c CA9600181 A AECL EACL AECL Research EACL Recherche

AECL-11118,COG-l-94-299

Alkali-Iodide/Urania Systems at High Temperatures I. Cesium Uranate Chemistry

Systèmes d'iodure alcalin/bioxyde d9uranium à hautes températures L Chimie des uranates de césium

J. McFarlane, J.C. LeBlanc

VOL 2 7 NS 1 7

September 1994 septembre AECL RESEARCH

ALKALI-IODIDE/URANIA SYSTEMS AT HIGH TEMPERATURES I. CESIUM URANATE CHEMISTRY

by

J. McFarlane and J.C. LeBlanc

Whitesheli Laboratories Pinawa, Manitoba ROE 1LO 1994 AECL-11118 COG-I-94-299 SYSTÈMES D'IODURE ALCALIN/BIOKYDR D'URANIUM k HAUTES TEMPERATURES I. CHIMIE DES URANATES DE CÉSIUM

par

J. McFarlane et J.C. LeBlanc

II est probable que les composés d'uranates se forment dans le combustible

Irradié à partir de la réaction entre l'UO2 et les produits de fission, le long des joints de grains d'UO2 et dans l'intervalle entre le combustible et la gaine. On a étudié la documentation sur la chimie à haute tempéra- ture des uranates de césium et rubidium. On examine les résultats des essais en cellules de Knudsen à une température de 900 à 2 600 K sur des uranates de césium. Ces études indiquent que les phases uranates dépendent du potentiel chimique de l'oxygène du système qui varie en fonction de la composition de la phase solide ou liquide.

Service de Chimie de recherche Laboratoires de Vhiteshell Plnava (Manitoba) ROE 1L0 1994 AECL-11118 COG-I-94-299 ALKALI-IODIDE/URANIA SYSTEMS AT HIGH TEMPERATURES I. CESIUM URANATE CHEMISTRY

by

J. McFarlane and J.C. LeBlanc

ABSTRACT

Uranate compounds are likely to form in irradiated fuel from the reaction between UO2 and fission products along UO2 grain boundaries and in the fuel-cladding gap. Literature on the high-temperature chemistry of cesium and rubidium uranates was reviewed. Results from Knudsen cell experiments from 900 to 2600 K on cesium uranates are discussed. These studies indicate that the uranate phases formed depend on the oxygen potential of the system, which varies with the composition of the condensed phase.

Research Chemistry Branch Whiteshell Laboratories Pinawa, Manitoba ROE 1LO 1994 AECL-11118 COG-I-94-299 CONTENTS

Page

1. INTRODUCTION 1

2. A REVIEW OF CESIUM URANATE CHEMISTRY 1

2.1 THERMODYNAMICS I 2.2 DIFFRACTOMETRY AND SPECTROSCOPY 5 2.3 DIFFERENCES BETWEEN CESIUM AND RUBIDIUM 7 URANATE CHEMISTRY 2.4 REACTIVITY OF Csl WITH URANIA^ 7

3. EXPERIMENTAL 8

4. RESULTS 10

5. DISCUSSION AND CONCLUSIONS 18

REFERENCES 20 1. INTRODUCTION

Speciation of fission-product cesium and iodine at high temperatures would affect their release from the fuel matrix, out of the fuel elements, and from the reactor coolant system into the containment building in the event of a reactor accident. The objective of this work is to study the chemistry that occurs in simple simulated fuel/fission-product systems and to determine accurate thermodynamic quantities for compounds that are formed at high temperatures. This thermodynamic data can be input into more sophisticated equilibrium chemistry models (Garisto 19S2, Imoto 1991) to predict chemical speciation in reactor accident scenarios.

2. A REVIEW OF CESIUM URANATE CHEMISTRY

2.1 THERMODYNAMICS

Phases of the cesium-uranium-oxygen system have a complex dependence on temperature and oxygen potential (Cordfunke et al. 1975). Ternary compounds produced in this system are called uranates, and are of interest to nuclear fuel chemistry (Cordfunke and Konings 1990). Fission-product cesium can react with fuel to form uranates, which are low-density compounds that can swell and deform the fuel cladding if they occur in sufficiently large amounts in the fuel-cladding gap (Fee et al. 1977, Johnson and Johnson 1974). Uranate formation may alter local oxygen potential in the fuel and affect the chemistry of fission- product release (Iyer et al. 1989). At temperatures and oxygen potentials in nuclear oxide fuel, the uranates of interest are the monouranate Cs2UO4, diuranate Cs2U2O7 and polyuranate Cs2U4O]2 (Cordfunke and Westrum 1979). As discussed later, more uranium-rich phases co- exist with U3Og rather than UO2+!l.

Experiments to determine the stability of uranate phases have yielded a number of conflicting results. The uranates are difficult to synthesize in a pure form (Prigent and Lucas 1965); many cycles of heating and then pulverizing the reagents are required, over a period of days. Examples of synthetic procedures are outlined in Table 1.

