Metallic phases precipitated in UO2 fuel

Journal of Nuclear Materials 151 (1988) 318-326 North-Holland. Amsterdam

318

METALLIC PHASES PRECIPITATED I. Phases in simulated fuel

T. MUROMURA, Department

of Chemistry,

T. ADACHI,

IN UO, FUEL

H. TAKEISHI,

Z. YOSHIDA,

Japan Atomic Energy Research Institute,

Tokai-mura,

T. YAMAMOTO Ibaraki-ken,

and K. UENO

319-11, Japan

Received 20 April 1987; accepted 15 October 1987

The chemical state of solid fission products (FPs) was studied using simulated spent fuels of 5-30% FIMA, which were made by the heat-treatment at 1273 to 2273 K under various oxygen potentials. The phases formed in the fuel were identified by X-ray diffraction. Under the oxygen potential below -350 kJ/mol 0, at 1673 K, fluorite and perovskite phases were produced with the metallic phases of (Yand c in the fuel of 10% FIMA. At the potential between - 250 kJ/mol 0, and - 340 kJ/mol 0, three oxide phases, fluorite+perovskite+ scheelite, were formed with three metallic phases, OL + c + 0. Above - 270 kJ/mol 0, the scheelite and metallic phases (n + z) were precipitated in the fuel matrix. The lattice parameter of the perovskite gradually decreased with oxygen potential from 0.420 nm at - 540 kJ/mol 0, to 0.412 nm at - 270 kJ/mol 0,. There was an abrupt change in lattice parameters of the < phase at the oxygen potential of - 300 kJ/mol 0, at 1673 K, which is that of MO-MOO, equilibrium. The possible role of MO on the phase formation was discussed in regard to oxygen potential.

1. Introduction Fission products consisting of more than 30 elements are formed during irradiation of nuclear fuel of which composition depends on burnup, specific power, average neutron energy, irradiation time and cooling period [1,2]. Although there is some difference between the fission yield in UO, fuel and that in (U, Pu)O, fuel, the chemical state of the fission products in the former is analogous to that in the latter [3]. Post irradiation examinations on the UO, 14-61 and (U, Pu)O, fuels [7-lo] with bumups of 3-S% FIMA revealed that most of the solid fission products are mainly incorporated into three phases: matrix fluorite, perovslcite and metallic phases. The phase behavior of simulated (U, Th)O, fuel [ll-131 was also analogous to those of the UO, and (U, Pu)O,. In the matrix fluorite phase, the actinide and rare earth elements are incorporated as solid solutions. Some amounts of Sr, Ba and Zr are also soluble in this phase. The majority of BaO, however, appears to precipitate with SrO as the perovskite (Ba, Sr)ZrO,, in which minor amount of Cs, rare earth elements, U and Pu would be soluble (141.

0022-3115/88/$03.50 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)

Formation of the metallic phases seems to be considerably complex. The metallic phases produced in the fuel are a mixture of two or more phases, which mainly consist of MO, Tc, Ru, Pd and Rh. The amounts of MO and Tc in the phases largely depend upon oxygen potential in the fuels [3]. In the dissolution of spent nuclear fuels in dilute HNO, solution for reprocessing, insoluble materials remain in the solution. The insoluble residue thus obtained consists of the metallic phases formed in the fuel during irradiation, reprecipitates from the solution of spent fuels, crud (radioactive corrosion product) adhering on the surface of fuel pins, fine chips of cladding formed during chopping of the fuel pins, etc. [15]. They would cause some problems during reprocessing such as degradation of the solvent due to their high radioactivity, deterioration of decontamination factor and clogging of the piping system of the equipments. The main purpose of this series of study is to clarify the chemical properties of the insoluble residues, especially those of the metallic phases in the fuel. In the present study, the oxide and metallic phases has been made by the reaction between UO, and simulated fission products corresponding to 5530% FIMA at the

B.V.

319

T. Muromura et al. / Metallic phases precipitated in rr0, fuel. I

of 1273-2273 K under various oxygen potentials, and the conditions for their formation have been examined. temperatures

2. Experimental 2.1. Experimental

conditions

Oxygen potential

of irradiated fuel.

The oxygen potential of fuel during irradiation has a significant effect on the formation of the secondary phases in the fuel matrix [14], redistribution of elements [16], and rate of the reaction between the fuel and cladding [17]. The oxygen potential does not necessarily change linearly with burnup. This is because MO, Tc and other elements act the role for oxygen sinks. The initial composition of LWR fuel is nearly stoichiometric UO, and the value of O/U ratio is kept very close to 2.0 during irradiation up to high bumups. Accordingly, the oxygen potential of the fuel slightly increases with burnup if the oxygen pickup by zircaloy cladding is negligible. It has been reported that the oxygen potential for 1% FIMA is around -320 kJ/mol 0, at 1673 K, and that for 10% FIMA -280 kJ/mol 0, [l].

