Japan:TEPCO retracts criticality call +Related Article
Laaska News Nov. 3 ,2011
The operator of the Fukushima nuclear power plant has retracted an earlier assessment that a continuous nuclear reaction, or a criticality, could have taken place in the damaged Number 2 reactor.
The Tokyo Electric Power Company, or TEPCO, said on Thursday that the small amount of xenon-135 it detected in gas taken from the reactor’s containment vessel was the result of the spontaneous nuclear fission of radioactive curium-242 and -244. The two substances are contained in nuclear fuel.
The amount of xenon-135 detected almost matched the amount that would have been produced if the radioactive curium in the fuel had undergone spontaneous fission.
TEPCO says a criticality event would have resulted in higher levels of xenon concentration.
Spontaneous fission refers to the nuclear fission of radioactive materials other than uranium, and it does not lead to criticality. Such fission is said to occur constantly.
The earlier detection of small amounts of Xenon-135 had suggested the possibility of a criticality occurrence in the melted fuel in the damaged reactor.
TEPCO sys it will send the assessment to the Nuclear and Industrial Safety Agency for reevaluation.
Shortcomings in nuclear safety assessment found
The organization in charge of assessing the safety of Japan’s nuclear plants has admitted it allowed nuclear fuel rods to pass quality checks using a faulty manual.
The manual was borrowed from the Japanese manufacturer of the rods.
The Japan Nuclear Energy Safety Organization conducted the checks in 2008 on 4 sets of fuel rods for reactors.
The government-backed organization says it approved 3 of the 4 sets even though the manual said the rods were 3 to 5 centimeters shorter than the actual length of 4 meters.
It says the examiners failed to notice the mistakes as they did not closely check the manual beforehand.
Industry watchdog, the Nuclear and Industrial Safety Agency, says it will order the organization to correct these shortcomings and improve its screening procedures.