Once formed, the monouranate, Cs2UO4, is hygroscopic and if exposed even to traces of moisture will react differently from the dry compound at high temperatures, an example being the following reaction that occurs above 1000 K (Bharadwaj et al. 1983).

2Cs2UO4 + H2O * Cs2U2O7 + 2CsOH (1)

Hence, many of the experiments leading to results reviewed here were carried out in an inert atmosphere and with an excess of cesium metal (Fee and Johnson 1978), a combination that served the additional purpose of minimizing the oxygen potential of the system. -2-

TABLE 1

SYNTHESIS OF CESIUM URANATES

Final uranate phase Synthesis Reference

Cs2UO4,Cs2UA(a,frY), VOi{m) + Cs2CO3 Cordfunke et al. 1975 Cs2U4OI3, Cs2U5O16, (air, 873 K, 1 week, Efremova et al. 1961 Cs2U7O22, Cs^.sO^ final phase dictated by Cs/U ratio of starting materials)

Cs2UO4 UOj»0.8H20 + CsNO3 O'Hare and Hoekstra (air, 1023 K, 7 d) 1974(a)

Cs2U4Ol2 Cs2U4O,3 decomposition Cordfunke and Westrum (argon, 1073 K) 1979

Cs2U4OI2 (a,p,y) UO3(am) + CsjSO^ Van Egmond 1975 (1423 K)

Cs2U4OI3 UO3(am) + CsCl{1) Van Egmond 1976(a) (oxygen, 1173 to 1223 K)

Cs4U5O17 UO3(8m) + CsCl(I) Van Egmond 1976(b) (air, 973 K, 120 h)

= amorphous, , = liquid

Most of the research into uranate thermodynamics has been carried out at the Netherlands Energy Research Foundation and at Argonne National Laboratories. Thermodynamic heat capacity data at low temperatures, 5 to 350 K, have been collected using an adiabatic calorimeter for Cs2U4O,2 (Cordfunke and Westrum 1979), Cs2U2O7 (O'Hare et al. 1981), and Cs2UO4 (Osborne et al. 1975, 1976). The entropy increment data, S°T - S°2,81jK, from the low- temperature adiabatic heat-capacity measurements, yield the standard entropy of formation of the uranates at 298.15 K, AS°f(2,g_,5K). Drop calorimetry has been used to collect heat-capacity and enthalpy-increment data at high temperatures: up to 1000 K for Cs2U4O12 (Cordfunke and Westrum 1979), to 1060.7 K for Cs2UO4 (Fredrickson and O'Hare 1976), between 1033 and 1105 K on the Cs2U4O,2/Cs2U4O13 system (Iyer et al. 1990) and the rubidium polyuranates, Rb2U4OI2 and Rb2U4O13 (Iyer et al. 1990).

From enthalpies of solution measurements and auxiliary thermodynamic values, standard heats o of formation at 298.15 K, AH f(29g)JK), have been determined for the uranates: for Cs2U4O12 3-

(Cordfunke and Westrum 1979), Cs2UO4 (O'Hare and Hoekstra 1974(a), Cordfunke et al. 1986) and Cs2U2O7 (O'Hare and Hoekstra 1975); and for Li2UO4, Rb2UO4 and K2UO4 (O'Hare and Hoekstra 1974(b)). The accuracy of the heats of formation depends on the reliability of the auxiliary thermodynamic data, and so the most reliable heats of formation have been arrived at by at least two independent reaction pathways (Cordfunke et al. 1986). Combined 0 with AS f{298l5K), AH°f(M8,JK) will give the standard free energies of formation at 298.15 K, AG°f<298i5K) (Osborne et al. 1975).

Another thermodynamic parameter, the temperature dependence of oxygen pressures above uranate mixtures in equilibrium, has been measured electrochemically. The electromotive force, (EMF) of a cell is a measure of the Gibbs free energy of reaction, which is related to the oxygen potential at equilibrium conditions. The pressure of oxygen can be determined even if the cesium partial pressure represents another degree of freedom of the system, as long as the activity of cesium metal is known, e.g., by performing the experiment in an excess of liquid cesium (Adamson et al. 1980). Measurements of EMF were recorded for a mixture of Cs2U4O12 and UO2 between 974 and 1024 K against a Ni/NiO reference electrode (Cordfunke 1974). The equilibrium

Rb2U4Ol3 - Rb2U4O12 + >/2O2 (2) was studied in a solid oxide electrolyte galvanic cell between 1019 and 1283 K (Iyer et al. 1989, 1990). The analogous cesium polyuranate equilibrium has also been investigated electrochemically between 1050 and 1200 K (Une 1985). Electrochemical measurements have accompanied thermogravimetric analysis, such as the measurement of oxygen potential using a yttria-stabilized zirconia electrode in a sample of UO2 being titrated with gaseous cesium at 1073 K (Fee et al. 1977). A similar apparatus has been used to study the kinetics of the reaction of Cs and UO2+X (Fee and Johnson 1981 (a)).

