Tokyo Electric Power Co. said Nov. 3 that naturally occurring "spontaneous nuclear fission"--not a state of criticality--created the xenon detected at the Fukushima No. 1 nuclear power plant.
Tokyo Electric Power Co. said Nov. 3 that naturally occurring "spontaneous nuclear fission"--not a state of criticality--created the xenon detected at the Fukushima No. 1 nuclear power plant.
The company said it reached that conclusion after evaluating the state of nuclear fission based on the concentration of xenon detected and temperatures and pressure levels at the No. 2 reactor of the crippled plant.
Instead of being caused by self-sustaining nuclear fission, the xenon was likely generated by spontaneous fission of curium and other radioactive substances derived from the reactor fuel, TEPCO said. Spontaneous fission takes place in suspended nuclear reactors under normal conditions.
The company also said the xenon discovery will not affect its schedule of having all of the damaged reactors reach a stable "cold shutdown" state by the end of this year.
Radioactive xenon-133 and xenon-135 were detected Nov. 1 during an analysis of gas purified by a control system in the containment vessel of the No. 2 reactor. Xenon-135 was again detected on Nov. 2.
Xenon-133 and xenon-135 have relatively short half-lives of about five days and about nine hours, respectively, which raised concerns that nuclear fission had taken place in recent days inside the reactor.
TEPCO officials initially said they could not rule out the possibility that some parts of the fuel rods had reached criticality, which can be caused when uranium fuel and water are positioned in a well-balanced manner under certain conditions.
Fuel rods in the No. 2 reactor are believed to have melted and fallen to the bottom of the pressure vessel, partially leaking to the containment vessel.
The Nuclear and Industrial Safety Agency (NISA) on Nov. 2 confirmed that radioactive xenon had been detected, but it raised doubts that criticality had been reached. NISA said xenon continued to be detected even after the injection of boric acid water, which absorbs neutrons. Neutrons are a requisite for criticality to occur.
NISA officials pointed to the possibility of spontaneous fission, which can occur even in the absence of neutrons.
TEPCO adopted that view on Nov. 3.
Yet questions remain about the interior state of the reactors. Scant data had been available, other than readings of thermometers and pressure gauges, that could accurately show the situation inside the reactor containment vessels.
TEPCO on Oct. 28 started operating equipment to block the discharge of radioactive substances from the containment vessel at the No. 2 reactor. This made it possible to analyze the composition of gas within the vessel.
However, neutron detectors to check for nuclear fission, water level gauges in the reactors, and other equipment indispensable in monitoring the interior state of the reactors have remained out of order since the March 11 tsunami swamped the nuclear plant.
Nuclear fission could also be taking place at the No. 1 and No. 3 reactors, where fuel is believed to have melted. But gas measurements are not immediately possible at those reactors because high radiation levels are hampering the installation of a device to control gas within the containment vessel. That device was used to detect the xenon at the No. 2 reactor.
TEPCO officials said large-scale criticality incidents were unlikely at the plant because the water-injection system was cooling down the reactors on a sustained basis.
Continued pumping of water into the No. 1, No. 2 and No. 3 reactors brought their temperatures below 100 degrees in late September.
The government defines a cold shutdown as a state in which the temperature at the bottom of the pressure vessel is 100 degrees or lower and the amount of radioactive substances discharged from the reactor is suppressed at low levels.
"We do not believe that there is any impact on achieving the goals in our road map toward bringing the crisis under control," Junichi Matsumoto, acting general manager of TEPCO's Nuclear Power and Plant Siting Division, said Nov. 2. "But how to monitor and control nuclear fission will be a challenge for the future."
Another challenge that keeps popping up is swiftly exchanging information about the Fukushima nuclear crisis.
Industry minister Yukio Edano on Nov. 2 admonished NISA Director-General Hiroyuki Fukano for a delayed report on the suspected nuclear fission at the plant, according to Chief Cabinet Secretary Osamu Fujimura.
Fujimura said TEPCO informed NISA of the situation on the night of Nov. 1, but the agency did not relay the information to the prime minister's office until after 7 a.m. the following day.
NISA informed Edano still later, and Fujimura himself learned about the situation only after he came to the prime minister's office past 9 a.m.
"We should have been informed immediately of any indication of possible nuclear fission," Fujimura said.
(This article was written by Hidenori Tsuboya and Tatsuyuki Kobori.)