PORV Block Valve Closed

PORV block valve closed, loss of coolant halted. 

The TMI accident demonstrated the need for reliable pressurizer equipment (e.g., the ability to open or close the PORVs as necessary). Moreover, power supplies used to provide power for the pressurizer PORVs, block valves and level indication may not have been qualified to present stringent post-accident requirements. Several concerns were identified regarding the adequacy of the power supplies for the pressurizer equipment particularly with respect to the loss-of-offsite power event [see NUREG-0737]. NUREG-0737 identifies new guidance to assure adequate power for the pressurizer equipment consistent with the requirements of 10 CFR 50 Appendix A, (Reference 2) (GDCs 10, 14, 15, 17 and 20). 

As long as the upper part of the reactor coolant system contained only steam, the bubble could have been condensed (collapsed) by refilling (with full HPI) and repressurizing (by closing the PORV block valve) the system. However, with large amounts of noncondensible hydrogen in the system, the bubble could no longer be collapsed. 

Core degradation continued after the PORV block valve was closed because there was still no way to cool the uncovered portion of the core. Although steam generator A contained 50% cold water, there was no circulation of reactor coolant through the steam generators. In some ways the situation was worse than before the PORV was closed. As the reactor coolant pressure increased, it took less energy to evaporate each pound of residual water covering the bottom portion of the core. 

At 5 h 15 min, the PORV block valve was closed to initiate the repressurization. Two makeup pumps were running throughout the repressurization so that a feed and bleed situation existed. By 5 h 43 min, the primary system was fully repressurized. The pressure was maintained between 2000 and 2200 psig by cycling the PORV block valve. 

The operators were reluctant to start a reactor coolant pump for fear of vibration-induced seal failure LCK2A. They recognized they had bubbles in both loops. They believed the reactor core was covered and considered the possibility of uncovering it as each option was reviewed. The concern that the PORV should remain closed was reevaluated leading to a decision to use the PORV block valve for pressure reductions. 

At 8 h 41 min, the reactor coolant pressure reached 600 psig, and the core flood check valves opened. Little water was injected from the core flood tanks into the reactor vessel. Some control room personnel interpreted this to mean the core was covered others concluded that the core had never been uncovered. At 9 h 10 min, plant personnel closed the PORV block valve, halting the depressurization. 

The operator did not know that the PORV had failed. He believed the RCS depressurization was due either to the fully open pressurizer spray valve or to the feedwater flow to the steam generators. He closed the spray valve and the PORV block valve as precautionary measures. But subsequent analyses showed that the failed PORV was responsible for the rapid RCS depressurization. Two minutes later, the reactor operator opened the PORV block valve to ensure that the PORV was available. Fortunately, the PORV had closed by itself during the time the block valve was closed. The failed PORV was the ninth abnormality that had occurred within 15 minutes after reactor trip. 

Because the inflow was now greater than the outflow through the PORV, the pressurizer level began to rise. The rate of rise was increased when the steam generators, starved of feedwater because of the closed block valves, boiled dry, causing the reactor coolant to heat up and expand. 

At about 6 22 am (2 h and 22 min into the accident), someone in the control room finally decided to close the blocking valve downstream of the PORV (Rogovin and Frampton, 1980). At long last, this action finally eliminated the leak path from the RCS. Unfortunately, this was the only action taken at this time and no additional mitigation steps were taken. Ideally, once operators realized that coolant had been lost, they should have added water into the RCS to ensure the fuel was covered. 

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