Study of the vaporization of Ba from UO2 nuclear fuel particles by high-temperature mass spectrometry

JOURNAL OF NUCLEAR MATE~~S

52 (1974) 89-94.0

NORTH-HOLLAND PUBLISHING COMPANY

STUDY OF THE VAPORIZATION OF Ba FROM UO, NUCLEAR FUEL PARTICLES BY HIGH-TEMPERATURE MASS SPECTROMETRY K. HILPERT Zentralinstitutfiir Analytische Chemie, ~ernf~rschung~nlage Jiilick, Germany

R. F~RTH~ANN

and H. NICKEL

Institut fur Reaktorwerkstoffe, Ke~f~~~c~ungsanlageJiilich, Germany Received 15 March 1974 In connection with improving the retention of solid fission products in gas-cooled high-temperature reactor fuels, the vaporization of Ba from UOz model nuclear fuel particles with and without a pyrocarbon coating was studied by high-temperature mass spectrometry using a Knudsen cell. The UOz kernels of the particles were doped with BaO. In addition, some of them contained A1203. Whereas BaO mainly evaporated from the surface of the kernels as BaO, only Ba could be observed over the coated particles. Moreover, the BaO vapor pressure over kernels with and without the addition of A1203 was determined. From this it was determined that the BaO vapor pressure could be diminished by approximately two orders of magnitude by the admixture of Ai2O3. Finally it was proved that the diminution of the BaO vapor pressure was caused by the formation of the compound BaA1204. Dans le but d’ambliorer la retention des produits de fission solide dam les combustibles de rdacteur &haute temphature refroidis par un gaz, la vaporisation de Ba $ partir de particules de combustible nucl6aire de type UO2 recouvertes ou non de pyrocarbone a 6tC CtudiBe par spectrom&rie de masse B haute temperature en utilisant une cellule de Knudsen. Les noyaux des particules de UO? Ctaient dogs par BaO. En’outre, quelques-uns de ces noyaux contenaient Al2O3. Tandis que BaO s%vaporait principalement des noyaux sous forme de BaO, seul Ba pouvait dtre observe sur les particules recouvertes d’un dkp6t de carbone. De plus la tension de vapeur de Baa au-dessus des noyaux dopes ou non par Al203 fut d6terminCe. De ces mesures il fut de’duit que la tension de vapeur de BaO pouvait 6tre diminude de deux ordres de grandeur environ par l’addition de Al2O3. Finalement il fut prouvk que la diminution de la tension de vapeur de BaO &it due A la formation du composd Ba A1204. Im Zusammenhang mit der Verbesserung der Spaltproduktriickhaltung im Brennstoff von gasgekiihlten Hochtemperaturreaktoren wurde die Verdampfung von Ba aus UOz-Modellbrennstoffkernen mit und ohne Pyrokohlenstoffbeschichtung untersucht. Hierbei wurde die Methode der Hochtemperaturmassenspektrometrie unter Verwendung einer Knudsenzelle eingesetzt. Die UOz-Kerne waren mit BaO ddtiert. ZusHtzlich enthielt ein Teil von ihnen noch A1203. W&rend von der Kernoberfliiche im wesentlichen nur BaO abdampft, konnte iiber den beschichteten Teilchen nur Ba beobachtet werden. Weiter wurde der BaO-Dampfdruck iiber Kernen mit und ohne AlzO3-Zusatz bestimmt. Hieraus ergab sich, dass durch eine Al~O3-Beimischu~g der BaO-Druck urn etwa zwei Gr~ssenordnun~n verkleinert werden kann. Es wird gezeigt, dass die Dampfdruckerniedrigung auf die Bildung von BaAl204 zuruckzufiihren ist..

1. Introduction Fuel elements of high-temperature gas cooled reactors [l] contain coated nuclear fuel particles [2]. The particles consist of a spherical ceramic fuel kernel which is coated with several different layers of pyrocarbon to improve fission product retention. Investigations on fissiqn product release, however, have shown that at h.& temperatures pyrocarbon is insufficient to retain the solid fission products (e.g. 137Cs, 8gSr, g”Sr and ?40Ba). The contamination of the pri-

mary gas circuit, caused by this phenomenon, is only admissible if the gas exit temperature is less than 750°C. In the future the temperature is to be raised further because of the development of high-temperature gas-cooled reactors with integrated gas-turbines and applications in the process heat field. Therefore, safety requires further ~provement of the retention ability of the coated particles for the solid fission products. One new possible way to do this is the addition of

