Japanese stress test results approved

13 February 2012

Japan's nuclear safety regulator has endorsed the results of stress tests at two idled nuclear power reactors, clearing the way for their possible restart. Meanwhile, a sharp rise in the temperature recorded in unit 2 of the Fukushima Daiichi plant could be the result of a faulty thermometer.

Ohi (Kepco)
Ohi (Image: Kansai Electric)

The Nuclear and Industrial Safety Agency (NISA) has submitted a report to Japan's Nuclear Safety Commission (NSC) evaluating the results of stress tests at units 3 and 4 of Kansai Electric Power Company's Ohi nuclear power plant in Fukui prefecture. The agency concluded that the utility has taken sufficient measures at the units to prevent a similar accident to the one at the Fukushima Daiichi plant, even if the plant was hit by a comparable earthquake and tsunami. These measures include new used fuel pool refill systems, mobile back-up generators and robust stores of emergency equipment.

NISA endorsed Kansai's evaluation that Ohi 3 and 4 would be able to withstand an earthquake with ground acceleration of up to 1260 gal (1.8 times its design basis of 700 gal) and a tsunami 11.4 metres in height (four times its design basis). In addition, should the units lose all off-site power supply, the reactors could still be kept cool for up to 16 days through the use of fire engines pumping water, while the used fuel pools could be cooled for up to 10 days. However, NISA said that it needs to confirm Kansai's further investigation of tsunami that have occurred in the area in the past, as well as its studies into how active fault lines in the region could be connected.

Two-phase stress tests were mandated by the Japanese government following the March 2011 Fukushima Daiichi nuclear power plant accident. Utilities are required to examine the safety margin of important pieces of equipment in accordance with guidelines set by NISA and NSC. Based on the results of these initial tests, carried out while units are shut down for inspections, the government is to decide whether a reactor can or cannot resume operation. No date has been slated for the first restart, and even with the go-ahead from NISA power companies would still require approval from local prefectural governments. Although this is not a legal requirement, the deference traditionally shown by power companies to local officials has become mandatory in post-Fukushima Japan.

Ohi unit 3 was the first Japanese nuclear power plant to complete the first step of the mandatory stress tests in October 2011. Ohi 3 and 4 would be the first reactors to restart following inspections, if given approval. Currently only one of Kansai’s eleven reactors - unit 3 at its Takahama plant - is in operation, but that is due to shut for inspections later this month.

A ten-strong mission from the International Atomic Energy Agency (IAEA) visited Japan in late January to review the methodology of the stress tests carried out at Kansai's Ohi nuclear power plant. The team concluded that Japan's stress tests are generally consistent with IAEA safety standards, but made a range of recommendations to NISA to ensure thorough and lasting improvements in safety are made.

Step two of the stress tests will involve a comprehensive safety assessment of all reactors and will be conducted to enhance the reliability of safety checks. This will be an overall evaluation that relates closely to the stress tests carried out in the European Union and elsewhere.

Faulty thermometer?

Tokyo Electric Power Company (Tepco) has increased the rate of water injection and added boron to unit 2 of the damaged Fukushima Daiichi plant due to a sensor indicating increasing temperatures at the lower part of the unit’s reactor pressure vessel (RPV).

Data from the thermometer suggests that the temperature in that part of the RPV had steadily risen from around 50°C to over 90°C over the past ten days. Today, the sensor indicated a temperature of over 285°C. However, sensors at two other locations in the lower part of unit 2's RPV continued to show temperatures of around 45°C. The increased water flow even brought these readings down to just above 30°C.

"Following our cooling efforts, temperatures at the two other locations are declining steadily while that at the location in question keeps rising," Tepco general manager Junichi Matsumoto was quoted as saying by Reuters. "This leads us to think that the thermometer at the location in question is not functioning properly, rather than the actual temperature rising." The absence of steam supports this hypothesis, he added.

Researched and written
by World Nuclear News