That's not true. There are huge investments needed to upgrade nuclear power plants over their lifetime, which typically will need to be run long past its estimated lifetimes. A lot of upgrades are even not possible. The powerplant also ages. The steel ages. The pipes age. The software and computers are no longer in production. France now needs billions just to keep their old fleet running, which is based on outdated technology and designs. "Grand Carénage" is estimated to cost 50 billion Euros. EDF, the owner would be bankrupt, if the French government hadn't taken over the whole industry, where the tax payer now finances the losses.

Plus most nuclear powerplants don't cover the costs after a shutdown, incl. dealing with the materials and their radiation. We have often seen that the money available is not covering the costs of dealing with waste, spent fuel, etc. -> tax payer covers it -> the French government can't increase the state controlled electricity prices, without huge public protests.