90

K. Hilpert et al., Vaporization of Ba from UO? nuclear fuel particles

certain refractory oxides to the UO, kernels such as Al,O, which can react with the solid fission products Sr and Ba, or Al,O, + SiO, which can form compounds with the fission product Cs. If the compounds formed are comparatively stable it might be possible in this way to diminish the fission product release. With respect to the immiscibility of AllO and UO, (and ThOz), solid-state reactions of Al,03 with solid fission products cannot be affected by UO, and ThO,. Al,O, is able to form refractory ternary oxides with SrO and BaO (e.g. SrA1204, SrAl,O,, SrAl1201g. BaAl,O,, BaAl,,OIg). The effect of formation of ternary oxides on fission product retention was studied out-of-pile by heating U02 kernels with and without addition of A1203 doped with SrO or BaO [3,4]. These experiments showed a considerable decrease of the Ba and Sr release from UO, kernels containing A1203. In studies of the Ba distribution between the two oxide phases of UO,/Al,O, kernels the Ba was found to be concentrated mainly in the Al,03 phase [3]. This can be explained by the formation of ternary compounds (e.g. BaAl,O,, BaAI1,OIg). In order to prove this explanation and to obtain a better understanding of the Ba transport in the coated particles the vaporization of the added BaO from kernels with and without Al,03 was studied by applying high-temperature mass spectrometry using a Knudsen cell. Additionally, the same kernel species were coated with various layers of pyrocarbon and the Ba-containing species which evaporate after their migration through the coating was identified.

2. Experimental 2.1. Instrument The measurements were carried out using the Knudsen cell-CH 5 mass spectrometer system supplied by Varian MAT, Bremen. The vapor beam effusing from the cell was directed towards an electron impact ion source. The ions produced by 22 eV electrons were magnetically deflected over an angle of 90” and detected either by a multiplier (amplification factor 106) or a Faraday cage. The vapor beam could be interrupted by a shutter. The cylindrical molybdenum Knudsen cell could be equipped with linings made of Pt and Al,O,. Its inner

diameter and height were 8 mm respectively. The temperatures were measured with a calibrated disappearing filament-type optical pyrometer supplied by PyroWerke, Hannover. The pyrometer was sighted through a flat glass window into a black-body hole laterally placed close to and below the bottom of the cell. The estimated maximum error of the temperature measurement (including window corrections) was + 1%

[51. 2.2. Samples Special model kernels were prepared which contain stable isotopes of Ba. It is important that these ‘artificial’ fission products have a chemical form similar to the fission products being generated in a nuclear reactor. Therefore, the ‘artificial’ fission product Ba must be present in the kernel as oxide. This is possible by preparation of the kernels from aqueous solutions according to the Hydrolysis Process (‘H-Process’) [6]. Applying this process, Ba and Al were added to the initial solutions of uranyl nitrate as barium nitrate and aluminium nitrate respectively. The added Ba contained the radionuclide 133Ba and hence the concentration of the Ba in the kernels could be determined easily by y-spectrometry. For vapor pressure measurements two different kernel species were prepared. They had the composi‘tion UO, + 0.80 at % Ba and UO, + 0.89 at % Ba + 5 at % Al corresponding to a burn-up of about 10% lima. The Ba content was determined after the preparation was completed. It is interesting to note that although the two kernel species had the same initial concentration of Ba, the species without Al,03 lost more Ba during the sintering process (13OO”C, 3 h) than the other one. This indicates a better Ba retention by kernels containing A1203.

3. Results and discussions 3.1. Gaseous phase The various kernel species were investigated with an ionization energy of 22 eV but no Ba containing ions could be observed other than Ba+ and BaO+. Their relative intensities, which were obtained using MO cells without lining, are given in table 1. For the

K. Hilpert et al., Vaporization of Ba from UOz nuclear fuel particles

91

Table 1 Relative ion intensities over various samples. Sample

TWI

I( ‘38Ba+)

1(ls4BaO+)

UOz + 0.80 at % Ba 1

1680

130

100

0.5

0.9

1.0

0.0

UOz + 0.80 at % Ba run no. 2

1868

118

100

0.41

0.84

1.3

0.0

UOz + 0.89 at % Ba + 5 at % Al

2050

360

100

40

90

135

8.5

2068

225

100

43

88

140

8.3

run no.

