Most employers offer on the job training. The training covers health hazards, personal safety and site safety. It also covers hazard identification and decontamination. Many times a worker will run into other hazards during a cleaning. This is why many workers take classes to learn about other hazards.
Employers like to hire workers who have many licenses.
Decommissioning and decontamination workers training at nuclear facilities is more involved. In addition to the training course listed above, workers must take courses about government rules on nuclear materials and radiation safety. This adds up to 3 months of training. Many agencies offer this training. Workers in all fields are required to take a refresher course every year.