Coal plant burns coal to heat water, makes steam, and the steam powers a turbine to produce electricity. A nuclear power plant uses nuclear fuel to heat water and produce steam similar to a coal plant. It may do this indirectly (e.g. second loop between the nuclear fuel and water loop to prevent the water becoming radioactive). This means that to build a nuclear plant you essentially need to build a coal plant, and then also the nuclear reactor and safety stuff, which makes them more expensive. Since coal plants are being turned off anyways, it might be more cost effective to just retrofit old coal plants so the only cost is the nuclear reactor side of things (plus any necessary maintenance and upgrades)