Bakker et a/.59 performed a critical review of the phase diagram and of the thermodynamic proper­ties of the UO2-ThO2 system. Solid UO2 and ThO2 form an ideal continuous solid solution. The phase diagram proposed by Bakker et a/.59 on the basis of the available experimental data is presented in Figure 9(a). The authors report the large uncer­tainties on the phase diagram because of the experi­mental difficulties. The thermodynamic properties of (Th1 _JU3,)O2 solid solutions have been recently investigated by Dash et a/.60 using a differential scanning calorimeter and a high-temperature drop calorimeter. The ternary compound ThUO5 was synthesized and characterized by X-ray diffraction. The thermodynamic data on this compound were estimated.

Like UO2, PuO2 forms a continuous solid solution with ThO2 in the whole composition range. Limited melting point data were measured by Freshley and Mattys.61 The results indicate a nearly constant melt­ing point up to 25wt.% ThO2. In view of the

instability of PuO2 at high temperature (high pO2 over PuO2), this behavior could be due to a change of the stoichiometry of the samples. The available liquidus temperature measurements do not repro­duce the recommended value for the melting point of PuO2. The full lines in Figure 9 give the solidus and liquidus curves considering an ideal behavior of the PuO2-ThO2 system.

D. Manara and F. De Bruycker

2.04.1 Introduction

Research on actinide carbides as nuclear fuel began in the 1950s. Then, uranium dioxide and mixed uranium-plutonium oxides began to be preferred as nuclear fuel in most of the Generation II and III power plants, due to the fact that the option of fast reactors for civil purposes had mostly been aban­doned. This led to an abrupt interruption in actinide carbide research between the first half of the 1970s and the second half of the 1990s. In the last decade, there has been renewed interest in actinide carbides in view of a nuclear fuel more suitable for high burnup and high-temperature operation with a reduced ‘margin to melting,’ in the framework of the ‘Generation IV’ nuclear systems development.1 Consequently, actinide carbides are now being stud­ied with more and more advanced methods, both experimental and computational.

The goal of the present monograph is to summa­rize the state-of-the-art knowledge of the most rele­vant physical and chemical properties of actinide carbides. This work is largely based on a few earlier reviews on the same subject: Storms,2 Rand,3 Holley et a/.,4 Matzke,5 the Gmelin Handbooks,6-9 and the OECD-NEA reviews.10-13 More detailed and/or more recent data are taken from single references.

The phase relation between actinides and alkaline — earth metals changes with the increase in the atomic number of the latter. Regarding the Be-related
systems, a NaZn13-type intermetallic compound (D23-structure) is observed in the Th-Be, U-Be, and Pu-Be phase diagrams.1,4 These intermetallic compounds melt congruently near the Be terminal and the decomposition temperatures are estimated to be 2203, 2273, and 2223 K for ThBe13, UBe13, and PuBe13, respectively. Since these data have at least ±50 K error, the decomposition temperatures are reasonably comparable. Figure 4 indicates the U-Be phase diagram as a typical example quoted from Okamoto,4 which was mainly constructed from the observations of Buzzard.14 The eutectic point appears near the U terminal at 1363 K, and a narrow liquid miscibility gap appears near the Be termi­nal in the U concentration region between ^0.7 and ^2.2 at.%. Although the latter was questioned by Hansen and Anderko,15 there is no other available experimental data for this system. According to Wilhelm et a/.,16 this miscibility gap is thermody­namically unlikely and its presence is possible only if there is a strong clustering in the liquid phase, and thus thermodynamic functions for the U-Be system are estimated by introducing a simple thermodynamic model. As for the solid phase, a few percent of solid solubility of Be in g-U was observed.14 Also, a spinodal composition was given in the central region.14 There are two different sources for the Pu-Be system.11,17 The significant differences between them are the congruent melting temperature of the PuBe13 and the shape of the liquidus. The latter phase diagram given in Konobeevsky17 was then modified based on the several unpublished results obtained at the Los Alamos National Laboratory, as shown by Ellinger et a/.18 The modified Pu-Be phase diagram by Ellinger et a/.18 was recommended by Okamoto,19 who showed phase relations that are mostly similar to those in the U-Be system, although the eutectic temperature lowered to ^903 K. There is also a small percent solubility of Be in e-Pu (bcc structure). The Th-Be phase diagram was mainly constructed by Okamoto19 from the observations by Badaeva and Kuznetsova.20 When neglecting the ThBe13 com­pound, the Th-Be phase diagram looks like that of a typical eutectic type, and the eutectic point was reported to be at 1503 K and at 65 at.% Th. However, according to Okamoto,19 due to the cursory nature of the work of Badaeva and Kuznetsova,20 the eutectic point is still only a rough estimation. Table 1 sum­marizes the thermodynamic functions for the Th-Be, U-Be, and Pu-Be systems, which were estimated by Okamoto19 with respect to the liquid phases. The NaZn13-type intermetallic compound was also

