Fukushima earthquake leaves nuclear plants unaffected

Tepco found no "abnormalities" at its Fukushima plants following an off-shore earthquake on Tuesday morning. The cooling system in the used fuel pool of Tepco's Fukushima Daini unit 3 automatically shut but was subsequently restarted. The Japan Meteorological Agency (JMA) issued tsunami warnings following the magnitude 7.4 quake.

The quake took place at 5.59am, with its epicentre off the Fukushima coast. It was originally classed as magnitude 7.3 and later upgraded to 7.4 by the JMA.

Cooling functions at unit 3's used fuel pool stopped automatically at 6:10 am. Tepco said at a press conference, broadcast by Japan's NHK, this was triggered by changes in water levels in the pool due to vibrations from the earthquake. Tepco restarted the cooling system at 7:47 am after verifying that it was undamaged.

The pool is used to store irradiated nuclear fuel rods removed from the Fukushima Daini reactors, which have not operated since 2011. Without cooling functions, the temperature of the pool would be expected to increase at a rate of 0.2 degrees Celsius per hour. This would mean that even in the absence of any cooling, it would take about a week for the temperature of the water to reach 65 degrees, the limit for safe operation.

Tepco said the temperature of the pool had been recorded at 28.7 degrees (measured at the cooling pump suction port) at the time cooling was stopped and had risen to 29.5 degrees (measured at the pool surface) by the time it restarted. Taking into account the different locations at which the measurements were taken, this equated to a pool temperature rise of 0.2 degrees, the company 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.

Work to transfer contaminated water at the Fukushima Daiichi site was temporarily suspended to allow equipment to be checked for damage. It was restarted after inspections found no abnormalities, Tepco said.

Fukushima Daini is a four-unit boiling water reactor plant about 11km south of Fukushima Daiichi. Fukushima Daini unit 3 was not damaged by the March 2011 earthquake and tsunami that caused the nuclear accident at the neighbouring Fukushima Daiichi plant.

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, 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.

The JMA issued tsunami warnings in Fukushima and Miyagi prefectures, and advisories for Iwate and Ibaraki prefectures, in anticipation of tsunami waves of up to three metres in height. According to the JMA a wave of 140 cm was recorded at Sendai port, with smaller waves recorded at other locations along the coastline. The JMA lifted the tsunami warnings and advisories at 12:50pm.

Today's earthquake was significantly smaller than the Great East Japan Earthquake of 11 March 2011, which measured 9.0 and created tsunami waves up to 15 metres high, devastating wide areas of Miyagi, Iwate and Fukushima prefectures.

Researched and written
by World Nuclear News