"Passive cooling" are words that people like to hear, so everyone's going to tell you that their favorite Gen IV is "passively cooled", but one has to approach

"Passive cooling" are words that people like to hear, so everyone's going to tell you that their favorite Gen IV is "passively cooled", but one has to approach these claims with a certain level of realism. I mean, the reactor's going to be producing about 1% of rated thermal power for a few days after shutdown, so 36 MW. The heat of vaporization of water is about 3 MJ/kg (heat of vaporization is more important than heat capacity, so we'll just use that). That means that the fuel inside a 1 GWe nuclear reactor, a day or two after shutdown, can boil 12 kg of water in a second. This is a fundamental property of nuclear reactors - no reactor design is going to make decay heat disappear. So I would be skeptical of designs that claim to be "passively cooled". What they mean is "passively cooled for a while", which is an important engineering achievement that I don't want to trivialize at all - I just think it puts the current designs a little in perspective, since some people apparently believe that you can scram a gen 4 reactor, go on a luxury 7-night cruise, and then deal with at your liesure, which isn't true - you've got to find a way to get water in there (or whatever), you just have a lot more time to do so because of the passive safety systems.