by Jeff Durbin
A fire and a later explosion on March 11 at the Tokai nuclear fuel reprocessing plant in Ibaraki Prefecture have released a small quantity of radiation, affecting 37 plant workers. The government has recognized it as Japan's worst-ever nuclear accident. Just 15 months after the Monju coolant leak, the Power Reactor and Nuclear Fuel Development Corporation (PNC, or "Donen"), the semi-governmental corporation that operates Tokai, once more faces criticism for plant design flaws, a sloppy and unsafe response, and failure to properly warn nearby residents and local authorities. The Tokai plant will be shut down for the remainder of the year.
Donen has announced that 22,600 becquerels (Bq) of alpha rays and 58.3 million Bq of beta rays were released into the air, about six percent of the legal limit. Some radiation, including Cesium 134, has been detected 20 kilometers from the plant in Oaraimachi. Of the 37 workers, the most seriously affected was exposed to 2700 Bq of cesium, less than one percent of the maximum allowable annual dose for Japanese nuclear workers. Three other workers received 1000-2000 Bq of cesium. Although the amounts involved are small, both cesium and plutonium, which produced some of the alpha rays, may have impacts on health proportional to the exposure; any radiation, that is, could increase the probability of cancer later in life.
The explosion happened at 8:04 pm on March 11, and blew apart windows and steel doors and shutters in a low-level waste facility, where reprocessed waste is mixed with high-temperature asphalt and stored in 200-liter drums. The explosion was probably caused by a buildup of gas after the building was sealed following a fire at 10:06 that morning. The fire was put out with water, instead of carbon dioxide or a water-carbon dioxide mixture, and workers also failed to close off incoming air as the safety manual prescribes. The only thermometer in the facility stopped working after the fire.
Thirty-four of the workers were exposed at the time of the morning fire, most of them in an adjacent building called the Z facility, which had no radiation alarm. A passageway between the buildings has double doors to block airflow, but they were open at the time of the fire. Despite four radiation alarms going off seven minutes after the fire started, there was no evacuation order for the building where the fire occurred until 20 minutes had passed. The Z facility was not evacuated until 74 minutes after the fire.
Some questionable decisions were also made around the time of the evening explosion: first, workers had been getting ready to enter the building around 8 pm to restart fans damaged in the fire, and narrowly missed being caught in the explosion; then, at about 11 pm, three hours after the explosion, three workers were sent to the site to inspect and videotape the damage before the building's safety was known.
On the same day, 64 people, including university students and foreign trainees, were on a tour of the Tokai plant between 9:30 am and 4 pm, but were not informed of the accident. They visited a building 100 meters from the location of the fire, but Donen officials had decided that with the fire apparently extinguished, there was no risk. Radiation levels around the tour route were not checked. Tokai village officials did not hear of the accident until 2 am that night, and emergency sirens in the village sounded the following morning at 6:30 am, about 10 hours after the explosion.
Since tests in the early 1970s, Donen had been aware that a mixture of nuclear waste and asphalt readily produces heat and is flammable. Nevertheless, no devices were in place to measure the temperature of the stored drums. Donen has already designed a facility which doesn't use asphalt, and for several years has considered replacing the existing plant.
With the Tokai plant closed, two tons of spent fuel remain in the separation facility awaiting reprocessing, and will emit radiation if left as is. The 16-year old Tokai site is Japan's only reprocessing facility and handles 12 percent of the nation's spent nuclear fuel. The remainder is sent to Europe, and in fact on March 18 a return shipment arrived in Rokkasho, Aomori Prefecture, after a two-month journey from France. Rokkasho is also the site of a reprocessing plant slated to go into operation in 2003. The Aomori prefectural government, however, says it will now postpone signing a safety agreement with Japan Nuclear Fuel Corporation, a consortium of power companies that is building the Rokkasho plant.
The accident will probably be classified as level 3 on the International Nuclear Event Scale. Until now, a radiation leak at Mihama plant in Fukui in 1991 had been Japan's most serious nuclear accident, at level 2. The Monju accident of December 8, 1995 has recently been graded level 1. On the scale of 0 to 7, Three Mile Island in the U.S. in 1979 was level 5, and Chernobyl in 1986 rated 7.
(Gathered from reports in The Japan Times and Daily Yomiuri.)