29 Dec (NucNet): Japan�s Chubu Electric Power Company plans to decommission two non-operational reactor units at its Hamaoka nuclear power plant in Shizuoka prefecture and to build a replacement sixth unit at the site.

The Japan Atomic Industrial Forum (JAIF) said the proposals were announced on 22 December.

The proposals are for the Hamaoka-1 and -2 boiling water reactor units to be decommissioned and for Hamaoka-6 to be constructed on the eastern side of the plant site. The new unit could start operation as early as 2018.

Chubu has estimated that it would need to invest some 300 billion Japanese yen (about 2.4 billion euro) in seismic protection measures at Hamaoka-1 and -2, which were both in operation for three decades. The company has therefore chosen to build a replacement unit instead.

JAIF said voluntary measures had already been completed to increase seismic tolerance margins at units 3, 4 and 5 of the Hamaoka plant to margins of about 1,000 Gal (centimetres per second squared, or cm/s2) on open foundation surfaces*. These three units are currently in operation, JAIF added.

Hamaoka-1, a 515-megawatt unit first connected to the grid in 1974, was shut down for its 19th periodic outage in April 2002. Hamaoka-2, an 806-megawatt unit first connected to the grid in 1978, was shut down for its 20th periodic outage in February 2004.

In January 2006, Chubu said that the outages of the two units would be extended by three years to March 2011. However, Chubu now says the units� performance in terms of economy and safety would not be sufficient to return them to service.

* In the earthquake that struck Tokyo Electric Power Company�s Kashiwazaki-Kariwa nuclear plant in Niigata prefecture in July 2007, the seismographs on unit 1 showed that maximum horizontal seismic acceleration ranged from 311 Gal to 680 Gal. The scram settings (the point at which a nuclear reactor unit will automatically safely shut itself down) at Kashiwazaki-Kariwa were 120 Gal for seismic movement in a horizontal direction and 100 Gal for vertical movements.

According to the US Geological Survey, the quake measured 6.6 on the Richter scale and subsequent data indicated that peak ground acceleration (PGA) exceeded design limits for the plant.

A report from an International Atomic Energy Agency�s mission on lessons learned from the Kashiwazaki-Kariwa experience is expected to be made publicly available in January 2009.

� by John Shepherd

>>Related reports in the NucNet database (available to subscribers)

Chubu Electric Extends Periodic Outages For Two Hamaoka Units (News No. 26, 31 January 2006)

IAEA Team Completes Third Kashiwazaki-Kariwa Mission (News in Brief No. 140, 11 December 2008)

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