Table 1 Thermodynamic functions for actinide-Be systems

GO(Be, liq) = 0 GO(Th, liq) = 0 GO(U, liq) = 0 GO(Pu, liq) = 0

GO(Be, bcc) = -12 600 + 8.067T GO(Be, HCP) = -14 700 + 9.428T GO(Th, bcc) = -13807 + 6.808T GO(Th, fcc) = -17 406 + 9.012T GO(U, bcc) = -9142 + 6.497T GO(Be13Th) = -14630 + 4.902T GO(Be13U) = -17100 + 6.470T GO(Be13Pu) = -24 980 + 9.400T Gex(Th-Be, liq) = xTh(1 — xTh) (13520 -830xTh)

Gex(U-Be, liq) = xU(1 — xU) (36100 — 5090xU — 580xU) Gex(Pu-Be, liq) = xPu(1 — xPu) (10160)

Source: Okamoto, H. In Phase Diagrams of Binary Actinide Alloys-, Kassner, M. E., Peterson, D. E., Eds.; Monograph Series on Alloy Phase Diagrams No. 11; ASM International: Materials Park, OH, 1995; pp 22-24, 146-151, 164-168, 207-208, 218-219, 246-247, 297-300, 411-412, 423.

Note: values are in J mol-1. Tis in K. x is mole fraction.

observed in Pa-Be, Np-Be, Am-Be, and Cm-Be sys­tems.21- The decomposition temperatures are pre­dicted to be at least higher than 1673 K for the Np-Be system and 1773 K for the Am-Be and Cm-Be systems.

Figure 5 shows the Np-Be phase diagram preliminar­ily estimated in the present study, assuming the inter­action parameter for the liquid phase and the Gibbs energy of formation for NpBe13 are the same as those for the Pu-Be system. It is speculated that the phase relations in the Np-Be system will have a reasonably
similar shape with the Pu-Be system, with the excep­tion of the Np terminal. By measuring the thermal arrests for several compositions, especially near the Be terminal, the speculated phase diagram will be modified efficiently.

Regarding the Th-Mg system, there are some conflicting issues among the available data.24-26 The tentatively assessed phase diagram was given in Nayeb-Hashemi and Clark.27 However, the phase relations related to the gas phase were not given in the phase diagram, although the boiling point of Mg is 1380 K, which is lower than the transition temperature between a-Th and p-Th. Two interme­tallic compounds, that is, Th6Mg23 and ThMg2, exist near the Mg terminal in the low-temperature region. At the least, these decomposition temperatures are much lower than that for the ThBe13, suggesting that the stability of the Th-Mg compounds is far lower than that of the Th-Be compounds. Thermodynamic functions for the ThMg2 were determined by Novotny and Smith28 between 692 and 812 K by means of vapor pressure measurement. The derived equation for the Gibbs energy of the formation is

DfG°(Mg2Th) = — 59.871 ± 12.979 + (63.639 ± 18.000) x 10-3T (kJmol-1)

Table 2 summarizes the thermodynamic values at 750 K. As for the enthalpy and entropy of formation, the higher values in the table were recommended by Nayeb-Hashemi and Clark.27 A similar phase rela­tion near the Mg terminal was reported for the Pu-Mg system,30 although the phase relation at high tempera­ture was different. There is a miscibility gap for the liquid phase in the high-temperature region ofthe Pu- Mg system. On the other hand, there are no interme­tallic compounds in the U-Mg system, and the limited solubility even for the liquid phase was shown.31 These facts on the phase relation between actinides and Mg suggest that the miscibility between actinides and Mg becomes far poorer than that between actinides
and Be. By assuming the systematic variation in the actinide-Mg systems, partial phase diagrams for Pa — Mg, Np-Mg, and Am-Mg systems were proposed by Gulyaev and Dvorshkaya32 and there is limited solubility for the solid and liquid phases.

