The discovery of xenon in the Fukushima No. 1 nuclear plant’s reactor 2 earlier this week was likely a result of “spontaneous fission,” a nuclear expert said Friday, agreeing with an earlier conclusion by Tokyo Electric Power Co. that some of the melted fuel did not reach the condition of “criticality,” or sustained chain reaction, of nuclear fission.
But Katsutada Aoki, an expert in nuclear engineering who headed the reactor physics division of the Atomic Energy Society of Japan, also criticized Tepco, saying the utility could have done a better job in analyzing the implications of the newly discovered xenon gases and avoided spreading needless fear that a nuclear chain reaction might have restarted.
“The discovery of xenon in the reactor is no reason to fear anything serious,” Aoki told The Japan Times.
Spontaneous fission should not be confused with nuclear criticality, especially since both the temperature and the pressure levels have remained stable in the reactor, Aoki said.
“(The) chances of a criticality taking place is not zero — but it is practically (unthinkable) at this point,” he added.
Tepco revealed Wednesday that it found one one-hundred-thousandth of a becquerel per 1 cubic centimeter of xenon-133 and xenon-135 in gas samples from reactor 2, saying it might indicate the melted fuel in the reactor could have briefly reached criticality since xenon can be generated through such nuclear fission.
But one day after the announcement, Tepco denied criticality had occurred, saying it found the amount of xenon was too small to be generated through fission via criticality.
Had the nuclear reaction attained that level, there would have been a concentration of approximately 10,000 times more xenon, Junichi Matsumoto told a news conference Thursday.
According to Aoki, spontaneous fission is a phenomenon in which radioactive decay occurs without a neutron or other particle striking the atom.
But during criticality, neutrons emitted from fission trigger two or more fissions and this reaction is repeated in a sustained way, generating considerable heat and raising pressure inside the reactor.
“Neutrons are released even in the case of spontaneous fission, but they are absorbed by other particles before the next reaction begins,” Aoki explained.
Spontaneous fission takes place regularly even inside an idle reactor, so it is also natural for Tepco to find traces of xenon in the reactor, Aoki said.
In the case of reactor 2, curium 242 and 244, which were among the nuclear fuel, are believed to have naturally decayed without a chain reaction occurring, releasing extremely small amounts of xenon.
According to Tepco, the amount of xenon detected corresponded with the theoretical amount of the gases that could have been created through spontaneous fission of the curium that was held inside reactor 2.
Aoki, who has also served as a researcher for Toshiba Corp.’s nuclear reactor division, said Tepco’s initial announcement and later downplaying of the possibility of a criticality event only adds to the impression that the firm does not have the situation under control, although the temperature and pressure inside the reactor remained stable.
Matsumoto, the Tepco spokesman, acknowledged that the explanation may have caused excessive concern to the public.
“We only mentioned (nuclear criticality) because the analysis was incomplete at that point and there was a possibility” of such a scenario, he said. “Its regrettable that it caused unease since the word ‘criticality’ is easily misinterpreted as a serious condition by the public.”
On Wednesday, Tepco took precautions and poured a mixture of water and boric acid into the reactor to prevent a chain reaction. It was the same measure as was taken in the middle of the meltdown crisis in March.
“Tepco went ahead and used boric acid as a preventive measure, but in my opinion even that was unnecessary,” Aoki said.
Detailed radiation map
The Environment Ministry said Friday it will conduct a detailed survey and compile a map on airborne radiation levels in parts of Fukushima Prefecture most affected by the nuclear crisis to help draw up decontamination measures.
Monitoring will begin Monday in Iitate and continue through February, covering mainly residential areas in the no-entry zone within 20-km of the Fukushima No. 1 plant as well as other designated evacuation areas with relatively high radiation levels.