The follow-on question is why aren't these plants retrofitted to be secure? I'd somewhat naively expect that its simpler to upgrade an existing plant than permit a new plant in.a separate location.
Understanding, improving, testing, and certification of a plan to protect against the huge risks involved is expensive and often in practice timelines and budgets often go significantly over.
There's numerous MUCH newer designs that: are much smaller, much easier to scale, absolutely identical, well understood, robust in the face of failure, and don't even need operators. Additionally since they are identical they get economies of scale and only need a finite number of experts on hand, not a group of them per site. You literally need a flat site, water, and electrical hookups. If you don't provide enough water for cooling they shut down. After their useful service life you put them back on a train car and ask for a replacement.
Some of these projects are ready to deliver, but the early customers have been cancelling. Bill Gates funded a project, and there's several around.
https://www.thegazette.com/subject/news/business/duane-arnol...
It should be quite clear that neither solution are even close to enough power to bridge the gap and require additional energy in the form of fossil fuels. It will be less fossil fuels than if the nuclear plant was exclusively replaced with fossil fuels, but it will be more fossil fuels than before and it will be adding to global warming in a time where the planet can't take more.
While I support expanding nuclear power capabilities in general, a straightforward rule like “don’t decommission old plants as long as fossil fuels are still being used” seems dangerous and irresponsible.