In the cases of the Ca-, Sr-, Ba-, and probably Ra — related systems, the miscibility between actinides and these heavy alkaline-earth metals is expected to be very poor even for the liquid phase, although the available information is very limited. Thorium metal was prepared by calciothermic reduction at around 1223 K, and the solubility of Ca in Th was found to be very low (<0.12 at.%). No binary compounds between U and Ba were observed in the determination of the U-Ba-C ternary phase dia — gram.34 The U-Ca, U-Sr, Pu-Ca, Pu-Sr, Pu-Ba, and Am-Ba systems were predicted to be immisci­ble, and the mutual solubility was extremely low even in the liquid phase.11,35-38 According to semi­empirical modeling,39,40 the limited mutual solubility and the absence of any intermetallic compounds were also predicted for the Th-Ba system, although there is no available experimental data. These suggest that, for the actinides-Ca, — Sr, — Ba, and possibly, -Ra sys­tems, the allotropic transformation temperatures of both the actinides and alkaline-earth metals will only appear in the phase diagrams. Figure 6 shows the Pu-Ca phase diagram as a typical example, which was calculated by taking very large positive values (^50 kJ mol-1) for the interaction parameters of each phase. Other systems are considered to have a similar tendency.

Based on the high thermal conductivity (see Table 6) and high corrosion resistance of nickel-copper alloys in seawater, Alloy 400 has been widely applied in boiler feed water heat exchanger tubes and shells, and Alloy 500 has found wide use for pump shafts and impellers in seawater pumps. Based on such industrial applications, Alloy 400 was used for SG tubes in some CANDU reactors.

Chemical compositions Ni > 99.0 Ni > 99.0, C<0.02 Ni-31Cu-2Fe (S £ 0.024) Ni-31Cu-2Fe-0.04S Ni-30Cu-2Fe-0.6Ti-2.7Al Ni-15Cr-8Fe Ni-23Cr-8Fe-1.4Al Ni-29Cr-9Fe Fe-33Ni-21Cr Ni-15Cr-7Fe-2.5Ti-1Nb-0.7Al Ni-19Cr-17Fe-3Mo-0.9Ti-0.5Al-5.1Nb Fe-25Ni-15Cr-1.3Mo-2.1Ti-0.3Al Ni-28Mo-5Fe-2Co Ni-28Mo-4Fe-2Co-Low Si, Low C Ni-30Mo-2Fe-2Co-2Cr-2W-2Mn Ni-17Mo-16.5Cr-4.5W-5.3Fe-0.3V Ni-16Mo-15.5Cr-5Fe-3.7W-2Co Ni-16Mo-16Cr-2Fe-1.5Co Ni-21Cr-13.5Mo-4Fe-3W-2Co Ni-21.5Cr-9Mo^Fe Ni-21.5Cr-9Mo^Fe Ni-21Cr-16Mo-4Fe-3.7W-1.2Al Ni-23Cr-15.7Mo-1Fe-0.3Al 42Ni-21.5Cr-25Fe-3Mo-2.2Cu-0.9Ti
Allox-system	Alloy no.	Features Commercial pure Ni. Applicable in caustic solution below 315 0C Low C commercial pure Ni. Applicable in caustic solution above 315 0C Applicable to the components for sea water, salt unit, crude distillation, etc. Free machining grade of Alloy 400 P. H. version of Alloy 400, up to 600 0C. Applicable to pump-shaft, impellers, scrapers, etc. Excellently resistant to in chloride SCC. Applicable to structural materials Excellently resistant to high-temperature oxidation. Applicable to oxidation-resistant parts Excellently resistant to many corrosive aqueous media, etc. Applicable to structural materials Highly resistant to high-temperature oxidation. Applicable to components for high-temperature use Typical P. H. Ni-based alloy. Applicable to parts which need high tensile, creep and creep rupture properties Higher strength level than Alloy X-750. Applicable to parts which need high tensile, creep and creep rupture, etc. Age-hardenable alloy. Good strength and oxidation resistance up to 700 0C Excellently resistant to hydrochloric acid. But, weak to solutions with mixing of oxidant Improved on corrosion resistance in heat affected zone after welding of Alloy B Minimized fabrication problems for Alloy B-2. Not applicable to the environment with ferric or cupric salt Excellent high resistance to oxidation, corrosion in chlorine, compounds with chlorine, oxidizing acid, etc. Improved on fabricability and long range aging characteristics of Alloy C Advanced Alloy B. Superior corrosion resistance to oxidizing environment compared to Alloy B Improved on corrosion resistance of Alloy C-276 in oxidizing environment High creep rupture strength and high resistance to corrosion and pitting in oxidizing environment Improved on low cycle fatigue properties and cold formability of Alloy 625 High Cr content of Alloy C-276. Excellent resistance to SCC, pitting and crevice corrosion in aggressive media Pure Ni-Cr-Mo alloy. Excellent corrosion resistance and thermal stability Improved on aqueous corrosion resistance in a wide variety of corrosion media, modified by Alloy 800
Figure 7 Nickel-based alloy systems and theirfeatures (dotted line: reference material, P. H.: precipitation hardened). In nuclear power plants, several of these nickel-based alloys have been applied or are suitable as materials for various components, pipes, tubes, and other parts. The main applications or candidates of nickel-based alloys for various nuclear reactors are summarized in Table 2.

