232Th, the only natural Th isotope, can absorb ther­mal neutrons to produce fissile 2 3U and is therefore used as fertile material in breeder reactors. Nowa­days, the thorium fuel cycle is mostly envisaged in India, which has about one-fourth of the total world thorium resources, but this option is kept open in other countries such as Norway and Australia, which also have abundant Th ores.33 Thorium dicar­bide is a candidate fertile material for the Generation IV high-temperature reactor (HTR) and VHTR systems, and it is also exploitable for accelerator — driven system (ADS) burners. Solid solutions of UC2-ThC2 were candidate fuels for the Dragon High Temperature Reactor-coated particle fuels.36 However, thorium-based fuel is difficult to recycle because of the radioprotection issues generated by the hard g-emission of 208Tl (2.6 MeV), formed in the 232Th-233U spent fuel.

2.04.2.1 Phase Relationships

Atmospheric pressure phase equilibria in the Th-C system are reported in Figure 4.

Thorium metal has an fcc (a) structure below 1633 K and a bcc structure (p) at higher temperatures. The first can accommodate carbon atoms as intersti­tials, resulting in the formation of thorium monocar­bide without any lattice change.5 The ThC1±x fcc solid solution range, extending from pure Th to ThC196 at high temperatures, is stable between ThC067 and ThC097 below 1300 K. The exact high carbon limit is still under debate.37 A miscibility gap seems to exist in the ThCi_x phase field, between ThC0.06 around 1000 K,38 ThC0.30 at 1413 (±40) K,39 and ThC0 67 at 1150 K,2 probably extending to room temperature with approximately the same composi­tion boundaries. At higher temperatures, single car­bon interstitials can be replaced by C2 groups up to ThC1.96. Thus, only two compounds have been observed in the Th-C system at atmospheric pres­sure: the fcc monocarbide with its broad nonstoichio­metry range and the dicarbide, more often observed

Figure 4 The Th-C phase diagram.

as hypostoichiometric (ThC2—x). Thorium sesqui — carbide Th2C3 has been observed only at pressures above 30 kbar.27 At low temperatures (below 1500 K), ThC2—x is a monoclinic line compound (a) with composition ThCi.94,40 observed in equilibrium with ThC098 at 1528 (±40) K in the presence of oxygen.41 Around 1528 (±40) K, ThC2—x converts eutectoidally to a tetragonal phase (p) with a homogeneity range between C/Th = 1.66 at 1528 K and 1.96 at 1713 K, the temperature at which the a! p ThC2 phase transition occurs at its C-rich phase boundary.40 Pialoux and Zaug42 reported a different phase diagram, with higher C/Th ratios for the Th-rich p-ThC2 phase boundary, extending from 1.96 at 1570 K to 1.85 at 1743 K. This phase diagram does not include the eutectoid decomposition of p-ThC2, but rather a a! p-phase transition in the line com­pound at 1570 K. All authors agree on the formation of a cubic fcc ThC2—x modification (g) as the tem­perature is raised above 1763 (±45). A solid miscibil­ity gap has been observed by Bowman et a/.40 in the ThC-ThC2—x domain, with a maximum at 2123 (±40) K and C/Th = 1.22. The same maximum was observed by Pialoux and Zaug42 at 2173 (±40) K and C/Th = 1.95. There exists a ThC2-C eutectic of proba­ble composition ThC238 and temperature 2718 K. Obvi­ously, some questions on the ThC2—x phase boundaries are still open, often in relation to the large uncertain­ties in the reported transition temperatures.

The commonly accepted melting point of pure Th is 2020 ± 10 K.6 In the low-carbon domain, a eutectic

(or peritectic) isotherm around 1980 K in the compo­sition range of 0.06 < C/Th < 0.13 has been observed.

Two congruent melting points were observed in the solid solution region with 0.13 < C/Th < 1.96, the first at T = 2773 ± 35 K and C/Th = 0.97 ± 0.05, the second at T = 2883 ± 35 K and C/Th = 1.90 ± 0.06.

The boiling point of ThC2 was extrapolated to be 5400 K at 1 atm.43