Nuclear reactor mishaps: less dangerous than thought?

Radioactive iodine and three Mile Island are the primary elements in the latest twist in the nuclear reactor safety drama. Radioactive iodine has been considered the most hazardous potential byproduct of a nuclear catastrophe. The conventional wisdom has been that about half of the total fatalities in the worst of accidents would be caused by this substance.

At the Three mile Island (TMI) nuclear power plant near Harrisburg, Pa., it was the possibility of large radio-iodine releases that caused panic and indecision at the Nuclear Regulatory Commission (NRC). However, only a minute amount of this lethal material escaped. In fact, the amount was so small that the team monitoring the accident with radioactivity detectors worried that something was seriously awry.

This unexpected turn of events has led a small cadre of Los Alamos and Oak Ridge National Laboratory experts to challenge some of the basic assumptions used to predict the consequences of accidents at nuclear reactors. And the Electric Power Research Institute (EPRI), the research arm of the electric utility industry, is pursuing this as part of a more sweeping argument that the health effects of major accidents at nuclear power plants have been substantially overestimated.

One of the first people to realize the significance of the low iodine release at TMI was William Stratton, a Los Alamos physicist who worked with the Kemeny Commission appointed by President Carter to probe the accident.

Generally, experts have assumed that iodine would behave like the chemically unreactive "noble" gases. This assumption was considered conservative -- the more reactive the iodine, the smaller the amount that would remain a gas and escape into the environment. But there was uncertainty on how iodine would behave in an accident.

During the Three Mile Island accident, a million times more of the noble gas xenon was released than iodine. "Radioactive iodine is a real 'baddy' because it concentrates in the body, but radioactive xenon is near-benign because it quickly diffuses and fades into the background," Dr. Stratton explains.

The physicist recruited two Oak Ridge chemists, Anthony Melanauskus and David Campbell, to investigate this further. They found that the large amounts of hydrogen and water present created chemical conditions that caused the radio-iodine to combine as a salt with cesium and other metals, rather than staying in its gaseous, elemental state.

This conclusion explains why little iodine was released in some other accidents at research reactors, says Dr. Stratton. He and his collaborators, while careful not to venture opinions on the policy implications of their work, have been urging that the Nuclear Regulatory Commission (NRC) give a high priority to reviewing this issue.

As a result of their activities, NRC has set up such a review. And the Nuclear Safety Oversight Committee late last month sent a carefully worded letter to the White House urging that the NRC and Department of Energy pursue this issue "more aggressively."

"If this assessment, which to our knowledge has not been refuted, proves correct, it would have major implications for such regulatory issues as plant siting and emergency planning, because the potential exposure of the neighboring population in the event of a major accident would be much lower than previously assumed," the commission's letter stated.

The policy implications of this work are not as clear cut as it might at first appear, however. Richard Denning of Battelle Memorial Institute, who is an expert in modeling nuclear accidents, for one, is quite cautious.

"The reason so little iodine was released at TMI was that the escape pathway was through water. Even if it had been elemental iodine, very little would have escaped. We have been looking at extremely improbable accident sequences, of the sort that might happen once every 200,000 reactor-years [or once every 1,300 years with the number of US reactors now operating or under construction], and there are cases where the iodine would not come into contact with water. With these sequences it makes very little difference which form the iodine is in," Dr. Denning asserts.

Herbert Kouts, a Brookhaven National Laboratory researcher who discovered the iodine descrepancy independently, agrees that "we are not in a position yet to change policy. In a case like this, you retain the conservatism as long as you must. We understand the chemistry now, but we don't know the accident sequences well enough."

Although enthusiastic about the possibilities, EPRI researcher Ian Wall is equally cautious. "There is strong circumstantial evidence that [radioactive] releases to the atmosphere may be a factor of 10 to 30 lower than previously assumed," Dr. Wall says. The effect of such a 10-fold reduction, particulary in iodine, during dominant accident sequences would be to reduce early fatalities and injuries to near zero and reduce the amount of contaminated land 30-fold, he has calculated.

"I intend to devote the next 12 to 18 months to this issue. In that time I should be able to prove the case or otherwise," Dr. Wall says.