ORNL-5388 - the Molten Salt Energy Technologies Web Site

4-50
where RF is the reprocessing recovery factor (0.98). While such an expression i s not
absolutely correct, it does provide a measure of the relative growth capability of each
reactor. Since the data sumnarized in Table 4.5-1 are based on three separate reference
LMFBRs operating with a variety of design differences and fuel management schemes, the
above expression was used simply to provide relative values for each system. It should
also be noted that sane reactor configurations listed have dissimilar core and axial
blanket materials and thus would probably require modifications to standard reprocessing
procedures.
The data presented in Table 4.5-1, although preliminary, do serve to indicate cer-
tain generic characteristics regarding the impact of the alternate LMFOR fuel options.
considering those cases in which similar core materials but different blanket materials
By
are utilized it is clear that the choice of the blanket material has only a rather small
effect on the reactor physics parameters. On the other hand, the impact of changes in the
core fissile and fertile materials is considerable, particularly on the breeding ratio.
Utilizing 233U as the fissile material results in a significant decrease in the breeding
ratio relative to the corresponding Pu-fueled case (ranging from 0.10 to 0.15, depending
on the system). This decrease is due primarily to the lower value of v (neutrons produced
per fission) of 2% relative to 239Pu and 241Pu. Somewhat compensating for the difference
in v is the fact that the capture-to-fission ratio of 233U is significantly less than that
of the two plutonium isotopes. The differences in breeding ratios given in Table 4.5-1
reflect the net result of these two effects, the decrease in v clearly dominating. Use of
233U as the fissile material also results in a slight decrease in the fissile inventory
required for criticality. This is due to two effects, the lower capture-to-fission ratio
of 233U relative to the plutonium isotopes, and the obvious decrease in the atomic weight
of 233U relative to Pu (% 2.5%).
. The replacement of 2j*U by zs2Th as the core fertile material also has a significant
impact on the overall breeding ratio regardless of the fissile material utilized. As the
data in Table 4.5-1 indicate, there is a substantial breeding ratio penalty associated
with the use of 232Th as a core material in an LMFBR. This penalty is due to the much
lower fast fission effect in 232Th relative to that in 2s*li (roughly a factor of 4 lower).
The fertile fast fission effect is reflected in the breeding ratio in two ways. First,
although the excess neutrons generated by the fission of a fertile nucleus can be sub-
sequently captured by fertile material. their production is not at the expense of a
fissile nucleus. Moreover, the fertile fission effect produces energy, thereby reducing
the fission rate required of the fissile material to maintain a given power level. Since
both these effects act to improve the breeding ratio, it is not surprising that use of
Th-based fuels result in significant degradation in the breeding ratio. A further
consequence of the reduced fast fission effect of 232Th is a marked increase in fissile
inventory required for criticality, evident from the values given in Table 4.5-1 for the
required initial loadings.
a
> 3
3
3
3
3
a
1
3
3
3
1
3
1
3