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184186
Tue, 05/24/2011 - 19:53
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https://oananews.org//node/184186
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Fukushima plant's Nos. 2, 3 reactors also suffered meltdown: TEPCO
TOKYO, May 24 Kyodo - The operator of the troubled Fukushima Daiichi nuclear power complex said Tuesday that meltdowns may have occurred in the cores of the Nos. 2 and 3 reactors in addition to the meltdown already found to have taken place at the No. 1 reactor, but stressed that the melted fuel is now being kept cool at the bottom of the pressure vessels.
Tokyo Electric Power Co. also maintained its view that it was only after the giant tsunami hit following the devastating March 11 earthquake that the plant lost all its power sources, eventually leading to the loss of the reactors' key cooling functions.
The government's Nuclear and Industrial Safety Agency, which received the report from the company Monday, will present its own assessment on the reactor cores ''in the near future,'' according to agency spokesman Hidehiko Nishiyama.
As workers struggle to bring the three crippled reactors under control, Tokyo Electric is at the same time stepping up efforts to look into what actually happened in the early days of the country's worst-ever nuclear crisis.
The government agreed during a Cabinet meeting the same day to set up a third-party panel to look into the causes of the crisis, with members expected to compile a medium-term report on their findings in December and the final report by summer 2012, government officials said.
In its assessment, Tokyo Electric estimated that the pressure vessels of the Nos. 2 and 3 reactors containing the nuclear fuel rods may have been damaged if it turns out that the levels of the water inside the vessels are lower than data now shown by measuring gauges. Accurate water levels are still unknown as adjustment work for the gauges has not yet finished.
If the water levels have dropped to a level low enough to expose all parts of the 4-meter-long fuel rods, then it can be assumed a large part of the fuel in the No. 2 reactor dropped to the bottom of the vessel about 101 hours after the reactor automatically shut down following the quake, while the same must have happened at the No. 3 reactor in about 60 hours, said the company known as TEPCO.
The firm also issued another estimate in the case the water levels have been maintained as shown on the gauges, which would result in less damage to the fuel. But the company believes ''the reality is closer to the case in which the water levels are not maintained,'' according to a TEPCO official.
But, regardless of the water levels, TEPCO said it is likely that damage to the reactor cores started from about 8 p.m. on March 14 for the No. 2 unit, and about 9 a.m. on March 13 for the No. 3 unit, with the temperature of the cores reaching some 2,800 C at those times.
If the pressure vessels were damaged by the melted fuel, water containing radioactive substances may have leaked out to the flask-shaped primary containers and eventually to the buildings housing the reactors as well as the adjacent reactor turbine buildings.
But TEPCO said the current temperatures around the pressure vessels suggest the melted fuel has settled at the bottom of the vessels and is being kept stably cool with water being injected into the reactor cores from outside as an emergency measure.
TEPCO also said it does not expect a situation in the future in which massive radioactive substances would be further released outside.
Commenting on the latest development at the plant as reported by the utility, Goshi Hosono, a special adviser to Prime Minister Naoto Kan, said, ''The government feels responsible for its (earlier) lax assessment of the accident.''
The government continues to aim to bring the reactors to a stable condition known as a ''cold shutdown,'' in which water temperatures inside the reactors are stably brought below 100 C, Hosono also said at a press conference.
Earlier in May, the company said it believes the No. 1 reactor suffered a meltdown about 16 hours after the 2:46 p.m. quake on March 11, based on data taken after workers adjusted water gauges to accurately measure the water level inside the pressure vessel.
The gauges had indicated before the adjustment that the No. 1 reactor fuel was half covered by water, but revealed afterward that the fuel had been fully exposed.
About its assessment on how the plant was affected by the quake and tsunami, TEPCO said emergency generators started operating normally after the six-reactor Fukushima plant lost its external power source in the wake of the magnitude 9.0 quake.
But the massive tsunami waves, believed to have been more than 14 meters high, flooded the emergency diesel generators and power distribution boards, leading the plant to basically lose all its power sources.
Echoing TEPCO's view that the loss of power was caused by the tsunami, rather than the quake, Nishiyama of the nuclear regulatory body said, ''So far, we do not think that the quake...had a major impact on the important parts (of the reactors).''
