May 23, 2013
The Latest From Japan’s Nuclear Disaster
Posted on Mar 12, 2011
The term core meltdown is not officially defined, but it can refer to a number of scenarios in which safety systems fail and the nuclear fuel or the metal cladding around the fuel reaches extreme temperatures. Most active reactors today are within a primary containment vessel, made out of metal, which is designed to encase even molten nuclear fuel.
All nuclear reactors employ a nuclear fuel that slowly undergoes nuclear decay via fission. The fuel atoms split, shooting neutron particles into neighboring atoms, leading to a controlled fission chain reaction. A byproduct of this reaction is a tremendous amount of thermal energy, which is then used to heat water and produce steam that drives a turbine linked to an electrical power generator.
The explosion at the Fukushima Daiichi power plant occurred about 3:30 p.m. local time Friday. Videos of the explosion showed a violent event with debris being thrown high into the air, followed by a substantial cloud.
Later, photos of the plant showed a skeletal structure remaining, with most of the building walls missing.
After Friday’s massive earthquake, the reactor was automatically placed in its lowest-power “shutdown” state. Even in this state, the nuclear fuel requires active water-cooling and a steady supply of electrical power. According to reports, several backup power generators failed, leaving the plant with only eight hours of battery power. The Unit 1 reactor has been without active cooling for a longer period than called for in its design.
Japanese officials have reportedly flooded the reactor core with salt water, with the goal of reducing the core temperature. This is considered to be an unusual emergency measure, which the company probably did only to avert greater disaster. The salt water may cause chemical corrosion of the fuel assembly, which would ruin the reactor core’s economic value. It is unclear how salt water was delivered to the core, or whether any of the potentially radioactive water was being released back into the sea after entering the core.
It is considered vital to keep the fuel assembly completely submerged, to ensure proper cooling, and to keep the assembly from burning in a chemical fire, which would release radioactive smoke. Some reports indicated that the core may have been partly uncovered before the infusion of the salt water.
The explosive damage to the containment building suggests that substantial portions of the water cooling loop were destroyed. Such an event would cause the water and steam to escape, leading to a release of radioactivity in the form of various gases that collect within the fuel assembly and cooling loops.
Most worst-case scenarios at the Japanese plant are less dire than the disaster at Chernobyl in 1986. The Chernobyl plant reactor core was not housed within a hardened primary containment vessel, a condition that allowed extremely radioactive materials to escape when the core reached extreme temperatures and destroyed the soft outer containment building. The situation at Chernobyl was exacerbated because the graphite control rods caught fire, resulting in the uncontrolled release of radioactive smoke.
The situation at the Japanese plant has officially been classified as a level four event, in a range of one to seven on the International Nuclear and Radiological Event Scale. Japanese nuclear officials have been criticized in the past for minimizing the seriousness of some nuclear accidents.
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