Workers have been trying to resolve cooling problems at four reactors at northeastern Japan’s Fukushima Daiichi nuclear power plant, whose cooling systems are said to have been disabled by Friday’s tsunami. The crisis has raised serious concerns about radiation.
Below is a short glossary of some terms often used in accounts of the Fukushima Daiichi crisis.
Boiling-water nuclear reactor: The type of nuclear reactor used at the Fukushima Daiichi plant. In these reactors, fuel rods – made of radioactive pellets that usually contain enriched uranium-235 and are encased in zirconium alloy – are placed into a reactor vessel, or large steel tank filled with water. Nuclear fission – the splitting of atoms – is initiated, giving off heat and sending free neutrons toward other atoms, causing more fission in chain reactions. The heat boils the water, which turns to steam. The steam is piped to turbines, making them turn, which creates electricity. A condenser turns the steam back to cool water, which is sent back to the reactor core. See an interactive explaining this reactor type and the Fukushima Daiichi crisis.
Boric acid: A material used to help slow down nuclear fission reactions. South Korea is sending more than 50 tons of boric acid to Japan for use at Tokyo Electric Power's damaged nuclear facilities, Yonhap News Agency reported. South Korea's Ministry of Knowledge Economy said it was supplying the boric acid at Japan’s request after Tokyo used its reserves of the material at the Fukushima Daiichi nuclear plant.
Chernobyl: The site, in northern Ukraine, of the world’s worst nuclear power plant disaster. A meltdown, explosion and fire at the site on April 26, 1986, sent a large radiation cloud over much of Europe and contaminated large areas of then-Soviet Ukraine, Russia and Belarus. Thousands of deaths are said to be attributed to the disaster, and Ukraine's health ministry estimated that one-sixteenth of its population of 49 million was suffering from grave health disorders related to the incident. The disaster led to thousands of cases of childhood thyroid cancer, according to Dr. Ira Helfand of Physicians for Social Responsibility, which opposes the use of nuclear power.
Control rod: A rod, separate from a fuel rod, that contains material that can absorb neutrons. When placed into a reactor, it slows the fission chain reactions. As the control rods are removed, the reactions increase.
Fission: The splitting of an atom, which releases energy - usually in the form of heat - that can be used to produce electricity, according to the U.S. Nuclear Regulatory Commission. To see how the Fukushima Daiichi plant uses fission to produce electricity, see the boiling-water nuclear reactor entry.
Fuel rod: A long, zirconium alloy tube containing radioactive pellets that fuel a nuclear reactor. A group of fuel rods make up a fuel assembly. To see how fuel rods are used at Fukushima Daiichi, see the boiling-water nuclear reactor entry.
International Atomic Energy Agency (IAEA): This agency, with 151 member states, inspects nuclear and related facilities under safeguard agreements. It was established in 1957 to facilitate the peaceful use of nuclear energy. Among other things, the agency monitors nuclear reactors to make sure nuclear material is not being diverted for making weapons, and it helps countries prepare and respond to emergencies.
International Nuclear and Radiological Event Scale (INES): A scale developed by the IAEA to identify the severity of nuclear incidents. It goes from level 1, which indicates very little danger to the general population, to level 7, a "major accident" with a large release of radioactive material and widespread health and environmental effects.
Japanese nuclear authorities initially rated the Fukushima Daiichi incident at level 4, which is characterized as a minor release of radioactive material that necessitates only measures to control food due to contamination. But Joseph Cirincione, president of the Ploughshares Fund and author of "Deadly Arsenals: Nuclear, Biological and Chemical Threats," said the level is far beyond 4. "We are way beyond Three Mile Island level and heading into Chernobyl territory," he said. "This is at least a 5, probably a 6 (a serious accident) and it could end up a 7."
"It's clear we are at level 6, that's to say we're at a level in between what happened at Three Mile Island and Chernobyl," Andre-Claude Lacoste, president of France's nuclear safety authority, told reporters Tuesday.
Level 6 events have broad consequences that require countermeasures to deal with the radioactive contamination. Level 7 events would constitute a larger release of radioactive material and would require further countermeasures.
Meltdown: This refers to the melting of nuclear fuel rods in the reactor core. The melting, which would release radiation from the pellets inside the rods, can happen if the control rods aren’t able to control the chain reactions (and therefore the temperature) inside the core, or if cool water isn’t pumped into the reactor continuously. High temperatures can also compromise the reactor vessel and surrounding containment systems; if those systems are breached, radioactive material could be released into the environment.
At the Fukushima Daiichi plant, three active reactors shut down automatically as intended because of Friday’s earthquake. But cool water still was needed to keep the fuel rods from overheating, and the earthquake caused the water pumps to lose power. Emergency generators then powered the pumps, but the subsequent tsunami damaged those generators. The pumps then operated on battery power, which lasted for eight hours.
Some Fukushima Daiichi reactors are believed to have experienced partial core meltdowns. Tom Cochran, a senior scientist in the nuclear program at the Natural Resources Defense Council, said one uncertainty is whether the reactors’ containment vessels will keep any molten nuclear material from entering the environment. If the containment system is breached, "You have more or less direct access to the environment,” he said. “At that point, you expect the volatile fission products and the gaseous fission products to get out of the system.”
Potassium iodide: A compound that scientists say may give some protection from exposure to radioactive iodine-131, which is one of the fission products that can be released in a meltdown. Potassium iodide would not give any protection against any other radioactive isotope that may be released in a meltdown at a nuclear power plant.
Reactor core: The part of the reactor where the fission takes place. In the case of Fukushima Daiichi plant, it’s the vessel that contains the fuel rods, water and the control rods.
Sievert (and millisievert): A unit of measurement for radiation dosage. According to the World Health Organization, the average person is exposed to about 3 millisieverts a year of radiation, from naturally occurring, medical and other sources. But monitoring at the Fukushima Daiichi site has recorded radiation as high as 400 millisieverts an hour - a level known to be a risk to human health. Exposure to 1,000 millisieverts (1 sievert) of radiation can cause radiation sickness.
Spent nuclear fuel: Nuclear reactor fuel that has been used to the extent that it can no longer effectively sustain a chain reaction, according to the U.S. Nuclear Regulatory Commission.
Spent fuel storage pool: An underwater storage and cooling facility for spent fuel assemblies – groups of fuel rods - that have been removed from a reactor, according to the U.S. Nuclear Regulatory Commission. The water is supposed to cool and shield the spent fuel rods.
On Wednesday, the head of the U.S. Nuclear Regulatory Commission told a House committee hearing that water had disappeared from a spent fuel storage pool in one of Fukushima Daiichi’s units, exposing spent fuel rods and leading to the emission of "extremely high" levels of radiation.
Three Mile Island: A nuclear power plant near Middletown, Pennsylvania, which was the site of the worst nuclear power disaster in the United States. A partial core meltdown occurred in 1979. The plant had containment units, so the release of radioactive material into the environment was minimal. The incident caused no injuries or deaths, and only low levels of radiation were found in plants and animals, experts said.
- CNN's Aaron Brodie contributed to this report.