Radiation and the Japanese Nuclear Reactors (Nuclear Energy Institute - NEI)

Nuclear Energy Institute (NEI) ^ | Updated 3/13/11 @ 3:30 p.m. EDT | Nuclear Energy Institute (NEI)

[SteveH]

Radiation and the Japanese Nuclear Reactors

March 2011

Key Facts

* An earthquake measuring 9.0 on the Richter scale struck off the northeastern coast of Japan on March 11, triggering a tsunami. Along with the loss of life and damage, Japan also faces the challenge of stabilizing nuclear power plants in the hardest-hit region.

* After the earthquake, all the operating reactors at the Fukushima Daiichi and Fukushima Daini nuclear plants shut down automatically, as they are designed to do. However, due to the loss of offsite power and failure of the backup diesel generators, there were difficulties powering the waste heat cooling systems at the Fukushima Daiichi plant.

* To reduce the resulting increase in containment pressure, Tokyo Electric Power Co., vented steam—containing small amounts of radioactive material—from the primary cooling circuit of reactors 1 and 3. The released vapor passed though filters designed to remove radioactive components such as iodine and cesium. Upon release, the slightly radioactive vapor dispersed into the atmosphere.

* Residents have been evacuated from a 12.5-mile radius around the Daiichi plant and about two miles around the Daini plant. The precautionary measures taken to evacuate residents near the sites are intended to prevent or mitigate any radiation dose from radiation releases that might occur as the situation develops.

* A buildup of hydrogen gas in the secondary containment structure at Daiichi Unit 1 led to an explosion at that reactor (see reactor diagram). However, the integrity of the primary containment structure was not compromised and there were no large leaks of radiation from the reactor core.

Radiation Measurements

The U.S. Nuclear Regulatory Commission’s (NRC) annual limit for worker exposure to radiation is 5,000 millirems (mrems). The average U.S. nuclear power plant worker receives 120 mrems annually. A typical X-ray provides 10 mrems per film.

The NRC’s public radiation dose limit is 100 mrems annually. The average U.S. public exposure from the commercial nuclear fuel cycle, including nuclear power plant operations, is less than 1 mrem per year.

The average American receives more than 600 millirems of radiation exposure annually—about half from naturally-occurring sources and the rest from medical applications, such as CT scans and X-rays. Although there is scientific evidence for health risks following high-dose radiation exposures, risks of health effects are either too small to be observed or are nonexistent at levels below 5,000–10,000 mrem. [1]

A small amount of radiation was released in the 1979 Three Mile Island accident in Pennsylvania, but studies found that it did not have an impact on health or the environment. About half of the reactor fuel at TMI Unit 2 melted during the early stages of that accident.

--- [1] Position Statement of the Health Physics Society, “Radiation Risk in Perspective,” U.S. Health Physics Society, July 2010.

[sefarkas]

This is simply not true. There is a pool in the reactor building that is used for rod storage during refueling but long term storage is done at another location on another part of the campus. Since there was no refueling going on for units 1 or 3 there is no fuel stored in these pools.

In the Mark I containments, the top of the spent fuel pool is at the same level as the refueling floor. With the sheet-metal walls torn away, the spent fuel pool is "outside"; however, the pool is 23 feet deep down to the top of the fuel bundles kept there. Note that the Japanese re-cycle used fuel; thus, there would not necessarily be as many fuel bundles in the pool as you would find in an American Mark I containment. New/unburned fuel is also kept in the "spent fuel pool" awaiting the next refueling outage. It is the spent fuel bundles with the decaying fission products that creates the 'residual heat' that the spent fuel pool cooling system has to remove. Other containment designs, particularly the ones for PWRs (e.g., Westinghouse, Combustion Engineering) have a separate 'fuel handling building' in which they keep spent fuel bundles. The location of the spent fuel pool is plant specific.