In the aftermath of the 7.6 magnitude earthquake that struck the west coast of Japan on New Year’s day, an oil slick, approximately five metres by 10 metres was observed on the sea's surface in front of the Shika NPP in Ishikawa, according to plant owner Hokuriku Electric.

The leak was treated with a neutralising agent and a small amount of oil film was also found in the gutter and on a road around the unit 2 reactor but external radiation levels were normal with no adverse impacts on human health or the environment.

Hokuriku Electric said the oil slick probably resulted from transformer insulating oil leakage during the earthquake, triggering the fire extinguishing system, which dispersed the oil and sprayed water around the transformer. The oil may then have entered the gutter through rainfall. Hokuriku Electric is continuing to investigate the situation. However, critical external power supplies, monitoring facilities, and cooling systems are operating normally.

The company told a press conference that approximately 3,500 litres of oil had leaked, rendering a portion of the external power supply system inoperable. The total oil leakage amounted some 19,800 litres including some 100 litres from a transformer at unit 2.

The two reactors at Shika NPP, the closest nuclear plant to the earthquake epicentre, have been shut since the Fukushima Daiichi accident in 2011. As well as the oil leak, Hokuriku Electric reported temporary power outages, and water spill-over from used nuclear fuel pools in the wake of the earthquake.

In March 2023, Japan’s Nuclear Regulation Authority (NRA) concluded that there are no active faults underneath the Shika NPP, contrary to a previous view presented by an NRA expert panel in 2016. NRA generally accepted Hokuriku Electric’s view that none of the faults are active.

Hokuriku Electric applied for a screening of the Shika 2 reactor under in August 2014 under the new stricter safety standards introduced after the Fukushima accident. In April 2016, the NRA team said that there was a possibility that the S-1 fault underneath the unit 1 reactor building and the S-6 fault underneath the pipes carrying coolant seawater to the unit 2 reactor could be active. The team requested more data. Hokuriku Electric then conducted borehole investigations at 420 locations. It analysed 10 faults, examining the ages of mineral veins crossing through the faults in addition to those of the strata on top of them. It concluded that none of the faults were “active faults with possibilities of moving in the future”.

Meanwhile, rescue efforts continue in the areas affected by the recent earthquake, with 195 people still unaccounted for and 560 reported injured. The scale of damage is not yet clear, especially in the cities of Wajima and Suzu, as rescuers are unable to reach some areas due to destroyed roads and communications.

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