BREAKING NEWS: TEPCO retracts criticality call (NHK, Nov 3, 2011 15:20 +0900 (JST)
The operator of the Fukushima nuclear power plant has retracted an earlier assessment (see earlier: “TEPCO: Reactor may have gone critical” report) 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.
Xenon at Fukushima not result of ‘critical’ nuclear reactions: TEPCO (Mainichi, Nov 3)
TOKYO (Kyodo) — Tokyo Electric Power Co. said Thursday the detection of radioactive xenon at its stricken Fukushima Daiichi power plant, indicating recent nuclear fission, was not the result of a sustained nuclear chain reaction known as a criticality, as feared, but a case of “spontaneous” fission.
“It is not leading to instability of the reactor or a rise of the radiation level outside,” the utility known as TEPCO said, adding that it does not expect the incident to impact its goal of attaining a stable condition of the plant’s troubled reactors called cold shutdown.
The plant operator said it has judged that spontaneous fission, which it says occurs at a constant rate, has generated xenon-133 and xenon-135 at its crisis-hit No. 2 reactor, as a criticality would have resulted in the recording of concentration levels 10,000 times higher.
The data at hand match estimated levels of xenon produced by sporadic fission of curium-242 and curium-244 inside melted fuel, it said.
It also cited as evidence supporting its view the detection of xenon even after it poured boric-acid solution to absorb neutrons necessary for a fission reaction, and a lack of abnormal rises in the reactor’s temperature or pressure.
Xenon, which was found Tuesday from gas collected from the No. 2 reactor’s primary containment vessel, is believed to have leaked out of the fuel’s cladding tubes due to their meltdown, it said, adding that it plans to continuously monitor gas inside the furnace.
The nuclear crisis at the plant, the world’s worst in 25 years, erupted in the wake of the March 11 earthquake and tsunami, and resulted in the meltdown of nuclear fuel in the six-reactor power complex’s Nos. 1 to 3 reactors.
When it revealed Wednesday that it had detected xenon at the No. 2 reactor, it touched on the possibility that melted fuel inside the reactor may have temporarily gone critical.
Bottle, radium-tainted soil removed in Tokyo Setagaya’s ward (Kyodo Nov 3) | Radioactive bottle removed from Setagaya hot spot (Asahi, Nov 3)
The science ministry on Nov. 2 removed a bottle containing a brown substance believed to be radium that likely caused radioactive hot spots around a supermarket in Tokyo’s Setagaya Ward.
The bottle was found 40 centimeters under the ground near the entrance of the supermarket in the ward’s Hachimanyama district. The partially damaged bottle was 20 centimeters long and 10 cm in diameter.
Ministry officials said the bottle looked very old, but there were no signs that specified a date. They also said they were trying to determine why the bottle was there.
About 40 millisieverts per hour of radiation was detected around the bottle. After it was taken out of the area, the radiation levels of the soil fell to 2 millisieverts/h, according to the Ministry of Education, Culture, Sports, Science and Technology.
The level fell further to about 25 microseiverts/h after bottle fragments and soil were removed, according to the ministry. The bottle and soil were sealed in a special container, officials said.
They concluded the radioactivity detected there was not related to the accident at the Fukushima No. 1 nuclear power plant.
The ministry remeasured levels in and around the supermarket on Nov. 2, and detected 0.2 to 12 microsieverts of radiation at 15 locations, including the ground’s surface and the supermarket floor.
The mystery of where the radioactive material discovered by a supermarket in Tokyo’s Setagaya Ward came from may have been cracked with the Nov. 2 discovery of a buried chemical reagent bottle.
Abnormally high levels of radiation were detected Oct. 28 on the property of a Power Larks supermarket during radiation checks spurred by a local resident’s warning. It was determined that the hot spot was not connected to the Fukushima nuclear plant, as radium was not one of the elements released in the disaster.
Work by the Ministry of Education, Culture, Sports, Science and Technology and other authorities to clear the hot spot and determine its cause has been ongoing for several days, resulting in the Nov. 2 find of the broken 500-milliliter bottle containing a mass of dark red-brown material believed to be radium-226.
After digging out the soil around where the bottle had been, radiation at the site dropped from 40 millisieverts per hour to 25 microsieverts per hour, leading the science ministry to conclude that radium in the bottle was indeed behind the hot spot.
Even after the bottle and surrounding soil were removed, however, a sweep of the supermarket property found 12 places both in- and outside the building emitting relatively high radiation of between 0.2 and 12 microsieverts per hour. Furthermore, there were three spots on the roads bordering the property which registered radiation doses of 0.3 to 2 microsieverts per hour. Airborne radiation around the supermarket hovered around 0.1 microsieverts per hour.
The science ministry and other authorities have said they will continue their examination of the area.
Residents wary about radiation in the wake of the ongoing the crisis at the Fukushima No. 1 Nuclear Power Plant have been uncovering abandoned radioactive substances, with repeated discoveries of radium-266 in Tokyo’s Setagaya Ward.
Radioactive materials were used widely in the past, including in medical products and luminous paints. After World War II, a law requiring people to obtain the government’s permission when using such materials was put into effect, but the law was not always strictly abided by. Now, with many residents armed with dosimeters in the wake of the Fukushima crisis, it is possible that more radioactive materials may be uncovered.
