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Tuesday, Feb. 14, 2012

Oi reactor stress tests approved by NISA


Staff writer

The Nuclear and Industrial Safety Agency, in its first approval of stress tests for nuclear facilities, said Monday the level of quake and tsunami resistance of two reactors at the Oi power plant in Fukui Prefecture is adequate.

NISA submitted its report, which basically recommends restarting units 3 and 4 at the plant operated by Kansai Electric Power Co., to the Nuclear Safety Commission of Japan. After the commission approves the report, NISA will ask local governments for permission to fire the reactors back up.

Whether or when the reactors will resume operations remains unclear because it will be difficult to gain local residents' support due to the Fukushima nuclear disaster continuing since last March.

Nonetheless, NISA's approval of the stress test, which Kepco conducted and submitted a report to NISA in October and November, is a necessary step to resume reactors suspended for periodic checks.

Only three of Japan's 54 commercial reactors are currently in operation and those three will be offline by May.

"We believe appropriate measures are currently being taken at reactors 3 and 4 of the Oi power plant to prevent . . . situations similar to what Fukushima No. 1 plant is facing, even if they experience quakes and tsunami equivalent to the ones that hit the Fukushima plant," NISA says in the report.

"Also, we acknowledge Kansai Electric is continuing to work on improving safety even more," it states.

In essence, NISA says in the report that reactors 3 and 4 are strong enough to resist quakes and tsunami bigger than those of March 11.

However, the report is unlikely to ease the concerns of local residents and antinuclear activists.

The report states that the level of shaking that hit the Fukushima No. 1 plant was 675 gals, or 12.5 percent more than the plant's designated resistance level of 600 gals, and the height of the tsunami was 15 meters, 9.5 meters more than the anticipated 4.5 meters.

One gal equals a change in rate of motion of 1 cm per second per second. It is a unit to measure peak ground acceleration in earthquakes, which basically means the level of shaking.Kansai Electric conducted the stress test under the assumption that the reactors could face shaking equal to of 1,260 gals, or 80 percent more than the designated resistance level of 700 gals, and a tsunami of 11.4 meters, 8.55 meters higher than the anticipated 2.85 meters.

NISA said the Oi reactors' cooling equipment and other facilities are properly located and maintained so that the assumed quake and tsunami would not knock them out.

The agency also noted that nuclear fuel rods inside the Oi reactors wouldn't suffer damage for 7.2 days even if power to their cooling systems is lost and left without external assistance.

"Providing that the gal figure that hit the Fukushima No. 1 plant was just about 10 percent more than anticipated, the assumption for the Oi plant — 1.8 times stronger than anticipated — is sufficient," NISA says in the report.

"Also, there is no tectonic plate boundary near Wakasa Bay that would cause large-scale tsunami such as the one that hit Fukushima No. 1 plant. Taking into account historical records of past tsunami in the area, the 11.4-meter tsunami assumption is sufficient," NISA said.

Besides preparedness of equipment and facilities, NISA checked that Kepco's assumptions in the stress tests left a big enough safety margin. For example, the assumptions included that other reactors at the Oi plant were also damaged and that the utility wouldn't get any outside help.

NISA, however, found that Kansai Electric did not conduct the stress tests under the assumption that the reactors would face tsunami higher than 11.4 meters and shaking of more than 1,260 gals.

NISA also said there are several points where Kansai Electric needs to improve.

For example, NISA recommended that the utility prepare for transporting necessary workers via helicopter in case employee dormitories and access routes are damaged.



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The Japan Times

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