Tokyo Electric Power Co. said cold shutdown conditions have been achieved at the Fukushima No. 1 nuclear power plant, and is awaiting government confirmation on what would be a huge step toward ending the crisis.
However, nuclear experts continue to raise doubts that such conditions accurately reflect the state of the damaged reactors at the crippled nuclear plant.
TEPCO and government officials on Nov. 17 released a revised road map for bringing the situation at the Fukushima plant under control. The document said a state close to cold shutdown had been achieved since the reactor cores were being cooled stably and the amount of radioactive materials released into the outer environment had also decreased substantially.
The revised road map reiterated earlier objectives of achieving cold shutdown before the end of the year.
"Cooling is proceeding in a stable manner," Goshi Hosono, the state minister in charge of the Fukushima nuclear accident, said. "We will carefully confirm over the next six weeks or so if this condition can be maintained."
Officials of the Nuclear and Industrial Safety Agency are continuing their appraisal by hearing the opinions of experts, although they have informed their U.S. counterparts that a situation equivalent to cold shutdown had been achieved, sources said.
The final decision on whether cold shutdown has been reached will be up to Japan's political leaders.
Through cooling of the No. 1 to No. 3 reactors that were hit by explosions and other accidents, the temperatures at the bottom of the pressure vessels were between 37 and 68 degrees, according to the document. Even if an error up to 20 degrees was included, the reactor temperatures still would be below 100 degrees, which is one of TEPCO's conditions for a cold shutdown state.
The temperatures of gases within the containment vessel, into which some melted nuclear fuel is believed to have leaked, were also between 39 and 70 degrees, the document said.
The release of new radioactive materials was measured at levels of 60 million becquerels per hour at the gates to the Fukushima plant facility. That was lower than the provisional level of 100 million becquerels per hour detected about one month ago.
The level of additional radiation exposure caused by the nuclear accident was at a level of 0.1 millisievert over the course of a year, lower than the government standard of 1 millisievert, the document said.
But some nuclear experts have cast doubts on using the 100-degree temperature at the bottom of the pressure vessel as a condition for cold shutdown.
That temperature level is used for reactors operating under normal conditions. At the Fukushima plant, the control rods and nuclear fuel in the reactors have melted, and some may have leaked into the containment vessels.
Even at a meeting in October held by NISA officials, experts questioned the use of the temperature condition. Akira Yamaguchi, a professor of reactor core engineering at Osaka University, warned against focusing excessively on the 100-degree condition.
One reason the bottom of the pressure vessel is being used for the temperature condition is that it is relatively close to where the nuclear fuel is likely located. Moreover, few other locations are available where temperatures can be measured.
TEPCO officials can only estimate the temperatures of the fuel in the containment vessel because it is unclear where it is located.
Kazuhiko Kudo, a nuclear engineering professor at Kyushu University, said the situation at the Fukushima plant should not be described as cold shutdown. Instead, he said, a more accurate description would be a shutdown achieved through a complicated cooling system.
Normal cooling equipment was rendered inoperable by the March 11 quake and tsunami, forcing TEPCO to install temporary equipment connected by about 4 kilometers of hoses to cool the reactors.
When TEPCO first compiled the road map on April 17, there was no clear definition of what would constitute cold shutdown.
Soon thereafter, the central government and TEPCO finally acknowledged that meltdowns had occurred in the No. 1 to No. 3 reactors.
Other nuclear plants achieved cold shutdown at their reactors following the March 11 disaster. The focus on the three damaged reactors at the Fukushima No. 1 plant moved to when cold shutdown would be achieved rather than understanding the actual conditions of the reactors.
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