Tokyo Electric Co. crews prepared Monday to pump seawater into a third reactor at the stricken Fukushima Daiichi nuclear plant in order to prevent or halt a meltdown of its fuel assemblies, hours after a second explosion of leaked hydrogen gas rocked another reactor building at the site on Japan's northeast coast.
Experts called the injection of seawater and neutron-absorbing boron into the site's three crippled reactors units a desperation move never attempted before in the industry. It amounted to sacrificing the reactors in an attempt to maintain the structural integrity of the reactor and its encasing concrete containment structure and prevent a potential uncontrolled major radiological release. Three other Fukushima Daiichi reactors had been shut down for planned work before Friday's 8.9 earthquake and were not part of the crisis.
"I would describe this measure as a Hail Mary Pass but if they succeed, there is plenty of water in the ocean and if they have the capability to pump this water in the necessary volume and at the necessary rates ... then they can stabilize the reactor," said former Energy Department official Robert Alvarez, according to press accounts of his press conference Saturday.
However, it was not clear how long the stabilization would take, or whether periodic releases of gases from the reactor vessels to relieve pressure could pose long-term radiation contamination threats to the area around the plant.
A buildup of hydrogen gas, formed by damage to the reactor core when cooling systems failed, was blamed for the explosion Monday at the plant's No. 3 unit building, a steel and concrete structure that forms the outer containment of the reactor. Authorities said the blast injured 11 people but apparently did not damage the reactor and main concrete containment shell.
"There is no massive radioactive leakage," Cabinet Chief Cabinet Secretary Yukio Edano said.
Over the three days, radioactive releases described as minor in scale were deliberately made from the plant's units to relieve gas pressures within the reactor structures. Japanese officials carried out a steadily advancing evacuation of areas surrounding the site, with nearly 200,000 people reported to have left the area as of Sunday, adding to an immense personal trauma of tens of thousands forced to flee the tsunami's destruction.
Seawater injection at Unit 1 was ordered on Saturday after a series of systems failures left the top part of the reactor fuel assemblies unprotected by cooling water, authorities said. Although the three working reactors had been shut down because of the earthquake, the fuel rods continued to give off heat.
Started by a failure of backup power generators
In normal operation, the heat is removed by pumping water through the vessel, creating steam that drives electricity-generating turbines. The loss of outside power and then backup power at the site prevented the cooling system from operating normally.
The same emergency water injection was planned Monday for Unit 2, after crews detected a buildup of pressure inside its reactor, evidence that its fuel assemblies also had become uncovered.
The power plant lost outside power after the earthquake, and the tsunami left large backup diesel generators out of service, Japanese officials said. Battery power, the third line of defense, apparently could not maintain the cooling water flow.
"Defense in depth" is the watchword of the nuclear industry, a strategy of backups for critical components, and backups to backup when needed. But defenses had failed to account for the impact of a tsunami's massive flooding
Reports from government and industry officials in Japan initially minimized the threat. The U.S. Nuclear Energy Institute distributed an assessment from the Federation of Electric Power Companies of Japan saying that the Unit 1 reactor core still had a sufficient amount of water for cooling, "with no danger of the nuclear fuel being exposed."