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8 6 4
7-
4=H20 SPECTRUM
2=D20 SPECTRUM
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86
NEUTRON ENERGY (MeV)
3 2 40 08 06
I 1
I
!
3=C4RBON SPECTRUM
4=FAST SPECTRUM (50% Na, 50% U-METAL, ~ o % * ~ ~ 80% U , 23aU)
spectra, and comparing it with experimental de-
terminations of the same quantities. The results
indicate that the model is useful at least for pre-
dicting relative damage rates in different spectra.
A useful simplification arises from the observa-
tion that the damage per unit time is closely pro-
portional to the total neutron flux above some
energy E,, where Eo has the same value for widely
different reactor spectra. We have reconfirmed this
observation, to our own satisfaction, by comparing
the (calculated) damage per unit flux above energy
E, as a function of E, for spectra appropriate to
three different moderators (H20, DLO, and C) and
for a “typical” fast reactor spectrum. The results,
plotted in Fig. 6.5, show that the flux above about
50 kev is a reliable indication of the relative dam-
age rate in graphite for quite different spectra.
Figure 6.6 shows the spectra for which these re-
sults were derived. The equivalence between
MSBR and DFR experiments is simply found by
equating the doses due to neutrons above 50 kev
in the two reactors. We have not yet calculated
the DFK spectrum explicitly, but we expect it to
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ORNI-OWG 61-44844
O.(O 0.08
I I
4 2 3 4
5
LETHARGY U
Fig. 6.6. Neutron Flux Per Unit Lethargy vs Lethargy. Normalized for equal dunioye in graphite.
be similar to the “fast reactor” spectrum of Fig.
6.6, in which 94% of the total flux lies above 50
kev. Since the damage flux in the MSBR is essentially
proportional to the local power density,
we postulate that the useful life of the graphite is
governed by the maximum power density rather than
by the average, and thus depends on the degree of
power flattening that can be achieved (see next
section). In the hlSBR the average flux above 50
kev is about 0.94 x 1014 neutrons cm-’ sec-’ at
a power density of 20 w/cm3. From the DFR irradiations
it has been concluded that a dose of
5.1 x 10‘’ nvt (> 50 kev) can be tolerated. The
lifetime of the graphite is then easily computed;
this useful life is shown in Table 6.1 for an assumed
plant factor of 0.8 and for various combinations
of average power density and peak-toaverage
power-density ratio.
It must be acknowledged that in applying the
results of DFK experiments to the MSBX, there
remain some uncertainties, including the possibility
of an appreciable dependence of the d a ~
age on the rate at which the dose is accumulated