IDAHO FALLS, Idaho (AP) — The federal government has fined a contractor for an Idaho nuclear plant after an electrical explosion there last year.

The Post Register reports that Battelle Energy Alliance will pay $60,000 and complete a number of safety assessments following the April 23, 2015 incident. U.S. Department of Energy documents say three Battelle workers were performing regular maintenance on a circuit breaker at the Idaho National Laboratory when they triggered a type of explosion called an arc flash.

According to the documents, it knocked the workers to the ground but they were uninjured. In a letter addressed to INL Director Mark Peters, the DOE said the explosion could have been prevented. Ed Anderson, INL's chief operating officer for facilities and site services, says power workers have made changes to improve safety.