Monday night, a power outage at the Main Anti-Earthquake Building at the Fukushima Daiichi nuclear power station suspended operations at three spent fuel pool cooling systems and other critical plant facilities. TEPCO was later criticized for a three hour delay between the power outage and their notification of the event to the press.
After further investigation, TEPCO announced that the switch gear in the process building, common fuel pool, and Unit 3 and Unit 4 switchgear had been found to be not operable. Workers were unable to determine what caused the boards to stop functioning, as no visible damage was found. They later repaired two of the boards, but are currently using an emergency power generator to restore cooling for the Unit 4 spent fuel pool while the Unit 3 spent fuel pool and common fuel pool are still not restored.
“We are still trying to identify the cause (of the power loss). We need to investigate further,” said Tepco executive Masayuki Ono, who served as spokesman at the news briefing Tuesday morning.
The blackout not only raised concerns about the resilience of the ad hoc cooling systems, but affected multiple critical systems, including the Kurion Cesium adsorption system at the water treatment facility, the Unit 3 and Unit 4 spent fuel pool primary and secondary cooling systems, the Unit 3 PCV gas control system, and the Common fuel pool cooling and purification system. Additionally, TEPCO found that the Unit 1 spent fuel pool alternative cooling system and the Nitrogen separator system were not operating, though not due to the switch gear problems.
The Unit 4 spent fuel pool (1,533 spent fuel assemblies) and the Common fuel pool (6,377 spent fuel assemblies) both house enough spent fuel pool assemblies that the decay heat still given off is capable of heating up over 40 degrees in less than a week. The Unit 4 spent fuel pool has the highest temperature increase rate, due to the full core offloaded in the already over-packed fuel pool, two to four times faster than that Unit 1 and Unit 3 spent fuel pools.
Source: The Japan Times