Table 1 Composition

of simulated

Element

Stand-in

spent fuels (ORIGEN role

(3) Nd (4) (5) (6) (7) (8) (9) (10)

Cs Rb Ba Sr Zr Mo Ru

(11) Rh (12) Pd (13) (14) (15) (16)

Ag Cd Sn Te

FP compositions

Pr, Pu, Np, Am, Cm La, Sm, Y, Eu, Gd, Pm

Tc

Composition of simulated UO, fuel. The irradiation state selected in the present study is as follows: The UO, fuels (235U = 3.42%) are assumed to have undergone bumups of S-308 FIMA in a PWR at a mean power of 36.3 MW/ton U. The value of neutron flux postulated is 10”/cm2s. The elemental compositions of the fuels for 5% FIMA and 10% FIMA were calculated using the ORIGEN-2 code [18]. Multiplying the composition of 10% FIMA, we provisionally obtained the compositions of 20 and 30% FIMA, respectively. The compositions obtained were simplified using some chemical stand-ins: The amounts of the transuranium elements and Pr, other rare earth elements and Tc were substituted by those of Ce, Nd and MO, respectively. The amounts of In, Sb, etc. were neglected because of their low fission yield. The compositions thus determined are shown in table 1.

2, PWR, 235U = 3.42%)

Bumup 5 (mg/gU

(1) u (2) Ce

In the present study, the bumup is designed in the range of S-30% FIMA, and the oxygen potential between - 160 and -540 kJ/mol 0,. The ranges of bumup and oxygen potential are so wide compared with those of the real spent fuel that these conditions would bring about some advantage for the identification of the phases.

(% FIMA)

Reagent

10

20

30

881.7 19.1 20.0

776.7 38.2 40.0

677.8 57.3 60.0

initial)

936.5 12.0 10.2 4.00 0.48 2.18 1.23 5.08 5.92 3.17 0.54 2.36 0.13 0.22 0.15 0.74

for 20 and 30% FIMA were tentatively

1.17 0.77 4.91 1.83 8.92 11.1 8.46 0.62 7.02 0.25 0.86 0.34 1.61

14.3 1.54 9.82 3.66 17.8 22.2 16.9 1.24 14.0 0.51 1.72 0.68 3.22

obtained

by multiplying

21.5 2.31 14.7 5.49 26.8 33.3 25.4 1.86 21.1 0.75 2.58 1.02 4.83 that for 10% FIMA.

UO, Ce(N03),.6H20 Nd(N0,),.6H,O CsCl RbCl BaC12.2H20 Sr(NO& ZrO(NO,).2H,O (NH,),Mo,O,,4H20 RuCl,.3H,O Rh(N03),~2H20 PdCl, AgNO, CdC12.2.5H,0 SnCl,.SH,O TeCl 4

320

T. Muromura et al. / Metallic phases precipitated in U02 fuel. I

2.2. Procedure

Gases and reagents. The gases, He, 50% CO + 50% CO,,

10% CO + 90% CO, and 4% Hz + 96% He, were used to control the oxygen pressure in the reaction atmosphere. They were purchased and supplied from cylinders. The relation between oxygen pressure and temperature was calculated based on the assumption that 10 ppm 0, was contained in both He and 4% H, + 96% He, and that thermodynamic equilibria were to be established in the respective reactions of 2H, + 0, ti 2H,O and 2C0 + 0, F? 2C0, [19]. The reagents used were shown in the last column of table 1. They were respectively dissolved in dilute HNO, solutions. Preparation of simulated fuels. The mixed solutions with

the compositions in table 1 were first prepared. They were dried up on a hot plate and the residues obtained were calcined at 1173 K in a stream of 4% H, + 96% He to avoid vaporization loss of Ru and MO. The calcines were pressed into pellets of 7 mm in diameter and 1.0 g in weight. They were then heat-treated in a stream of the mixed gas in an induction furnace [20]. The temperature of the furnace was measured by a two-color eye pyrometer or a calibrated optical pyrometer. The temperature was controlled constant within 20 K. X-ray diffraction analysis. After the heat-treatment, the pellets were pulverized and subjected to X-ray diffraction using Cu-Km radiation. The phases produced were identified using powder diffraction files [21]. The lattice parameter, aO, of the UO, phase was determined from the diffraction pattern between 90” and 145” (28) by least squares extrapolation: a value was plotted against cos’0 and a0 was determined at cos2B = 0. The diffraction pattern of the secondary oxide phases and metallic phases (alloys) were so weak and blurred that their lattice parameters were calculated from the diffraction lines at low angles calibrated using Si as a standard. The metallic phases were also identified in the insoluble residues: Sintered pellets were dissolved in 3M HNO, at 368 K for 2 h. The insoluble residues were filtered and then subjected to X-ray diffraction.

