The earthquake and tsunami that struck Japan’s northeast coast on 22 November tested the sea wall built around the Fukushima Daiichi NPP, which had been seriously damaged by the March 2011 earthquake and tsunami. The earthquake was centred off the coast of the Fukushima Prefecture at a depth of about 10 kilometres, according to the Japan Meteorological Agency (JMA). Some residents in coastal areas of Fukushima and nearby prefectures were evacuated to higher ground.
The Pacific Tsunami Warning Centre said the first wave of the tsunami was measured at 90 centimetres, then the waves died down. Some 90 minutes after the earthquake there was also a 1.4-metre wave that was large enough to cause some flooding. While the JMA calculated the earthquake’s magnitude at 7.4, the US Geological Survey (USGS) measured it at magnitude 6.9.
It was far less powerful than the 2011 earthquake, which generating enormous tsunami waves, some as high a 40 metres, killing 20,000 people and leading to the meltdown of three of the four reactors at Fukushima Daiichi, which is now undergoing clean-up and decommissioning. The four reactors at the nearby Fukushima Daiini NPP were undamaged in 2011, but have remained closed ever since.
At both Fukushima plants, Tokyo Electric Power Company (Tepco) maintains cooling systems to prevent the used nuclear material from overheating. Naohiro Masuda, head of Tepco’s Decommissioning Unit, said a one-metre tsunami was observed at both Fukushima plants but precautions were taken to prevent a possible breech. "With regards to Fukushima Daiichi, there appears to be no problem at the plant, but we proactively stopped operation of the contaminated water discharge system with a judgement that it could be problematic if (radioactive water) leaked out," he said.
In 2015, Tepco completed a 780-metre coastal sea wall around the heavily damaged reactor buildings at Fukushima Daiichi to reduce the amount of contaminated water flowing into the ocean. During the worst of the crisis it was leaking 400t of radioactive water a day. Work to transfer contaminated water at the Fukushima Daiichi site was temporarily suspended to allow equipment to be checked for damage, but restarted after inspections found no abnormalities, Tepco said.
Kendra Olrich, a Senior Global Energy Campaigner with Greenpeace in Japan, said the sea wall fared relatively well this time but noted that the recent earthquake illustrates that Japan is too geologically unstable to safely operate NPPs.
An underground ice wall has also been constructed around the damaged reactors to reduce the amount of groundwater flowing into the reactor buildings where it becomes contaminated. However, the wall has faced a series of technical problems, and it is not known whether the earthquake has caused any damage.
Tepco said there were no “abnormalities” at either of the Fukushima plants, and no radiation level changes had been detected or injuries reported after the earthquake. The cooling system for the used fuel pool at unit 3 of the Fukushima-Daini, which stopped working as a result of the earthquake, resumed operating after a short period. Tepco said the stoppage was caused by the tremor and “strong acceleration” at the power plant.
A total of 2,544 fuel assemblies are stored in the pool. The temperature of the water inside the pool was 28.7 C when the system stopped and warmed to 29.5 C before it restarted, well below the upper operational limit of 65 C. Tepco also said two dust monitors at Fukushima Daini temporarily stopped working because of power supply interruptions following the earthquake, but operated normally once the power was restored.
Seismometers at Fukushima Daiichi measured a horizontal ground motion of 54.2 Gal with vertical motion 45.5 Gal, with horizontal motion of 68.4 Gal and vertical motion of 65.6 Gal measured at Fukushima Daini. According to the Japanese Atomic Industrial Forum (Jaif), both plants are designed to withstand a maximum ground motion of 600 Gal in the horizontal direction and 400 Gal in the vertical direction. "Given this, it is readily understood that there would be no effects from this earthquake," Jaif said.
At the time of the earthquake, some 30 vessels were operating at sea near the Fukushima Daiichi NPP, collecting underwater debris resulting from the 2011 disaster, but there were no reports of any damage to them from the tsunami.