TEPCO: Melted fuel ate into containment vessel (NHK, Nov 30, 2011)
The operator of the damaged Fukushima Daiichi nuclear power plant has announced the results of an analysis on the state of melted fuel in the plant’s Number 1 unit.
The Tokyo Electric Power Company, or TEPCO, and several research institutes made public their analyses on the melting of fuel rods at 3 of the plant’s units at a government-sponsored study meeting on Wednesday. The analyses were based on temperatures, amounts of cooling water and other data.
TEPCO said that in the worse case, all fuel rods in the plant’s Number 1 reactor may have melted and dropped through its bottom into a containment vessel. The bottom of the vessel is concrete covered with a steel plate.
The utility said the fuel may have eroded the bottom to a depth of 65 centimeters. The thinnest part of the section is only 37 centimeters thick.
TEPCO also said as much as 57 percent of the fuel in the plant’s Number 2 reactor and 63 percent in the Number 3 reactor may have melted, and that some of the melted fuel may have fallen through reactor vessels.
Melted nuclear fuel eroded reactor container by up to 65 cm: TEPCO (Mainichi, Nov 30)
TOKYO (Kyodo) — The operator of the crippled Fukushima Daiichi nuclear power plant said Wednesday that the concrete base of the No. 1 reactor container had been eroded by up to 65 centimeters when the fuel inside melted, although the steel container itself was left intact.
According to Tokyo Electric Power Co.’s analysis, all of the fuel inside the No. 1 reactor melted after cooling functions failed in the wake of the March 11 earthquake and tsunami, with a substantial amount of the fuel melting through the base of the reactor pressure vessel and dropping into the outer primary container.
If the erosion had expanded another 37 cm, the damage would have reached the steel wall, according to the utility known as TEPCO.
As for the Nos. 2 and 3 reactors, which also experienced meltdowns, the amount of fuel that dropped to the bottom of the pressure vessel is estimated to be around 60 percent.
Fukushima No. 1 nuclear plant’s No. 1 reactor building, with its nearly-completed cover, is seen on Oct. 8 in this photo provided by Tokyo Electric Power Co.
The bottom of the two reactors’ pressure vessels is unlikely to have been damaged on a large scale. But if the fuel had melted through the vessels, the primary container of the No. 2 reactor could have been eroded by 12 cm and that of the No. 3 reactor by 20 cm, TEPCO said.
Currently, the melted fuel inside the Nos. 1 to 3 reactors is believed to be cooled by water injection and no further erosion is occurring, it said.
Study shows deeper meltdown than thought at Fukushima nuclear reactor (Japan Today, Dec 1)
TOKYO — Radioactive debris from melted fuel rods may have seeped deeper into the floor of a Japan’s tsunami-hit nuclear reactor than previously thought, to within a foot from breaching the crucial steel barrier, a new simulation showed Wednesday.
The findings will not change the ongoing efforts to stabilize the reactors more than eight months after the Fukushima Daiichi plant was disabled, but they harshly depict the meltdowns that occurred and conditions within the reactors, which will be off-limits for years.
The plant operator Tokyo Electric Power Co said its latest simulation showed fuel at the No. 1 reactor may have eroded part of the primary containment vessel’s thick concrete floor. The vessel is a beaker-shaped steel container, set into the floor. A concrete foundation below that is the last manmade barrier before earth.
The fuel came within a foot of the container’s steel bottom in the worst-case scenario but has been somewhat cooled, TEPCO’s nuclear safety official Yoshihiro Oyama said at a government workshop. He said fuel rods in the No. 1 reactor were the worst damaged because it lost cooling capacity before the other two reactors, leaving its rods dry and overheated for hours before water was pumped in.
The nuclear crisis following the March 11 earthquake and tsunami caused massive radiation leaks and the relocation of some 100,000 people.
Another simulation on the structure released by the government-funded Japan Nuclear Energy Safety Organization, or JNES, said the erosion of the concrete could be deeper and the possibility of structural damage to the reactor’s foundation needs to be studied.
JNES official Masanori Naito said the melting fuel rods lost their shape as they collapsed to the bottom of the vessel, then deteriorated into drops when water pumping resumed, and the fuel drops spattered and smashed against the concrete as they fell, Naito said.
TEPCO and government officials are aiming to achieve “cold shutdown” by the end of the year _ a first step toward creating a stable enough environment for work to proceed on removing the reactors’ nuclear fuel and closing the plant altogether.
The government estimates it will take 30 years or more to safely decommission Fukushima Daiichi.
Wednesday’s simulations depict what happened early in the crisis and do not mean a recent deterioration of the No. 1 reactor. Oyama said, however, the results are based only on available data and may not match the actual conditions inside the reactors, which cannot be opened for years.
Some experts have raised questions about achieving the “cold shutdown,” which means bringing the temperature of the pressure vessel containing healthy fuel rods to way below the benchmark 100 Celsius (212 Fahrenheit). They say the fuel is no longer there and measuring the temperature of empty cores is meaningless, while nobody knows where and how hot the melted fuel really is.
Kiyoharu Abe, a nuclear expert at JNES, said it’s too early to make a conclusion and more simulations should be done to get accurate estimates.
