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I = H,O SPECTRUM
2 = D20 SPECTRUM
3 = CARBON SPECTRUM
4 = FAST SPECTRUM
2 5
in terms of an equivalent Pluto dose; the total DFR
dose, that is,
is 2.16 times the equivalent Pluto dose.) From an
inspection of all the available data, we have concluded
that a dose of about 2.5 x neutrons/
cni (equivalent Pluto dose) could be sustained
without any significant deterioration of the physical
properties of the graphite, and this has been
adopted as an allowable dose, pending further detailed
consideration of mechanical design problems
that might be associated with dimensional changes
in the graphite.
In order to interpret these experiments to obtain
predictions of graphite damage vs time in the
MSBR, it is necessary to take into account the difference
in neutron spectra in the two reactors.
This, in turn, requires assumptions regarding the
Fig. 6.5. Fast Flux os a Measure of Radiation Damoge.
ORNL DWG 67-11810
5 1 o6
effectiveness of neutrons of different energies €or
producing the observable effects with which one
is concerned. At present the best approach available
is to base one’s estimates of neutron damage
effectiveness on the theoretical calculations of
graphite lattice displacements vs carbon recoil
energy carried out by Thompson and Wright. The
Thompson and Wright “damage function” is integrated
over the distribution of carbon recoil energies
resulting frotn the scattering of a neutron
of a given energy, and the result is then multiplied
by the energy-depcndcnt scattering cross section
and integrated over the neutron spectrum in the reactor.
Tests of the model have been made by
Thompson and Wright by calculating the rate of
electrical resistivity change in graphite relative to
the *N~(n,p)~ ‘Co reaction, in different reactor
’M. W. Thompson and S. €3. Wright, J. Nucl. Mater. 16,
146-54 (1965).