The company that runs the Sellafield decommissioning has been fined £75,000 and ordered to pay £26,100 in costs after two workers were exposed to radiation.
The contractors were drilling through a concrete floor contaminated with plutonium in a waste facility that Sellafield was converting into offices when the July 2007 incident occurred.
Carlisle Crown Court was told that the men were exposed to higher than anticipated levels of radiation, though neither workers suffered any immediate illness or injury.
His Honour Judge Batty QC handed down the fine warning that Sellafield needed to have adequate controls in place to protect workers and that the outcome could have been much worse.
But he did note the site had a good safety record and the company had acted on the problem as soon as it was identified.
The prosecution was brought by the Health and Safety Executive and the firm pleaded guilty at a previous hearing to failing to discharge its duty under the Health and Safety at Work Act.