1(238U’)

1(254uo+)

1(270uo?+)

1(*%03+)

run no. 1

UO2 + 0.89 at % Ba + 5 at % Al run no. 2 -

intensity of BaO+ a value of 100 was chosen. From ionization efficiency measurements and from the resulting appearing potentials it follows that only Ba and BaO exist in the gaseous phase and that Bat and BaO’ result from simple ionization. Table 1 shows that most Ba’/BaO+ ratios, which were obtained by measuring the various kernel species, are clearly different. Obviously, this is true even for samples of the same composition U02 + 0.89 at % Ba t 5 at % Al. It could be shown that in that case the different ratios were caused by different Ba pressures and not by the BaO pressures which are equal within the accuracy of measurement (viz. table 2). This observation indicates that the Ba in the gaseous phase was mainly generated by the reduction of BaO. The conclusion is proved by measurements which were carried out with molybdenum cells lined completely with Al2O3. In the course of these investigations, especially at the beginning, comparatively small Ba’/BaO’ ratios could be observed. Evidence of the formation of Ba by reduction of BaO is’tinally supported by mass spectrometric measurements according to the Knudsen method on BaO [5]. Though BaO sublimes congruently [7], using MO cells without: lining the same Ba+/BaO+ ratios could be observed as those which were obtained over various kernel species. In contrast to the above-mentioned result, according to which the BaO in the UO2 kernels evaporates mainly from the surface of the kernels as BaO, no BaO could be observed over the same kernel species coated with different layers of pyrocarbon. It was shown that after the migration through the coating

only Ba evaporates from its surface. No other Bacontaining species could then be found in the gaseous phase. 3.2. Vapor pressures The ion currents I(i) for individual species i were converted to corresponding partial pressures p at temperatures T by the relation p=E

o(lo7Ag+)

y(‘07Ag+)

o(i)

y(i)

h(lo7Ag) h(i)

I(i) T ’

(1)

where E is a sensitivity factor, a(lo7Ag’)/a(i) the ratio of cross sections for Ag and i, r(lo7Ag’)/r(i) the ratio of the amplification factors for the multiplier, and h(lo7Ag)/h(i) the ratio of the isotopic abundancies. Using the maximum ionization cross sections [B] linear intereolation yields for the u-ratio at an electron impact energy of 22 eV u(Ag)/u(BaO)

= 0.42 .

The sensitivity factor was determined by calibration with silver (purity 99.999%) according to the quantitative evaporation method [9]. BaO vapor pressures over various kernel species were determined by applying eq. (1). Three different Knudsen cells were tested for the measurements: (i) a MO cell lined completely with Al203 (type l), (ii) a MO cell lined completely with Pt (type 2) and (iii) a MO cell without lining (type 3). If type 1 was used the BaO vapor pressures were substantially lower than

92

K. Hi!pert et al., ~ap~rizat~o~ of Ba from UOz nuclear fuel particles

Table 2 BaO vapor pressures. Sample

BaO vapor pressure curve (p in torr, Tin K)

Temperature range (K)

BaO pressure at 1818 K (torr)

1607 to 1793

1.0 x lod

1571 to 1819

1.2 x lo4

1571 to 1819

1.1 x 1o-4

4 UO2 + 0.8 at % Ba (run no. 1)

loglop = -(2.526t0.039)

‘+

+ (9.9hO.23)

UOz + 0.8 at % Ba (run no. 2)

loglop = -(2.367*0,035)

I$+

mean value

loglop = -2.441$+

UOz + 0.89 at % Ba + 5 at % Al (run No. 1)

loglop = -(3.026+0.075)

$

+ (10.37t0.39)

1855 to 2055

5.3 x lo-’

UO2 + 0.89 at % Ba + 5 at % Al (run No. 2)

loglop = -(2.986+0.081)

‘$ + (10.27t0.41)

1840 to 2065

7.0 x lo-’

mean value

loglop = -3.006

1840 to 2065

6.2 x 1O-7

(9.09iO.21)

