Japan’s Nuclear Regulation Authority (NRA) said no irregularities have been confirmed at NPPs along the Sea of Japan, including five operating reactors at Kansai Electric Power Co’s Ohi and Takahama plants in Fukui Prefecture. Two reactors at Hokuriku's Shika NPP in Ishikawa, the closest nuclear plant to the epicentre, have been shut since the Fukushima Daiichi accident in 2011. Regular inspections are taking place at the plant as aftershocks continue, but Hokuriku said no major damage has been reported, with cooling and monitoring systems all operating, although the electricity supply was compromised.

NRA reported that Shika plant workers heard what sounded like an explosion, and then smelled something burning near a transformer that received electricity for use at unit 2. Staff checking the area discovered the fire extinguishing system had been activated. No fire was detected but the reactor's electricity supply was partially disabled. Hokuriku officials believe pressure inside the transformer increased due to the tremor. This triggered a device to release the pressure, causing a loud noise and activation of the fire extinguishing system.

Electric power for equipment critical for the plant's safety was secured through other systems, and cooling pools where used fuel is stored continued to operate. Hokuriku officials are assessing the damage but said there was no change in the readings of the plant’s monitoring posts, and that no external radiation had been detected. However, of the 101 radiation monitoring posts in the plant's vicinity, 13 are no longer operational.

At Tokyo Electric Power Company Holdings’ (Tepco’s) Kashiwazaki Kariwa NPP in Niigata Prefecture, which hosts seven reactors – all offline – plant officials said water from a used fuel pool had spilled, but that no abnormalities in operation had been detected. About 10 litres spilled from the unit 2 reactor and about 4 litres from the unit 7 reactor. Tepco said it is measuring the radiation levels.

In Fukui Prefecture, Kansai Electric Power reported no problems at its 11 reactors at the Mihama, Ohi and Takahama plants, seven of which are currently in operation. The other four reactors are in the process of being decommissioned. No problems were reported at Japan Atomic Power’s Tsuruga NPP in Fukui Prefecture. One of its two reactors is being decommissioned, while the other is undergoing inspections.

While the nuclear power plants avoided damage, the New Year’s Day quake and tsunami warnings brought back memories of the devastating Great East Japan earthquake.

The 7.6 magnitude earthquake, which struck Ishikawa Prefecture, triggered waves of about 1 metre along Japan's west coast and neighbouring South Korea. The Japan Meteorological Agency (JMA) initially issued a major tsunami warning for Ishikawa prefecture but later downgraded it to an advisory. The earthquake was the largest since 2011, was reported to have caused the deaths of at least 55 people, with rescuers continuing to search rubble for survivors. The Japanese government ordered more than 97,000 people in nine prefectures on the western coast of Japan's main island Honshu to evacuate.

Before the March 2011 accident at the Fukushima Daiichi NPP, Japan had 54 reactors in operation providing around 30% of its electricity. However, within 14 months of the accident, all the plants were closed pending regulatory change in line with more stringent safety checks and regulations. Subsequently a number of nuclear units were closed permanently. To date, 12 of the remaining 33 operable reactors have cleared inspections confirming they meet the new regulatory safety standards and have resumed operation. Another 17 have applied to restart. The restarted plants are Sendai 1&2, Genkai 3&4, Ikata 3, Mihama 3, Ohi 3&4 and Takahama 1-4.

Image: The Shika nuclear power plant in Ishikawa (courtesy of Hokuriku)