2.05.3.1 Actinide-Alkali Metals

There is very limited experimental information on the relation between actinides and alkali metals. The solubility of U in liquid Li is 0.0015, 0.00087, 0.00018, and 0.000058 at.% at 1273, 1173, 1073, and 973 K, respectively8: that in liquid Na is <0.005 at.% at

CO(N) = compounds existing in the phase diagram, where N is the number of the compounds LS = wide range of liquid and solid solutions EP = eutectic or peritectic type MG = liquid phase miscible but miscibility gap exists IM = immiscible, where very limited mutual solubility even in liquid [ ] = estimated

0 He IM Ne IM Ar IM Kr IM Xe IM Rn IM

Figure 2 (Continued)

CO(N) = compounds existing in the phase diagram, where N is the number of the compounds LS = wide range of liquid and solid solutions

EP = eutectic or peritectic type *1

MG = liquid phase miscible but miscibility gap exists *2

IM = immiscible, where very limited mutual solubility even in liquid *3

[ ] = estimated *4

*5

(d)

Figure 2 (Continued)

Am-X

EP = eutectic or peritectic type

MG = liquid phase miscible but miscibility gap exists

IM = immiscible, where very limited mutual solubility even in liquid

[ ] = estimated

: close to the eutectic type : miscibility gap observed in the liquid *3: miscibility gap observed in the solid *4: close to the solution type *5: several percent of solid solubility

(e)

CO(N) = compounds existing in the phase diagram, where N is the number of the compounds LS = wide range of liquid and solid solutions

EP = eutectic or peritectic type *1: close to the eutectic type

MG = liquid phase miscible but miscibility gap exists

IM = immiscible, where very limited mutual solubility even in liquid

[ ] = estimated

(f)

: miscibility gap observed in the liquid miscibility gap observed in the solid 4: close to the solution type 5: several percent of solid solubility

Figure 2 (a) Phase diagram type of Th-X system. (b) Phase diagram type of U-X system. (c) Phase diagram type of Np-X system. (d) Phase diagram type of Pu-X system. (e) Phase diagram type of Am-X system. (f) Phase diagram type of Ce-X system.

371 K.9 Na does not react with U at 973 K.10 Accord­ing to Schonfeld eta/.,11 the Pu-Li, Pu-Na, and Pu-K systems are completely immiscible and the mutual solubility is extremely low even in the liquid phase. These suggest that in the U-alkali and Pu-alkali metal systems, allotropic transformation tempera­tures of both actinides and alkali metals are observed only in the phase diagrams. Systematically, similar reactions are expected for the Np-alkali metal sys­tems. Figure 3 indicates the Pu-Li phase diagram as a typical example, which was calculated using the regular solution model by taking very large positive values (^50 kJ mol-1) for the interaction parameters of each phase. There is no available information on the relation between Am and alkali metals. The Nd-Li phase diagram is reported in Ganiev et a/.,12 in which there is a limited miscibility gap for the liquid phase and several percent of Nd solubility in liquid Li. This may suggest that the miscibility between Am and alkali metals becomes slightly better than the other light acti­nide series elements. Regarding the Th-Na, Th-K, and U-Na systems, the existence of several compounds is reported elsewhere.13 However, it is claimed that these data are unreliable.1 Th-alkali metal systems are reasonably predicted to be immiscible.

2.08.2.1.1 Chemical compositions, physical properties, and mechanical properties

The chemical compositions of nickel and typical nickel-copper alloys are shown in Table 3, along with those of other nickel-based alloys.

Alloy 200 (UNS N02200) is a commercially pure (99.6%) wrought nickel. Alloy 201 (UNS N02201) is the low-carbon version of Alloy 200. These alloys have good mechanical properties and good resistance to corrosion at low to moderate temperatures in

Figure 4 Nickel-molybdenum binary phase diagram.

