Workers strung a banner decorated with smiley faces across the entrance to the Department of Energy's uranium plant here today in preparation for their annual family picnic.

But the mood was somber behind the plant's razor-wire fence, where hundreds of workers observed a 24-hour "stand down" to review safety practices at the facility.

"All but the most critical operations were suspended," said Jimmy Massey, who manages operations at the plant for Bechtel Jacobs Co., a Department of Energy contractor. "Employees were reminded of the existence of plutonium on site."

Plant officials acknowledged that they were taken by surprise Wednesday when Energy Secretary Bill Richardson ordered the day of review and assessment. His decision was made after a preliminary investigation found lapses in programs designed to shield workers from radiation.

The safety operation idled about 400 employees of the Energy Department and its contractors at the site but did not directly affect the plant's uranium operations, which are now managed by a private corporation.

Managers said they drilled workers on safety procedures and invited them to raise concerns. Some wanted quicker reports on how much radiation they were exposed to on a daily basis, Massey said.

Others wanted more details on the results of periodic air tests on the site.

"They wanted more feedback," Massey said.

Richardson's decision came after a month of bad news for the Paducah facility. A Washington Post investigation revealed in August that radioactive material was seeping out of the site, and that thousands of workers had unknowingly been exposed to plutonium and other highly radioactive metals.

Since then, workers have found more contaminated sites and safety lapses, and a $10 billion class action lawsuit has been filed against the plant's former operators.

During the stand down, plant managers said, some construction workers expressed concerns that, for the first time, they had to wear radiation monitoring badges. But overall, they said, workers told their supervisors they felt the plant was a safe place to work.

Managers also reviewed contaminated sites in and around the facility, to see if they were properly labeled and roped off. Officials discovered no major problems, Massey said, but did find some signs that needed repainting and others that listed outdated telephone numbers.

"We found out operations continue to be carried out safely on site," said Robert Poe, an assistant manager in the region for the Energy Department. "We feel pretty good from that standpoint."

By midday, many of the workers had squeezed into tiny metal buildings just outside the plant, where their supervisors recited to them from safety manuals.

"It's good to take a break and think about these things," said one worker, who declined to give his name. "Everyone's taking it seriously."

The stoppage, however, did not affect Kyle Gore, a subcontractor at the plant who is helping to build a holding facility for thousands of rusting cylinders filled with radioactive gas.

"I don't know what it's all about," Gore said with a shrug as he climbed into a white pickup. "I did hear something about it on the news."

Yet Gore said wider accounts of problems at the plant have taken a toll on workers, who fear for their health and their jobs.

"Everyone's panicking out here," he said.

Throughout the event, planning continued for the weekend festivities. But safety concerns even intruded there, as emphasized by a huge electronic sign flashing at the plant entrance with a message about radiation monitors.

"Going on a tour during the picnic?" the sign asked. "Wear your badge!"

Built in 1952, the Paducah plant produced enriched uranium for nuclear weapons, Navy submarines and commercial power plants. Workers say they were not told about the presence of plutonium, which entered the plant over two decades in contaminated shipments of uranium. Many workers believed they were handling only uranium, which is only mildly radioactive.

In Washington, a Kentucky congressman announced a Sept. 16 date for the first formal hearing on the Paducah contamination. Rep. Edward Whitfield (R), whose district includes Paducah, said he would press for answers from the plant's former contractors as well as from the investigators.

The House Commerce Committee's panel on oversight and investigations also will hear testimony from employees at the plant, including plaintiffs in a lawsuit filed with the Justice Department that alleges dumping of radioactive material outside the plant's fence.

"These allegations deserve complete answers from all parties that were ever involved with the operation of the Paducah Gaseous Diffusion Plant," said Whitfield, who plans to tour the facility next week. "The answers we get at this first hearing will determine which areas we need to further investigate. This is the first hearing, but it won't be the last."