23 Mar, 11 | by BMJ Group
The Fukushima Daiichi nuclear power plant has six reactors. It is located on Japan’s northeast coast, close to the earthquake’s epicenter. A tsunami higher than any anticipated took the plant’s generators out of service.
In a nuclear power plant, the core, where the nuclear reactions take place, generates heat, which boils water and further heats the steam, which turns turbines to generate electricity, just as electricity is generated in a coal or natural gas plant. Heat exchangers separate steam that contacts the core and therefore contains radionuclides from clean steam that drives the turbines.
What happened at Fukushima Daiichi (I’ll refer to it simply as Fukushima after this) is that the earthquake cut external electrical power, which pumps the water that cools the reactor core. Fuel that has seen its useful life in the core is called spent, or used, fuel and is stored in cooling pools in the reactor buildings. Pumps for cooling water to these pools also depend on external power. The purpose of the generators that the tsunami destroyed is to back up the external power in case of a blackout.
Units 1, 2, and 3 were operating when the earthquake hit; units 4, 5, and 6 were down for a planned outage. Units 1, 2, and 3 were successfully shut down in response to the loss of external power. But in a nuclear reactor, the fuel rods continue to generate heat after the fission reactions are shut down. This is because the atoms produced by fission are unstable and decay by emitting radiation and heat. Many of these unstable species decay over a period of days or weeks, emitting less heat over time. But just after shutdown, enough heat is generated to melt the fuel elements, and some cooling is needed for months.
Power has been restored to all units. Units 5 and 6 are now in cold shutdown, which means that coolant water is circulating with a temperature below 100 C. The spent fuel pools appear to be stable. This is good news, but there is some distance to go. There have been some radiation releases, apparently small, and there may be more. In reactors 1, 2, and 3, the fuel rods are probably damaged. There have been difficulties with keeping water in the spent fuel pools, but their situation is quite unclear.
Anything beyond this and the radiation readings being provided is a matter of surmise or wild leap of imagination. Information is not as plentiful as any of us would like. Further, there seem to be problems in translation from Japanese to English and from tech-speak to everyday talk. In my posts, I’ll try to make responsible surmises and help with translation from tech-speak.
I’ve written more about Fukushima on my home blog. Scroll down to see the last week’s posts. If you have a particular topic you would like me to address, please list it in a comment here. I’ll do my best to answer your questions, and I’ll be honest about what I know and don’t know.
Some helpful links:
- A timeline of events at Fukushima.
- The cartoonist XKCD has developed an excellent chart illustrating radiation doses and their effects.
- World Nuclear News is providing updates, as is the International Atomic Energy Agency.
- A short video explaining nuclear fission and how reactors work.
Cheryl Rofer holds an A.B. from Ripon College and an M.S. from the University of California at Berkeley, both in chemistry. She is retired from the Los Alamos National Laboratory, where she worked from 1965 through 2001 on tthe nuclear fuel cycle, management of environmental cleanups, and other topics. She has also been involved with cleanups in Estonia and Kazakhstan of former nuclear sites. She is immediate past president of the Los Alamos Committee on Arms Control and International Security and a member of the Board of Trustees of Ripon College (Ripon, Wisconsin). She also blogs at Phronesisaical (http://phronesisaical.blogspot.com/)