Cr Figure 5 Iron-nickel-chromium ternary phase diagram (at 650°C).

caustic solutions such as NaOH or dilute deaerated solutions of common nonoxidizing mineral acids such as HCl, H2SO4, or H3PO4.7

The mechanical properties of Alloy 200 at ele­vated temperatures are shown in Figures 8 and 9.3 Alloy 200 is typically limited to use at temperatures below 315 °C. At higher temperatures, Alloy 200 products can suffer from graphitization, which can severely compromise the properties of the material. Alloy 200 is susceptible to embrittlement after long­term heating in the range of 425-760 ° C, due to carbide precipitation along grain boundaries.4 For service above 315 °C, Alloy 201 is preferred.7

The reason for the good corrosion resistance of Alloys 200 and 201 is the fact that the standard oxidation-reduction potential of nickel is more noble than that of iron and less noble than that of copper. Due to nickel’s high overpotential for hydro­gen evolution, hydrogen is not easily discharged from any of the common nonoxidizing acids, and a supply of oxygen is necessary for rapid corrosion to occur. Hence, in the presence of oxidizing species such as ferric ions, cupric ions, nitrates, peroxides, or oxy­gen, nickel can corrode rapidly. The outstanding corrosion-resistance characteristics of Alloy 200 to
caustic soda and other alkalis have led to its success­ful use in caustic evaporator tubes.7

The nickel-copper Alloy 400 is a complete solid — solution alloy that can be hardened only by cold­working. Alloy 400 contains about 30-33% copper in a nickel matrix and has similar characteristics as those of Alloy 200. It has high strength and toughness over a wide temperature range and good resistance to many corrosive environments. Alloy 400 exhibits excellent resistance to corrosion in many reducing media. It is also generally more resistant to attack by oxidizing environments compared to higher copper — content alloys. It is also widely used in marine appli­cations. Alloy 400 products exhibit low corrosion rates in flowing seawater, whereas in stagnant condi­tions, crevice and pitting corrosion can be induced. It is also resistant to SCC and pitting in most fresh and industrial waters. Alloy 400 is highly resistant to hydrofluoric acid at all concentrations and at all temperatures up to their boiling points. It is therefore widely used in components for seawater applications, salt units, crude distillation, and as a structural mate­rial in chemical plants.8

Alloy K-500 (UNS N05500) is a precipitation — hardened version of Alloy 400. It contains aluminum

and titanium, and is hardened by the formation of submicroscopic particles of intermetallic compounds, Ni3(Ti, Al). The formation of intermetallic com­pounds occurs as a solid-state reaction during the thermal aging (precipitation hardening) treatment. Prior to the aging treatment, the alloy component needs to be solution-annealed to dissolve any phases that may have formed during previous processing. The solution annealing and aging are normally car­ried out in the temperature range 980-1040 °C and 540-590 °C, respectively. Alloy K-500 has the excel­lent corrosion-resistant features of Alloy 400 with the added benefits of increased strength up to 600 °C and hardness. The alloy has low magnetic permeability and is nonmagnetic up to 134 °C.

Some typical applications of Alloy K-500 include pump shafts, impellers, medical blades and scrapers, oil well drill collars and instruments, nonmagnetic housings and other complementary tools, electronic components, springs, and valve trains.9

The mechanical and various physical properties of nickel and typical nickel-copper alloys are shown in Tables 4 and 5, respectively, along with those of other nickel-based alloys. Physical properties at ele­vated temperature are shown in Tables 6-8.

