From the link it looks like it had many changes implemented to improve safety: The EPR is a so-called “evolutionary” reactor, that is to say that its design is based on that of existing reactors, the French N4 type nuclear reactors and the German Konvoi . It thus benefits from proven technologies and operating feedback from its predecessors. It is a powerful reactor with a production capacity of 1,600 megawatts (MWe) compared to 1,450 MWe for the latest reactors built in France (type N4). It is designed for a service life of 60 years . Significant changes have however been introduced compared to existing reactors

Perhaps more importantly, there's a pretty long engineering history of assuming that "similar" means "don't need to test as much" not working out. Any time you make a change, you can and should be testing the parts as though they were a new design. I mean the most recent example of that was the Boeing MCAS.

Also Ariane 5 maiden launch failure: https://en.wikipedia.org/wiki/Ariane_flight_V88

I wouldn't call Ariane 5 an evolution of Ariane IV.

Code and digital system re-use in aerospace systems is not uncommon. After all, the fly-by-wire computer system on board the Space Shuttle was derived from the original Apollo flight computer, and they are two very different space vehicles..

Right but the point is it doesn't let you make assumptions that tests aren't needed, just that you expect them to be likely to pass. The design still has to be tested as though its a new system, it's just the re-use hopefully saved some development time and the testing hopefully finds fewer issues.

The MCAS was not thoroughly tested by design.

A new system requiring extensive testing would have alerted the FAA that something was off, and possibly led to a more costly re-certification they were trying to avoid.

That aircraft should never have been allowed to fly.

It doesn't matter if it's an evolution or a revolution - a change of any sort means new tooling to produce the components, new procedures to assemble them, new analysis to certify that they will all work together, and new training for everyone involved to do all the above.

All this was known before starting the building process, and the builder planned to deliver the reactor in 2012. It is late (right now: not delivered) mainly because project management and quality insurance were abysmal: https://en.wikipedia.org/wiki/Flamanville_Nuclear_Power_Plan...