[neutronicus]

"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.

[jacques_chester]

So safer, but not failsafe past 72 hours.

An incremental improvement, not a quantum leap.

[neutronicus]

Right. The number may be longer for Gen IV as opposed to Gen 3.

Rejecting 36 MW of heat for weeks at a time without some intervention is a very difficult, if not impossible engineering problem.

[DennisP]

How about liquid thorium? Reactor gets too hot, a plug melts in the bottom and all the fuel drains into a wide basin.

[iwwr]

How about submerged reactors? They'd use ocean currents to cool passively.

[neutronicus]

Probably yes, although possibly no, since rate of heat transfer depends on flow rate past the heat transfer surface.

Given that the AP1000 manages for 72 hours solely with "hot water rises, cold water sinks" levels of flow, a submerged reactor could probably remain cooled indefinitely, although salt water corrosion would then become a part of your life.