> Nuclear plants can, in a worst case scanario, be given the SCRAM treatment to shut them down hard. If that procedure is successful then the reactor is off. I

> Nuclear plants can, in a worst case scanario, be given the SCRAM treatment to shut them down hard. If that procedure is successful then the reactor is off. It isn't as simple as that, with a lot of reactors. The ones at Fukushima were SCRAMmed. A running nuclear reactor will build up a collection of fission products that have various half-lives that can cause problems. In Fukushima's case, these fission products continued to decay and produced lots of heat (about 6.5% of the heat produced during normal operation, according to https://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disaster https://en.wikipedia.org/wiki/Fukushima_Daiichi_nuclear_disa...). It was this heat that raised the temperature of the fuel and boiled off the cooling water and caused the disaster. So basically, with lots of reactors, SCRAMming is not enough - you need to maintain cooling for quite a long time afterwards. It is entirely possible to build reactors where this is not the case.