Rockville, Md., Dec. 23 - Eighteen years after it promised to decide how best to protect the public in a reactor accident, the Nuclear Regulatory Commission said on Friday that it would require the states to consider stockpiling a drug to protect against thyroid cancer and that it would pay for the pills for any state that wanted them. 

The commission has apparently spent more on considering whether to pay for the pills than on what the pills themselves would cost. Last year the commission told Congress that it had spent $2.6 million studying the use of the drug, potassium iodide. With an average of 80,000 people living within 10 miles of each of 70 reactor sites around the country, and with the maximum anticipated use being two pills, at 20 cents each, per person, the pills would cost less than $2.4 million if every state wanted them. Several states, however, have decided against them. 

The commission has budgeted $400,000 for the first year. 

The decision on Friday reversed a vote in April 1999 not to pay for states to stockpile potassium iodide. That vote reversed a decision in June 1997 to provide the drug. 

But the argument goes back much further, to the investigation of the Three Mile Island reactor accident in 1979. That accident set off a scramble by federal officials and pharmaceutical companies to manufacture the drug, which is only effective if taken before exposure to radiation or in the first few hours afterward. Potassium iodide saturates the thyroid gland and prevents the body from absorbing the radioactive iodine that is sometimes present in the air after a reactor accident. 

After the commission that investigated the Three Mile Island accident strongly recommended the creation of regional stockpiles of the drug, the N.R.C. promised Congress it would develop a policy by September 1982. 

It took until 1985 to develop a policy, which generally discouraged the use of potassium iodide. But that came just a few months before the Chernobyl nuclear accident in Ukraine, which changed the minds of many Western experts. The authorities in Poland distributed potassium iodide, and experts said there were fewer cases of thyroid cancer there as a result. 

In 1988, a staff lawyer at the commission, Peter G. Crane, who had thyroid cancer caused by exposure to radiation as a child, filed an official dissent on the N.R.C.'s position. Later, acting as a member of the public, he filed a petition for it to be reconsidered. That petition was the basis of Friday's action. 

A decision to use potassium iodide in this country would be up to local or state officials. Some public health officials fear that if they provide it, people might refuse to evacuate in the belief that they were immune to radiation. Potassium iodide protects only the thyroid. 

Some accidents release iodine 131, which is readily absorbed by the human body and concentrated in the thyroid. Other radioactive isotopes emit different kinds of radiation, which can damage the lung, the bone marrow or other organs. 

Participants in the debate say they are surprised at how long the debate on stockpiling has taken. A commission staff paper suggested in 1994 that the debate might cost more than the drug. 

That position was endorsed by William F. McNutt, who is a senior policy adviser at the Federal Emergency Management Agency, which works with the states on emergency plans, and the chairman of a multi agency subcommittee on potassium iodide. "There have been comments made: Why don't we just decide to buy it, it's a cheap drug and we're spending an awful lot of money analyzing it," Mr. McNutt said.

The chairman of the commission, Richard A. Meserve, said he could not explain why the commission had reversed itself twice in the last three years, except that the composition of the five-member panel had changed over that time. He said that his panel had resolved many issues much faster, including renewal of licenses on nuclear power plants and regulation of companies that make nuclear fuel. 

"It has taken an awfully large amount of attention by the commissioners," he acknowledged. 

So much, in fact, that half of Friday's decision to require states to consider putting potassium iodide in their emergency plans is old hat for most of the commissioners. 

"Oh, they flip-flopped," said Andrew Simpson, a planning supervisor in the Pennsylvania Emergency Management Agency in Harrisburg on Friday, when told of the vote. "Clearly, requiring the states to consider whether to use it or not is a non sequitur; all states have continually done that." 

All states are supposed to have supplies for emergency workers who would be called upon to stay in a radiation zone, and some have stocks for populations considered difficult to move, like prisoners or nursing home residents. Tennessee, Arizona and Alabama have supplies for the general population. Officials in some states, including New Jersey, said Friday that they would probably re-evaluate their positions in light of the latest vote. 

But some experts say that the drug would be difficult to distribute in an emergency, and that if pills were distributed in advance, to homes and institutions, members of the public would lose track of them before they were needed. 

Copyright 2000 The New York Times Company