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4.4. GAS-COOLED THERMAL REACTORS
J. C. Cleveland
Oak Ridge National Laboratory
4.4.1. High-Temperature Gas-Cooled Reactors
The High-Temperature Gas-Cooled Reactor (HTGR) is another candidate for implemen ing
alternate fuel cycle options, particularly the denatured 233U cycle. Unlike other reactor
types that generally have been optimized for either LEU or mixed oxide (Pu/*~*U) fuel, the
HTGR has a design based on utilization of a thorium fuel cycle, and although current-
design HTGRs may not meet potential proliferation-based fuel cycle restrictions, the refer-
ence design involves both 232Th and 233U, which are the primary materials in the denatured
fuel cycle.
In contrast to the fuel for water-cooled reactors and fast breeder reactors, the
fuel for HTGRs is not in the form of metal-clad rods but rather is composed of coated fuel
particles bonded together by a graphite matrix into a fuel stick. The coatings on the individual
fuel particles provide fission-product containment. The fuel sticks are loaded
in fuel holes in hexagonal graphite fuel blocks. These blocks also contain hexagonal arrays
of coolant channels through which the helium flows.
particles are of two types:
of pyrocarbon and silicon carbide; and fertile particles consisting of Tho2 kernels coated
only with pyrocarbon. The pyrocarbon coating on the fertile particles can be burned off
hot demonstrations of the hcad-ecd processing operations unique to this reactor fuel, the
crushing and burning of the fuel elements, the mechanical particle separation, and the
particle crushing and burning are needed to ensure that low-loss reprocessing can take
place.
In the conventional HTGR the fuel
fissile particles consisting of UC2 kernels coated with layers
while the Sic coating on the fissile particles cannot. Therefore the two particle types
can be physically separated prior to any chemical reprocessing. As indicated in Chapter 5,
An inherent feature of the HTGR which results in uraniurri resource conservation is
its high (% 40%) thermal efficiency.
All else being equal, this fact alone results in a
15% reduction in uranium resource requirements compared to LWRs, which achieve a 34%
thermal efficiency.
This larger thermal efficiency also leads to reduced thermal
discharges that provide significant siting advantages for HTGRs, especially if many reac-
tors are to be deployed in central locations such as energy centers.
Other factors inherent in HTGR design that lead to improved Ug8 utilization due
to the improved neutrol: economy are:
1.
Absorption of only % 1.6% of the neutrons by HTGR particle ccatings, graphite
moderator, and helium coolant, compared to ar: absorption of % 5.€% of the neu-
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