Other techniques used to study uranate systems e.g., UO2+x/Cs2UO4, include differential thermal analysis to look at phase transitions (Iyer et al. 1989) and Knudsen cells to investigate equilibria at high temperatures, (Johnson and Steidl 1972).

Cs2UO4(s) - 2Cs(g) + (1 - 72)O2(g) + UO2+X(S) (3)

As early reviews (Fee et al. 1977, Fuger 1985) and a recent thermodynamic tabulation of uranate chemistry already exist (Cordfunke and Konings 1990), the discussion here is limited to an overview of those uranate phases that are expected to participate in chemistry occurring at the fuel-cladding interface, at oxygen potentials less negative than -360 -4-

Phase diagrams presented in the literature (Fee and Johnson 1978, Lorenzelli et al. 1980) show the relationships between cesium and uranium and their dependence on oxygen potential and temperature of the system. These studies assume the behaviour of the system is independent of applied pressure. Both two and three phases can exist between solids in equilibrium (Fee et al. 1977). The regions that show two solid phases in co-existence, joined by a tie line, have two degrees of freedom (i.e., two of temperature, oxygen partial pressure or cesium partial pressure). The regions that have three solid phases in equilibrium only have one degree of freedom. For instance, specifying the temperature also specifies the oxygen potential of the system. Two uranate phases can be in equilibrium with UO2+X and gaseous cesium in oxide fuel between 873 and 1273 K, Cs2UO4 and Cs2U4Oi2 (Cordfunke and Westrum 1979), at lower and higher oxygen potentials respectively (Fee et al. 1977, Fee and Johnson 1981(b)). The cesium diuranate does not have a tie line with UO2+I, and so must convert to another uranate phase before decomposing to urania. A tie line exists between Cs2U4O12 and U3Og, and hence there is no equilibrium between UO2+![ and a cesium uranate with a higher U/Cs ratio than this polyuranate (Fee and Johnson 1978). Examples of two- and three-phase equilibria are given below.

2-phase equilibria

Cs UO «« U0 2Cs (4) 2 4 2 + (g) + o2

Cs2U4O12 •* 4UO2 + 2Cs(g) + 2O2 (5)

3-phase equilibria

Cs2U4O12 «» Cs2UO4 + 3UO2 + 0, (6)

Cs2U2O7 •* Cs2UO4 + uo2 + VlO (7)

Cs2U4O12 * Cs2U2O7 + 2UO2 + ViO2 (8)

Most stable cesium uranate phases can be considered as (Cs2O),,»(UO3)yt where x and y are integers. A notable exception is Cs2U4O,2, which can be written as (Cs2O)»(UO2MUO3)3. There has been much controversy concerning the stoichiometry of reduced cesium uranate compounds, i.e., phases containing uranium(V) or (IV)) that exist in equilibrium with liquid cesium and urania at very low oxygen potentials. Earlier studies suggested the presence of Cs, 3UO3 (Aitken et al. 1974), Cs^O^ (Cordfunke and Westrum 1979) where x = 0.5, or Cs05+xUO3 (Fee et al. 1977) where 0 < x < 0.5. These discrepancies have been attributed to oxidation of the uranate phase, after distillation of excess liquid cesium (Cordfunke et al. 1975). A consensus was reached that a Cs2UO3J6 (approximately Cs4U2O7) phase in equilibrium with liquid cesium could be formed, but at oxygen potentials more negative than would be expected in most used oxide fuel (Fee and Johnson 1981(a),1981(b)), except that used in liquid metal fast breeder reactors (LMFBRs) (Schumacher and Wright 1985). A pure 5- cesium-uranium(IV) oxide is unlikely to occur because of the thermodynamic stability of the UO2 lattice.

Studies of cesium and cesium uranates in oxide reactor fuels, UO^ and (tyPiOO^, have been performed. In hyperstoichiometric urania, the uranate phases that form will be in equilibrium with a uranium-oxygen phase, UO2+X where 0.000 < x < 0.010 (Fee 1978), unlike in a pure uranate system where decomposition of one uranate to another may be observed as it is heated, i.e., Cs2UO4-> Cs2U2O7-» Cs2U4OI2 (Cordfunke 1974). A reduced cesium uranate was observed in hypostoichiometric (U, Pu)O2.x above 923 K below an oxygen potential of -577 kJ»mol' (Aitken et al. 1974), and also in LMFBR fuel by Adamson and co-workers (1980). At higher oxygen potentials, Cs2UO4 was observed. The region of stability of Cs2UO3 56 between 800 and 3000 K was calculated and compared with the equilibrium oxygen potential of hypostoichiometric mixed oxide fuel (Schumacher and Wright 1985). The effect of gadolinium in solid solution with UO2 on uranate formation was studied between 673 and 973 K (Une 1987). Rare-earth solutes can change the oxygen potential of urania of a given O/U ratio.