R. J. M. Konings, O. Benes, and J.-C. Griveau

2.01.1 Introduction

The actinides are the 15 elements with atomic numbers 89-103 in the periodic system. The International Union of Pure and Applied Chemistry (IUPAC) has recom­mended that these elements are named actinoids (meaning ‘like actinium’), but this has never found gen­eral acceptance. In these elements, the 5f electron sub­shell is progressively filled, leading to the generalized [Rn 7s25fn] configuration. Unlike the lanthanides, in which the 4f electrons lie in the interior of the xenon core region and thus hardly contribute to the chemical bonds (called ‘localized’), the 5f electrons show a much more diverse character, particularly in the metallic state.1 The 5f electrons in the elements thorium to neptunium are placed in the valence shell (often called ‘itinerant’ or ‘delocalized’) and show substantial covalent bonding, whereas the 5f electrons in the elements amer­icium to lawrencium are localized. Plutonium and americium have a transition position, showing both localized and delocalized behavior depending on tem­perature, pressure, and magnetic field.2

The actinides are radioactive elements, their iso­topes having strongly variable half-lives. Owing to the short half-life, compared with the age of the earth, majority of the actinides have decayed and cannot be found in nature. Only the long-lived isotopes 232Th, 235U, and 238U are of primordial origin, and possibly 244Pu. Also, 231Pa is found in very low concentrations in natural minerals (e. g., pitchblende ores), but it is a product of the 235U (4n + 3) decay chain.3 Most other actinides are man-made elements. They were synthe­sized by nuclear reactions using reactors and accelera­tors in the period 1940 (Np) to 1961 (Lr). The metals from Th to Cm are available in gram quantities that have allowed experimental determination of (some of) their physicochemical properties; Bk and Cf metals have been prepared in milligram quantities and Es in microgram quantities and therefore only limited inves­tigations have been possible. The metals Fm and beyond have not been prepared in pure form.

The main technological relevance of the actinides is their use as fuel for nuclear fission reactors, partic­ularly the nuclides 233U, 235U, and 239Pu, which fis­sion with thermal neutrons. 235U and 239Pu occur in the so-called U/Pu fuel cycle. 235U is present in 0.7% in natural uranium; 39Pu is formed when uranium is irradiated in a reactor as a result of neutron capture by 238U. 233U is formed by neutron capture of 232Th in the Th/U fuel cycle. The vast majority of nuclear power reactors use oxide fuel, but carbide and nitride as well metallic alloys fuels have been studied since the early days of reactor development.4

In this chapter, we discuss the physicochemical properties of the actinide metals, with emphasis on the elements Th to Cm for which experimental data on bulk samples generally exist. The trends and sys — tematics in the properties of the actinide series will be emphasized and compared with those of the 4f series. These physicochemical data are essential for understanding and describing the properties of mul­tielement alloys (see Chapter 2.05, Phase Diagrams of Actinide Alloys) and actinide containing com­pounds (Chapter 2.02, Thermodynamic and Ther­mophysical Properties of the Actinide Oxides).

The known gaseous actinide oxide molecules are listed in Table 15. Experimental data exist only from Th to Cm oxides. The thermochemical properties of these

gaseous species reviewed and compiled by Konings eta/.38 are listed in Tables 16 and 17.

The recommended vapor pressures over solid UO2, ThO2, PuO2, and liquid UO2 are given in the review by IAEA.2 2 The equation of state of uranium dioxide was investigated by Ronchi eta/.21

The main application of uranium carbides is as a fuel for nuclear reactors, usually in the form of pellets or

tablets, but also in nuclear thermal rockets, where their high thermal conductivity and fissile atom den­sity could be entirely exploited.

2.04.4.1 Phase Relationships

The most recent thermodynamic optimization of the U-C phase diagram is due to Chevalier and Fischer.10 An assessment of the uranium-carbon phase diagram is reported in Figure 11.

Blumenthal102 studied the constitution of low — carbon alloys in the uranium-carbon system and proposed three different structures for the pure metal. The observed transition temperatures are 940 ± 1.3, 1047.8 ± 1.6, and 1405.3 ± 0.8 K for the a-p, p-у transitions and melting point, respectively. The low-temperature solubility of carbon in uranium is low: <3ppm in a-uranium, <10 ppm in the p-U, and between 0.07 and 0.09 at.% in у-U. In the pres­ence of carbon, the system has a eutectic point at 1390 K and two eutectoid reactions at tem­peratures slightly lower than the pure crystal struc­ture transition. The solubility of carbon in uranium increases with temperature. A few studies on the solubility of carbon in liquid uranium between 1500 and 2800 K have been assessed in the following equation103:

