When bats migrate back to Hanford this spring, those that live in the 1950s-era 105DR nuclear reactor will find some improvements to their quarters, thanks to the Department of Energy.

It's one facility on the nuclear reservation where various environmental interests conflict.

On one side are the possibly hundreds of bats that have found cracks and openings in the reactor and use them to get into a system of dank, dark underground pipes.

Scientists believe most of them are from a tiny brown bat species called small-footed myotis. The state considers it a priority species, meaning it should be protected, if possible.

On the other side is DOE's mission to seal up the defunct plutonium producer, remove all structures around it and put on a new roof to entomb any radioactive contamination within it.

"The project was to start with the underground tunnels," said Ken Gano, senior scientist for Bechtel Hanford. "We were going to seal off the pipes right at the building." That's still the plan. But Bechtel also is putting in a new entrance to the pipes, an 8-foot-tall "bat gate" that will rise over the sagebrush closer to the other end of the pipes. Bats will be able to fly between the bars of the tower into the tunnel below, which is not contaminated.

Bechtel will put up the gate in the next few weeks, expecting that by the time the entrance to the pipe from the reactor is sealed in the year 2000, the bats will have gotten used to the new route.

"In this situation, we don't have to create another habitat - just open another door," Gano said. "From all the experts I've talked to, they've said, 'If you build it, they will come.' " Of primary concern is a maternity colony biologists have known about since 1993. In many bat species, the females separate from the males to raise their young.

Female bats are nesting in the reactor piping underneath a concrete hatch cover near the reac tor end. Scientists speculate they like the warmth from the sun hitting the hatch - similar bats elsewhere have been found rearing their young at temperatures of 122 degrees.

The hatch also is made with 1/4-to 1 1/2-inch-wide cracks that make cozy homes. The bats can crawl in, then turn around to face out, Gano said.

At night, they fly through the pipes and out the reactor to forage. One little brown bat can catch 1,200 bugs in an hour, often two in a second, according to Bat Conservation International.

Gano's not sure how many bats are using the maternity roost, but maternity colonies typically have 30 to 100 mothers, each raising a single baby each year.

That slow reproductive rate, common to most bats, is among the reasons the bats are vulnerable to extinction, even though the small-footed myotis, along with many other species of little brown bats, are fairly widespread in the western United States.

They're also finding fewer places to roost in the West, as attics are built tighter, mine shafts collapse or are destroyed and caves are vandalized.

"Those things alone make it very sensitive," Gano said. "You wipe out a population in an area, and it takes a long time to recover." The bat gate will go up about 600 feet down the pipe from the maternity colony. Although humans aren't likely to disturb the bat habitat on the far reaches of the Hanford reservation, the gate is being built with steel pieces spaced 5 3/4 inches apart, too small for a human head to fit through.

The drop to the floor of the pipe will be 12 to 15 feet, so coyotes or raccoons may investigate but aren't likely to enter the tunnel.

Part of the reason Hanford's defunct reactors are being sealed up - put in interim safe storage in Hanford lingo - is to keep animals safe from radioactive contamination.

Researchers have long known from droppings found in Hanford reactors that bats are living in them. "Old reactor buildings have essentially become caves for them," Gano said.

Piles of yellow-jacket wings at other old reactors also indicate bats have been snacking in the reactors. A slightly larger bat, the pallid species, catches the yellow jackets, eating the bodies and letting the wings flutter to the floor below its roost.

Bats are considered a potential problem for spreading radioactive contamination from the reactors, Gano said.

But because most contamination was spread by water on the floors of the reactors, bats are less likely to be contaminated than animals like field mice.

In any case, the pipes they're roosting in from the spring to the fall should be clean. They were installed to carry river water to the reactor.

The pipe with the maternity colony was a backup source of river water and may never have been used, Gano said.

However, as part of the Tri-Party cleanup agreement, Hanford eventually must be cleaned up enough for residential use. That includes taking out pipes near the surface of the ground, such as those home to the small-footed myotis.

That work is not yet scheduled.

In the meantime, "we plan to preserve it and let the decision be made somewhere down the road," Gano said.