Nuclear experts largely agree with Tokyo Electric Power Co.'s assessment that a state of criticality was not reached at the No. 2 reactor of the crippled Fukushima No. 1 nuclear power plant.
But some raised questions about the company's change in stance, saying it underscores TEPCO's struggles to assess the actual conditions inside the damaged reactors at the plant.
TEPCO said Nov. 3 that spontaneous nuclear fission, which can occur naturally in reactors that have been shut down, generated radioactive xenon that was detected in the containment vessel of the No. 2 reactor.
The previous day, TEPCO officials said they could not rule out the possibility that the damaged fuel reached a self-sustaining chain reaction known as criticality, creating the xenon. That announcement led to widespread concerns that fuel in the reactor may have recently been out of control, given the relatively short half lives of the two types of xenon found.
However, TEPCO later said the amount of xenon detected was just 1/10,000th the level that is normally generated in a criticality incident. Instead, the xenon amount was more in line with the spontaneous fission of curium, TEPCO officials said.
Koji Okamoto, a professor of nuclear engineering at the University of Tokyo, agreed with TEPCO's latest assessment.
"I also calculated the amount of xenon from the data that TEPCO released and found it was about the same as the amount produced due to spontaneous fission," Okamoto said.
Kenji Sumita, a professor emeritus of nuclear engineering at Osaka University, said the results of the process that TEPCO used to absorb neutrons and prevent criticiality in the reactor supported the view that spontaneous fission had occurred.
"They injected boric acid solution, but the amount of xenon did not change. It is proof that the xenon was the result of spontaneous fission, not of criticality," Sumita said.
Hiroaki Koide, an assistant professor at the Research Reactor Institute of Kyoto University, also pointed out that specific conditions are generally required for nuclear fuel to reach criticality.
"In principle, the more the fuel's shape and location change due to an accident, the more unlikely criticality would occur," he said.
The fuel rods in the No. 2 reactor are believed to have melted, forming a misshapen lump at the bottom of the pressure vessel.
But how the fuel has melted or whether it has leaked through to the containment vessel remains unknown.
High radiation levels have prevented TEPCO workers from installing gauges and other equipment that could more accurately determine the internal conditions of the reactors.
TEPCO, for now, is depending largely on computer simulations based on temperatures and pressure levels in the reactors to estimate what is going on inside.
"The problem is that TEPCO pointed to the possibility of 'criticality' taking place, if only temporarily," said Fumiya Tanabe, director of the Institute of Nuclear Safety System Inc. "It eventually showed that the utility failed to predict the situation of fuel rods or the present internal state of the reactor. It should imagine the worst possible situation and deal with it for the recovery."
The Nuclear and Industrial Safety Agency of the Ministry of Economy, Trade and Industry said it will examine TEPCO's data and make its own analysis. NISA has already said it suspects spontaneous fission took place, not criticality.
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