The Yomiuri Shimbun
Can Tokyo Electric Power Co. bring the four troubled reactors at its Fukushima No. 1 nuclear power plant under control within six to nine months as it announced Sunday? What challenges must it overcome before achieving its avowed goals?
Under the announced timetable, TEPCO aims to bring the crippled reactors to a stable condition known as a "cold shutdown"--a situation in which water temperatures inside the reactors are stabilized below 100 C--within six to nine months.
It will take this amount of time to achieve cold shutdown because the reactors, fuel rods and containment vessels are all in terrible conditions, experts said.
In the aftermath of a series of accidents at the Fukushima Prefecture power plant that was triggered by the March 11 earthquake and a massive tsunami, TEPCO studied measures such as purifying ever-increasing amounts of contaminated water and using it to cool the reactors and cooling water inside the reactors through heat exchangers by running seawater through narrow pipes.
But under its new timetable, the utility decided to give precedence to a method of pouring water to fill reactor containment vessels to cool pressure vessels as suggested by U.S. nuclear experts.
This water-filling method is able to bring about a cooling effect within a short period, TEPCO official Junichi Matsumoto said, adding that the utility determined that filling containment vessels with water would be "the fastest way to cool fuel rods."
Under the timetable, the water-filling operation for the Nos. 1 and 3 reactors requires three months for completion. As for the No. 2 reactor, TEPCO is aiming to finish related work within six to nine months because it is also necessary to repair the damaged pressure suppression chamber.
Each containment vessel has a water capacity of about 6,000 to 7,400 tons. Experts have pointed out risks such as the vessels breaking down after being loaded with such an enormous amount of water or being unable to withstand a series of aftershocks.
If it takes a long time before the reactors are cooled to a state of cold shutdown, the amount of radiation-generated hydrogen will gradually increase, boosting the possibility of a hydrogen explosion, experts said.
TEPCO Vice President Sakae Muto told a news conference that if the amount of water to be poured into the No. 2 reactor's containment vessel increases, so will the chances of radiation-contaminated water leaking.
===
New water treatment facilities
As for the handling of contaminated water, there are plans to establish new water treatment facilities and create new reservoirs, according to TEPCO's timetable.
However, TEPCO has admitted the risk of possible delays in establishing such facilities and that they may not work as planned.
When transferring high-level contaminated water, careful handling is required to prevent the water from leaking from hose joints.
Meanwhile, it is highly likely that contaminated water is leaking from the No. 2 reactor, where a pressure suppression chamber is feared to have been damaged. Radiation levels around the reactor are too high for workers to approach and "seal" the reactor building.
At the No. 4 reactor, it is necessary to reinforce the structure below the temporary storage pool for spent nuclear fuel rods to prevent any possible spillage. However, this task is also expected to be fraught with difficulty.
===
A race against time
The Fukushima crisis requires a much longer timeframe compared with previous nuclear power plant accidents to achieve cold shutdown.
In case of the 1979 Three Mile Island nuclear power plant accident in the United States, it took the reactors about a month to reach this state.
In the 1986 Chernobyl accident in the former Soviet Union, a radioactive fire caused by an explosion ended after 10 days. Construction of a concrete-walled "sarcophagus" to enclose the nuclear reactor was carried out over a six-month period.
According to TEPCO's timetable, Step 1 is expected to continue for about three months and radioactive substances will need to be discharged to the atmosphere and sea.
It also will take at least three months for the company to be able to control the leak of radioactive materials during Step 2.
The Cabinet Office's Nuclear Safety Commission has estimated that even in April, up to 1 terabecquerel per hour of radioactive substances are spreading into the air. Even if TEPCO's operations proceed as scheduled, several thousand terabecquerels of radioactive substances will be emitted by the time the situation is brought under control.
It is especially crucial during the first three months to suppress radioactivity by using underwater curtains and giant reactor building covers equipped with special filters. It is also feared that the upcoming typhoon season will impact on plans to enclose the damaged reactor buildings.
===
Specialists skeptical
Specialists are skeptical of TEPCO's timetable for bring the reactors at the Fukushima plant to cold shutdown within nine months.
"Cooling the nuclear reactors should be the top priority, but TEPCO's timetable shows they will instead be doing many things at once," said Michio Ishikawa, a top advisor to the Japan Nuclear Technology Institute. "I'm worried that realizing the timetable might be pie in the sky."
Ishikawa emphasized that setting up new heat exchangers to cool the reactors was the most important step. Referring to TEPCO's plan to externally cool the reactors by filling the containment vessels with water, he said, "Cooling the reactors with impounded water is not such an effective strategy."
"Judging from difficulties experienced thus far [in efforts to bring the situation at the plant under control], I doubt that TEPCO can stabilize the reactors within nine months," said Ishikawa. "I think TEPCO also needs to review its organization, for example, by having someone who can exert strong leadership [to spearhead the timetable's implementation]."
Shojiro Matsuura, former chairman of the Cabinet Office's Nuclear Safety Commission, pointed out some difficulties involved in blanketing the reactor buildings with special covers.
"Temperatures and humidity will rise toward summer. It's already difficult to work wearing protective gear and the situation will be even worse in covered buildings," Matsuura said. "TEPCO needs to take measures against possible heat stroke [among workers]."
TEPCO also has said its goal is for evacuees to return home once the situation is stabilized.
However, Atsushi Kasai, former chief researcher at the Japan Atomic Energy Research Institute, expressed concern over soil contaminated with radioactive substances.
"I'm concerned about soil contamination by radioactive cesium--it has a long half-life of about 30 years," Kasai said. "These are highly contaminated areas. Careful consideration must be given as to whether residents be allowed to return home so early."
(Apr. 19, 2011)
No comments:
Post a Comment