The remaining Nos. 4 to 6 reactors were under maintenance at the time of the earthquake. The No. 4 unit has all the fuel in the spent-fuel pool, which also lost its cooling function. The Nos. 5 and 6 reactors achieved a state of ''cold shutdown,'' helped by one diesel generator which escaped from being flooded.
Given the limited availability of data, however, TEPCO has yet to unravel full details of the accident.
Tokyo Electric Power Co. also maintained its view that it was only after the giant tsunami hit following the devastating March 11 earthquake that the plant lost all its power sources, eventually leading to the loss of the reactors' key cooling functions.
The government's Nuclear and Industrial Safety Agency, which received the report from the company Monday, will present its own assessment on the reactor cores ''in the near future,'' according to agency spokesman Hidehiko Nishiyama.
As workers struggle to bring the three crippled reactors under control, Tokyo Electric is at the same time stepping up efforts to look into what actually happened in the early days of the country's worst-ever nuclear crisis.
The government agreed during a Cabinet meeting the same day to set up a third-party panel to look into the causes of the crisis, with members expected to compile a medium-term report on their findings in December and the final report by summer 2012, government officials said.
In its assessment, Tokyo Electric estimated that the pressure vessels of the Nos. 2 and 3 reactors containing the nuclear fuel rods may have been damaged if it turns out that the levels of the water inside the vessels are lower than data now shown by measuring gauges. Accurate water levels are still unknown as adjustment work for the gauges has not yet finished.
If the water levels have dropped to a level low enough to expose all parts of the 4-meter-long fuel rods, then it can be assumed a large part of the fuel in the No. 2 reactor dropped to the bottom of the vessel about 101 hours after the reactor automatically shut down following the quake, while the same must have happened at the No. 3 reactor in about 60 hours, said the company known as TEPCO.
The firm also issued another estimate in the case the water levels have been maintained as shown on the gauges, which would result in less damage to the fuel. But the company believes ''the reality is closer to the case in which the water levels are not maintained,'' according to a TEPCO official.
But, regardless of the water levels, TEPCO said it is likely that damage to the reactor cores started from about 8 p.m. on March 14 for the No. 2 unit, and about 9 a.m. on March 13 for the No. 3 unit, with the temperature of the cores reaching some 2,800 C at those times.
If the pressure vessels were damaged by the melted fuel, water containing radioactive substances may have leaked out to the flask-shaped primary containers and eventually to the buildings housing the reactors as well as the adjacent reactor turbine buildings.
But TEPCO said the current temperatures around the pressure vessels suggest the melted fuel has settled at the bottom of the vessels and is being kept stably cool with water being injected into the reactor cores from outside as an emergency measure.
TEPCO also said it does not expect a situation in the future in which massive radioactive substances would be further released outside.
Commenting on the latest development at the plant as reported by the utility, Goshi Hosono, a special adviser to Prime Minister Naoto Kan, said, ''The government feels responsible for its (earlier) lax assessment of the accident.''
The government continues to aim to bring the reactors to a stable condition known as a ''cold shutdown,'' in which water temperatures inside the reactors are stably brought below 100 C, Hosono also said at a press conference.
Earlier in May, the company said it believes the No. 1 reactor suffered a meltdown about 16 hours after the 2:46 p.m. quake on March 11, based on data taken after workers adjusted water gauges to accurately measure the water level inside the pressure vessel.
The gauges had indicated before the adjustment that the No. 1 reactor fuel was half covered by water, but revealed afterward that the fuel had been fully exposed.
About its assessment on how the plant was affected by the quake and tsunami, TEPCO said emergency generators started operating normally after the six-reactor Fukushima plant lost its external power source in the wake of the magnitude 9.0 quake.
But the massive tsunami waves, believed to have been more than 14 meters high, flooded the emergency diesel generators and power distribution boards, leading the plant to basically lose all its power sources.
Echoing TEPCO's view that the loss of power was caused by the tsunami, rather than the quake, Nishiyama of the nuclear regulatory body said, ''So far, we do not think that the quake...had a major impact on the important parts (of the reactors).''
The remaining Nos. 4 to 6 reactors were under maintenance at the time of the earthquake. The No. 4 unit has all the fuel in the spent-fuel pool, which also lost its cooling function. The Nos. 5 and 6 reactors achieved a state of ''cold shutdown,'' helped by one diesel generator which escaped from being flooded.
Given the limited availability of data, however, TEPCO has yet to unravel full details of the accident.
