Venting failed twice at Fukushima plant, possibly damaging reactor+

TOKYO, May 18 Kyodo -
The operator of the crippled Fukushima Daiichi nuclear power plant failed twice to vent steam out of the containment vessel of the No. 2 reactor, most likely resulting in damage done to part of it, a plant operator source said Wednesday.
The failure in the immediate aftermath of the March 11 earthquake and tsunami and subsequent trouble at the plant, such as the release of massive amounts of radioactive material into the air and the discharge of contaminated water, call into question Tokyo Electric Power Co.'s crisis management, observers say.
The world's worst nuclear crisis since the 1986 Chernobyl disaster has yet to be contained, with the government and the utility struggling to restart key cooling functions at most of the six reactors there.
The venting is meant to relieve pressure that is building up inside a reactor's containment vessel by letting steam escape into the air.
While the steam contains radioactive material, the maneuver is designed to prevent undue pressure from damaging the containment vessel, a scenario in which extraordinary amounts of radioactive material would be spewed into the environment.
The fear of catastrophic explosions was so great during the initial phase of the crisis that the government even ordered the utility, known as TEPCO, to conduct the venting as quickly as possible.
But by failing to relieve the pressure inside the reactor's containment vessel, TEPCO is most likely to have allowed damage to be done to a part called the suppression chamber, which is linked to the containment vessel.
In a venting operation, two valves inside pipes that lead from the containment vessel to the outside are opened to let steam escape into the air.
According to TEPCO, the valves were opened at 11 a.m. on March 13, two days after the natural disasters struck mostly in the northeast, including the plant's home, Fukushima Prefecture. But the pressure inside the containment vessel did not drop, nor did radiation levels go up in the surrounding area.
Two more valves in a separate system were opened at 12:02 a.m. on March 15, but the pressure did not fall then either, according to the utility.
Documents disclosed by TEPCO on Monday also show that the pressure did not drop inside the containment vessel. While the utility stops short of concluding in the documents that the venting had failed, the TEPCO source said the maneuver is deemed to have failed on both occasions.
In the second attempt, the valves closed after several minutes, raising the possibility that they had run out of battery power because they need electricity to be kept open.
An explosion was heard around the reactor's suppression chamber at 6:10 a.m. on March 15, with radiation levels at the plant's main gate skyrocketing to 11,930 microsieverts per hour at around 9 a.m. from 73.2 microsieverts three hours earlier.
The Nuclear Safety Commission of Japan has indicated that the explosion occurred because the suppression chamber was damaged.
Venting was also conducted at reactors Nos. 1 and 3. Radiation levels at the main gate measured between 281.7 and 385.5 microsieverts per hour after the maneuvers.
''We are looking into the cause of venting failures and their possible connection to damage done to the suppression chamber,'' a TEPCO official said.
While the government and the utility are struggling to bring the crisis under control, many residents and businesses in surrounding areas have been forced to flee for safety, with elevated levels of radiation still being measured there.