ORNL-5388 - the Molten Salt Energy Technologies Web Site

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4-16
since in each case the one fertile isotope is not significantly diluted by the presence of
the other. For MEU(20% 235U/U)/Th fuel, the 238U density is reduced by a factor of s6
(relative to LEU fuel), causing the 238U resonance integral to increase due to the reduced
self-shielding. The decrease in the 232Th density for the MEU/Th fuel (relative to the
HEU/Th) fuel is only a factor of s0.8 - resulting in a much smaller increase in the 232Th
resonance integral. Thus, although the 238U number density is roughly six times less in
MEU/Th fuel than in LEU fuel, the fissile Pu production in the MEU/Th fuel is still 40% of
that for the LEU fuel as shown in Table 4.1-1 (Cases A and B) due to the increase in the
238U resonance integral :
The presence in denatured uranium-thorium fuels of two fertile isotopes having
resonances at different energy levels has a significant effect on the initial loading
requirement. The initial 235U requirement for the HEU/Th and MEU/Th cases is 2375 and
2538 kg/GWe, respectively, reflecting the penalty associated with the presence of the two
fertile isotopes in the MEU/Th fuel.
The large increase in initial 235(5 requirements shown in Table 4.1-1 for the thoriumbased
HEU/Th and MEU/Th fuels compared to the LEU fuel results primarily from the larger
thermal-absorption cross section of 232Th relative to 238U as shown in Table 4.1-2. Also
contributing to the increased 235U requirements is the lower value of 11 of 235U which re-
sults from the harder neutron energy spectrum in thorium-based fuels.
Table 4.1-2. Thermal Absorption Cross Sections and Resonance
Integrals for 232Th and 238U in PWRs
Isotope u (0.025 eV)
a (barns)
Resonance Integrala (barns)
Infinitely
Di 1 Ute
In LEU
Fuel
In HEU/Th
Fuel
In irlEU(235U/U)/Th
Fuel
23 2Th 7.40 85.8 -
17 19
23 8u 2.73 273.6 21 -22 - 59-54
~~~ ~ ~~
‘For absorption from 0.625 eV to 10 MeV; oxide fuels.
A further consideration regarding MEU(235U/U)/Th fuel with uranium recycle must also
be noted. Since the fissile enrichment of the recovered uranium decreases with each generation
of recycle fuel, the thorium loadings must continually decrease. (As pointed out above,
it is assumed that the recovered uranium is not reenriched by isotopic separation techniques.)
The initial core 232Th/238U ratio is ~5.8 and the first reload 232Th/238U ratio is 4.4, but
by the fourth recycle generation the 232Th/238U ratio has declined to ~1.4.~ An alternative
is to use HEU (93.15 w/o 235U) as a fissile topping for the recovered uranium. In this way
the recovered uranium could be reenriched to an allowed denaturing limit prior to recycle,
thus minimizing the core 238U component and therefore minimizing the production of plutonium.
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