2.01.2.1 Crystal Structure

The stable crystallographic modifications of the acti­nides at atmospheric pressure are listed in Table 1. Compared to the lanthanide series in which the hex­agonal close-packed (hcp) and the face-centered cubic (fcc) structures dominate, the actinide metals show a remarkable variation in the structural

properties at room temperature, as shown in Figure 1. Particularly, the elements Pa-Pu have unusual low symmetry (distorted) crystal structures. a-Pa is body-centered tetragonal, and a-U and a-Np are orthorhombic but with slightly different space groups. a-Pu has a monoclinic crystal structure with 16 atoms in the unit cell at room temperature. Plutonium is unique in the periodic table of the elements with six allotropes at atmospheric pressure and one more at elevated pressure.

This complexity of the structural properties of the actinides is also evident from Figure 2, which shows the variation of the molar volume of the a-phases of the actinides at room temperature and atmospheric pressure, indicating that the actinides Pa to Pu follow the trend in the (itinerant) d-transition
metals, whereas the actinides Am to Bk follow that of the (localized) 4f metals. It is generally accepted that this complex behavior is due to the active role of the f-electron in the metallic bond and the changes in temperature and pressure by which the f-electron bonding character is affected. Experimen­tal observations and electronic structure calculations have indeed shown that the bonding in the transition metals is dominated by d-electron contributions, that in the lanthanides there is a lack of f-electron contri­bution, and that the actinides fall in between.5

In the system Pu-O, the partial pressures of the gaseous species Pu(g), PuO(g), and PuO2(g) vary with the O/Pu ratio and temperature as a large composition range exists for PuO2 _ x. The variation of the gaseous species calculated at 1970 K is shown in Figure 26(a) as a function of the O/Pu ratio using the CALPHAD model developed by Gueneau eta/.28 In the two-phase region [Pu2O3 + PuO2 _ x], the major species are PuO(g), Pu(g), and PuO2(g). With increasing O/Pu ratio in PuO2 _ x phase, the partial pressures of PuO(g) and Pu(g) decrease whereas PuO2(g) partial pressure slightly increases. Close to the stoichiometry, O(g) and O2(g) become the major gaseous species. The curve giving the total pressure of the gas as a function of the O/Pu ratio shows a minimum that corresponds to the congruent vapori­zation of plutonia occurring for a slightly hypostoi- chiometric oxygen composition.

In the U-O system, the vaporization is more com­plex due to the existence of both the hypo — and hyper­stoichiometric composition ranges of uranium dioxide. According to the measurements by Pattoret213 using mass spectrometry, reported in Figure 26(b), a sample with a composition UO2 _ x heated at 2250 K will tend to lose preferentially U via UO(g) to reach the

congruent composition UO2 _ x with a O/U ratio equal to 1.987 ± 0.01 corresponding to the minimum in total pressure, that is, the congruency. A sample with a com­position UO2 or UO2 + x will lose oxygen via UO3(g) to reach the same congruent composition. Above solid UO2, the largest contribution is from UO2(g).

