[welterde]

At least pebble bed reactors (like the already mentioned THTR-300) are not really safe. Get a bit of water into the primary cooling cycle, while the reactor is running at capacity and you won't have that power plant for long (happened at the AVR.. luckily it was running at lower temperature at the time so they only had to run the reactor quite a lot reduced capacity for a year to get the water out).

As for molten salt reactors.. still lots of problems to be solved related to the combination of molten salt and high temperature (I recall there being an US based molten salt reactor that got it's cooling cycle contaminated somehow.. but can't seem to find it right now)