Variations in oxygen potential have been used to explain the heterogeneous distribution of fission product cesium across a fuel pellet. At the outer perimeter of the pellet a high concentration of cesium is observed. This has been attributed to uranate and molybdate formation in a region of higher oxygen potential (Schumacher and Wright 1985) However, the distribution has also been explained by diffusion models: atomic diffusion inside a grain to cracks, pores and boundaries, followed by thermal migration in defects to the coolest region of the fuel pellets, the perimeter (Imoto 1991).

2.2 DIFFRACTOMETRY AND SPECTROSCOPY

X-ray diffraction (XRD) has been a valuable tool in the study of the cesium-uranium-oxygen phase diagram, both to confirm synthesis and to identify phases after isothermal heating experiments (Fee et al. 1977, Fee and Johnson 1978). A systematic study of the synthesis of stable cesium uranate phases, followed by identification by XRD, has been carried out by Van Egmond (1976(d)). These have been published for Cs2U4O12 (Van Egmond 1975), Cs2U5O,6 and Cs2U4OB (Van Egmond 1976(a)), Cs2UO4, Cs4UjOn, Cs2U7O22 and Cs^O^ (Van Egmond I976(b)), and Cs2U2O7 (Van Egmond 1976(c)). Crystallographic d-spacings have been documented for rubidium uranate phases by Ippolitova and co-workers (1961), and by Kovba and Trunova (1971). Investigation of the rubidium uranates was later repeated by Van Egmond and Cordfunke (1976). They also studied the potassium uranates.

Based on XRD data, most cesium uranate phases have structures related to U3Og, with pseudo-hexagonal uranium-oxygen layers sandwiching cesium atoms. The reduced uranate, Cs2U4O12, however, has a structure closer to that of UO2, with cesium atoms substituted for half of the uranium atoms (Van Egmond 1975) and the uranium-oxygen coordination being octahedral rather than cubic. The monouranates, Cs2UO4, Rb2UO4, and K2UO4, form -6- tetragonal uranium-oxygen layers. The structures of stable Cs2U2O7 phases, a and p, contain uranyl-oxygen layers of both hexagonal (Kovba et al. 1974) and tetragonal arranged uranium atoms (Van Egmond 1976(c)).

X-ray photoelectron spectroscopy (XPS) can be used to analyze for uranium (VI) uranates in the presence of excess uranium (IV), i.e., UO2+X, because the uranates do not have a uranium 5f electron (Al Rayyes and Ronneau 1991). An early XPS study of several uranium (VI) 6 compounds, most containing UO^, showed that the U P3/2 crystal field splittings were inversely dependent on the uranium-oxygen bond length (Veal et al. 1975), and were little affected by changes in ligand. Indeed, the XPS spectra of Cs2UO4 and Cs2U2O7 are very similar (Al Rayyes and Ronneau 1991). However, substituted cesium uranates, i.e., Cs2UO2X2 where X = Cl or Br, were distinguished by the spectra of the inner shell uranium orbitals (Baev et al. 1987).

Infrared spectra have been collected on solid uranate phases, primarily the monouranate, associated with a number of different metal cations. Assigned infrared bands are listed in Table 2 below. No vibrational spectroscopic information was found for high-temperature gas- phase or matrix-isolated uranates.

TABLE 2

VIBRATIONAL MODES FOR URANATE COMPOUNDS

Uranate Compound IR Frequencies Notes Reference (cm1)

Cs,UO4 727, 435, 280 A lattice vibration expected Hoekstra 1965. Ohwada 1970 below 200 cm-1

Rb2UO4 726, 490, 270 A lattice vibration expected Hoekstra 1965. Ohwada 1970 below 200 cm-1

KjUO, 732, 520, 265 A lattice vibration expected Hoekstra 1965, Ohwada 1970 below 200 cm-1

BaUO4 770, 728, 682. 490 415. 395, Appel et al. 1990 370, 305 295. 287. 280, 270 262

SrUO, 790,754,698.495 434,412, Assigned Bickel and Kanellakopulos 370, 280 270, 255, 245, 240 frequencies based 1990 on Appel above

Cs2U2O, (a,P) Unassigned frequencies not Mijlhoffetal. 1993 listed

mixed uraniumfV) uranates 4000 to 40000 Kemmler-Sack 1968 cm"' -7-

Mass spectroscopy has been used to observe the vapour above Cs2UO4 contained in an indium effusion cell (Bose et al. 1985). Between 1000 and 1573 K cesium ions + + + predominated, with small amounts of CsO , Cs2O and Cs2 . The measured second-law 1 apparent heat of vaporization over this temperature range, 340 kJ'mol' Cs(g), was explained by the following reaction