On Oct. 3, a resident with a dosimeter in Tokyo’s Setagaya Ward reported a high level of radiation in an area of the ward to ward officials. When the Ministry of Education, Culture, Sports, Science and Technology and other parties investigated, dozens of glass bottles containing radium-266 were found under the floor of a home in the ward. In Tokyo’s Shinjuku Ward, meanwhile, a former junior high school teacher told school officials that radioactive substances had probably been left at the school, and a small amount of uranyl sulfate was found.
Radium, which is found in nature, was the most widely used radioactive substance in Japan. From before the war, it was used in luminous paint to make watches and measuring equipment glow, and it was also used in radium needles which were inserted into the body on the grounds that they could treat cancer.
However, in 1958, after the harmful effects of radium were uncovered, regulations including the law on prevention of radiation hazards from radioisotopes required people to report the use of such substances to the government, or get permission to use them, depending on the condition of the radioactive material. However, there were repeated cases in which people continued to possess radioactive substances without getting permission, with the materials being left untouched when their owners died.
“In some cases people stored them away carefully, saying they were family treasures. It remained a fact that the law hadn’t seeped in,” said Yoshihide Nakamura, an official from the Japan Radioisotope Association.
Nakamura said the association launched a campaign around the period from 1965 to 1974 — conducted at the request of the former Science and Technology Agency — to have people report unused radioactive substances. This led to many discoveries of radium.
In spite of the campaign, however, not all radium could be collected. In 2000, illegally dumped radium needles were uncovered at a scrap-processing factory in Kobe.
Other radioactive substances have been used at universities and other research organizations without the government’s permission. In 2004, more than 1,000 bottles of waste fluid containing tritium and other substances that were used in previous experiments at Tokushima University were uncovered. Several similar findings have been made over the past few years. In 2005 and between 2009 and 2010, the Ministry of Education, Culture, Sports, Science and Technology asked about 3,000 universities and organizations that handled radioactive substances to conduct inspections. In the second investigation, 60 institutions reported 201 discoveries of such substances.
“There are a lot of old substances, such as those that university professors purchased overseas and left behind when they retired without others realizing,” said Takao Nakaya, head of the Radiation Regulation Office of the Ministry of Education, Culture, Sports, Science and Technology. The government now requires reports to be filed every year to prevent other cases of materials being left undetected.
Since gamma rays are easily released from radium, residents with dosimeters could find more abandoned radium. But Nakamura says people shouldn’t overreact.
“If such substances are found, people shouldn’t overreact. That could lead to the people who discover radioactive substances finding it more difficult to speak out about their discoveries,” he said.
No uncontrolled chain reaction at Fukushima nuclear plant: TEPCO (Japantoday, Nov 3)
The government and Tokyo Electric Power Co., the operator of the tsunami-hit Fukushima No. 1 Nuclear Power Plant, should step up their monitoring of the plant’s reactors to check whether nuclear fission is occurring and provide good explanations of the situation in the reactors to the public.
Xenon was detected in the containment vessel of the plant’s No. 2 reactor, and the finding was confirmed by the semi-governmental Japan Atomic Energy Agency.
Xenon is a radioactive substance with a short half-life generated in the process of nuclear fission of uranium, a nuclear plant fuel. The finding suggests that nuclear fission is occurring in the reactor, and it was initially feared that the reactor temporarily reached criticality — a chain of nuclear fission reactions.
Small-scale criticality temporarily occurring in a reactor would not mean that the reactor is in a dangerous situation. Criticality can be sustained only if certain conditions, such as the right ratio of water and fuel inside the reactor, are met.
Still, we mustn’t let down our guard. The government and TEPCO need to find out why the fission has occurred and take appropriate responses. TEPCO has injected boric acid into the No. 2 reactor to prevent it from reaching criticality, but more may need to be done.
Even if the reactor is not in a dangerous condition, the possibility that it might reach criticality has surely caused anxiety to the public. It is important for TEPCO and the government’s Nuclear and Industrial Safety Agency (NISA) to clearly explain the condition of the reactors and the implications of the discovery of xenon.
The xenon was detected from gas inside the reactor’s containment vessel using a containment vessel gas management system that began operations in late October. It was the first time that such a measurement was conducted, and at least one expert says it would be no surprise if nuclear fission has been occurring since well before these latest findings.
The gas inside the containment vessels of the Fukushima plant’s No. 1 and 3 reactors has not yet been analyzed. The government and TEPCO should carry out similar examinations of the inside of their containment vessels as soon as possible to check for nuclear fission occurring in them.
More than these individual measurements, however, what is needed is a system that constantly monitors the conditions of all the nuclear reactors at the plant. Officials should consider equipment that can detect neutrons, as they can be used as a direct indicator of whether criticality has occurred.
The government and TEPCO have announced their intention to bring forward the timing of achieving a so-called “cold shutdown” of the crippled reactors in their roadmap to bringing the nuclear plant under control. They define a cold shutdown as a situation in which the temperature inside the pressure vessels of the reactors are kept sufficiently low and the emissions of radioactive substances are under control.
However, the discovery of xenon in the No. 2 reactor’s containment vessel suggests nuclear fission is occurring even though the temperature at the bottom of the pressure vessel is thought to be below 100 degrees Celsius. It raises questions as to whether such a reactor can be considered stable, even if it is under that temperature threshold.
The conditions and locations of melted fuel in the plant’s No. 1 to 3 reactors as well as the details of the damage to the reactors remain unclear, and after this most recent finding, the government and TEPCO must step up their monitoring.
For earlier spot updates see this page.