Unit 1 isolation condensers stopped working after tsunami

28 November 2011

TEPCO has published a study of the functioning of the main emergency core cooling system for the damaged Fukushima Daiichi unit 1.

Fukushima Daiichi unit 1 IC system
Fukushima Daiichi unit 1 IC system

Based on review of records and site inspections in October, it concludes that the system essentially performed its function after the earthquake, but stopped functioning once the tsunami hit several hours later. It concludes that motor-operated valves that control the flow of water into the system were partly to blame, but that hydrogen accumulation might also have played a role.

The emergency isolation condenser system removes heat from the reactor by passing it through pipes cooled in contact with low-pressure cooling water. Unit 1 has two redundant systems, A and B, each operated with four motor-operated valves: two inside the pressurised containment vessel (1 and 4) and two outside (2 and 3). In normal conditions, all valves but valve 3 are open, and the system is actuated by opening valve 3.

The earthquake struck at 2:46 pm. Six minutes later (2:52 pm), the IC started automatically due to high pressure in the reactor. Until the tsunami hit about half an hour later, the reactor pressure was controlled by the actuation and shut-off of only system A. Operators shut off system B manually at 3:03 pm (although in this report TEPCO has not stated why this was done). An IC coolant data chart showed that the system A coolant temperature increased from about 25°C to approximately 100°C by the time the tsunami hit (although TEPCO said that it was not likely to have boiled off significantly in this period); the system B coolant temperature increased to approximately 70°C until it was manually shut off. TEPCO concludes from this data that the IC functioned adequately until the tsunami hit.

Once the tsunami hit, all sources of DC power were lost. On loss of power, valves 1 and 4 were designed to shut automatically. TEPCO has concluded that they did not shut fully, because some water was lost from the isolation condenser system A tank (it was measured to be 65% full in October, a decline from the previous, and normal, level of 80%). No leaks were found in an October inspection. The system B tank was found to be about 80% full, consistent with the conclusion that it did not operate after shut-off at 3:03.

DC power was temporarily restored in the hours after the tsunami hit. Operators discovered that the indicator lamps for valves 2A and 3A were on; the valves were closed at that point. At 6:18 pm, the valves were opened and steam was generated. But seven minutes later, the operator decided to turn off Isolation Condenser system A by closing valve 3A for three reasons, TEPCO said: 1. steam evaporation had ceased, suggesting that valves 1A and 4A had shut because of the isolation signal; 2. there might not be sufficient water in the IC tank to run the system; 3. IC was not working, and there was no water injection line set up to supply coolant to the IC.

At 9:30, the operator opened valve 3A and confirmed the generation of steam. In the intervening three hours, the situation had changed. First, the IC was sorely needed. The high-pressure coolant injection pump that was supposed to cool down the pressurised core was 'in no condition to be relied on'. Second, a fire engine pump was available to provide make-up water to the IC system. Third, the indicator lamp for valve 3A was unstable, and the operator was concerned about the effect of losing it. (In the report, TEPCO said that the system B valves were not working).

By 24 March, power was restored to I&C systems recording IC tank temperatures. The return water temperature from the IC to the reactor was found to be 140°C in system A, and 38°C in system B. TEPCO concludes that system A, because of the incomplete closure of valves 1A and 4A, had a slight inflow of steam that raised the IC water temperature. However, in system B, temperatures were relatively low because valves 2B and 3B were fully closed, so there was no flow of water from the core. An October site investigation reinforced these data points; valves 2A and 3A were found to be fully open, and valves 2B and 3B were found to be fully closed.

In the report, TEPCO goes on to propose reasons why the IC system A tank was not drained fully, since its valves were either intentionally (2A and 3A) or perhaps unintentionally (1A and 4A) left open. First, it said that hydrogen generated from exposed zirconium might have clogged the cooling pipes; second, that the reactor pressure dropped at 3:00 am on 12 March, reducing the amount of steam flow.

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