Cs2UO4 - Cs2U4O12 + 6 Cs(g) + 2O2 (9)

2.3 DIFFERENCES BETWEEN CESIUM AND RUBIDIUM URANATE CHEMISTRY

Cesium and rubidium are expected to behave in a similar manner chemically at high temperatures in irradiated fuel; however, there are differences in the phase diagrams of the rubidium-uranium-oxygen and cesium-uranium-oxygen systems. The uranium(V) species, CsUO3, has not been observed, unlike the other alkali uranates, MU03, where M = Li, Na, K, and Rb (Van Egmond 1976(d), Fee and Johnson 1978, Cordfunke and Ouweltjes 1981, Van Egmond and Cordfunke 1976). The apparent instability of CsUO, has been explained by the large size of the cesium ion (Fee 1978). The reduced uranate Cs2U4O,2 can exist in equilibrium with UO2+J(, and is synthesized by reduction of Cs2U4O,j at 1073 K. Attempts to synthesize the corresponding Rb2U4Ol2 were unsuccessful (Van Egmond and Cordfunke 1976), until Chawla and co-workers (Chawla et al. 1988) prepared M2U4OI2 compounds of potassium, rubidium and thallium. These compounds, analyzed as solids by XRD and in solution by infrared spectroscopy, are not isostructural with Cs2U4O12. Since then, the free energy of formation of Rb2U4O,2 has been obtained at 1000 and 1200 K (Iyer et al 1989, 1990).

2.4 REACTIVITY OF Csl WITH URANIA

In comparison with the extensive literature that exists on the reactivity of cesium metal with urania, few studies have focussed on the interaction between cesium compounds and urania at high temperatures. Mixtures of Cs2CO3, CsOH and urania were heated in a Knudsen cell and the vapour-phase cesium ions were detected using a mass spectrometer (Collins et al. 1976). Uranate formation was suggested to explain the low vapour pressure of cesium observed in this study.

Recently, threshold oxygen potentials for reaction of UO2+X and Csl were reported at 1073 K (Ugajin 1992). Mixtures were heated in contact with CO2/CO and product phases were analyzed using XRD. Below an oxygen potential of -255 kJ«mor\ Cs2UO4 was observed. At less negative oxygen potentials, the reduced polyuranate Cs2U4O12 was produced. The 1 polyuranate is stable to an oxygen potential of -301 klrniol" in the presence of MoO2. Oxygen pressures for the following equilibria were calculated from 573 to 1373 K. -8-

4UO2 -i- 2CsI •f 2O2 -- Cs2U4O12 + 21 (10)

U02 •i- 2CsI •f 02 •- Cs2UO4 + 21 (11)

4UO2 + 2CsI + Mo(U-saturated) + 3O2 * Cs2U4O12(Mo-saturated) + MoO2(U-saturated) + 21 (12)

The iodine pressures were assumed to be those above a binary cesium-iodine system. The calculated oxygen potentials were in reasonable agreement with the uranate phases observed to be formed at 1273 K.

Cesium is a much more abundant fission product than iodine (Aitken et al. 1974). In irradiated fuel, cesium may combine with other species in the following order: iodine, tellurium, urania and molybdenum (Cordfunke 1974). Fission-product iodine is more likely to form Csl in the fuel grain boundaries and cladding gap than iodides of barium or strontium

(Rest and Cronenberg 1987). Based on thermodynamic arguments, Cs2MoO4 should be the most abundant cesium compound; however, fission-product molybdenum is mostly located in metallic precipitates and not as a stable oxide available for reaction with cesium metal (Imoto 1991). The formation of cesium molybdates and uranates would affect the stability of Csl at high temperatures (Rest 1983), and in turn the release of volatile iodine as I2 (Kohli 1986). The objective of this current study is to determine the links between Csl/urania chemistry and the detailed knowledge of the Cs-U-0 phase diagram.

3. EXPERIMENTAL

Second-law heats of vaporization were measured in a Knudsen cell apparatus shown in Figure 1 (McFarlane et al. 1991). The apparatus was evacuated with a corrosion-resistant turbomolecular pump with magnetically levitated bearings to below 10s Pa. Samples were contained in an inductively heated tantalum cell supported on tungsten rods within an RF coil made with 6-mm-diameter copper tubing. This assembly can be heated to 3000 K by a Radyne 17-kW power supply, but was rarely operated above 2800 K. The tantalum cell was constructed with a removable lid, into which an orifice 0.6 mm in diameter was drilled. Temperature measurements were recorded on the side of the cell through a sapphire window by three Ircon dual-wavelength optical pyrometers, over the temperature range from 900 to 2800 K. The sapphire window was shuttered to prevent residue accumulation from affecting the temperature readings. -9-

