The reason nuclear regulation in the US is stricter these days is because companies used to do things like routing all their supposedly-redundant control and monitoring wiring through the same shared duct, stuffing it full of flammable foam, and then testing that for air leaks using a bare candle whilst the plant is operating. This is not a hypothetical problem - it was apparently standard practice in the era you're talking about and nearly caused a major catastrophe, and the fire regulations that stop practices like this are cited as a major cost making it hard to build nuclear power plants by companies in the business.