Direct use of nuclear heat had nothing to do with PURPA. PURPA wouldn't have made it more likely to be considered, since it wasn't putting power onto the grid. If nuclear wasn't being used for industrial heat before that, it wouldn't suddenly have become reasonable afterwards. Instead, PURPA would have made that less likely, since it would have improved the economics of the competing technology of burning fuels for process heat (since a valuable and saleable additional product, power at very high marginal efficiency, could now be produced and sold to the grid.)

There is the question of why nuclear had not and has not penetrated the market for industrial heat. I suspect it has something to do with nuclear not scaling down well, not being suited to industrial applications that may require more flexibility, and concerns about nuclear requirements bleeding over into design and operation of otherwise non-nuclear industrial processes.

Where will process heat come from in the future? Possibly heat pumps. The thermal energy from these (or from resistive heaters, at high temperature) is much cheaper to store, per unit of energy, than electrical energy in batteries, so this would provide a large, cheap source of dispatchable demand to ease renewable integration.