They crashed yes, but not for too long and they did recover. And was it ever confirmed it was because of vibe coding? Not sure how much if any it even impacted their stock.
I can confirm that nuclear power plant software has much higher quality level than normal commercial software and extremly extensive testing. In addition, lot of nuclear safety is checked below the software level on hardware level.
Developement of nuclear power plant software is very conservative, it will use LLMs maybe in 10-15 years.