More on passive safety in nuclear reactors

I just found this at Technology Review (talking initially about the Japanese reactors):

The reactors at the nuclear plant, built in the early 1970s, rely on active cooling systems that require electricity. Newer plant designs would lessen or eliminate the need for active cooling, making use of natural convection or a "gravity feed" system to cool reactors in the event of an emergency.

In one design, for example, the relatively new Westinghouse AP1000, water is suspended over the reactor housing. If pressure within the system drops, this allows the water to fall into the reactor area, submerging it in enough water to keep it cool.

While passive systems could be better in the event of electrical failures, they might not always be the safest systems. Kadak says that in an active system, it's easier to ensure that coolant gets exactly where it needs to be—it's simply pumped to the right location. Designing passive systems, on the other hand, requires complex models of how fluids will behave in a system that could be rendered incorrect if the system is damaged.

Kadak says that even more advanced reactor designs could overcome these issues. Some advanced reactors use molten metals to cool the reactor—the mass of these systems is enough to provide cooling in an emergency, he says, although if the molten metal were displaced by an earthquake, that could be a problem of its own. He's devoted much of his career to another advanced alternative, the "pebble bed reactor," which is designed to make it impossible for the fuel to get hot enough for a meltdown. The tradeoff is that the reactor has to be much larger--for a given amount of power--than a conventional reactor.