Shika 1 criticality rated at INES 2

23 April 2007

The historic criticality at Japan's Shika 1 has been provisionally categorised at Level 2 on the International Nuclear Event Scale (INES). The rating defines the event as an 'incident'. No workers were hurt and there was no release of radiation.

The 15-minute criticality event of 18 June 1999 was the most significant unreported safety-related event to come to light after the Nuclear and Industrial Safety Authority ordered companies operating all types of power plants to purge their records on November 2006. That action was prompted by the revelation that some data relating to hydroelectric dam operation had been mishandled, and some hydroelectric construction had occurred outside of River Law regulations.

The Shika 1 incident occurred during an inspection test in which control rods, which limit the rate of nuclear reactions and therefore the power of the reactor, were rapidly inserted. Three of the reactor's 89 rods moved out of position due to the mistaken operation of the water pressure valves that drive them.

In a boiling water reactor such as Shika 1, control rods enter the reactor core from below and feature emergency stop mechanisms to prevent them falling from the core under gravity for any reason. However, these mechanisms did not prevent the rods moving a number of centimeters. The height of the reactor core is 3.71 m.

Shika 1's owner, Hokuriku Electric Power Company, has put forward two scenarios for the incident. The first is that the reactor reached 2% of operational capacity (~32 MWt) six seconds after the control rods came to a halt. The second envisaged a prompt criticality of 15% of output (~239 MWt) which would have led to a sudden increase in temperature. Hokuriku said that both scenarios showed that output spiked, but never surpassed safe levels and would not have led to damage to nuclear fuel. NISA ordered the reactor to be shut down for checks after the revelation.

The most worrisome aspect of the event for the nuclear industry is the lack of communication between Japanese nuclear operators. After Hokuriku admitted the event - which the company's Toshihiko Takahashi said was covered up - two further companies, Chubu and Tohoku, admitted similar incidents.

Further information

Hokuriku Electric Power Company

Nuclear Industrial Safety Agency

WNN: Reactions to Shika 1 criticality
WNN: Shika 1 ordered to shut down

WNN Event Report: Criticality accident during periodic inspection
WNN Event Reports: The International Nuclear Event Scale (INES) Rating System

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