Comment by pfdietz

That's a problem, but it's not necessarily even the biggest problem. Other huge problems include the shear size of the machines per MW of output (and hence cost per MW), coupled with their dreadful complexity and the difficulty of keeping them operating when they become too radioactive for hands-on maintenance. Designs typically just assume the reactors will be reliable enough, when there's no empirical evidence to support that (and the one study that tried to estimate uptime based on analogies with other technologies found the reactor would have an uptime percentage of just 4%!)