Nuclear Reactors have a certain Limit on Powercycles. While you can power up an down with up to 10%/min that puts a huge amount of stress on all parts which leads to cracks and leaks. So you can’t actually replace the flexibility of gas powerplants with nuclear, because the risk associated with leaks is so much worse.
The reliability issue of regenerative energy can be lessend with better grids, because somwhere is always wind or sun. And then adding different storage technologies like battery, pressurised air or thermal storage should get us to a reliable power supply without emissions in the next decades.