A Level 7 Incident
After the explosion involving reactor facility 4, the decision was made to evacuate all “non-essential” personnel and 650 workers left the site, leaving just 70 to continue the stabilization work.
In addition to the three reactors, there was also concern that the spent fuel pools might be losing water thus allowing the fuel to overheat. Additional fire apparatus and helicopters were used, without much effect, in an attempt to keep the pools full. Eventually, concrete pumping trucks were flown in and used to maintain safe water levels though spent fuel was never in serious danger.
Despite the valiant efforts of plant workers, the fuel in all three operating reactors was uncovered for significant amounts of time and melted as a result. In some cases fuel rods were exposed early in the incident and eventually this occurred at all three reactors. The extremely high temperatures associated with the uncovered fuel contributed to the liberation of hydrogen gas which resulted in three separate explosions. These explosions resulted in much of the area being highly contaminated from debris strewn around the site.
Over 100 plant worker received doses in excess of 10 rem and two received doses over 65 rem. Capability to monitor plant or personnel radiation levels was lost early in the incident as the fixed system was either swept away in the tsunami or rendered inoperable because of the lack of power. Contamination was widespread, including at the Emergency Response Center. (Radiation levels were so high there that the windows had to be covered with lead.)
It is interesting to note that the only method to be successfully employed to inject water into the three failed reactors was using fire apparatus though this was ultimately ineffective and was hampered by pressure differentials and the inability to obtain a constant water supply. In addition, radiation levels were high enough that apparatus was left to run unattended and at one point ran out of fuel.
The Fukushima disaster is run-through with irony. The original bluff on which the plant is located was 35 meters high but was reduced to 10 meters in order to position the plant on bedrock to better withstand earthquakes and to reduce the cost of pumping seawater for cooling, thus making it vulnerable to a quake-induced tsunami. A plant that produced massive amounts of electricity was undone when it lost that very power. Key back-up systems were so intricately entwined with the systems that were designed to replace that the loss of one meant the loss of the other.
In the most highly regulated industry in the world, rules and procedures were necessarily discarded as humans fought to overcome design limitations and the effects of nature and physics.
Nature won, again.
Institute of Nuclear Power Operations
New York Times