Particularly in the last 48 hours, the focus of the Japanese earthquake/tsunami catastrophe has shifted to the ensuing fate of the Fukushima Dai-ichi Nuclear Power Plant (福島第一原子力発電所) and whether a nuclear disaster may be imminent.
The world first became aware of the troubles at the Fukushima Dai-ichi shortly after the earthquake struck on Friday, 11 March 2011. Concerns were raised about the increasing pressure inside reactor 1 of the complex. On Saturday 12 march 2011, the increasing pressure caused reactor 1 to explode. On Monday 14 March 2011, reactor 3 experienced a hydrogen explosion after cooling problems. The next day, a new explosion occurred at reactor 2. It was also announced that an explosion and ensuing fire occurred at reactor 4.
There is increasing concern that radiation is leaking freely into the air.
Reuters’ has collated an extensive timeline of the unfolding nuclear crisis.
Fukushima Dai-ichi is operated by Tokyo Electric Power Co (TEPCO). It began commercial operation in 1971. The facility contains six nuclear reactors, with two further reactors scheduled for construction in 2012 and completion in 2016-2017. Reactor 1 was scheduled for decommissioning this year, but the Japanese government granted an extension in operation of ten years.
Simply, creating electricity by way of nuclear means involves the use of nuclear fuel rods, typically composed of uranium or plutonium isotopes. Through a process called nuclear fission, a neutron is absorbed by the nuclei of these heavy isotopes, causing it to become unstable and split releasing more neutrons and vast amounts of heat in a self-sustaining cycle. A coolant, usually water, is circulated past the reactor core to absorb the generated heat. In normal electricity generation operation, this water becomes steam, which then spins turbines that generates electricity.
The earthquake caused the immediate shutdown of all nuclear power plants in Japan as a precautionary measure. Despite the plant’s shutdown and the termination of electricity generation, the nuclear fuel rods inside the reactor core continues to undergo nuclear fission. Normally, despite the shutdown, backup generators from a separate electrical source operate to continue circulating coolant to the reactor core in order remove decay heat.
In this instance, the backup generators failed and coolant was not circulated to cool the reactor cores. As a result, the reactor core began to overheat. This caused pressure to increase inside reactor vessel (which encapsulates the reactor core). In order to contain the pressure, authorities were forced to release this radioactive steam into the air.
At the same time, water levels inside the reactor vessel decreased markedly, at times exposing the nuclear fuel rods. As a result, hydrogen began producing through chemical oxidation between the fuel rods and the steam. This hydrogen leaked into the containment building and ignited causing the hydrogen explosions.
In order to cool the reactor core and contain the above problems, the decision was made by authorities to manually pump a combination of sea water and boric acid into the reactor vessel. Boric acid was used to absorb neutrons so as to slow down the fission process. Sea water was used to provide relief as a coolant. However, in doing so it contaminated the pure water inside the nuclear vessel. Should this reactor be used in the future, a process of decontamination will need to occur.
Also, above a certain temperature, the nuclear fuel rods begin to melt and compromise the integrity of the reactor vessel. For uranium fuel rods, this temperature is about 2,200°C. There are reports of partial meltdowns at the Fukushima Dai-ichi’s reactors.
For further information about nuclear reactors, Wikipedia provides a good read.