3. I. Formation of oxide phases Fig. 1 shows the oxide phases produced at 1673 K as a function of bumup and oxygen potential. The circles in the figure indicate the conditions, i.e. burnup and oxygen potential of sample preparation. When the burnup is less than 5% FIMA, the UO, phase with fluorite structure is observed as the only oxide phase by X-ray diffraction. In the samples above 10% FIMA, an assemblage of two or three phases was produced depending on the oxygen potential. A mixture of threeoxide phases of fluorite + cubic perovskite + tetragonal scheelite was produced in the atmosphere of around - 300 kJ/mol 0,. In the range of low oxygen potentials below -340 kJ/mol 0, there exist two phases of the fluorite and perovskite, and above -270 kJ/mol 0, two phases of the fluorite and scheelite. The phase behavior at 2073 K is also shown in fig. 2 in the same manner. A two-phase mixture of fluorite and scheelite is produced at a burnup of 30% FIMA around -280 kJ/mol 0,. The fluorite and perovskite coprecipitate at 20-30% FIMA below about -450 kJ/mol 0,. The solid solubility of the secondary phases in the fluorite appears to increase remarkably with temperature. The arrow (A) in figs. 1 and 2 shows the oxygen potential of the MO-MOO, equilibrium, and the arrow (B) that of the BaMoO,-BaMoO, equilibrium, as described below. Fluorite.

It is well-known that the UO, phase with fluorite structure is able to dissolve a large amount of

-100

0”

zE

0 - 200

2 E .-

0

- -3oo-

z g

(A)

I---0

.\ 0 Ft PvtS 0 ---

.-._. Ft

F -5oo-

‘-2 \

(‘\o

-4oo-

z

G

o

Y

‘1.

5

0

0

0

F+S

-z

0 -.-

Pv

I 01

0

0

0

-60000 Burnup,

3. Results and discussion The simulated fuels prepared consist of some oxide and metallic phases depending on temperature, burnup and oxygen potential.

% FIMA

Fig. 1. Formation of oxide phases at 1673 K. F: UO, phase with fluorite structure, Pv: Perovskite phase with cubic structure, S: Scheelite phase with tetragonal structure, (A): Oxygen potential of MO-MOO,, (B): Oxygen potential of the BaMoO, -BaMoO, equilibrium.

321

T. Muromura et al. / Metallic phases precipitated in UO, fuel. I

-_(B) 0” -200 -_(*)o

s C

0 0

-3oo-

0

0 -

4; s

0

(

0

2 -

F

;

-4oo-

z *

-5oo-

/

5 z s

0

‘\._

0

-600

/-----F t

0

/

I IO

0

Pv

O

0

I 30

I

/

20

Burnup,

% FIMA

Fig. 2. Formation of oxide phases at 2073 K. F: UO, phase with fluorite structure, Pv: Perovskite phase with cubic structure, S: Scheelite phase with tetragonal structure, (A): Oxygen potential of MO-MOO,, (B): Oxygen potential of the BaMoO, -BaMoO, equilibrium.

the actinide and rare earth elements in its crystal lattice. To some extent, Sr, Zr, and Nb are also soluble in this phase depending on temperature and O/U ratio. Solubility of Cs, Ba and Te is limited in small values [3]. The lattice parameter of the UO, phase decreased as the bumup increased [9]. Fig. 3 shows the variation of the lattice parameter with bumup for various oxygen potentials at 1673 K. The lattice parameter decreases from 0.5470 nm with bumup, as seen in curves (l)-(3). Curve (4) shows the

calculated values from the relation presented by Davies and Ewart [9]. The burnup dependences of curves (2) and (3) below 10% FIMA are in fair agreement with that of curve (4) calculated. Comparison of the curves in fig. 3 shows that the lattice parameter increases as the oxygen potential decreases. This would be predominantly due to formation of low valency ions with larger ionic radii under low oxygen potentials [22]. Furthermore, these results are also influenced by the transition of elements between the matrix UO, and secondary phases. The transition mainly occurs due to change of distribution coefficients of the elements depending on the oxygen potential. In the samples of 5% FIMA in the present study, formation of secondary oxide phases was not found by X-ray diffraction. In the irradiated UO, fuel of 4.6% FIMA at around 1773 K, however, the perovskite phase (Ba, Sr)ZrO, was detected by electron probe microanalysis [6,23]. This would be due to concentration gradient of BaO caused by its vaporization at high temperature fuel zone and precipitation at the cold zone [24]. A low solubility of BaO in UO, has experimentally been shown at low temperature [25]. Perovskite

phase. The cubic perovskite phase precipitates in the simulated spent fuel of bumup more than 10% FIMA at 1673 K, and more than 20% FIMA at 2073 K under low oxygen potentials, as seen in figs. 1 and 2. Fig. 4 shows the lattice parameter variation of the perovskite with oxygen potential at 1673 K. The lattice

