I was just reading about those a couple days ago. Here's a link to the
wikipedia entry: http://en.wikipedia.org/wiki/Pebble_bed_reactor

The basic idea is that each "pebble" contains only a small amount of
nuclear material, and it's encased in materials that can handle the
reactor's maximum temperature *without cooling*. Therefore meltdown cannot
happen. The use of helium gas as a coolant/working gas is apparently
related to helium's tiny nucleus, it's so small that the neutrons in the
reactor don't interact with it (it doesn't become radioactive from
exposure).
At least that's how I understand it :o)

-Denny



> To change the subject - slightly... Anybody read the Wired Mag article
(Sept
> 2004, p158) about "pebble-bed" nuclear reactors?  They're claimed to be
> "intrinisically safe".  Thermal energy transfer to generators is via
Helium
> gas. If all the Helium leaks out, reactor temp goes up a bit then cools
down
> because of some thermal expansion effect with the fuel which are billiard
> ball size graphite balls with silicon carbide coated fuel "specks" packed
> inside each one.  They claim they've actually done this test and it is
true.
> Depleted fuel balls "can go straight into lead-lined steel bins in the
> basement."  Anyway, good article - is it TRUE?
> Carey Fisher
>
>
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