Although the HTGR is not being widely used commercially, its high thermal efficiency, its ability to produce process heat at high temperatures, and its relatively efficient use of natural uranium resources when fed with thorium as fertile material give it potential future importance.

The core of the HTGR consists of hexagonal blocks of graphite pierced with two sets of longitudinal holes. One set of holes permits flow of helium coolant, whose outlet temperature may reach 1500°F, thus making possible high thermal efficiency. The other set of holes is filled with rods in which microspheres of nuclear fuel are imbedded in a graphite matrix. When the reactor first goes into operation, before a supply of 233 U has been accumulated, two kinds of microspheres are used.

In one kind, microspheres of fully enriched (93.5 w/o 233U) uranium carbide (UC2), about 200 цт in diameter, are coated with three concentric layers, called a TR1SO coating. The inner coating consists of porous graphite, to accommodate fuel swelling and fission-product gases. The intermediate coating, about 500 цт in outside diameter, consists of silicon carbide, to provide mechanical integrity for the spheres when the fuel is processed after discharge. The outer coating consists of impervious pyrolytic graphite, to retain fission products.

In the second kind, microspheres of thorium dioxide (Th02), about 500 цт in diameter, are coated with two concentric layers, called a BISO coating: an inner layer of porous graphite, to accommodate swelling and fission-product gases, and an outer layer of impervious pyrolytic graphite, to retain fission products.

During irradiation, about three-fourths of the TRISO-coated 235 U is consumed, leaving a residue of fission products and uranium whose isotopic content is around 20 percent 233 U, 25 percent 238 U, and 55 percent 236 U, formed by nonfission neutron capture in 233 U. At the same time, about 8 percent of BISO-coated thorium is converted to 233 U, some of which then undergoes fission.

When the first charge of fuel ceases to support a chain reaction, one-fourth of the fuel assemblies that have reacted most fully are replaced with fresh fuel. The spent assemblies are stored (“cooled”) for 150 days to permit some fission products to decay, 6.75-day 237 U to change to 237 Np, and 233 Th and 27-day 233 Pa formed by neutron capture in 233 Th to change to 233 U.

In processing the fuel, the first steps are to crush the graphite blocks and then bum them. The BISO-coated particles lose their graphite coating and become spheres of mixed uranium oxide consisting mostly of the 233 U isotope, and fission-product oxides. The TRISO-coated spheres lose their outer graphite coating, but the silicon carbide and inner graphite coating remain intact. The product then is a mixture of dense oxide spheres from BISO particles and less dense silicon carbide-coated graphite and U02 spheres from TRISO particles, both about 500 цт in diameter. The two kinds of particles are separated by elutriation with C02 gas, so that they can be processed separately with minimal mixing.

The residue of BISO particles is dissolved in mixed HN03 and HF and then separated by the Thorex solvent extraction process (Chap. 10) into a decontaminated M3U-rich uranium fraction, a thorium fraction containing 1.9-year radioactive 228 Th, and fission-product wastes.

The residue of TRISO particles is crushed to expose the remaining uranium and fission products. These are then dissolved in nitric acid and separated by a simplified version of the Purex process (Chap. 10) into a decontaminated uranium fraction containing around 20 percent 233 U and fission-product wastes.

Cooled Fuel, Totals 6734 kg Th 0.5 kg Po 462 kg U

26.1 kg Np

16.2 kg Pu 0.4 kg Am 0.2 kg Cm 792 kg FR

High Level Waste

0.1 kg Rj

26.1 kg Np

16.1 kgPu 0.4 kg Am 0.2 kg Cm

792 kg. FP

Figure 3.33 Fuel-cycle flow sheet for 1000-MWe HTGR fueled with thorium, enriched 23SU, once-recycled 23SU, and fully recycled 233U. Basis 1 year, 80 percent capacity factor.

In one possible fuel-cycle flow sheet for the HTGR, shown in Fig. 3.33, ^U-rich uranium from the BISO particles and the 20 percent 235 U from the TRISO particles are recycled as part of the fissile fuel for a later HTGR fuel cycle. When this is done, some of the graphite fuel blocks are charged with TRISO-coated fully enriched 235 U and BISO-coated thorium (point 1, Fig. 3.33), others are charged with TRISO-coated 20 percent 235U recycle uranium and BISO-coated thorium (point 2), and the rest are charged with BISO-coated 233 U-rich uranium and BISO-coated thorium (point 3). At the end of the cycle the spent fuel from the TRISO-coated fully enriched 23SU (point 4) is processed to recover uranium containing 20 percent 235 U to be recycled (point 2). The spent fuel from the TRISO-coated second-cycle 23S U (point 5) contains only 2 percent 235 U and is so highly contaminated by 236 U and fission products as to be discarded without reprocessing. The spent fuel from the BISO-coated recycle 235 U (point 6) and the BISO-coated thorium is processed to recover 233 U, to be recycled (point 3), and radioactive thorium, which is stored until 1.9-year 228 Th has decayed.

The quantities of uranium and thorium shown in the flow sheet Fig. 3.33 have been adapted from a study by Pigford [P2] of the fuel-cycle performance of the HTGR after a sufficient number of cycles have been operated to permit buildup of steady-state amounts of recycle 233 U and diluent 236 U. In earlier cycles the amounts of these two isotopes are lower.

The specific consumption of U308 and separative work in this HTGR cycle with 233 U recycle is compared in Table 3.16 with corresponding quantities for the LWR without or with plutonium recycle.

The HTGR with 233U recycle thus consumes about the same amount of separative work as the LWR with plutonium recycle, but uses only 65 percent as much natural uranium.