Long-term radiation effects difficult to gauge, experts say

TOKAI, Ibaraki Pref. — While facts continued to trickle out about the nation’s worst radiation leakage, which occurred at the nuclear fuel reprocessing plant here last week, officials of the governmental Power Reactor and Nuclear Fuel Development Corp. (PNC) kept claiming the leak posed no serious bodily or environmental harm.

By the weekend, all 37 workers believed exposed to radiation at the PNC-run Tokai plant had returned to normal duties, and PNC officials said none has shown symptoms of abnormalities. Experts agree that the amount of exposure — if the PNC-released data are correct — is not large enough to pose any immediate risk to the workers’ health. But some say longer-term effects cannot be ruled out.

Radiation first leaked after a fire broke out at the bituminization facility at 10 a.m. Still more leaked following an explosion 10 hours later. The 37 were among 112 workers who were known to have been in and around the facility at the time. The PNC has announced that a total of 22,600 becquerels of alpha rays — including those from highly toxic plutonium, which can cause cancer — and 58.3 million becquerels of beta rays — including those from cesium, which can pose a genetic hazard — were released into the air after the accident. A becquerel is a unit for measuring radioactive decay and shows the number of atoms that disintegrate per second.

These amounts are less than 6 percent of the legal safety ceiling, the PNC says, stressing that there will be no major damage to the environment. The PNC says its precise body counter detected more than 50 becquerels of cesium — PNC’s minimum measurable amount — in each of the 37 workers.

The 2,700 becquerels of cesium detected in the heaviest-contaminated worker is less than two-thousandths of the maximum annual intake limit for plant workers set by the government. Three others had between 1,000 to 2,000 becquerels of cesium, and the rest were detected with less than 1,000 becquerels.

The PNC has conducted medical checkups on the workers since Mar. 14 and says none have complained of any physical problems. An average human being is usually detected with 20 becquerels of cesium and can take up to about 15,000 becquerels a year, or 1 millisievert, according to Ryushi Ichikawa, a radioactivity ecologist at the Japan Chemical Analysis Center in Chiba.

A sievert is a unit of intensity of radiation’s impact on the human body. A radiation exposure of 1 sievert is enough to make one feel nausea and fatigue; a 7-sievert exposure is lethal. The amount of leakage and exposure in the latest accident, therefore, poses no major threat, Ichikawa said.

According to a United Nations report, an average person is naturally exposed to about 2.4 millisieverts of radiation a year, more than half of which comes from breathing the air, 15 percent from food intake, 16 percent from the earth and 15 percent from outer space. In addition, people on average expose themselves to about 1 millisievert of medical radiation — through X-rays for example — per year, the report says.