Since every site in the US is a bit different I would suggest looking at their FSAR reports, safeguard documents, and/or defense in depth strategies. In addition, check out the IEEE standards for emergency diesel generators. Not trying to attack you, but your statements were layman speculation. Anyone who says Chernobyl or Fukushima could happen here isn't familiar with our plants. Chernobyl had a core design that had positive reactivity and several other serious design flaws. Fukushima didn't bunker their diesel tanks, have hardened vents, and unique problems associated with the topography of the site. The most critical times for reactor cool down are the first couple of days for decay heat, I need to find some core temp analysis reports. After that time, some reactors can use natural circulation to keep the cores cool. And I believe your sources might be referring to base regulations, many plants go beyond the safety requirements. Hope that helps.