2.02.5.1 U-Pu-O

The partial pressures of the different gaseous actinide oxide molecules were calculated by Rand and Markin52 over U0 85Pu0.15O2 ± x at 2000 K using the thermodynamic data on the solid and gas phases (Figure 27). UO3(g) is the predominant gas species in the hyperstoichiometric region. This is due to the fact that in the MOX, the oxygen potentials are higher than in UO2 (see Section 2.02.4.2.2). The composi­tion of the vapor is enriched in uranium and oxygen in comparison to the solid. It means that the solid will lose uranium and oxygen. In the hypostoichiometric region, the uranium species are less in the vapor than in the solid for O/metal ratio below 1.96. It means that the solid will preferentially lose plutonium. A review of the previous experimental studies on vaporization of (U, Pu)O2 oxides was reported by Viswanathan and Krishnnaiah.214 The authors derived a thermochemi­cal model to calculate the partial and total pressures

Figure 27 Calculated partial pressures over Uo.85Puo.15O2 ±x at 2000 K according to Rand and Markin.52

above MOX with up to 40% mol. PuO2. The different studies show that a quasi-congruent vaporization is reached where (O/Metal)vapor=(O/Metal)solid which corresponds to a slightly hypostoichiometric mixed oxide in oxygen like in the binary oxides UO2 and PuO2.

2.02.5.2 U-Pu-Am-O

Calculations ofthe same type were recently performed by Maeda eta/.215 above a mixed oxide with the compo­sition (U0.69Pu0.29Am0.02)O2 ±x at 2073 and 2273 K (see Figure 28). The results show that in the hyperstoichio­metric region, UO3(g) remains the predominant gas species. But for O/metal ratio below 1.96 corresponding to the congruent composition, the AmO(g) species becomes the majormolecule in the vapor. Very recently, an experimental study on vaporization of AmO2 and (Pu, Am)O2 oxides was performed by Gotcu-Freis

216,42

et a/. using mass spectrometry.

The lattice parameter of cubic U2C3 was studied up to 2073 K by XRD, and no anomalies were detected either at low or high temperature. Its values vary from 807.3 pm at 10 K123 to 825.6 pm at 2073 K.114

Oetting et a/.124 determined the energy of forma­tion for vacancies in the U2C3 lattice to be ^0.8 eV, from the heat capacity increase above 1000 K.

The temperature coefficient of the electronic heat capacity was estimated to be g«84mJK-2mol-1 from low T heat capacity measurements, in agree­ment with the metallic character of uranium sesqui — carbide. U2C3 is antiferromagnetic below the Neel temperature TN « 55 ± 4 K.8

2.04.4.2.1.2 Uranium dicarbide UC2

Tagawa et a/.125 showed that the lattice parameter of a-UC2 increases linearly between UC180 and UC196 according to the following relation:

a = 352.45 + 0.75 x (C/U — 1.80) [7]

An uncertainty of ±0.01 pm stems from the different sample preparation methods. Tagawa et a/.126 also showed that the c/a ratio is 1.702 at room tempera­ture, and does not detectably vary as a function of the C/U ratio between UC180 and UC196, where c stays approximately constant and close to 600 pm. Atoji127 measured the lattice parameters of UC186 at 5 K by neutron diffraction, finding a = 351.7 ± 0.1pm and c = 598.9 ± 0.1pm. No phase transitions were de­tected between 5 and 300 K. The c/a ratio decreases with increasing temperature above 1473 K. Whereas a increases from 353.6 pm at 1073 K to 362.5 pm at 1973 K, there is no complete agreement about the behavior of c. Laugier and Blum108 suggested that c decreases from 605.6pm at 1073 K to 594.9pm at 1700 K on the U-rich side of the tetragonal UO2-x phase field, whereas it varies from 605.6 to 603.9 pm on the C-rich side.

