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.

New analysis of Fukushima core status

 

The likely states of Fukushima Daiichi 1 (top), as well as 2 and 3 (below). Water is injected via the main feedwater line, while units 2 and 3 benefit from the core spray that has helped reduce temperatures considerably

A new analysis of the accident at Fukushima Daiichi indicates more extensive melting probably occurred at unit 1 than previously thought, although the predicted status of units 2 and 3 remains about the same.

The bulk of unit 1’s nuclear fuel went through the bottom of the reactor vessel as well as about 70 centimetres of the drywell concrete below, according to the analysis released today. However, the corium did not breach the steel containment vessel 1.9 metres further down within the concrete, or the boundary of secondary containment some 7.6 metres further still.

Of the 10.2 metres of solid concrete that makes up the floor of the reactor building, the corium is thought to have melted and mixed with the first 70 centimetres only. The natural spreading and expansion of the corium, plus the addition of compounds of concrete, would have reduced the intensity of the heat produced until it reached an equilibrium and solidified in place. Tepco said it was confident the melting had ceased due to the absence of gases that would be released by the high-temperature reactions between corium and concrete.
The latest analysis was done to supercede one from May due to the emergence of some information that contradicted its predictions. Because this analysis takes into account some of this data, Tepco expect this model to be more accurate, although the company cautioned that its scenarios remain uncertain.

Unit 1 was the oldest of the three Fukushima Daiichi reactors operating at full power before the earthquake of 11 March, and was hit hardest by the loss of power following the tsunami and the flooding of diesel generators. For units 2 and 3 the analysis gave similar results to a simulation released in May, actually suggesting that the better of two scenarios presented then is more likely. Nevertheless, the cores of units 2 and 3 are thought to have overheated badly, with a large portion having melted or softened enough to slump to the bottom of the reactor vessel. A relatively small amount is thought to have passed through holes in the pressure vessel and fall to the drywell floor.

Tepco continues to inject cooling water to all three of the reactor vessels, and has succeeded in reducing temperatures from a range of sensors to well below the landmark 100ºC point usually known as cold shutdown. Official recognition of this goal in the roadmap to stabilisation, however, remains outstanding.

The extensive fuel damage means that injected water becomes highly contaminated, while holes in the pressure vessels allow this to accumulate in the building basements. Tepco operates a comprehensive set-up to treat this to a potable standard.

Courtesy: WNN

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.”

Source: Japan Today

Tepco’s Leader of ‘Fukushima Fifty’ Resigns Due to Illness

Masao Yoshida, who led the fight to bring Japan’s crippled Fukushima nuclear station under control, steps down tomorrow for medical treatment after almost nine months directing the disaster response from inside the plant.

Yoshida, 56, was hospitalized on Nov. 24. His employer, Tokyo Electric Power Co., didn’t specify the ailment in a statement on Nov. 28, citing privacy reasons. The company declined to say where Yoshida is being treated.

The nuclear engineer was manager of the Fukushima Dai-Ichi station on March 11 when Japan’s strongest earthquake on record and an ensuing tsunami hit the plant, causing the worst nuclear disaster since Chernobyl 25 years ago.

“Yoshida certainly won the respect of his men,” said Shigeharu Aoyama, who met him on April 22 at Fukushima Dai-Ichi after gaining access as a special member of Japan Atomic Energy Commission. “His decisions and courage saved lives.”

Radiation exposure is unlikely to be the cause of Yoshida’s illness, Tetsuo Ito, the head of Kinki University’s Atomic Energy Research Institute, said in a phone interview. Heavy radiation exposure would bring a quick onset of acute diseases, while late-onset diseases from radiation, such as cancers, take several years to develop, Ito said.

“Yoshida has been under tremendous stress given he’s taken the lead since the accident,” Ito said.

Yoshida informed Tepco President Toshio Nishizawa on Nov. 21 that he needs treatment after undergoing medical checks earlier this month, Junichi Matsumoto, a general manager at the company, said on Nov. 28.

Fukushima Fifty

After studying nuclear engineering at the Tokyo Institute of Technology, Yoshida joined Tepco in 1979, according to the utility. He was appointed head of the Fukushima Dai-Ichi plant in June 2010.

As radiation levels spiked in the early days of the crisis, workers were pulled out of the plant leaving behind what became known as the Fukushima Fifty who risked their lives to bring the reactors under control.

Spain awarded the Fukushima Fifty the Prince of Asturias Concord prize in September, calling them the “heroes of Fukushima.”

“Yoshida has made a huge contribution and I want him to focus on curing his illness,” Goshi Hosono, minister in charge of the response to the nuclear disaster, told reporters in Tokyo yesterday. “Yoshida’s leadership deserves rich praise and he has my sincere respect.”

Order Ignored

On March 12, a day after the tsunami, Yoshida ignored an order from Tepco headquarters to stop pumping seawater into a reactor to try and cool it.

Tepco said it may penalize Yoshida even though Sakae Muto, then a vice president at the utility, said it was a technically appropriate decision. Yoshida received a verbal reprimand after then Prime Minister Naoto Kan defended the plant chief, the Yomiuri newspaper reported.

“If Yoshida stopped pumping seawater, workers could have died,” said Aoyama. “They won’t forget that.”

“It’s our mission to ensure stable operations of the plant with safety as the top priority,” Yoshida said in a New Year greeting about two months before the nuclear disaster.

Yoshida thought several times those at the plant were going to die, he told reporters who visited the Fukushima station on Nov. 12, the Mainichi newspaper said. Yoshida told reporters he thought plant operators may completely lose control as the meltdowns accelerated, the Mainichi said.

Beyond Imagination

“Yoshida told me that he couldn’t imagine such a big tsunami would come after an earthquake,” Yotaro Hatamura, an engineering professor at the University of Tokyo who is investigating the accident, said in June. “From our discussions, I gathered that no one at the plant could imagine that such a tsunami would occur.”

Takeshi Takahashi, 54, general manager of Tepco’s nuclear power plant management department, will replace Yoshida, the utility said in a statement. Yoshida will remain as an executive officer of the nuclear power and plant siting division, it said.

“I feel as if my heart is breaking, leaving everyone I’ve worked with since the disaster this way,” Yoshida wrote in a message to workers at the plant when he left for hospital, according to a faxed statement from the utility. “I will concentrate on the treatment and do my best to be at work with you again as soon as possible.”

Source: SF Gate