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4-38
All of the above fuel cycles are for a 3360-MWt, 1344-MWe HTGR with a core power den-
sity of 7.1 Wt/cm3. Table 4.4-1 provides a summary, obtained from the detailed mass balance
information in ref. 1, of the conversion ratio, fissile requirements, fissile discharge, and
U3O8 and separative work requirements.
an equilibrium cycle enrichment of 7.4 w/o and 8.0 w/o, respectively.
preferred for no-recycle conditions.
Cases 1-a and 1-b involve the use of LEU fuel with
Case 1-b would be
In Case 2 thorium is used with 20% 235U/U (MEU/Th) for no-recycle conditions.
that while the initial U308 and fissile loading requirements are higher for the MEU/Th case
than for the LEU cases, due to the larger thermal absorption cross section of thorium and the
partial unshielding of the 238U resonances resulting from its reduced density, the cumulative
U308 requirements are slightly less for the MEU/Th case.
attainable in HTGRs and the res+ltant large amount of bred 23% which is burned in situ.
Other converter and advanced converter reactors (LWRs , SSCRs , and HWRs ) typically require
less U308 for the LEU case than for the MEU/Th case with no recycle.
Note
This results from the high burnup
Case 3 also uses the MEU/Th feed but with recycle of 233U. The unburned 235U and
The bred 233U re-
plutonium discharged in the denatured 235U particles is not recycled.
covered from the fertile particle, however, is denatured, combined with thorium, and
recycled. In the calculations for all cases involving recycle of denatured 233U, GA assumed
that an isotopic mix of 15% 233U and 85% 238U provided adequate denaturing. Due to the high
burnup and the fact that the thermal-neutron spectrum in HTGRs peaks near the 239Pu and 241Pu
resonances, a large amount of the fissile plutonium bred in the denatured fuel is burned
in situ, thus resulting in the low fissile plutonium content of the fuel at discharge.
siderable 238U self-shielding is obtained by the lumping of the 238U in the coated particle
kernels.
Studies are currently underway at GA concerning the use of larger diameter fissile
particles, thereby lowering the 238U resonance integral and, consequently, the amount of
bred plutonium di~charged.~
Case 4 employs a denatured 233U feed and includes uranium recycle. It represents a
feasible successor to Case 3 once an exogenous source of 233U is available.
Case 5 involves Pu/Th Fuel. Since no 238U is present in the core, no plutonium is
bred; only 233U is bred.
resulting in enhanced neutron economy.
essentially achieves the "Phoenix" fuel cycle effect, i .e., the decrease in 239Pu content
is largely compensated for by buildup of 241Pu from 240Pu capture and by buildup of 233U
from 232Th capture, resulting in a nearly constant ratio of fissile concentration to 240Pu
concentration. Therefore the fuel reactivity is relatively constant over a long burnup
period, reducing the need for control poison. This allows the core to be batch loaded;
i.e., the entire core is reloaded at approximately 5-yr intervals.
minimizes down time for refueling and eliminates problems of power sharing between fuel
elements of different ages.
This reactor has greatly reduced requirements for control poison,
This results from the fact that this Pu/Th HTGR
This reload scheme
Furthermore, it allows easy conversion to a U/Th HTGR after
any cycle. It is important to note that the Pu/Th case presented in Table 4.4-1 is not
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