On July 16, 2007, at 10:13, a major earthquake measuring 6.6 shook the Niigata region of Japan. The depth of the earthquake was assessed at 10 km (refer to the data sheet dated July 27, 2007).
The epicentre of the earthquake was situated approximately ten kilometres from the Kashiwazaki-Kariwa nuclear power plant operated by TEPCO. The power plant comprises 7 production units with a total installed power of 8212 MWe.
Production units 3, 4 and 7, in operation at the time of the earthquake, were automatically shut down. Production unit 2 was starting up at the time of the earthquake; it was also shut down automatically. Production units 1, 5 and 6 were already shut down for inspection.
The operator is currently assessing the quake-related incidents. By July 19, 67 quake-related incidents had been noted throughout the site. According to TEPCO, the most notable amongst these are the following:
- Following the earthquake, a fire began in an electrical transformer outside the reactor building of production unit 3. This was the first known case of a fire in a nuclear power plant resulting from an earthquake. This fire probably resulted from soil subsidence around a pylon carrying electric cables, causing a short circuit and ignition of oil by the produced sparks. The extinguishing system also appears to have been damaged by the subsidence. The fire was brought under control at 12:02, i.e. almost 2 hours after the outbreak.
- A water leak in production unit 6 was noticed on July 16 around 12:50, in an "unmonitored zone" (unmonitored with regard to worker radiation protection).This leak came from an overflow of the used fuel storage pool, but its radioactive nature was not detected until five and a half hours later (at 18:20). This led to a release into the sea of 90 000 Bq via the collection system; the associated doses are certainly very low.
- Several hundred storage drums in the solid waste storage building were tipped over and some lost their lids. These drums contained low activity waste, such as the residue from the incineration of gloves or masks worn by operators when entering the controlled zone. Traces of radioactivity were measured on the ground of the storage area.
- Traces of radioactivity (iodine, chrome 51, cobalt 60) were also detected in the ventilation system filter of reactor 7. These elements would therefore have been discharged into the atmosphere, with corresponding doses for the population well below a nanosievert (10-9 Sv). The source of these discharges is related to a delay in the shut down of the steam elimination systems on the floor of the turbine (system specific to boiling water reactors).
The acceleration values recorded by the power plant instruments are generally higher than those selected for the design. This may explain the large number of recorded quake-related incident, which were essentially water or oil leaks, displacement of ventilation conduits, etc... At this stage of the investigation, there does not seem to be any incident that questions safe operation and maintenance of the installations. However, the necessary inspections, repairs and studies before re-starting (including the seismic reassessment) will certainly continue for several months. In particular, investigations will be carried out to find, more accurately, the position of the fault at the origin of the earthquake.
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