Obviously we don’t want this to happen but claims of deaths from cancer are overblown.
From the NYTimes:
But there were probably less than 1,000 “excess” cancer deaths (out of 120,000 survivors) from the atomic bombings in 1945. It’s doubtful that a terrorist attack on a nuclear facility could come close to that sort of event.
From the 2006 NRC report on the atomic bomb survivors, “Health Risks from Exposure to Low Levels of Ionizing Radiation“: