Both employees and contractors at the mine near Gillette have been working towards the milestone and marked one year accident-free in September.

“I want to congratulate all those at Cordero Rojo on achieving this significant milestone,” president and chief executive officer Colin Marshall said.

“Great effort and dedication by each and every person at the mine was required.

“Cloud Peak Energy’s value of safety is an underlying principle that guides our operations.”

General mine manager Joe Vaccari added: “Since our achievement of one year with no lost-time incidents in September, everyone working at Cordero Rojo mine kept the momentum going to reach this substantial record.

“I appreciate all the hard work and focus that our employees have shown. If everyone goes home safe at the end of each shift, we are one step closer to achieving zero injuries.”

The operation is consistently a top US performer, with Cordero Rojo mining and shipping approximately 39.2 million short tons of coal in 2012, putting it in third place in the nation for the year.

Coal mined from Cordero Rojo is primarily shipped to electric utilities in the western, midwestern and southeastern US.

According to federal data, Cloud Peak Energy had one of the lowest all injury frequency rates of the top 25 coal producers in the US in 2012.

Two of its mines, Cordero Rojo and Spring Creek, have surpassed the million-hour mark without reportable injuries.