105 109

68.129 — 5.2922 + 1.5347

T T2

1012 1014

— 1.9721— + 9.2191—

Stoichiometric uranium monocarbide is stable from room temperature to its melting point (2780 K). However, at high temperature (>1400 K), UC can exist in both hypostoichiometric and hyperstoichio­metric forms.104 It can accommodate both carbon vacancies and excess atoms by substituting a single carbon with two carbons. This behavior implies some variations in its lattice parameter.

At a higher carbon content, two more compounds are known to exist in the U-C system: U2C3 and

UC2-x.

If U2C3 is the thermodynamically stable phase until its peritectoid decomposition temperature (2106 K), it is normally not found in samples quenched from above this temperature, where UC and UC2 are identified instead. On the other hand, as explained in Section 2.04.1.2.3, U2C3, once produced, can be easily quenched to room tempera­ture. However, its thermodynamic stability below 1250 K is still controversial as some authors reported

3200

3000

2800

2600

2400

2200

2000

1800

1600

1400

1200

1000

800

600

400

Figure 11 The equilibrium U-C Phase diagram based on calculated and experimental data. p-UC2_x and UC1+x have the same face-centered cubic Fm 3m structure, and are completely miscible at high temperature, but display a miscibility gap up to 2323 K. Some authors identify these modifications as UC1+x’ and UC1+x" to distinguish the high and low — carbon boundaries of the miscibility gap.

the decomposition of UC + C at lower tempera­ture.105 This sesquicarbide has a body-centered (bcc) cubic structure of the Pu2C3 type (Table 1). The study of U2C3 presents important experimental issues, and results are often controversial and affected by low accuracy. Above the peritectoid temperature, U2C3 decomposes into UC1+x and p-UC2_:), A miscibility gap between these two phases has been determined by Sears106 by microstructure anal­ysis on quenched samples. Its low-temperature boundary corresponds to the peritectoid (2106 K) delimited by UC11 and UC17 and its maximum tem­perature is 2323 K at a composition close to UC13. The complex mechanisms of these transformations were described by Ashbee eta/.107 At higher temper­ature, UC1+x and p-UC2_j, are fully miscible, so that some authors108 identify them rather as UC1+x’ and UC1+x". Uranium dicarbide exists in two different structures, a a tetragonal form between 1753 and 2050 K, and a b cubic form at higher temperatures. UC2 decomposes so slowly upon cooling that it is normally observed as the stable phase in equilibrium with pure carbon at room temperature. It was
therefore decided to establish a ‘metastable’ ura­nium-carbon phase diagram, where U2C3 is left out and a-UC2 is the stable phase in equilibrium with UC and C at room temperature10 (Figure 12).

UC2 is hypostoichiometric. Its phase boundary in equilibrium with C varies from UC189 at the lowest temperatures to UC192 at the highest.8 Laugier108 based on some high-temperature XRD studies, pro­posed the decomposition of tetragonal UC2 into U2C3 below 1753 K and redefined the transition domain between UC2 and U2C3. The hypostoichio — metry domain of a-UC2 extends from the carbon- rich boundary to a phase limit in equilibrium with U2C3, which reaches UC177 at its maximum temper­ature (2057 K — Figure 11). At higher temperature, U2C3 is in equilibrium with p-UC2_x The martensi­tic transformation from a — to p-UC2 occurs at 2050± 20K. Bowman eta/.109 investigated the dicar­bide behavior by high-temperature neutron diffrac­tion. They showed that p-UC2 is of the type B1 KCN. This result rules out the CaF2 structure previously proposed by Wilson (based on high-temperature XRD analysis)110 and agree with the complete

3200

3000

2800

2600

2400

2200

2000

1800

1600

1400

1200

1000

800

600

400

Figure 12 The metastable U-C phase diagram.

