“But the nuclear fuel in a power reactor won’t explode under any circumstances.”
No, but nuclear reactors, even low powered ones running on natural or low-enriched uranium (LEU) have a tendency to produce some nasty radioactive by-products which could make quite a mess if spread around. I suspect extracting those by-products for use in a dirty bomb would be beyond most, if not all, groups capabilities, but blowing the reactor up could certain create at least a local contamination problem. But, then, we’re not talking about using these in heavily populated areas.