The water in the tank was first noticed in December, she said, during inspections near the graphite reactor.

But the graphite reactor was a crude tool for experiments on humans.

The graphite reactor has been operating for about two years, using natural uranium fuel and 1950's technology, Government experts said .

Brookhaven officials said they believed that some of the plutonium might have come from a graphite nuclear reactor the laboratory closed in 1969.

Cleanup of the graphite reactor actually began many years ago, but only in pieces.

That was prompted mainly by high levels of mercury, which came from many lab facilities, including the graphite reactor.

The graphite reactor began operation in 1950, the first nuclear reactor built exclusively for scientific research.

"One of the earliest incentives to build the graphite reactor was to fill in those details."

Although not all effects are well understood yet, more than 100 graphite reactors have successfully operated in the last 60 years.

General Kimstach did not offer a timetable for gradually eliminating the graphite reactor.