The Yomiuri Shimbun
Government officials and nuclear experts were still investigating Friday the source of the leak that exposed three workers to high levels of radiation at the Fukushima No. 1 nuclear power plant.
High levels of radioactive substances--10,000 times normal levels--were detected in water in the basement of the turbine building next to the plant's No. 3 reactor. On Thursday, the three workers were exposed to these excessive amounts from an unknown source in the basement.
While suspecting the radioactive substances in the water could have come from broken nuclear fuel rods, the experts were not able to determine the exact source. The water likely came from the reactor or the storage pool for spent fuel rods, but they did not know which one yet, an official of the Economy, Trade and Industry Ministry's Nuclear and Industrial Safety Agency said at a press conference Friday.
"As far as we can tell by the data, the reactor is still able to contain radioactivity," the official said. "But those radioactive substances leaked out of somewhere so we have to investigate what happened. It's possible the reactor itself is broken."
Nuclear experts cited three possible ways radiation could have leaked:
-- Spent nuclear fuel rods stored in a temporary pool on the fourth and fifth floors of the reactor building broke and were pushed out by the recent water spraying operations.
-- Steam gradually leaked from the main pipe connecting the reactor to the turbine building due to a valve malfunction.
-- Large amounts of radioactive substances discharged into the air were absorbed by the water.
In the No. 3 reactor, the exterior building that accommodates the storage pool was severely damaged by a hydrogen explosion on March 14. Because the reactor's control system was left unable to cool the storage pool or add water to it, nuclear experts said it is possible the spent nuclear fuel rods overheated and were partially damaged.
Regarding the first possibility, because a special team from the Tokyo Fire Department and Self-Defense Forces personnel had sprayed 4,050 tons of seawater on the building as of Thursday, the nuclear experts suspect this water could have scattered radioactive materials in the pool.
It has been difficult to confirm whether the pool contains water, as radiation levels around the building have been very high and access to the site is obstructed by debris from the explosion.
About 1,425 tons of water have been sprayed at the pool, nearly triple its volume.
When the pool is full, the surface of the water is about 40 meters aboveground, and the first basement of the turbine building is about nine meters underground, a difference of nearly 50 meters. It is possible water that had overflowed from the pool leaked through the broken exterior building and flowed into the turbine building.
As for the second possibility, TEPCO officials said functions keeping out steam in the main pipe had worked properly until the reactor lost all outside power in the tsunami. But, one official said, "We can't rule out the possibility that pipes connecting the reactor and the turbine or other parts have broken."
The experts did not believe the third possibility was likely, considering the density of radioactive substances in the air around the nuclear power plant.
(Mar. 26, 2011)
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