1 6

1 CL z & 0.416 -

0.545 -

0

E

z 3

0.543 I 0

I 10 Burnup,

I

I

20

1

I

30

% FIMA

B 0.414 .-8 A: 10%FIMA z 4 o,4, 2 _ 0: 20%FIMA .: 30%FIMA -600

-400 Oxygen

Fig. 3. Variation of lattice parameter of fluorite phase (UO,) with bumup under various oxygen potentials at 1673 K. (1): - 540 kJ/mol 0, in 4% H, +96X He, (2): - 335 kJ/mol 0, in 10% CO, + 90% CO, (3): - 274 kJ/mol 0, in 50% CO, + 50% CO, (4): calculated using ref. [9].

(EJ) i -200

potent io I , kJ/molO2

Fig. 4. Variation of lattice parameter of perovskite with oxygen potential at 1673 K. (A): Oxygen potential of the MO-MOO, equilibrium, (B): Oxygen potential of the BaMoO,-BaMoO, equilibrium.

322

T. Muromura et al. / Metallic phases precipitated in UO,

parameter decreases from 0.420 to 0.412 nm as the oxygen potential increases from - 540 to - 270 kJ/mol 0,. It also decreases with bumup, though the dependence is relatively small, as seen in the figure. This would be due to the variation of distribution coefficients of the constituents with oxygen potential. Thus the lattice parameter could become an indicator of the local oxygen potential in the fuel. The lattice parameters in the figure are in good agreement with those of the perovskite (Ba, Sr)(U, MO, Zr)O, reported in the phase relation study: About 45 mol% BaMoO, (a = 0.414 nm) was soluble in BaZrO, (a = 0.419 nm) and that a complete solid solution was made between BaUO, (a = 0.439 nm) and BaZrO, [26]. The decrease of the lattice parameter with oxygen potential is likely to be attributed to substitution of MO(W) in the Zr site of the (Ba, Sr)ZrO,. The Mo(IV) would be formed by the oxidation of the metallic phases under high oxygen potentials by the reaction, [Mel

metallicphase

+

[ 0,

] fuel

=

[MO%

Iperovshte.

(1)

The oxygen potential of eq. (1) at high temperatures is approximately obtained by extrapolation and assuming unit activity of MO and MOO, (271:

AG(O,)(kJ/mol

0,)

= -588

- 0.01922’ log T

+0.2337.

(2)

The arrow (A) in figs. 1, 2 and 4 shows the oxygen potential of the MO-MOO, equilibrium at 1673 K. There seems to be a continuous transition of MO between the metallic phases and the perovskite due to similar oxygen potentials of these phases. The lattice parameter variation of the perovskite is closely related to that of the hexagonal phase in the metallic phases, as described in section 3.2. The amount of the perovskite decreased with oxygen 0, and disappeared potential above - 340 kJ/mol above -240 kJ /mol O,, as seen in figs. 1 and 4. On the other hand, the scheelite was produced under high oxygen potentials, as is described later. It has been shown that many kinds of elements are soluble in the perovskite. The elements which can substitute for Ba in BaZrO, include the actinide and rare earth elements in trivalent state together with Cs and Rb for charge compensation. Zirconium can also be replaced by the actinide and rare earth elements in quadrivalent state [28,29]. Accordingly, it is likely that some amount of the alkali metals, actinide and rare earth elements are incorporated in this phase. The formation of the perovskite, (Ba,,,,Sr,,,Cs,,,) (Zr,,,, Mo,~,,U,,,,Pu,,+,)0, [12] was reported in irradiated (U, Pu)O, [14].

fuel.1

Scheelite phase. The scheelite type phase is found in the fuel prepared under higher oxygen potentials than - 340 kJ/mol 0, at 1673 K, in fig. 1. Formation of this phase would be due to oxidation of MO in the perovskite phase. The MOO, thus produced would react with BaO and SrO in UO, and/or in the perovskite, and the scheelite type phase, e.g. (Ba, Sr)MoO, would be produced in the fuel. The reaction may be expressed in the form, [BaMoOs lperovstite + : Lo* Irue, = rBaMoO,

lschee,lle. (3)

As a first approximation, the oxygen potential of the reaction is obtained using the extrapolated values of free energy of formation of the molybdates [30] and assuming unit activity of the solid constituents:

AG(O,)(kJ/mol

0,)

= - 302 + 0.0656T.

