Because of legal individual exposure limits, including monthly, quarterly, and annual limits, they need lots of workers to cycle in and out of service. There is such an immense amount of work to do, that if you only have a limited number of workers, the amount of time the cleanup is going to take is only that much longer. Also, more workers will mean a lower average exposure to each worker. The work has to get done, so importing workers is perfectly legitimate here.
Actually a great opportunity for nuclear workers.
Was going to post a link demonstrating that, but that site has been down the past few hours.