miscibility of UC and UC2 at high temperature, already proven by many authors.4,111,112

The liquidus line presents two maxima between UC and UC2 at 2780 ± 20 K and 2730 ± 20 K cor­responding to the melting point of UC and UC19, respectively. A minimum temperature around 2675 K is observed between UC15 and UC16. Although the literature melting temperature data show some disper­sion, probably due to the sample impurities and alter­ation during the heat treatment, the points assessed by Chevalier and Fischer101 and confirmed by Utton eta/.113 seem reliable within the reported uncertainties. The liquidus and solidus lines are very close together at all compositions and can hardly be distinguished experimentally.

2.06.3.2.1 Properties

Although UIV is not stable in aqueous solution, UF4 is a very stable ionic solid that melts at 1309 K when many covalent tetrafluorides melt below 573 K and polymeric tetrafluorides (ZrF4, HfF4, SnF4, PbF4, …) melt around 1100 K. This is due to its high lattice energy of 12 970 kJ mol-1. UF4 is a green solid withmonoclinic symmetry, a = 12.73 A, b = 10.75 A,

c = 8.43 JA, and b = 126°20,(32) (Figure 6) and a den­sity of 6700 kg m-3. UF4 has a thermal conductivity of 1.96 W m-1 C — . In the granular form, the effec­tive conductivity can be as low as 0.2 W m °C-1. It has a small solubility in water (0.1 g l-1at 298 K). When exposed to air, it will form an hydrate UF4-2.5H2O after several days.

UF4 can be pyrohydrolyzed with water. The equi­librated reaction

UO2 + 4HF ^ UF4 + 2H2O

has been studied33 and is very significant above 673 K (Figure 7).

A compromise must be found for the synthesis of UF4 from UO2 with respect to the temperature. To obtain good kinetics, the temperature must be increased but to avoid pyrohydrolysis and UF4 sintering, it must be lowered.

2.06.3.2.1.1 Density

The density of liquid UF4 has been measured by Kirshenbaum and Cahill34 from 1309 to 1445 K. The results can be represented by:

p(kg m-3) = 7784 — 0.92 T T(K)

2.06.3.2.1.2 Viscosity

The viscosity of pure liquid UF4 was measured from 1323 to 1428 K by Desyatnik eta/.35 and from 1138 to 1618 K by Kulifeev and Panchishnyi.36 The results are scattered.35 They can be presented by the equation:

2.06.3.2.2 Thermodynamic properties

The thermodynamic properties of crystalline UF4 have been analyzed in detail by Fuger et a/.,37 Grenthe et a/.,24 and updated by Guillaumont et a/.39 The recommended values are reported in Table 2.

The thermal conductivity of the actinide metals varies strongly within the series. This is particularly true at low temperatures for which the data for a-Th and a-Pu differ by two orders of magnitude,
as shown in Figure 20. This trend is opposite to that for the electrical conductivity and is in line with the Wiedemann-Franz law that states that the ratio between thermal conductivity and electrical conductivity (s = 1/p) is a constant for any tem­perature (l/s = LT, where L is the Lorenz number, 2.44 x10—8W O K—2). One can notice that thermal conductivity of Pu at 100 K is the lowest reported for any pure metal (3.5Wm-1K-1).

Experimental data for high temperatures are known only for the major actinides Th, U, and Pu in a reasonable temperature range, whereas the mea­surement for Np is made close to room temperature

180 160 140 120

1100

q

80

Q.

60 40 20 0

(Figure 21). The recommended equations are given in Table 10. The values for Th, taken from the assessment by Touloukian and coworkers,76 show a slight increase with temperature. It should be noted that our graphs show a discrepancy between the low — and high-temperature data near T = 300 K, which is probably related to the purity of the samples, as it is known that the properties of thorium metal are highly sensitive to carbon impurities.73 The values for U, also from the assessment by Touloukian and coworkers,76 are based on a set of several concordant

measurements and cover the temperature range for the a-, p-, and g-phases but do not show distinct differences.

