Basin Electric Power Cooperative

A power plant has overexposed its workers to radiation, and the Nuclear Regulatory Commission is proposing a fine. The plant, though, is not a reactor; it runs on coal.

The commission said on Monday that it was proposing a fine of $24,700 against the Basin Electric Power Cooperative, which exposed 17 workers, six of them above regulatory limits, at a three-unit coal plant in Laramie near Wheatland, Wyo.

The workers were exposed to a radioactive element, cesium 137, that is common in nuclear plants because it is produced when uranium atoms are split. The Laramie River Station, completed in the early 1980s, does not split atoms, but it does use cesium, as many coal plants do.

Cesium 137 emits gamma rays. In coal plants and other industrial plants, the amount of radiation that passes through the material being measured gives an indication of its content, just as an X-ray gives an indication of what is inside a human body. Typically, the coal plants are measuring ash and moisture content of the fuel they burn, and the quantity passing through a coal chute.

Laramie River Station has 216 monitors that use radiation, some of them in the pollution control system. Daryl Hill, a spokesman for the co-op, said the monitor in question measures the flow from a coal bunker down to a feeder, which spits the coal into a boiler. “If the flow stops, it detects there’s no fuel and sends a signal to the control room,’’ he said.

The gauge has a shutter mechanism, akin to the shutter on a camera, and it was supposed to be closed and locked before the workers — welders — were sent into the area. But the shutter was left open.

Unlike workers at a nuclear plant, the welders did not have equipment to measure the amount of radiation they were exposed to. The calculated dose was 647 millirems, about what the average American receives in two years from natural sources. That amount would not be a violation in a nuclear plant, although inadvertently exposing workers there would have been a problem. For non-nuclear workers, considered members of the general public, the limit is 100 millirems.

The commission said the violation was significant because the workers could easily have been sent there for a longer period and could have absorbed a bigger dose.

The exposures occurred in September 2009, and managers say they have made a variety of changes to prevent a repetition. The co-op has until Sept. 24 to decide whether to contest the fine, Mr. Hill said.