The transformation a! p is diffusionless of the martensitic type. It occurs without movement of
the U atoms, and with a slight deformation of the C sublattice. The transformation shear angle is between 4° and 6°. p-UC2-x crystallizes in a fcc structure of the KCN-type with a0 = 548.8 pm.109

UC2-x is a metal. The UC2 electronic state den­sity at the Fermi level was recently calculated by Shi et al., 9 in reasonable agreement with the tempera­ture coefficient g of the electronic heat capacity. This was estimated to be 16.3 mJ K-2 mol-1, to yield N (Ef) ~ 3.45 eV-1 atom-1 for UC190 and 16.7 mJK-2 mol-1, to yield N(Ef) « 3.53 eV-1 atom-1 for UC194.

Atoji127 showed that a-UC2-x is paramagnetic down to 5 K, without superconductivity.

2.04.4.2.2 Thermodynamic properties

UF4 has been studied extensively. The heat of forma­tion of UF4 has been measured using fluorine bomb calorimetry by Hayman40 at 315 K (without tak­ing into account the impurities in the samples), by Wijbenga,41 and by Johnson.42 The value obtained by Hayman was recalculated by Wijbenga considering the impurities and the value of the heat of formation of UF6(s) measured by Johnson.26

Mal’tsev et a/43 determined the heat of forma­tion of the highest crystal hydrate of uranium tetrafluoride, UF4-2.5H2O, as the sum of the heats of nine reactions. Using the heat of hydration of UF4-2.5H2O given by Popov et a/.,44 Mal’tesev et a/. deduced the heat of formation of the anhydrous tet — rafluoride. Several works reported the enthalpy of formation by solution calorimetry:

• At first, Khanaev et a/45 used various (HCl + H3BO3 + FeCl3) aqueous solutions at 323 K.

• A solution of hydrofluoric acid, hydrochloric acid, and aqueous aluminum chlorate AlCl3-6H2O was used by Hu et a/.46

• Finally, Cordfunke et a/47 have chosen a mixed aqueous solvent containing sulfuric acid, boric acid, and ceric sulfate.

The results obtained are widely scattered from (—1884.9 ± 2.9) to (—1921.3 ± 4.2) kJ mol-1.

The selected value came from the IAEA review,37 which is a weighted average of the values published

Table 2 Thermodynamic properties of the crystalline uranium tetrafluoride

AfH° (UF4, cr, 298.15 K) (kJ mol-1) S0 (UF4, cr, 298.15 K) (J K-1 mol-1) Cp (UF4, cr, 298.15) (J K-1 mol-1) Cp (UF4, cr, T) (J K-1 mol-1)

by Johnson42 using fluorine combustion calorimetry (with the selected enthalpy of formation of UF6(s)) and Cordfunke et a/.47 using solution calorimetry (with the selected enthalpies of formation of HF (aq), U3O8(s), and g-UO3)24 The values differ by 10.7 kJ mol-1, therefore the NEA-TDB24 recom­mended further measurements to resolve the discre­pancies in the experimental values.

2.06.3.2.2.1 Heat capacity

The low-temperature heat capacity of crystalline uranium tetrafluoride was measured by:

• adiabatic calorimetry by Brickwedde et a/25 from 20 to 350 K;

• adiabatic calorimetry by Osborne et a/.48 between 5 and 300 K;

• and isothermal calorimetry by Burns et a/.49 in the 1.3-20 K temperature range.

The values are very close except for the values of Brickwedde eta/.25 extrapolated at T< 15 K (Figure 8).

Only data available for the thermal diffusivity and conductivity of the liquid state of plutonium have been reported. Wittenberg and coworkers77,78 mea­sured the thermal diffusivity (D) from which they derived the thermal conductivity, which is constant in the measured range (973 to 1073 K). As discussed above, the two publications by these authors are not consistent. In the early one,78 Wittenberg gave 0.017-0.021 and 0.022-0.023 cm2 s-1 for the thermal diffusivity in two experiments with different heating

rates, yielding to l = 5.4 Wm-1 K-1 and 6.3 Wm-1 K-1, respectively. In the later publication,77 Wittenberg reports D = 0.057-0.056 cm2 s-1 for the temperature range 948 to 1073 K, yielding l = (17±1) Wm-1 K-1. This latter value is recommended here.

