[ RadSafe ] Graphite-moderated reactors

George J. Vargo vargo at physicist.net
Thu Nov 10 20:46:59 CST 2005

Graphite reactors can sustain criticality with natural uranium (0.072%
U-235).  Also, they have a large fraction of neutron spectrum is in the
resonance region, which is favorable for 1/v resonance capture by U-238,
hence their utility for Pu production.  At low temperatures, the graphite
retains a lot of energy in the lattice structure (Wigner effect), which can
result in severe geometric distortion of the graphite stack.  The Wigner
growth can be mitigated by higher operating temperatures where the graphite
stack becomes self-annealing (This was a BIG problem in the early years at
Hanford and was alleviated by significant increases in the operating
temperature)  Uncontrolled release of the Wigner energy in the air-cooled
Windscale pile reactor resulted in a fire and large release of fission

High-temperature gas reactors such as the former Ft. St. Vrain reactor can
produce superheated steam and reach thermodynamic cycle efficiencies over
45% (compared to 34% in BWR and 33% in PWR).  I don't know much more about
the PBMR beyond what I've read in the media.  Perhaps someone else can
enlighten on that.

George J. Vargo, Ph.D., CHP
Senior Scientist
MJW Corporation
610-925-5545 (fax)
vargo at physicist.net

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