Optical Pyrometers

1H

Sapphire Window- lonizatlon Gauge Shutter-

Quadrupole Mass Spectrometer 10-s pa

f H Diffusion Pump 1.6-mm-dla. Aperture Tantalum Knudsen Cell ^ Mirror Sapphire Window Shutter ^->. Cold Cathode y_ Gauge

"Cooled Shield To Induction Heater

FIGURE 1 : Schematic of Knudsen Cell Apparatus - 10-

Volatiles effusing from the cell were ionized by electron bombardment at 45 eV, and detected with a Finnigan 3200 quadrupole mass analyzer. The mass spectrometer was evacuated with a diffusion pump and was separated from the Knudsen cell by a 2-mm-diameter orifice. This orifice was blocked by a shutter for background scans. Mass spectra were collected on a Compaq 386 computer using Shrader data acquisition software.

The cesium diuranate sample was prepared from a mixture of Cs2CO3 and UO3 that underwent four cycles of heating to 1073 K, cooling and powdering, over a period of two weeks. An XRD analysis of the product gave a composition of 47% p%Cs2U2O7 and 26% (X-Cs2U2O7, with the remainder being a mixture of unidentified uranate phases.

4. RESULTS

In a Knudsen cell experiment, the gas phase in equilibrium with the vapour is sampled. Second-law heats of vaporization were computed using the integrated form of the Clausius- Clapeyron equation (Atkins 1978).

1 1 1 ln(P,-P2) =-AHïap.R .(T1 -T2 ) (13)

In the experiment involving the volatilization of cesium diuranate, Cs2U2O7, single and doubly charged ions of cesium only were observed between 1150 and 1600 K. As in the case of the volatilization of Cs2UO4 by Bose and co-workers (1985), no uranium species were seen until higher temperatures were reached. Experimental results are presented in Table 3. Plots of log(IT), where I is the ion signal, proportional to the partial pressure of the vapour phase are presented in Figures 2 and 3, as a function of T\ for cesium and uranium-containing ions respectively.

TABLE 3

EXPERIMENTAL RESULTS FOR VAPOUR PHASE ABOVE Cs,U?O7

Ions Observed Apparent AHvap Appearance (kJ-moï1 ) Temperatures (K) •Cs+, Cs+ 260 ± 10 1150-1600 "U0,\ U0+, U\ 506 ±6 2240-2820 UO , U 2, • Cesium isotope observed was l"Cs. " Uranium isotope observed was 238U. -11 -

10000/T(K) + FIGURE 2: Arrhenius Plot of Cs (IT 133) and Cs~(IT 66.5) Seen from Heating Cs2U2O7 in a Knudsen Cell. The slope of log(IT) as a function of 1/T, where I is the ion current, is proportional to the apparent heat of vaporization of neutral species giving rise to the ions.

102 3.8 3.9 4 10000/T(K) + + + FIGURE 3: Arrhenius Plot of UO2 (IT 270), UO (IT 254), U (IT 238), UO^IT 135), UCr(IT 127) and U^(IT 119) Seen from Heating Cs2U2O7 in a Knudsen Cell. The slope of log(IT) as a function of 1/T is proportional to the apparent heat of vaporization of neutral species giving rise to the ions. - 12-

Using the phase diagrams and thermodynamic data presented earlier, it was possible to predict the cesium vapour pressure and apparent heat of vaporization that should be observed for various uranate interconversions as a function of temperature. In this manner, the dominant chemical process occurring in the experiment was identified. A series of calculations have been performed for Equilibria (4) through (8) to give cesium pressure as a function of temperature. Assumptions accompanying these calculations are that the solubility of cesium in UOJ+Il or the uranates is very low and that corrections in the AG°,(T) of UO2 because of non-stoichiometry are very small, within the estimated uncertainty of thermodyn'amic values for cesium uranates (Fee et al. 1977). In none of these systems is a uranate molecule volatilized intact. The results are plotted in Figures 4 through 8.

In the first group of Figures, 4 through 6, depicting three-phase equilibria, the calculated oxygen potential is plotted as a function of temperature. Along with the oxygen potential of the uranate equilibrium, the oxygen potential of UO2+X, 0.001 < x < 0.010, is plotted as a function of temperature (Lindemer and Besmann 1985). In Knudsen cell studies with Csl/urania mixtures, UO2+Il is in excess, and hence should establish the oxygen potential ins.de the cell. The range of O/U ratio used in these calculations is that expected in used fuel; hence, these plots allow a prediction of which urania equilibria are important in Knudsen cell studies and in reactor fuel.

