Critical cooling systems for four pools containing thousands of nuclear fuel assemblies at the Fukushima No. 1 power plant shut down due to a loss of power overnight Monday, highlighting the vulnerability of the ad hoc equipment set up after the meltdowns two years ago.

Tokyo Electric Power Co. said repairs to fully recover the cooling functions of the fuel pools at reactors 1, 3 and 4 were completed on Tuesday night. Meanwhile, the utility said another huge common fuel pool would be fully functional by 8 a.m. Wednesday.

Tepco was still trying to pinpoint the cause of the power loss Tuesday afternoon, raising concern about the soundness of the facilities at the badly damaged nuclear plant, where decommissioning work will take decades.