(4)

Under the oxygen potential higher than that of the BaMoOs-BaMoO, equilibrium, therefore, the amount of the perovskite rapidly decreases in the fuel, as seen in fig. 1. The ZrO, separated from the perovskite would dissolve in the UO, phase. Though the solubility of ZrO, in UO, is limited and temperature dependent [31,32], it has been reported that the rare earth elements of tri- and quadrivalent states dissolved in UO, phase increase the solubility of ZrO,, because they are able to stabilize the fluorite type phase as seen in the systems of ZrO,-Ce,O, [33], ZrO,-CeO, [34], ZrO,-Nd,O, [35], UO,-(Zr, La, Ce, Nd)O, [36], etc. At 2073 K, precipitation of the scheelite was observed in the sample of 30% FIMA under a high oxygen potential of -280 kJ/mol 0, alone, as seen in fig. 2. This would be because of the solubility increase of the scheelite in the UO, with temperature and vaporization loss of the oxides of Ba, Sr and MO at high temperatures under high oxygen potentials. The lattice parameters of this phase, which was found in the fuel of 30% FIMA heat-treated at 1673 K in He (- 160 kJ/mol 0,) for 8 h, were a = 0.5474 nm and c = 1.234 nm. From the phase relation of the CaMoO,-SrMoO,-BaMoO, system [37], its composition appears to be around (Ba,,,Sr,,)MoO,. The scheelite is capable of taking some elements into its crystal lattice. To a certain extent, the rare earth elements in trivalent state can substitute for the alkaline earth metals without charge compensation. The alkali metals are also capable of substituting into the site of the alkaline earth metals with charge compensation by rare earth elements of trivalent state [38-401. Accordingly, it may be said that some amount of alkali metals and rare earth elements are soluble in this phase.

T. Muromura et al. / Metallic phases precipitated in UO, fuel. I A ternary oxide, CszMoO,, was observed on the fuel side of the fuel-cladding gap at high oxygen potentials [41]. This phase appears to exist in a gaseous state in FBR fuel and to condense in the cold fuel zone [42,43]. In the present study, however, this phase was not found probably due to its evaporation loss under the experimental conditions. From eqs. (3) and (4), it is suggested that the scheelite would be formed instead of the perovskite under the oxygen potential above -190 kJ/mol 0, at 1673 K, which is higher than that of 10% FIMA (- 280 kJ/mol 0,) of UO, fuel [l]. From the experimental results in fig. 1, however, it could be assumed that the scheelite phase coprecipitates with the perovskite in the fuel center under high temperatures and high bumups. 3.2. Formation of metallic phases The metallic phases formed were identified by X-ray diffraction both in the heat-treated simulated spent fuel and in the residues obtained at the dissolution of the fuel. Fig. 5 shows a typical example on the formation of the metallic phases found as a function of bumup and oxygen potential at 1673 K. In all the simulated fuels before dissolution, a hexagonal structure phase (E phase) and a cubic structure phase (a phase) are found. After dissolution, the E phase is still identified but the (Y phase is totally disappeared. On the other hand, a tetragonal structure phase (u phase) is identified only in

-100

0” 72 E -200 \

2 - -300 0.-

0

0

IB1

Eta

0

O

/---------

-O (A) o

(/”

0

\o

0

Eta 0

-600

I

0 I

10

0 /

,

20

0 1

I

30

Burnup, % FIMA

Fig. 5. Formation of metallic phases at 1673 K. a: Pd based alloy with cubic structure, c: Ru based alloy with hexagonal structure, U: Intermetallic compound MosRu, with tetragonal structure, (A): Oxygen potential of the MO-MOO, equilibrium, (B): Oxygen potential of the BaMoO,-BaMoO, equilibrium.

