[_cenw]

I was actually talking about spent fuel rods, the kind you put in concrete casks (and yet they will outlive those). Because that's the stuff that will stay dangerous for time beyond human comprehension.

Radioactive water is not usually a concern. There are heat exchangers between an inner loop with a very small amount of water that is radioactive (but not forever or highly radioactive) and an outer loop with just... seawater. Maybe you're thinking about Fukushima, where they dumped a lot of water on an open core?