As (U, Pu)N were some of the most promising candidates for the first breeder reactors, they are the best studied nitride solid solution fuels. UN and PuN form a continuous solid solution, and the lattice param­eter increases with an increase in the plutonium con­tent, and is accompanied by a large deviation from Vegard’s law, as shown in Figure 7,26 suggesting the nonideality of the solution. A diagram of the calculated U-Pu-N ternary phase at 1000 °C, shown in Figure 8,1 suggests that there is a relatively narrow range of pos­sible (U, Pu)N compositions, as is the case with U-N and Pu-N binary systems. It is suggested that the sesquinitride solid solution (U, Pu)N15 exists in a sys­tem in which PuN may constitute up to 15mol%27, although this is not depicted in Figure 8.

As uranium monocarbide and plutonium mono­carbide, as well as other actinide carbides, have an NaCl-type fcc structure, actinide nitrides and acti­nide carbides form solid solutions. Some research performed on actinide nitride carbides, for example, U-N-C, Pu-N-C,28-30 have investigated the suit­ability of these carbonitride fuels and the impurities in nitride fuels after carbothermic reduction. Phase

UN NpN

NpN PuN

Composition

stability graphs of U and/or Pu-N-C, both with and without oxygen, also have been constructed in order to make pure nitride fuels.31, 2 The irradiation behav­ior of (U and/or Pu)-N-C fuels also has been reported,33 but the details of this data are out of the scope of this chapter.

As MAs are usually burnt with uranium and plu­tonium for transmutation, and as Am originally exists in Pu, (MA, U)N or (MA, Pu)N have also been well studied. As mentioned above, the vaporization behav­ior of (Pu, Am)N has been studied34, and abnormal vaporization of Pu and Am was observed. The lattice parameters of (U, Np)N and (Np, Pu)N increase with increase in Np and Pu content, and with a small deviation from ideality, as shown in Figure 7.24 Although scarcely any data for pure CmN has been obtained, X-ray diffraction data for (Cm04Pu06)N has been reported, as shown in Figure 9.24

Inert matrix fuels, where MA as well as uranium and plutonium are embedded in a matrix, are also being considered for use in ADS for transmutation. Recent research in MAs has focused on using various nitride solid solutions and nitride mixtures as inert matrix fuels. For ADS targets, matrices have been designed and selected so as to avoid the formation of hot spots and to increase the thermal stability, especially in the case of Americium nitride. Con­sidering their chemical stability and thermal conduc­tivity, ZrN, YN, TiN, and AlN were chosen as candidates for the matrix.16,35 ZrN has an NaCl-type

0.48

0 0.2 0.4 0.6 0.8 1

PuN CmN

PUO2 Composition cmO2

Figure 9 Lattice parameter of (Pu, Cm)N and (Pu, Cm)O2. Reproduced from Minato, K.; etal. J. Nucl. Mater. 2003, 320, 18-24, with permission from Elsevier.

fcc structure with а = 4.580 A and has nearly the same thermal conductivity as UN, has a high melting point, good chemical stability in air, and a tolerable dissolu­tion rate in nitric acid. Recently, abundant data have been made available for ZrN-based inert matrix fuels. It is planned that (Pu, Zr)N, with about 20-25% Pu, will be used to burn Pu in a closed fuel cycle.36 The lattice parameter of (Pu, Zr)N decreases with an increase in the Zr content, and is between that of PuN and ZrN, in accordance with Vegard’s law.24 It has also been estimated, using a model, that (Pu, Zr)N with 20-40 mol% PuN, does not melt till up to 2773 K; this is based on experimental thermody­namic data which show that U0.9Zr0 8N does not melt till up to 3073 K.37 In the case of (Am, Zr)N, it is reported that two solid solutions are obtained when Am content is over 30%24, as shown in Figure 10. The Am content of the two phases have been estimated, from the lattice parameter, to be 14.5 and 43.1 mol%. A thermodynamic modeling of a uranium-free inert
matrix fuel, for example, (Am0.20Np0.04Pu0.26Zr0.60), has also been accomplished.38

In contrast to ZrN, TiN does not dissolve MA nitrides even though TiN also has an NaCl-type fcc structure. This is explained by the differences in lattice parameter, which was estimated by Benedict.39 A mixture of PuN and TiN was obtained by several heat treatments above 1673 K, and the product, in which one phase was formed, did not contain the other phase.40 TiN, as well as ZrN, have nonstoichio­metry. It is also reported that a TiN + PuN mixture may be hypostoichiometric although (Pu, Zr)N is hyperstoichiometric.