30

323

I0

0

$

20-

0

/ IO O 1 &+attJ

&+a

s

9 E

IO-

2

0 0

-

o

I I

t

1300

0

1500

IO

0

oio

1700

1900

Temperature,

0 0

2100

2300

K

Fig. 6. Formation of metallic phases in 4% H, +96% He, a: Pd based alloy with cubic structure, r: Ru based alloy with hexagonal structure, e: Intermetallic compound MosRu, with tetragonal structure.

the insoluble residue derived from the fuel produced in an atmosphere of around - 300 kJ/mol 0,. The fact that the u phase is not identified in the fuel before dissolution may be due to its lesser amount of formation in the fuel matrix. Fig. 6 shows the temperature dependence for formation of metallic phases in the atmosphere of 4% H, + 96% He. The tetragonal phase is found with the hexagonal e and cubic (II phases in the fuels prepared at high temperatures above 2073 K. Hexagonal phase. The metallic phases found in the post irradiation studies were mostly the hexagonal E phase. It has been reported by many workers that this phase is a Ru-based alloy, and that its main components are Ru, MO, Tc and Pd [3]. The lattice parameters of this phase sharply increase with temperature from 1173 to 1273 K, as seen in fig. 7. This would be a rapid increase of reaction rate with temperature on the formation of this phase. In the range of 1273-1873 K, the lattice parameters of the e phase are almost constant. It seems that the phase relation of the metallic phases would not vary appreciably in this temperature range. Above 2073 K the lattice parameters gradually decrease. This would be attributed to the phase relation of the metallic phases and vaporization loss of constituents. The relation of lattice parameter of the hexagonal E phase and oxygen potential is shown in fig. 8 for 1673 K and in fig. 9 for 2073 K. Though the lattice parameters were little influenced by the reaction temperature

324

T. Muromura et al. / Metallic phases precipitated in UO, fuel. I

0.278 g

0.276 5

0450-

B

E

g- 0445. _tz ," 0.440.

0.274 ; a w 0.272 ‘ij

z 04351 E 3 0.430 0 0425" 1100 1300

% 0 0.270 ' 0 1500

1700

1900 2100 2300

0.448

-

% 0.274 g w

0.444

_

0.272 'Z

0.440

-

0.436

I 0 0.270 ' D

’

W z % 5 ”

(A) (El

/

-600 .-500

Temperature , K Fig. 7. Variation of lattice parameters of hexagonal phase with temperature in 4% H, +96% He. (1): Variation of a , (2): Variation of c, 0 and 0: 5% FIMA, 0 and n : 10% FIMA, A and A: 20% FIMA.

1

0.278

0.276 g -

%

5 0.274 ~a

r” LI 0.448 w

2____

.0.436

w

_

(2)

I

z 0.444 S P 0.440 v

0.272 z s 0.270 5 0

0

I

I

-500

+I00

Oxygen potent ial

I

-300

I.

/

-200

-100

0

, k J/m0102

Fig. 8. Variation of lattice parameters of hexagonal phase with oxygen potential at 1673 K. (1): Variation of a, (2): Variation of c, 0 and 0: 5% FIMA, 0 and W: 10% FIMA, A and A: 20% FIMA, @ and 0: 30% FIMA, x: from refs. [8] and [26].

I I 1,1

1 -400

Oxygen

-300

potential,

-200 kJ/mol

, -100

0

O2

Fig. 9. Variation of lattice parameters of hexagonal phase with oxygen potential at 2073 K. (1): Variation of a, (2): Variation of c, 0 and 0: 5% FIMA, 0 and n : 10% FIMA, A and A: 20% FIMA,

and burnup below -330 kJ/mol O,, they sharply decrease with oxygen potential at 1673 K: The lattice parameters are a = 0.2762,0.2772 nm and c = 0.4436-0.4458 nm under -540 kJ/mol O,, and a = 0.2728-0.2750 nm and c = 0.4364-0.4398 nm under - 160 kJ/mol 0, in fig. 8. The variation of the lattice parameters in fig. 8 is analogous to that found in the simulated fuel (Th, U)O,: The lattice parameters of the hexagonal phase produced in the (Th, U)O, fuel of a bumup 21.5% FIMA under - 326 kJ/mol 0, at 1773 K were a = 0.2754 nm and c = 0.4433 nm and under - 122 kJ/mol 0, a = 0.2707 nm and c = 0.4290 nm. respectively [ 111.

0.276 :

Q

(I 1

0

is 2

fi

-:

c

0 and 0: 30% FIMA.

Considering the oxygen-potential calculated for UO, fuel is in the range between - 320 kJ/mol O,(l% FIMA) and - 290 kJ/molO,(5% FIMA) at 1673 K [l], it is suggested from the results in fig. 8 that the hexagonal E phase formed during irradiation will have as lattice parameters a = 0.2745-2765 nm and c = 0.4400-0.4445 nm. The hexagonal phase found in the irradiated UO, had as lattice parameters a = 0.2752 nm and c = 0.4411-4415 nm [26]. This type of phase with lattice parameters of a = 0.2746-0.2761 nm and c = 0.44154431 nm was also identified in the insoluble residues from (U, Pu)O, fuel [26]. The oxygen potentials of the MO-MOO, and BaMoO,-BaMoO, equilibria appear to influence the lattice parameter variations, as described in section 3.1. The MO content in the hexagonal phase seems to decrease as the oxygen potential increases by the reaction in eq. (1) and the lattice parameters also seem to decrease with decrease of the MO content. This deduction is consistent with the variation of lattice parameters shown in the phase relation study of the systems Ru-MO-Rh [44], Ru-Mo-Pd [45] and Ru-MO-Rh,, Pd,., [461. In contrast to the results in fig. 8, distinct changes of the lattice parameters at the oxygen potentials of the MO-MOO, and BaMoO,-BaMoO, equilibria are not found at 2073 K, as seen in fig. 9. This may be attributed to uncertainty of the thermodynamic values used in extrapolation of the oxygen potentials above. This also suggests that the solubility of MO in this phase at 2073 K is smaller than that at 1673 K. To clarify this behavior, further study will be needed in the phase

