Reactors shut down safely at Japan's Hamaoka nuclear power plant this morning as the Shizuoka region was hit by a 7.1 magnitude earthquake.
The tremors originated about 26 kilometres below the surface of Suruga Bay, on the south-western side of Hunshu, Japan's main island.
At the Hamaoka plant about 20 kilometres away, two large reactors were in operation but shut down automatically. Another reactor was already shut down for a periodic inspection, while the two other units at the plant are permanently out of service for decommissioning.
The shutdown reactors, units 4 and 5, are both boiling water reactors rated at 1092 MWe and 1325 MWe respectively.
Owners Chubu Electric Power Company issued regular reports after the event which detailed the steps being taken at the plant to ensure safety. Only minor damage to some masonry and fittings in the plant buildings has been found so far, although water in a fuel cooling pond at unit 5 was shaken enough to trigger a radiation alarm for a time. This occurred inside the plant's sealed containment area and monitors in the building's exhaust stack recorded no release of radiation. Workers are currently checking over the reactors, which are expected to return to service quickly.
Ground acceleration recorded at the plant was in the range of 100-160 gal for four of the reactors, with unit 5 getting the worst of the vibrations with 425 gal. Hamaoka reactors 3 to 5 are engineered to withstand up to 1000 gal, although upgrading the older units 1 and 2 to this standard proved uneconomic for Chubu, which decided to retire them early and replace them instead.
Earthquake regulations were revised after the Niigata-Chetsu offshore earthquake which hit Kashiwazaki-Kariwa nuclear power plant in July 2007. The seven reactors at Kashiwazaki-Kariwa all stayed safe during the earthquake, but its strength came as a surprise to seismologists and led to a simplification and strenghening of requirements for nuclear plants. One of the seven reactors is now back in operation, with another expected to follow soon.
Chubu said that power demand dropped by 200 MWe as blackouts hit 9500 homes in Aoi Ward and Shikuoka City as well as other parts of Shizuoka prefecture. There was widespread disruption, with around 70 injuries reported.