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"A pebble-bed reactor thus can have all of its supporting machinery fail, and the reactor will not crack, melt, explode or spew hazardous wastes. It simply goes up to a designed "idle" temperature, and stays there. In that state, the reactor vessel radiates heat, but the vessel and fuel spheres remain intact and undamaged.

I dunno about that - this report suggests that although the fuel might not melt, the fuel spheres can still be damaged by heat spikes during normal operation and should water leak in (like, from the primary steam circuit that you'd use to generate power), you might get a big oldschool Chernobyl-style graphite-steam reaction.

Which would be kinda bad, wouldn't it? Especially since PBRs seem to be designed without gastight containment.

Thorium / Pebble Bed Reactors have had some issues to I believe. Personally, I would have to do some more reading, but some folks on slashdot were talking about that a week ago.
http://hardware.slashdot.org/story/11/03/21/2312211/A-New-Class-of-Nuclear-Reactors?from=rss

"Germany ran a pebble bed reactor at the Nuclear Research Facility at Juelich. The Juelich post-mortem report concluded that pebble bed reactors have severe problems in practice (at least some of them base design flaws), in the specific case of the Julich AVR reactor leading to Strontium-90 contamination of the soil and aquifer beneath the reactor."

The post-mortem report is posted here http://www.eskom.co.za/content/AVR-Report-Press.PDF

Actually, the pebble reactor in Julich, Germany (I'll assume that's what you are referring to) had severe problems leading to long half-life fission products contaminating the soil and water around the reactor.

The flaws are not based on the particular design of the AVR facility, but seem to be flaws in the whole pebble-bed idea. You can read the Julich Research Facilities own post-mortem here: http://www.eskom.co.za/content/AVR-Report-Press.PDF

I've always wondered about using the potential energy of water (that is, raising it to a higher height), to store that energy to smooth out production versus demand issues for electricity.

Yes - it's being done in South Africa, the Drakensberg Pumped Storage Scheme.

I read an article in Wired that really piqued my interest in a form of Nuclear Power plant that the Chinese had been working on. These are called Pebble Core Reactors, and basically look like a big tylenol capsule stood on its side. I am by no means an expert in this area, but in layman's terms the system works like this. Instead of radioactive rods, the system uses radioactive pellets. As the pellets heat up they fall through a grating system to the storage below. If the thing overheats, the system collapses onto itself. MIT continues to work on this, along side the Chinese and many others. One interesting thing from the Wired article, this technology goes all the way back to the 50s. The decision to go with the water cooled system was partly based on the Navy's desire for Nuclear submarines and ships. Of course the others were waaay more expensive, so naturally business wanted those.

• ESKOM PBMR Info