Originally Answered: What would be the cost of converting nuclear plants into solar electric plants?
Why would we do such a thing? The two technologies are totally different. There is little commonality upon which to build (or rebuild) when converting from one to the other.
Case in point...solar electricity comes from photo-electric cells. These are cells that produce electricity directly from the solar energy that strikes them. There are no intermediate steps between sun and electricity. Further, it would take acres of photoelectric cells to create the electrical power equivalent to a typical nuclear power plant. That is, solar plants require a lot of land (which is why many are in open desert areas), while nuclear power plants are much more compact.
On the other hand, nuclear power plants start with the source of energy, the nuclear pile, and convert the heat produced by that pile into some sort of propulsion energy. Typically, that means creating superheated steam. But to avoid irradiating the water, there is usually a step where the pile's heat heats up another liquid, like a form of liquid sodium, which is then carried to where the water is. The steam then propels generators that, finally, generate the electricity. In other words, there are several intermediate steps between the nuclear power and the electrical power. And photo-electric cells need none of those steps.
The bottom line is this...photo-electric power plants are so different from nuclear power plants that it would be far cheaper to build new solar plants on new grounds than to rebuild the nuclear power plants on the same grounds.