WASHINGTON (AP) _ Potentially enough plutonium to start a chain reaction was recorded in a radioactive waste tank at the Hanford Nuclear Reservation in Washington state, but the reading appears to be based on ''calculational errors,'' the Energy Department said Tuesday.

Energy Secretary James Watkins wrote to interested members of Congress that a departmental review team ''will thoroughly investigate the potential infraction and root causes'' to ensure the safety of the tanks at Richland, Wash., where plutonium first was processed for nuclear weapons during World War II.

''Preliminary indications are that calculational errors resulted in this reporting'' of excessive plutonium, he wrote.

A spokesman for Westinghouse Hanford Co. in Richland, operator of the installation, said the results were skewed by a faulty computer program.

''It turns out there was a problem with the spreadsheet program and that kicked out what turned out to be bad numbers,'' said Eric Campbell.

''So yeah, we cried wolf, but we thought it was the responsible thing to do - to say we may have a problem, then later show it wasn't a problem,'' Campbell said.

Nevertheless, the disclosure alarmed a congressman and independent scientists who have been monitoring the aging tanks.

''It is very troubling,'' Rep. Ron Wyden, D-Ore., said. ''It's important not to make rash charges, but excess plutonium has the potential to be a very serious problem. When you see that you have to ask about radiation releases.''

Campbell said a lack of accurate records of wastes in the 40-and 50-year- old tanks keeps the department from saying definitively that there are no safety problems.

''No one is exactly sure what is in the tanks,'' he said.

Watkins mentioned the potential danger regarding plutonium in a letter to members of Congress, dated July 16, that accompanied a report on the status of 177 underground storage tanks at the reservation.

The report warned that some of the aging tanks could begin to leak high- level radiation before the department can move the wastes sometime after the turn of the century.

However, the report did not mention the plutonium buildup.

''The department determined that a potential plutonium mass limit could have been exceeded,'' Watkins wrote in the cover letter.

Campbell said the faulty computer program extrapolated across the half- million gallon tank showed the tank contained between 397 and 408 pounds of plutonium.

That was more than the 276 pounds the department has set as a safety level but well short of the 551 pounds that could - but not necessarily would - result in a critical mass, he said.

Recalculations showed the actual amount of plutonium in the tank to be only 123 pounds, he said, and at less than 3 percent of the density needed to begin a chain reation.

The concentration in the tank was one-tenth of a gram per liter compared with the 4 grams per liter necessary to result in critical mass, he said.

The term ''critical mass'' is used by scientists to denote the amount where nuclear fission becomes self-sustaining with the release of energy. A strong case would be necessary to confine the reaction long enough to let it build up into a nuclear explosion, scientists say.

Tom Cochran, a scientist specializing in nuclear issues for the National Resources Defense Council, said it is not possible to determine the safety threat without reviewing the actual documents. But he said ''my guess is yes'' that there is a serious safety concern.

''The issue appears to be criticality - the possibility of a criticality condition in the tank due to excess concentration of plutonium,'' he said.

''You could get a sudden release of heat and fission products. The heat could rupture a tank,'' he said.

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