The maximum hazard from a crippled nuclear power plant depends on how much radioactive fuel is on site, both in the reactors and in the storage pools. And the Daiichi complex in Fukushima, Japan, damaged by the 11 March earthquake and tsunami contains more fuel than was at risk at Three Mile Island.
The Daiichi complex had a total of 1760 metric tons of fresh and used nuclear fuel on site last year, according to a presentation by its owners, the Tokyo Electric Power Company (Tepco). The most damaged Daiichi reactor, number 3, contains about 90 tons of fuel, and the storage pool above reactor 4, which the Nuclear Regulatory Commission's (NRC's) Gregory Jaczko reported yesterday had lost its cooling water, contains 135 tons of spent fuel. The amount of fuel lost in the core melt at Three Mile Island in 1979 was about 30 tons; the Chernobyl reactors had about 180 tons when the accident occurred in 1986.
When the 9.0 earthquake struck, three of the six nuclear reactors at the Daiichi complex were running and were immediately shut down. The other three were already down for inspection, and their fuel had been unloaded.
The three machines in service—only one of which appears to have been seriously damaged in the quake—contain partly burned fuel that could harm the environment and endanger public health. Storage ponds that contain used fuel also pose a risk—as Jaczko, the NRC chair, emphasized in dramatic testimony yesterday to the U.S. Congress. Citing the deterioration of Daiichi's cooling systems, he recommended that people evacuate the area around the plant within a 50-mile radius.
Some saw Jaczko's comments as an overreaction. But it raised the question of what a worst-case scenario would look like.
The answer must take into account fuel rods held in standby in the reactors plus used fuel in the seven storage pools—one co-located with each reactor and a central holding facility. Although cooling and fuel containment systems have done their jobs as designed in most cases, one reactor appears to be leaking from its containment structure. And one holding pool—according to Jaczko, but not Tepco—may have run out of water. The temperature of some other pools is elevated.