A 7.3-magnitude earthquake caused some deficiencies at two of the three nuclear power plants (NPPs) located in relatively close proximity to the fault, but it did not cause any safety concerns, according to Japan’s Nuclear Regulation Authority (NRA).

The quake struck about 60 kilometers off the coast of Japan on March 16 at 2:36 p.m. UTC. The NPPs affected include the Fukushima Daiichi facility, site of the infamous nuclear disaster in February 2011. Other potentially vulnerable facilities were the Fukushima Daiini and Onagawa nuclear power stations.

At the Daiichi site, hundreds of seismically qualified tanks store treated water from the 2011 accident. The force of the quake reportedly moved some of these tanks up to 10 centimeters, but there was no leakage, according to the NRA. Also at the Daiichi site, a cooling pump for the Unit 5 reactor’s spent fuel pool temporarily stopped working, but later resumed operation and cooling functions at the site have been maintained, the regulator said.

At the Daiini site, which is about 11 kilometers south of the Daiichi site in Fukushima Prefecture, the spent fuel pool pumps for Units 1 and 3 stopped for a period, but they have reportedly been returned to service. One of the four transmission lines that supply off-site power to the Daiini site was also lost as a result of the quake. However, the three remaining transmission lines were said to be adequate to keep the power plant stable. No damage was reported at the Onagawa station.

Both the Daiichi and Daiini facilities are being decommissioned. Tohoku Electric Power Co., operator of the three-unit Onagawa station, decided in 2018 to decommission Unit 1, but it continued to pursue bringing Unit 2 back online. In November 2020, the company received approval from Murai Yoshihiro, the governor of Miyagi Prefecture, to restart the unit, and it seemed destined to do so sometime this year. The company was also considering a restart of Unit 3 in the future. It’s unclear if yesterday’s earthquake will alter any of those plans.

Onagawa was actually closer than the Daiichi plant to the epicenter of the 2011 Tohoku earthquake—less than half the distance. The earthquake knocked out four of Onagawa’s five external power lines, but the remaining line provided sufficient power for its three boiling water reactors (BWRs) to be brought to cold shutdown. The Onagawa station was also affected by the Miyagi earthquake, a 7.2-magnitude tremor in August 2005. Greater-than-design-basis vibrations were reportedly recorded at the plant, but analysis after the event found no damage to reactor systems.

None of the Onagawa units have operated since February 2011. If Unit 2 was to restart, it would be the first BWR to operate in Japan since the Fukushima accident. The Daiichi units were also BWRs.

Aaron Larson is POWER’s executive editor (@AaronL_Power, @POWERmagazine).