Hello Michiel!

08 Oct 03 16:10, you wrote to me:

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

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

Yes I would.

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

.. and it's free's alot of energy I guess.

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

(U235? Sorry I don't own a periodic system anymore)

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

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

I see!

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

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

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

Light water contains just H2O, I mean with the origin isotope?

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

Indeed!

 MV> Hope this helps.

Yes Thanks alot Michiel!

Regards,
Bo

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