-200 on) s 2.010 -220 O/U s 2.005 -240 -260 i -280 s. -300 I -320 -340 O/U s 2.001 -360 200 700 1200 1700 2200

Temperature (K)

FIGURE 4: Oxygen Potential is Calculated as a Function of Temperature for the Equilibrium Cs2U4OI2 « Cs2UO4 + 3UO2 + O2 and for UO2+l, 0.001 < x < 0.010 - 13-

-100

-150 - o -200 O/U = 2.010

To ^^^^^ O/U a ZOOS -250 -^—""__^»-

^^- I -300 ^-r-^~. (D z.——-~ . — o/u=z901 2 -350 .—. • X O -400 200 700 1200 1700 2200 Temperature (K) FIGURE 5: Oxygen Potential is Calculated as a Function of Temperature for the Equilibrium Cs2U2O7 «* Cs2UO4 + UO2 + 14O2 and for UO2tx, 0.001 < x < 0.010

-200 O/U s 2.010 -220 O/U = 2.005 -240 -260 I -280 § a. -300 œ I -320 -340 O/U = 2.001 -360 200 700 1200 1700 2200 Temperature (K) FIGURE 6: Oxygen Potential is Calculated as a Function of Temperature for the Equilibrium Cs2U4O12 * Cs2U2O7 + 2UO2 + J*O2 and for UO2+X, 0.001 < x < 0.010 - 14-

Plots for two-phase systems, Figures 7 and 8, show calculated cesium pressures as a function of temperature for three different controls of oxygen potential. Depending on experimental conditions of oxygen potential, the cesium pressure and temperature coefficient of the Arrhenius plot can vary significantly. Plots are drawn here for the oxygen potential established by UO,™ 9 UO and for a background of 2.6 x 10" kPa O2 in the cell. In the Knudsen cell experiment the detectable range of a vapour species starts at about 10"5 kPa. It can be seen from these plots that the temperature at which this pressure is reached is very dependent on oxygen potential

102

10°

CD 2 O/U = 2.001 Q. io- (0 / O/U = 2.010 O 4 o io- l ID

10' pO2 = 2.6x10 "9 kPa 10r10 500 1000 1500 2000

Temperature (K)

FIGURE 7: Cesium Pressure is Calculated as a Function of Temperature for the Equilibrium Cs2UO4 « UO2 + Cs ,g) + O2. The oxygen pressure is set by UO2+I, x = 0.001 and 0.010, or set at 2.6 x 109 kPa. - 15-

102

10° O/U = 2.001 O/U = 2.010 C 102 S, 8 10"4 s 1 10"6 10'8 pO2 =2.6x10" kPa 10"10 200 700 1200 1700 2200 Temperature (K) FIGURE 8: Cesium Pressure is Calculated as a Function of Temperature for the Equilibrium Cs2U4Oi2 * 4UO2 + 2Cs w + 2O2. The oxygen pressure is set by UO2+X, x = 0.001 and 0.010, or set at 2.6 x 10"9 kPa.

It is worth noting that the cesium ions observed from a reacted Csl/urania mixture may appear at a different temperature than the cesium ions observed in these diuranate studies. In some two- phase regions, e.g., Equilibria (4) and (5), cesium pressure will be lower under more oxidizing conditions. For instance, calculations show that above 1300 K, the partial pressure of oxygen for Equilibrium (4) is higher where control of the oxygen pressure is by an excess of UO2008 and not by the ambient O2 in the evacuated cell.

The appearance temperature of the cesium ions corresponds to that predicted by decomposition of either Cs2UO4 or Cs2U4O12 to UO2. The cesium diuranate likely disproportionates to these uranate phases by Equilibrium (14).

3Cs2U2O7 Cs2U4O,2 + 2Cs2UO4 (H)

Calculated equilibrium oxygen potential as a function of temperature has been plotted in Figure 9 for this process. Equilibrium (14) would be allowed to shift to the right above 1100 K under Knudsen cell ambient vacuum conditions, an oxygen pressure of 2.6 x 10"9 kPa. The uranate phases formed would subsequently decompose to UO2, Cs(g) and O2(g). At an oxygen pressure of 2.6 x 10"9 kPa, the monouranate and polyuranate would be able to establish the Equilibria (4) and (5) respectively. -la-

I o

-400 200 700 1200 1700 2200 Temperature (K) FIGURE 9: Oxygen Potential is Calculated as a Function of Temperature for the Equilibrium 3Cs2U2O7 - Cs2U4Ol2 + 2Cs2UO4 + 0.5O2

The above explanation of diuranate decomposition was tested by comparing measured to calculated second-law heats of vaporization. Table 4 gives the apparent heats of vaporization that would be observed from several cesium uranate equilibria around 1100 K. If calculations were performed in specific conditions of oxygen potential, these conditions are also noted. It is evident that the oxygen potential would affect the cesium vapour pressure, and, hence, the measured second-law apparent heat of vaporization. This is likely the reason why the second-law heat measured for the uranate mixture is less than all of those listed in the table. - 17-

