Hanford researchers twice have tried to count the bats that have made a home in an underground concrete structure near the Columbia River.

They set up a video camera with an infrared light outside the hatch to the structure, called a clearwell, and let it roll.

"It's like popcorn coming out," said Ken Gano, a natural resource specialist for Washington Closure Hanford. "They come and go all night long."

Both times they've counted about 2,000 bats, which they say probably is a low estimate. But that number makes the colony the largest identified in the state, rivaled only by a colony that roosts under an Olympia pier.

This time next year, Washington Closure and the Department of Energy expect to know much more. As part of the Hanford cleanup effort, the clearwell is scheduled to be demolished in fiscal year 2009, which begins next October.

"That (gives) us some time to figure out how to deal with it," Gano said. "We can look at the impact to demolishing it and what we can do to provide an alternate roost site."

The colony qualifies as a priority species designation for the state because it's a maternity colony, with females spending the spring and summer roosting in the clearwell while each raise a single pup. It's so large that there is a possibility it's populating the entire region.

Researchers believe the bats are a type called Yuma myotis. But species can be difficult to distinguish from each other, and that's one question the study will answer.

They have furry brown bodies with black wings and are smaller than the silver-haired bat that is more commonly seen occasionally roosting in attics in the Tri-Cities.

Each Yuma myotis weighs about 6 to 8 grams - less than two nickels - and has a body smaller than a mouse. But they look bigger in flight because of a wing span that stretches 6 to 8 inches. In a night they might eat their weight in small insects, such as the mosquitoes and midges that are plentiful along the nearby Columbia River.

Bats are common at Hanford. They like warm temperatures and roost in reactors and other unused concrete buildings that heat up in the sun.

But this colony has found what amounts to a bat palace inside the large clearwell once used to hold filtered water for Hanford's F Reactor when it produced plutonium for the nation's nuclear weapons program. Sometime after it stopped operating about 30 years ago, one of its six hatches was left open, providing a doorway for the bat colony.

From above ground the roof of the clearwell looks like a parking lot a little larger than a football field. Inside, it's 15 feet deep and has 98 pillars holding up the roof.

Hanford researchers went inside a couple weeks ago, knowing they might not find many bats this time of year. The bats migrate to hibernate when the weather gets too cool for them to find the insects they need. Although Hanford researchers don't know where the colony has gone, they suspect it is not far.

They found about 30 bats still in the clearwell at the end of summer, but plenty of evidence that more had been there.

Around the base of the pillars, guano is piled more than an inch deep. The bats prefer temperatures of 90 to 110 degrees so they don't have to expend energy to stay warm and to help keep their pups, which are born hairless, warm. They evidently have used the top of the pillars, moving up and down the columns to find the optimum temperature, and also have roosted along the edges of the underside of the closed steel hatches.

Researchers found more bats when they entered a 700-foot-long flume adjacent to the clearwell that was used to carry water in and out. In video they shot of their entry into the flume, bats dodged around their heads in the narrow passageway and clung in clusters to the roof.

"They like togetherness," said Dana Ward, DOE environmental scientist.

Researchers also spent an evening catching bats in a fine net near the exit of the flume.

Each bat was carefully untangled from the net, then measured.

"They're squeaking at you the whole time," Gano said. "You learn how to handle them and not squeeze too hard."

Some also had a tiny hole punched in their wing to collect tissue for DNA samples.

"It doesn't seem to hurt," Gano said. "It grows back in a couple weeks."

During the year of research, researchers hope to learn more about the genetic relationships and diversity within the colony, providing information about the colony's regional importance. The research also should answer what temperature and humidity the Yuma myotis requires for roosting with data from sensors placed inside the clearwell and flume.

"There's not a lot of information about bats and what their habitat requirements are," said Jon Lucas, an environmental specialist for Areva who is working on the research as part of his work to earn a master's degree.

Acoustic sensors will provide information on when the bats show up next spring and also information about when they come and go daily.

In about a year DOE should be ready to make a decision on what to do with the colony.

"Our hope is to turn the flume into the new roost site," Gano said.

It may be more acceptable to keep and manage the smaller structure than the clearwell, Ward said.

Although they are small animals, it's a big issue for DOE. Its policy is to manage the Hanford cleanup with as little impact to plants and animals as possible. And under a presidential order, Hanford must protect animals and other natural resources to allow more of the site to possibly be included in time in the Hanford Reach National Monument.