Same thing happened to a certain extent with French nuclear. This article talks vaguely about new requirements whilst large components were manufactured, but one of the big problems with Flamanville 3 was that one of the largest and most critical components - the main pressure vessel that formed the primary containment for radioactive materials - didn't meet the existing material specifications and was at increased risk of failing, and the manufacturer had basically faked the testing and certification on it. After discovering this, the regulators went back and looked at the reactor pressure vessels built by the same supplier during the golden era of French nuclear, and a bunch of them turned out to be defective in the same way.