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Monju nuclear reactor reaches criticality after 14-year shutdown

Japan Atomic Energy Agency workers, under the watch of safety inspectors, check that the Monju nuclear reactor has reached criticality, on Saturday, May 8, in Tsuruga, Fukui Prefecture. (Pool photo)
Japan Atomic Energy Agency workers, under the watch of safety inspectors, check that the Monju nuclear reactor has reached criticality, on Saturday, May 8, in Tsuruga, Fukui Prefecture. (Pool photo)
The Monju nuclear reactor is pictured in Tsuruga, Fukui Prefecture, on Thursday, May 6. (Mainichi)
The Monju nuclear reactor is pictured in Tsuruga, Fukui Prefecture, on Thursday, May 6. (Mainichi)

TSURUGA, Fukui -- The Japan Atomic Energy Agency's Monju fast-breeder nuclear reactor reached criticality on Saturday, two days after resuming operations following an accident over 14 years ago.

Agency officials began pulling out the last of the prototype reactor's 19 control rods at 8:49 a.m. on Saturday. At 10:36 a.m., the output of the reactor reached 0.03 percent of capacity as calculated, and officials announced that it had reached criticality -- the point where a nuclear reaction is sustained.

A total of five start-up and stoppage tests will be performed until late July to examine the reactor's performance, and its output will be raised to 1.3 percent of capacity.

Between Thursday and Friday, an alarm on a device to measure the amount of radiation sounded six times. The warnings were judged to be caused by a fault, but officials from the Japan Atomic Energy Agency and the Ministry of Economy, Trade and Industry's Nuclear and Industrial Safety Agency deemed that the process to bring the reactor to criticality would not be affected, and work proceeded as scheduled.

However, the fact that the announcement of the first alarm sounding was delayed by half a day prompted Nobuaki Terasaka, director-general of the Nuclear and Industrial Safety Agency, to summon Japan Atomic Energy Agency President Toshio Okazaki and issue him a verbal warning.

The agency will inspect the radiation detection device between May 16 and 20, when the reactor is stopped, and investigate the cause of the problem.

(Mainichi Japan) May 8, 2010

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