What's caused the closure of Nuclear plants and the lack of building new ones is simply cost. Yes, some of it is regulatory hurdles, but a lot of it is simply cost. Nuclear is expensive up front. Coal has LOT's of secondary problems. Cancer clusters downwind of coal plants. Water pollution, etc, etc. Yes, Coal and Nat gas will be backup for Solar and Wind. Use the cheapest source of generation when it's generating and fire up the rest when it's not. The redundancy will still be cheaper than running the most expensive sources all the time.