"Backup power initially worked, but failed when the sea wall protecting the site was found to be no where near high enough to stop the tsunami from flooding the generators," said Mary Olson, a nuclear waste specialist at the Nuclear Information and Resource Service. "Of course, the generators should not have been placed in low-lying areas behind the sea wall--that was clearly a human error."
Once the emergency generators were knocked out, eight sets of backup batteries were brought online to keep the pumps going. Initially that worked too, but after about eight hours and hundreds of damaging aftershocks, their power was exhausted and plant operators began loosing control of the rising temperature inside the containment vessels.
"For every single nuclear reactor in the world, 50 percent of the risk comes from loss of power to the site. Reactors do not power themselves, but depend on external sources of electricity for their control rooms, pumps and other auxiliary equipment," said Olson. "A nuclear reactor does not have a switch. You can stop it generating electricity, but you can't turn off the heat."
The only option left to plant operators was pumping seawater into the containment buildings in a last-ditch attempt to cool off the inner containment vessel, which is made from concrete and steel. Nevertheless, steam pressure building up in three reactors have now caused explosions and deepening concern about the integrity of the reactor containment vessels.
"Unfortunately, it is extremely difficult to pump seawater into the containment vessel because there is too much pressure inside. There are pipes that can allows water to be pumped in, but right now the pumps are not working, and its not clear that the pipes are even connected," said Olson.
According to the International Atomic Energy Agency, the No. 2 reactor lost is normal cooling capability late Monday (March 14), prompting emergency workers to start pumping sea water into the damaged reactor. On Tuesday morning, a hydrogen explosion rocked the No. 2 reactor’s containment building, causing a release of radioactive steam clouds that coincided with a pressure drop inside the inner containment vessel, indicating that its outside shell had possibly cracked.
Also on Tuesday, a fire broke out in the No. 4 reactor containment building, which had been shut down before the tsunami hit last Friday. Spent fuel rods in its storage pool were apparently exposed while workers concentrated on the three other damaged reactors. Large amounts of radiation were reportedly released into the air before the fire was extinguished.
According to Japanese officials, radiation levels in and around the Fukushima plant spiked to 400 millisieverts per hour after the No. 2 reactor explosion and the No. 4 fire, prompting Japanese Prime Minister Naoto Kan to expand the evacuation order from 12 to 18 miles from the plant.
Wind shifted inland on Tuesday, causing radiation levels to temporarily jump 10 times higher than normal as far away as Tokyo, according to Japanese officials, who claimed the rise was not major health hazard. By nightfall local time, according to the U.N. weather agency, winds had shifted back out to sea, lowering radiation levels in Tokyo and around the Fukushima plant. That could indicate that the inner containment vessel at the No. 2 reactor remained intact and that the spent fuel rods stored in No. 4’s pool were again covered with water.
After fueling reactors for up to six years, spent fuel rods are stored underwater for 10 to 20 years in pools at the bottom on containment buildings. All six Fukushima reactors have spent fuel rod pools at the base of their containment buildings. The water both cools the fuel rods and provides shielding from radiation. These storage pools are now exposed to the air at the Nos. 1 and 3 reactors, but officials said they will not release radioactive materials as long as they are covered with water. Workers were said to be using fire trucks to pump in sea water.
It remains uncertain whether or not molten uranium and plutonium inside the reactors will burn through the containment vessels since rising temperatures can no longer be controlled. If the containment vessels are breeched, the Chernobyl scenario becomes a possibility, some experts think.
If the containment vessels hold--either through luck or the process of venting steam to relieve the mounting pressures inside--then the remaining hazards will be the venting of radioactive air from inside the containment vessels and from the fuel pools exposed by the hydrogen explosions.
Most of the Fukushima reactors have operated well beyond their expected 25-year life expectancy. The plant began operating in 1971. The No. 1 reactor was to have been decommissioned in 2011, but Japanese regulators recently gave operator Tokyo Electric Power a ten-year extension of its operating license for the reactor.
Early Wednesday morning local time, yet another fire broke out inside the containment building at the No. 4 reactor, according to Reuters. Tokyo Electric Power did not immediately release information on the level of radiation being released, but warned that the cooling systems were also failing in the Nos. 5 and 6 reactors, which also were off-line when the tsunami hit.