William Sames: Summarizing nuclear situation in Japan

On March 11, Japan suffered a 9.0 magnitude earthquake followed by a tsunami. The occurrence of these natural disasters led to death and destruction across the country, and it also led to problems at some of Japan's older nuclear reactors.

The reactors that are experiencing difficulties are at the Fukushima Dai-ichi site, located along Japan's eastern coast. There are six reactors at the site. At a separate site, Fukushima Daini, four plants safely reached cold shutdown conditions.

When the earthquake hit, the reactors were shut down, or "scrammed," which took them down to approximately 8 percent power production. Reactor power cannot be turned off immediately because heat is still being produced from the decay of radioactive material inside the fueled region of the reactor — the reactor core. Cooling of the core is necessary to remove this residual heat.

This cooling is provided by operating water circulation pumps that move cooling water through the reactor core. The electrical power necessary to operate these circulation pumps needs to be provided from another source — it cannot come from the reactor after it is shut down. This power must come from backup generators or off-site sources.  

According to Japan's information updates, failure of recirculation pumps occurred at three of the Dai-ichi reactors following the tsunami. These failures led to problems cooling the core, and to the formation of gas pressure in the primary containment.

Hydrogen gas is formed when the fuel is not fully cooled due to a chemical reaction between water and zirconium. Hydrogen explosions occurred when hydrogen gas vented to the secondary containment reached critical concentration levels, causing damage to secondary containment of four of the six reactors.

 Venting was a necessary action to reduce pressure in the primary containment. Currently available information shows no damage to any of the primary containment structures protecting reactor vessels from these explosions.

Spent fuel pools located on-site at these reactor locations contain used nuclear fuel. Water levels and circulations need to be maintained to remove decay heat and prevent fuel damage.

Substantial resources are being focused on this task and there have been reports that the situation is stabilizing.

As of March 20, the remaining reactor sites affected by the earthquake have been shut down or cooling has been restored. The radiation dose rate at the site boundary ranges from 1-34 millirem per hour. Dose rates between 20-40 kilometers from the plant have been reported as "marginally above background." Spent fuel pool cooling continues to remain a top priority to response teams. Seawater and boric acid are being used as methods to cool the reactors and the spent fuel pools.

"I would say optimistically that things appear to be on the verge of stabilizing," said William Borchardt, executive director for operations of the Nuclear Regulatory Commission.

According to John Poston, professor of nuclear engineering at Texas A&M University, the largest dose to an on-site worker has been .10 Sievert. While this dose is five times the legal annual limit in Japan, this dose is still considered to be safe and no health effects are anticipated.

There have been reports of milk and spinach exceeding radioactivity limits in some areas of Japan. The levels are still so low that Japanese Chief Cabinet Secretary Yukio Edano felt the need to put things into perspective.

"This is not at the level that would have any direct effect on your health," Edano said.

He went on to say that drinking the milk for a year would result in the equivalent dose of one CT scan and eating the spinach for a year would result in one-fifth of a CT scan. Eight other tested food products had radioactivity levels below allowable limits.

As Japan continues to monitor the reactors, status updates will continue to be released.

Technical analysis of these events will provide a basis for improved safety in nuclear plants worldwide. In the United States, the NRC will use this information to conduct additional safety reviews of all plants in the country. Based on the lessons learned from this natural disaster, additional safety features will be implemented.