Thermal conductivity data above ambient tem­perature exist for all crystal phases of plutonium. The data for a-Pu from 100 to about 400 K were reported by Sandenaw and Gibney.40 However, the agreement with other values at ambient temperature is poor, which might be due to the differences in purity and to the accumulated radiation damage. Wittenberg and coworkers77,78 measured the thermal diffusivity (D) of the 8, 8′, and e phases from which they derived the thermal conductivity, which was found to be constant in all three cases. However, the numbers in the early publication78 for the thermal diffusivity are different from those in the later publi — cation.77 The values in Table 10 are taken from the latter work, which we consider to be the final results. Note that only the early values are cited in the

Gmelin review from 197 6.66 As discussed by Witten­berg, the data indicate that the thermal conductivity of the g — and 8-phases are nearly the same (13 ± 1) Wm-1 K-1. These trends are in qualitative agree­ment with the electrical resistivity measurements, as discussed in Section 2.01.4.2. Wittenberg also noted that the large decrease in the thermal conductivity of the e-phase is not expected to be comparable with the electrical resistivity measurements, and he sug­gested that this value may be too low as a result of the difficulty in maintaining good thermal contact after the volume contraction during the 8- to e-phase transformation.

Although the Wiedemann-Franz law states that the ratio between thermal conductivity and electrical conductivity is almost constant for metals, it was shown that the value for l/aT at T = 298 K varies regularly in the lanthanide series, as shown in Figure 22. The values for Th, U, and Np are close to the Lorenz value, and that of Pu is slightly higher. The values for Am and Cm in this figure are sugges — tions,79 assuming that the thermal conductivity of Cm is close to that of Gd.

These nitrides are also usually prepared by car — bothermic reduction of the oxides.22-24 As it is very difficult to prepare bulk samples due to their high radioactivity, there have been no systematic studies on their phase stability. However, it has been estab­lished that there is only mono nitride in these systems from the fact that no nitrogen absorption occurred upon cooling in nitrogen atmospheres during car — bothermic reduction. These mono nitrides have an NaCl-type face-centered cubic structure, and their lattice parameter ranges from 0.4899 to 0.5041 A, as shown in Table 1.24 The similarity in the crystal structure of these three nitrides, as well as uranium and plutonium nitrides, is advantageous as nuclear

fuels, especially as accelerator-driven system (ADS) targets of nitride solid solutions that contain a large amount of minor actinides (MAs). Experimental research on their vaporization behavior has revealed that the congruent melting temperature of NpN was 2830 °C.25 There are scarcely any data on the phase stability and other properties of pure CmN. Some data on Cm and U or Pu solid solutions have been reported, and these will be discussed in the next section.

bcc Pu2C3 is probably the most stable phase in the Pu-C system. Its lattice parameter was already measured to be a = 812.9 ± 0.1pm by Zachariasen in 1952.217 However, a certain scatter of data between samples in equilibrium with PuC1-x (minimum a = 812.10 ± 0.1pm21 ) and samples in equilibrium with graphite (maximum a = 813.4 ± 0.1 pm219) sug­gest the existence of a narrow nonstoichiometric homogeneity range, estimated as about 1 at.%.2 The high-temperature lattice constant was reported to be 834.0 pm at 1773 K.9 C2 pairs distance was established to be 139.5 ± 0.5 pm.213

Unlike PuC1-x, PuC15 is paramagnetic.211 Photoelectron spectroscopy showed that the main difference between the electronic structures of the
two carbides essentially consists of the partial occu­pancy of the 5f6 states in Pu2C3.211

The heat capacity of Pu2C3 was measured by Danan221 and Haines et a/.214 at low temperatures, starting from 10 K. Lower temperature measure­ments were hindered by the self-irradiation heating of Pu. For this reason, Einstein or Debye tempera­tures could not be obtained. Holley et a/4 assessed low — and high-temperature values based on these experimental data, plus the enthalpy data of Oetting215 (Figure 14 and Table 9).