“I don’t think the simulation today was wrong, but we should look at this from various viewpoints rather than making a conclusion from one simulation,” Abe said. “It’s just the beginning of a long process.”
Rice shipments banned from part of Fukushima Pref. for contamination (Kyodo, Nov 29) | Radioactive rice reaches public; more shipments suspended(Asahi, Nov 30) FUKUSHIMA–Nine kilograms of rice exceeding safety standards for radiation were sold to consumers, the Fukushima prefectural government said, as shipments of rice from two more districts in the prefecture were suspended….
Related: Farmer who unknowingly shipped radioactive rice says prefecture’s testing not enough (Mainichi, Nov 30)
DATE, Fukushima — “I shipped my crops trusting Fukushima Prefecture’s declaration about crop safety. It’s really a shame that things turned out as they have,” says farmer Ichiko Takahashi, whose rice was found to be over the government’s radiation limit after part of it was sold to consumers.
“If the testing methods are not changed, the same problem (of crops over the government limit making it through) will happen again.”
Even after the prefecture made its announcement that crops were safe in October, Takahashi had doubts that the radiation tests were sufficient and brought her crops in herself for testing. The results she received on Nov. 18 were under the provisional government limit of 500, but not by much, at 476 becquerels of cesium.
She had already sent 18 kilograms of mochi rice for sale to customers. Afraid she would lose customers’ confidence by selling the rice as it was, she asked for its sale to be halted and she took it to be tested at Fukushima University. A few days later she got news that it was over 700 becquerels. On Nov. 28, she also got news from the prefectural government that testing on her rice there had found it over the government limit.
Of the 18 kilograms of rice, 10.5 kilograms had already been sold. Nine of those kilograms were to unknown buyers, something that worries her.
Takahashi has a reputation in the area as a knowledgeable vegetable farmer and has taught elementary school students how to grow sweet potatoes and rice.
Says Takahashi: “I’ve farmed until now under the philosophy that you mustn’t make anything dangerous. I tried to keep radiation out by not using water from the mountain rivers. What more could I have done?”
Fukushima plant head told workers to disregard order on water injection (Mainichi, Nov 30) | Fukushima plant chief takes sick leave, to be replaced(Asahi, Nov 30)
The head of the stricken Fukushima No. 1 nuclear power plant is stepping down for undisclosed health reasons and will be replaced in December.
Nuclear cleanup volunteers’ invisible enemy (Yomuri, Dec 1) Excerpts below…
“DATE, Fukushima–Some local governments in Fukushima Prefecture have started decontamination operations using volunteers to remove radioactive substances released after the accident at the Fukushima No. 1 nuclear power plant.
One work site in particular had a peculiar atmosphere, in which volunteers fought invisible, potentially hazardous radiation particles with simple equipment, making them appear to be cleaning a garden.
I participated in the decontamination volunteer work at a specific spot recommended for evacuation in Date, about 50 kilometers from the crippled nuclear plant.
At 9:30 a.m. Saturday, a bus carrying 33 participants arrived in the city’s Tsukidate-Kitanosawayama district. Most of them were middle-aged or elderly men from other prefectures. There were six women including me.
In the basin in which rice paddies after the harvest could be seen in abundance, houses stood as if they were being supported by the mountain slopes.
We were provided with body warmers, dust masks and dosimeters. The radiation level I measured above the ground was about three microsieverts per hour.
Yearly exposure at this rate could exceed 20 millisieverts, which requires that the site be designated as a specific spot recommended for evacuation.
“We know there’s radiation here,” one participant said.
And yet the place was situated in such serene surroundings. We were surprised again.
We conducted the day’s decontamination work at sites near two barns. We collected fallen leaves and put them into garbage bags, tying the top of the bags with adhesive tape.
We then scraped off topsoil with shovels and sickles. The bags were heavier than I expected as the contents contained a great deal of moisture. I carried the bags, shaky on my feet, to a temporary storage site on the premises.
When a city government official measured radiation levels below the eaves troughs of the barns and greenhouses, the meter read more than 10 microsieverts.
We were surprised again to see the wide range of radiation levels at different spots at the same location.
An hour after beginning the work, the mask made my face feel sweaty and itchy, but I could not remove it for fear of the radiation stirred up from the mud.
Just removing fallen leaves and scraping off the surface soil cut the radiation levels in half. Further digging up the soil to a depth of five centimeters brought the level down to less than two microsieverts.”… Read more here.
Couple commits suicide in car with jar of radioactive thorium in Niigata (Japan Today, Dec 1)
NIIGATA — Police said Wednesday that a couple in their 60s was found dead in a car in Niigata Prefecture, having apparently committed suicide.
According to NTV, the couple was found dead in a car in a wooded area in Nishi Ward at about 9:30 a.m. Inside the car, police found a jar containing powder and a note reading “Thorium.” Some media reported that the couple had been hanged.
Police said an autopsy will be carried out Thursday to determine the cause of death.
The Niigata Center for Environmental Radiation Monitoring confirmed that the powder contained radioactive thorium, which was measured to be emitting a peak radiation level of 21.4 microsieverts per hour, NTV said.
Local residents expressed concern that there were radioactive materials nearby in an area where children are living.