- Quote from: nspunx4 on Mar 03, 2012, 12:35 ---I think I understand now. The initial over pressure condition would have been present with or without immediate introduction of EFW. Thank You!

--- End quote ---

I am not sure if this is correct.  I am betting that the trip and initial SG blow down did not lift the PORV.  The PZR Spray should have been enough to stay away from the PORV lift setpoint.  This is all just an educated guess from me and should in no way be construed as a fact.

Fermi2:
Actually in the 70s a turbine/reactor trip on a BW tended to operate the PORVS 100% of the time.

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