Earthquake has impact on Fukushima Daiichi plant

February 23, 2021, 6:58AMNuclear News

The black star represents the epicenter of the February 13 earthquake. Image: USGS

There has been no off-site impact from the February 13 earthquake that struck off the east coast of Japan near the Fukushima Daiichi nuclear power plant, Tokyo Electric Power Company (TEPCO) reported on February 19. The earthquake, however, has caused a water leakage from two of the site's primary containment vessels (PCVs).

A nuclear alert order was issued by the plant about 20 minutes after the earthquake, and the water treatment and transfer facilities were shut down. Inspections after the event revealed no anomalies and the nuclear alert order was rescinded on February 14.

The nuclear plant in Fukushima Prefecture in northeastern Japan is now undergoing decommissioning.

Water levels: Following the earthquake, water levels in the primary containment vessels (PCVs) of Units 1 and 3, which were crippled by the Fukushima Daiichi nuclear accident in March 2011, had dropped by several tens of centimeters and have continued to fall each day, according to TEPCO. In Unit 1, the water level fell by up to 70 centimeters (about 27.5 inches) beginning on February 15, while Unit 3 began leaking water on February 14, lowering the water level by 30 centimeters (about 9 inches).

TEPCO said it is continuing to pump water into the vessels and that there should be no safety problem. Nitrogen is also being added to raise the pressure.

“Since no significant fluctuations have been seen with temperatures at the bottom of the primary containment vessels . . . we have determined that there is no off-site impact,” TEPCO said.

The cause: The water level decreases are likely due to the changes to damaged portions of the PCVs, according to Lake Barrett, a senior advisor to TEPCO and Japan’s International Research Institute for Nuclear Decommissioning.

“The earthquake did something in the lower sections of the PCVs to enlarge existing or create new leakage paths that is causing a new balance with a lower water elevation for the same flow out onto the reactor building basement floor,” Barrett told Newswire. “The increase was relatively small and not surprising as events and changes such as this will take place inside as time goes by."

Based on the TEPCO data so far, the leakages are well within containment, while the processing capabilities of monitoring equipment, such as temperature sensors, are working. “The decrease in one of the Unit 3 temperature sensors is likely because it was underwater near the water surface,” said Barrett, a 50-year emeritus ANS member who was the Nuclear Regulatory Commission’s site director for the cleanup of the Three Mile Island accident. “When the water level dropped, it was no longer under the slightly warmer water and became exposed to the cooler gas space above the water.”

The gas atmosphere inside the containment is mostly nitrogen—not air—and is slightly cooler, which is likely why that sensor temperature went down slightly when the water level dropped.

Barrett added that as TEPCO gathers information from further physical investigations and analyses, such as water volumes and radiological study of samples, more will be learned.

The earthquake: The February 13 earthquake struck offshore east of Tōhoku, Japan, at 11:07 p.m. (JST) with a magnitude of 7.1. It was followed by multiple aftershocks less than an hour later, with the strongest shock being at magnitude 5.3. There was no tsunami threat from the earthquake or aftershocks, it was reported.

The earthquake is considered to be an aftershock of the 2011 earthquake that ultimately resulted in the Fukushima plant accident. “The February 13 earthquake occurred in the vicinity of the rupture area of the March 11, 2011, magnitude 9.1 Tohoku earthquake,” according to the U.S. Geologic Survey. The USGA added that the earthquake was the result of "thrust faulting" near the "subduction zone interface plate boundary between the Pacific and North America plates."

Broken seismometers: TEPCO reported on February 22 that two seismometers at one of its three melted reactors have been out of order since 2020 and did not collect data when the February 13 earthquake struck, according to the Associated Press.

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