Passive safety systems are based on natural forces (convection, gravity) and do not need any external input (operator / energy) to work. I think the idea is really great - why design complex "active" systems when you could let nature do what it does? However, in reading reviews about passive safety systems I find that most are constructed as backups to the active systems. For example, the EPP-1000, ESBWR, CANDU 6 & 9, AC-600 and AP-600 (among others) all have a large water tank over the reactor. The design is that after the active system fails, gravity just dumps water all over the reactor, cooling it. Another common design is to build a large "containment pool" that collects the reactor after it melts.
I don't understand why the passive systems are being used as a last attempt backup instead of the first. It would just seem more natural to use a system that works off of nature instead of pumps and machines to prevent accidents.
There are problems with passive systems. They tend to be slower to respond / alleviate the problem (gravity driven and heat transfer can be much faster with pumps), and it they might not work outside a given problem / setup, while active ones have more flexibility. (You can always pump fluid, but with a passive system you need something to drive the flow).
I may be wrong, but I was under the impression that there exist some, admittedly few, first-line passive cooling systems. I seem to remember using helium as the working fluid was one of these, though I'm not sure I remember why. I think it was something to the tune of "as it heats, it expands and its heat capacity rises or drops or something and it finds its own equilibrium". Tell me if I made something up in here...
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