RICHLAND — About 600 workers were sent home early from a construction site at the Hanford nuclear reservation after a third safety problem in a week.

The incident was the latest in a string of problems associated with the waste-treatment plant under construction at the south-central Washington site. Construction was halted indefinitely this summer on a large portion of the project because of seismic problems, rising costs and delays.

The plant is being built to turn millions of gallons of radioactive waste into glasslike logs for permanent disposal in a nuclear-waste repository. The waste, the remnants of Cold War-era plutonium production for the nation's nuclear weapons arsenal, is stored in 177 underground tanks nearby.

In the Thursday incident that caused managers to shut down construction, a worker in an excavated hole cut a gas line without ensuring there was no propane in the line, said John Britton, a spokesman for Bechtel National, the contractor for the project.

The line held only residual propane. But had more propane been in the line, the worker could have been killed, Britton said. Propane, which is heavier than air, would have settled at the bottom of the hole, possibly asphyxiating the worker.

Earlier in the week, one worker was shocked when a metal pole was driven into the ground and touched a buried electrical line. Another worker failed to shut off one of three conveyor belts — assuming it was already off — while sampling gravel at the construction project's plant for making concrete.

No one was seriously injured, but managers were concerned the incidents had the potential to harm or kill workers. All were caused by human error and workers not following safety procedures, Britton said.

Bechtel National officials met with labor leaders yesterday to try to ensure that the site is safe so work can resume, Britton said. The meetings were to continue through the weekend.

The Energy Department has levied fines against and withheld part of the fee for Bechtel over safety concerns, although the project's injury statistics have improved in recent months.