T. Muromura et al. / Metallic phases precipitated in UO, fuel. I

relation of the metallic phases and thermodynamic ues of the compounds formed.

val-

Tetrugonal phase. The tetragonal u phase was identified in the residue derived from the fuel produced after the heat-treatment in the atmosphere around the MO-MOO, equilibrium at 1673 K, as shown in fig. 5. This phase was also found in the samples heat-treated above 2073 K under low oxygen potentials, as seen in fig. 6. The lattice parameters of this phase produced in the present study are a = 0.953-6 nm and c = 0.493-7 nm. These values are in good agreement with those of MosRu, (a = 0.954 nm and c = 0.495 nm) [47]. The dependences of the lattice parameters on temperature and oxygen potential were relatively small. This could be due to a narrow composition range of the MosRu,: Up to 5 at%, both Rh and Pd can be soluble in this phase at 1973 K

t461. The phase study of the Mo-Ru-Rh-Pd system revealed that a high MO content in FPs results in the formation of the tetragonal a phase and a cubic MO phase at temperatures around 2173 K [46]. In the post irradiation study of the UO, fuel with 4.3% FIMA, formation of the L and (I phases was found by electron probe microanalysis. The occurrence of these phases seems to be enhanced by increase of MO content in the metallic phases under low oxygen potentials [48,49]. In the atmosphere of higher oxygen potentials than that of the BaMoO,-BaMoO, equilibrium, the tetragonal phase was not found in the metallic phases probably due to depletion of MO in the metallic phases by the formation of the scheelite, as described in section 3.1. Cubic phase. X-ray diffraction shows that the cubic (Y phase is introduced in all the samples prepared in various conditions. The lattice parameter of this phase was in the range of 0.383-0.392 nm, which is reasonably in agreement with that of Pd metal (0.388 nm) [50]. Any definite relation between lattice parameter and temperature or oxygen potential was not obtained in the present study. In most of the insoluble residues obtained at dissolution of the simulated fuel, the cubic a phase was not found by X-ray diffraction. It appears that the cubic (Y phase is more soluble in HNO, solution than the other metallic phases. The cubic (Y phase formed in the present study would be identical with the Pd alloy containing such as Sn, Sb, Te and Ag found in the gap between the fuel surface and the cladding and at the central void of FBR fuel pins [3]. The phase relation studies on the metallic phases [51,52] suggest that a monotectic reaction would occur

325

in the metallic phase system in the irradiated fuel. The hexagonal c and tetragonal u phases are first precipitated due to their higher melting temperatures than that of the cubic a! phase. In some studies of the irradiated fuel, formation of a cubic AuCu,-type structure phase was reported [3]. This phase, which had a chemical formula (U, Pu) (Pd, Rh), with lattice parameters of 0.408-0.412 nm, was found in the outer surface of the fuels under the atmosphere of low oxygen potentials. In the present study, however, this phase was not found by X-ray diffraction.

Acknowledgements The authors would like to thank Dr. H. Okashita and Dr. T. Fujino for their valuable comments and discussions and Messrs. Usuda and Shinohara for calculating the composition of FP elements at different burnups.

References

111S. Imoto, J. Nucl. Mater. 140 (1986) 19. PI C.E. Johnson et al., Reactor Technol. 15 (1972/73)