TABLE 4

HEATS OF VAPORIZATION OF CESIUM FROM URANATE EOUILIBRIA

Equilibrium AHvap Conditions(pO2, Temp(K)) (kJ-mol1 )

Cs2U4O12 •• 4UO2 + 2Cs(g) + 2O2(g) 336 pO2 = pCs, HOOK

Cs2U4O12 * 4UO2 + 2Cs(g) + 2O2(g) 323 excess UO2+X, O/U =2.010, 1100- 1600 K

Cs2U4Ol2 * 4UO2 + 2Cs(g) + 202{g) 280 excess UO2+,, O/U = 2.001 800- HOOK

9 Cs2U4Ol2 * 4UO2 + 2Cs(g) + 2O2(g) 664 pO2 = 2.6xl0 kPa 1100- 1400 K

Cs2UO4 - UO2 + 2Cs(g) + O2(g) 327 pO2 = 0.5(pCs), HOOK

Cs2UO4 «• UO2 + 2Cs(g) + O2(g) 316 excess UO2+X, O/U = 2.010 1100- 1400 K

Cs2UO4 * UO2 + 2Cs(g) + 02{g) 310 excess UO2+X, O/U = 2.001 1000 - 1200 K

9 Cs2UO4 «* UO2 + 2Cs(g) + O2(g) 489 pO2 = 2.6xlO kPa 1200 - 1400 K

2Cs2U2O7 « Cs2U4O12 + 2Cs(g) + O2(g) 339 pO2 = 0.5(pCs), HOOK

4Cs2UO4 « Cs2U4O,2 + 6Cs(g) + 2O2(g) 322 pO2 = 0.33(pCs), HOOK

2Cs2UO4 * Cs2U2O7 + 2Cs(g) + 0.5O2(£) 312 pO2 = 0.25(pCs), HOOK

Above 2240 K, ions from the volatilization of UO2 are observed as shown in Figure 3. The second-law heat of vaporization measured for these ions is lower than the 592 kJTnol'1 calculated for

UO'2(s. ) UO2(g) (15) - 18- at 2000 K using the data from the compilation of Cordfunke and Konings (1990). The low value probably arose from a shift from effusive toward hydrodynamic flow from the Knudsen cell as the pressure increased in the cell at the highest temperatures.

5. DISCUSSION AND CONCLUSIONS

The literature review of uranate chemistry at high temperatures yielded two implications for further experiments into alkali iodide/urania chemistry. One is the importance of controlling the oxygen potential in the system under study. Metal/metal oxide buffers (Cr/Cr?O3, CrxC/Cr203, Fe/FeO, Mo/Mo02, Ni/NiO, and Cu/Cu2O) have been used to control the oxygen potential in experiments involving cesium and mixed oxide fuel (U, Pu)O2 (Aitken et al. 1974).

Another consideration is the importance of reaching chemical equilibrium in the Knudsen cell. The transformation of one uranate phase to another can be very slow even at high temperatures, requiring up to a week at 873 K (Cordfunke 1974). Iyer and co-workers (1990) found equilibration kinetics were slow in the Rb2U4O12 and Rb2U4OI3 system. However, the kinetics of the reaction between cesium and UO2+X

xCs(g) + UO2+X(C) -> x/2Cs2UO4 + ((2-x)/2)UO2(c) (16) were found to be rapid (Fee and Johnson 1981 (a)) between 600 and 1070 K. This result was used to explain the inhomogeneous distribution of cesium found in used fuel. Although diffusional release of cesium (Imoto 1991) from the hot central region of fuel is an alternate explanation for the observed cesium redistribution during irradiation. Because of the conflicting reports in the literature, experiments that monitor the approach to equilibrium in alkali iodide/urania systems heated in an inert atmosphere are planned.

While cesium diuranate can exist in conditions of temperature and oxygen potential found in reactor fuel, it is likely a minor component. The monouranate, rather than the diuranate would be the first uranate phase to form from gaseous fission-product cesium and UO2+]l (O'Hare et al. 1981). In addition, at high temperatures, the diuranate would decompose to the mono and polyuranates, as suggested by our experimental results.

Recently, the Knudsen cell has been calibrated for absolute vapour pressure measurements, a process that has involved considerable effort. The value of being able to record partial pressures directly is indicated in the results for the diuranate experiments. Cesium pressures depend on the oxygen potential, in addition to the temperature, in two-phase systems. Hence, absolute pressure measurements are required for accurate determination of the temperature dependence of the Arrhenius plot, knowledge that is critical to a full understanding of the chemical system being studied. -19-

ACKNOWLEDGEMENTS

The authors would like to thank Dr. Norman H. Sagert, who initiated this project, and Drs. Peter Taylor and William Hocking for reading and commenting on the manuscript. The project was partially funded by the CANDU Owners Group (COG). -20-

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