Johnson eta/222 measured the formation enthalpies. Other thermodynamic parameters based on these and Fischer’s131 reviews are reported in Table 8.

2.04.6.2.2 Plutonium dicarbide

Plutonium dicarbide exists only at high temperature, between 1923 and 2513 K. It is difficult to quench the dicarbide to room temperature because the transfor­mation of PuC2 to Pu2C3 + C is extremely rapid. Nonetheless, its structure was identified to be tetrag­onal of the CaC2 type (I4/mmm), with a = 363 pm and c = 609.4 pm.206 Harper et a/.205 observed by high — temperature XRD that PuC2 undergoes a martensitic phase transition similar to a-UC2 !p-UC2 around 1983 K. The high-temperature structure was fcc Fm 3m (KCN) with a lattice parameter a = 570 ± 1 pm. Previous investigation performed on quenched samples could only identify the tetragonal phase.

Standard enthalpy and entropy of a-PuC2 at 298 K reported in Table 8 are extrapolated.9 AfG° for a-PuC2 (Figure 15) is calculated using the data recommended by Fischer and Holley et al. No data are available for |3-PuC2.

2.07.3.1 Nuclear Grade Zr Base Metal

The most frequently used ore is zircon (ZrSiO4), with a worldwide production of about 1 million metric tons per year, out of which only 5% is processed into zirconium metal and alloys.

The processing of Zr alloy industrial components is rather difficult because of the high reactivity of the Zr metal with oxygen. It consists of several steps to obtain the Hf-free Zr base metal for alloy prepa­ration: decomposition of the ore to separate Zr and Si, Hf purification, and Zr chloride or fluoride reduction.

2.07.3.1.1 Ore decomposition

Three different processes are currently used for the Zr-Si separation:

• In alkali fusion, where the zircon is molten in a NaOH bath at 600 °C, the following reaction takes place:

ZrSiO4 + 4NaOH! Na2ZrO3 + Na2SiO3 + 2H2O

Water or acid leaching allows the precipitation of ZrO2.

• The fluo-silicate fusion:

ZrSiO4 + K2SiF6 ! K2ZrF6 + 2SiO2

It produces a potassium hexafluorozirconate which, reacting with ammonia, leads to Zr hydroxide.

• The carbo-chlorination process is performed in a fluidized bed furnace at 1200 °C. The reaction scheme is the following:

ZrO2 (+SiO2 + HfOz) + 2C + 2Cl2

! ZrCU(+SiCl4 + HfCU) + 2C

The controlled condensation of the gaseous tetra­chloride allows the separation of Zr and Si, but not of Hf from Zr.

2.07.3.1.2 Hf purification and removal

The processes described above separate Si from Zr, but the Zr compounds remain contaminated with the initial Hf concentration. The high neutron capture cross-section of Hf (sa ~ 105 barn, compared to 0.185 barn for Zr) requires its suppression in Zr alloys for nuclear application. Two major pro­cesses are used for this step: the MIBK-thiocyanate solvent extraction and the extractive distillation of tetrachlorides.

• In the first case, after reaction of zirconyl chloride (ZrOCl2), obtained by hydrolysis of ZrCl4, with ammonium thiocyanate (SCN-NH4), a solution of hafnyl-zirconyl-thiocyanate (Zr/Hf)O(SCN)2 is obtained. A liquid-liquid extraction is per­formed with methyl-isobutyl-ketone (MIBK, name of the process). Hf is extracted into the organic phase, while Zr remains in the aqueous one. Hf-free ZrO2 is obtained after several other chemical steps: hydrochlorination, sulphation, neu­tralization with NH3, and calcination.

• In the dry route, after the transformation of zircon into its chloride ZrCl4, through the carbo — chlorination process, Zr and Hfare separated using a vapor phase distillation, at 350 °C, within a mixture of KCl-AlCl3, where the liquid phase is enriched in Zr, and the vapor in Hf.