303. [31 H. KIeykamp, J. Nucl. Mater. 131 (1985) 221. of Nuclear Materials [41 R. Benz et al., Thermodynamics (IAEA, Vienna, 1979) Vol. 1, P. 565. [51 M. Koizumi, M. Satoh and K. Noro, J. Nucl. Mater. 51 (1974) 90. [61 B.T. Bradbury, J.T. Demant and P.M. Martin, Proc. British Ceram. Sot. 7 (1967) 311. Nuclear Entsorgung (Verlag Chemie, 171 H. KIeykamp, Weinheim, 1983) Band 2, p. 151. PI J.I. Bramman et al., J. Nucl. Mater. 25 (1968) 201. [91 J.H. Davies and F.T. Ewart, J. Nucl. Mater. 41 (1971) 143. PO1 F.T. Ewart et al, J. Nucl. Mater. 61 (1976) 254. P11 M. Ugajin and K. Shiba, J. Nucl. Mater. 105 (1982) 211. PI M. Ugajin, T. Shiratori and K. Shiba, J. Nucl. Mater. 84 (1979) 26. [I31 M. Ugajin and K. Shiba, J. Nucl. Mater. 91 (1980) 227. P41 C. Sari, C.T. Walker and G. Schumacher, J. Nucl. Mater. 79 (1979) 255. P51 K. Gonda, K. Oka and K. Hayashi, Nucl. Technol. 65 (1984) 102. of Nuclear WI M.H. Rand and T.L. Markin, Thermodynamics Materials (IAEA, Vienna, 1968) p. 637. 1171 S. Yamanaka et al., J. Nucl. Mater. 130 (1985) 524. (1980). WI A.G. Croff, ORNL/TM-7175 1191 A. Glassner, ANL-5750 (1957). and H. Tagawa, J. Am. Ceram. Sot. 61 WI T. Muromura (1978) 30.

326

T. Muromura

et al. / Metallic phases precipitated

(211 Powder Diffraction Files, Inorganic Materials, JCPOS, International Center for Diffraction Data, Penn. (1985). [22] R.D. Shannon, Acta Crystallogr. A32 (1976) 751. [23] B.M. Jeffery, J. Nucl. Mater. 22 (1967) 33. [24] H. Kleykamp, KfK-3394 (1983). [25] E.J. McIver, AERE-M-1612 (1966). [26] H. Kleykamp et al., J. Nucl. Mater. 130 (1985) 426. [27] 0. Kubaschewski, E. Ll. Evans and C.B. Alcock, Metallurgical Thermochemistry, 4th Ed. (Pergamon, New York, 1967). [28] R.W.G. Wyckoff, Crystal Structure, 2nd Ed. (Interscience, London, 1964) Vol. 2, p. 390. [29] H.D. Megaww, Phys. Sot. 58 (1946) 133. [30] 0. Kubaschewski, High Temp.-High Press. 4 (1972) 1. [31] I. Cohen and B.E. Schaner, J. Nucl. Mater. 9 (1963) 18. [32] K. Une and M. Ogumaa, J. Am. Ceram. Sot. 66 (1983) c179. [33] M. Yoshimura and T. Sata, Bull. Tokyo Inst. Technol. 108 (1972) 25. [34] E. Tani, M. Yoshimura and S. Somia, J. Am. Ceram. Sot. 66 (1983) 506. [35] A. Rouanet, Rev. Inst. Hautes Temp. Refract. 8 (1971) 161. [36] F. Schleifer, A. Naoumidis and H. Nickel, J. Nucl. Mater. 101 (1981) 150. [37] E. Barry et al., The Scientific Basis for Nuclear Waste

[38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52]

in UO, fuel. I

Management, Vol. IV, Ed. S.V. Topp (Elsevier, Amsterdam, 1982) p. 55. Ibid. ref. [28], Vol. 3, p. 21. L.G. van Uitert, J. Chem. Phys. 37 (1962) 981. M.M. Schieber, Inorg. Chem. 4 (1965) 762. I. Johnson and C.E. Johnson, Thermodynamics of Nuclear Materials, 1975 (IAEA, Vienna, 1975) Vol. 1, p. 99. T.B. Lindemer, T.M. Besmann and C.E. Johnson, J. Nucl. Mater. 100 (1981) 178. I. Johnson, J. Phys. Chem. 79 (1975) 722. A.E. Dwight and O.R. O’Boyle, J. Nucl. Mater. 136 (1985) 280. T. Fukuzawa et al., Tech. Rept. Osaka Univ. (Japan) 34 (1984) 219. J.O.A. Paschoal, H. Kleykamp and F. Thiimmler, Z. Metallkd. 74 (1983) 652. Ibid. ref. [21], Card no. 16-76. H. Kleykamp, J. Nucl. Mater. 84 (1979) 109. R. Forthann et al., Thermodynamics of Nuclear Materials, 1974 (IAEA, Vienna, 1975), Vol. 1, p. 147. Ibid. ref. [21], Card no. 5-681. R.P. Elliot, Constitution of Binary Alloys, 1st Suppl. (McGraw-Hill, New York, 1965) p. 730. F.A. Shunk, Constitution of Binary Alloys, 2nd Suppl. (McGraw-Hill, New York. 1969) p. 520.