Hi Bo,

 >  MvdV>>> Modern reactors use (heavy) water as the moderator.

Ok, you wanted to know about the graphite in reactors.

Natural consists of apr. 0.7% Uranium 235 and 99.3% U238.
When a U238 is hit by a neutron it transforms into Plutonium.
When a Uranium 235 nuleous is hit by a slow (thermal) neutron it splits
into two more or less equal parts plus two to three neutrons. But these
neutrons are so called "fast" neitrons.

In other to get a chain reaction going in natural Uranium one has to:

1) Slow doen the fast neutrons
2) see that not too many get absoired by the U238.

This is achieved by putting rods of uranium in a medium called the
moderator. For effective slowing down the moderator must contain elements
with a light nucleous and it must have a low cross section for neutron
absortion. The lightest element is hydrogen, but that easily absorbs
neutrons (forming deuterium and tritium). The next best is deuterium. For
reactors it is used in the form of heavy water. The oxygen is not in the
way as it hardly absorbs neutrons.

Another suitable material is graphite. That was used in Enroco Fermi's
first nuclear reactor.

If one does not use natural but /enriched/ uranium (3% U235) one can also
use ordinary or light water as the moderator.

The light water reactor is the most common type these days.

Contrary to a grpahite reactor, it is considered inherently safe. If for
some reason the moderater diapppears (leak, boiling of by overheating) the
reaction stops. In a graphite reactor the reaction does not stop when
overheating and the graphite can catch fire. A painfull lesson learned at
Chernobyl.

Hope this helps.

Cheers, Michiel

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