Two failed attempts to vent steam within days of the March 11 quake and tsunami most likely resulted in damage to the containment vessel of the Fukushima No. 1 power plant’s No. 2 reactor, a Tokyo Electric Power Co. source said Wednesday.
This latest revelation casts further doubt on the crisis-management capabilities of the plant’s operator, already put to shame by the release of massive amounts of radioactive material into the air and contaminated water into the ocean.
Venting is a procedure used to relieve pressure building up inside a reactor’s containment vessel. Although the steam contains radioactive material, venting it prevents excessive pressure from damaging the containment vessel — a hazardous situation that would release extraordinary amounts of radioactive material 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, damage was most likely done to the suppression chamber, a doughnut-shaped vessel underneath the containment vessel built to take excess steam from the pressure vessel.
Venting is carried out by opening two valves in pipes leading from the containment vessel to the outside.
According to Tepco, the valves were opened at 11 a.m. on March 13, two days after the natural disasters struck. But the pressure inside the containment vessel did not drop, nor did radiation levels rise 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 the batteries supplying electricity to keep them open had died.
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 the explosion occurred because the suppression chamber was damaged.
Venting was also conducted at reactors 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 the 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.
The world’s worst nuclear crisis since the 1986 Chernobyl disaster has yet to be contained, as the government and the utility struggle to install new cooling systems in four of the plant’s six reactors.
Onagawa to get levee
Tohoku Electric Power Co. said Wednesday it will construct a coastal levee about 800 meters long near its Onagawa nuclear power plant in Miyagi Prefecture, which shut down in the immediate aftermath of the March 11 earthquake and tsunami.
As part of emergency safety measures submitted to the government, Tohoku Electric will also build a tide barrier around rooms housing seawater pumps there by the end of next April.
Tohoku Electric will also strengthen doors at the buildings housing nuclear reactors to prevent seawater from entering important facilities.