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4-21
Case B" is a modification of Case B' in that in addition to the four Tho2 corner
pins, the other peripheral pins in the assembly are composed of MEU(235)/Th.
remainder of the pins contain LEU.
requirements by 12% relative to the reference BWR U02 design.
The
In the once-through mode this design increases U308
Both Case B' and Case B" would offer 'operational benefits to the BWR since they
have a less negative dynamic void coefficient than the reference U02 design.8 This is
desirable since the sensitivity to pressure transients is reduced. As shown in Table
4.14, in equilibrium conditions a BWR employing the Tho2 corner pin once-through de-
sign would discharge 24 kg 233U/Gl.le annually while the BWR employing the peripheral Tho2
mixed lattice design would discharge 125 kg 233U/GWe annually.
Use of these ciptions in the once-through mode not only could improve the operational
performance of the BWR but also would build up a supply of 233U. This supply would then
be available if a denatured 233U cycle (together with reprocessing) were adopted at a later
time. Furthermore, use of the mixed lattice designs could be used to acquire experience
on the performance of thorium-based fuels in BWRs.
in the once-through mode may also be feasible in PWRs.
Similar schemes for the use of thorium
Although only limited scoping analysis of the safety parameters involved in the
use of alternate fuels in BWRs has been performed,8 the BWR thorium fuel designs appear
to offer some advantageous trends over U02 designs relative to BWR operations and safety.
Uranium/thorium fuels have a less negative steam void reactivity coefficient than the
U02 reference design at equilibrium. This effect tends to reduce the severity of
overpressurization accidents and improve the reactor stability.
reactivity coefficient for the denatured 23%/Th fuel indicates that the core will have a
flatter axial. oower shape than the reference U02 design.
increase in kW/ft margin and increase the maximum average planar heat generation ratio
(MAPLHGR). Alternatively, if current margins are maintained, the flatter axial power
shape could be utilized to increase the power density or to allow refueling patterns
aimed at improved fuel utilization.
References for Section 4.1
The less negative void
This could result in an
1. N. L. Shapiro, J. R. Rec, and R. A. Ilatzie (Combustion Engineering), "Assessment of
Thorium Fuel Cycles in Pressurized Water Reactors," EPRI NP-359 (Feb. 1977).
2. "Thorium Assessment Study Quarterly Progress Report for Second Quarter Fiscal 1977,"
ORNL/TEl-5949 (June 1977).
3. R. A. Hatzie, J. R. Rec, and A. N. Terney, "An Evaluation of Denatured Thorium Fuel
Cycles in Pressurized Water Reactors,' paper presented at the Annual Meeting of the
American Nuclear Society, June 12-16, 1977, New York, Mew Yorl:.