9.50

lo4 T + 10.32

those which were determined by using type 2. This can be explained by the reaction of the gaseous BaO with the Al203 of the lining forming the compound BaAl,O,, thereby diminish~g the density of the gaseous BaO molecules in the cell In contrast to this, the BaO pressures obtained with type 2 are in agreement with those which were determined with type 3. Type 2, however, must also be rejected since after the measurements a certain amount of the kernels sticks to the Pt indicating some interactions between Pt and the kernels. Moreover, the melting temperature of Pt is comparatively low. No disturbing interactions could be observed if type 3 was used. Therefore, type 3 was applied for the investi~tion of the kernels. The measured BaO vapor pressures are given by the equations tabulated in table 2. Each equation was calculated according to the method of least squares from about 30 BaO pressure points. The vapor pressure at 18 18 K is also given in order to allow a better comparison of the various results. It was calculated by substitution of the temperature into the corresponding relation for the vapor pressure. The BaO vapor pressures over kernels of the corn-. position UO, + 0.8 at % Ba, which were obtained in the course of run Nos. 1 and 2, agree within the accuracy of measurement. It is possible that they represent the dissociation pressure of a compound such as BaUO3 which according to ref. [lo] is generated by heating BaO and U02 at temperatures above 1000°C.

The BaO vapor pressures, obtained in the course of the two runs, over kernels of the composition UO, + 0.89 at % Ba + 5 at % Al are in agreement within the accuracy of measurement. They are somewhat more than two orders of magnitude smaller than the pressures over kernels without A1203. Mean BaO vapor pressure curves for kernels with and without A1203 (see table 2) are depicted in fig. 1. Also given are the BaO pressures over the barium aluminate compounds and over BaO [S, 111. The BaO Tf*K) 2200 2100

2000

1900

1800

1700

1600

Fig. 1. BaO vapor pressures versus temperature.

1500

K. Hilpert et al., Vaporizationof l3afrom U02 nuclear fuel particles

5

pressures of the barium aluminates lowing equations of evaporation: (Ba,Al,O,)s

Wa+O,), (BaAl,,O,,),

t’

refer to the fol-

2(BaO>g + @aA$O,), ,

f WaO)p + (Ba~,,O,,), , 2

(BaO)s + 6(Al203)s

.

(2) (3) (4)

Fig. 1 shows that the BaO pressures over kernels containing A1,03 and the BaO dissociation pressure of BaAl,O, agree within the accuracy of measurement. From this follows that the compound BaAI, was generated in the kernels. This, however, does not imply that the BaO contained in the kernels was completely present in the form of the compound BaAl,O,. As can be calculated from the added quantities of BaO and A1,03, BaAI, and Al,O, would exist side by side in the kernels if all the BaO had reacted producing BaAl,O,. However, according to ref. [ 121 the two compounds BaA1204 and Al,O, react at temperatures above 1673 K generating BaAl1201g. This reaction is finished when the Al,O, is completely consumed. Thus, it can be concluded that the BaO contained in the kernels is present in the two compounds BaAl12019 and BaAl,O,. This cannot be derived from the BaO vapor pressures because the pressure over BaAl1201g is only slightly lower than the pressure over BaAl,O, (fig. 1). Different BaO vapor pressures were obtained over UO, kernels with and without Al,O, (fig. 1). In contrast to this, the Ba vapor pressures, which were observed over the same kernel species coated with pyrocarbon, are equal [5]. This result is not surprising and must be expected for thermodynamic reasons because there are chemical interactions between Ba and the pyrocarbon. On account of the small Ba loss by effusion through the orifice of the Knudsen cell the Ba supply from the kernels remains relatively high, even if they contain Al,O,. Hence, the Ba which is present in the coating of the two different coated-particle species must exist in the same chemical form (for instance a compound between Ba and C). The Ba pressures over the different coated-particle species with and without A1203 are therefore equal and represent dissociation pressures. However, this does not signify that Al,O, is ineffective for fission product retention in coated particles. This statement is supported by the application of a dynamic method of measurement in the form of

93

heating experiments [3,4]. As must be expected for the fuel elements of high-temperature reactors, there was no equilibrium during the heating experiments. Ba which evaporates from the coating could now escape completely in contrast to the situation in the course of Knudsen evaporation. Therefore, the Ba release from the kernels determined the overall release from the particles and a comparatively high retention of particles, on the addition’ of A!203, became observable in the heating experiments.

4. Conclusion The improved retention of BaO in UO, kernels containing Al,O,, which was observed in out-of-pile heating experiments [3,4] is caused by the formation of barium aluminate compounds. BaO reacts with Al,O, to a spine1 present as precipitations [3] in the kernels. Hence, the number of BaO molecules which can migrate to the surface of the kernels is diminished. The fraction of diffusible BaO molecules depends on the equilibrium constant of the reaction given by eq. (3) i.e. the dissociation pressure of the compound BaAl,04. This quantity determines the Ba release of the coated particles if one assumes that the tissionproduct migration through the coating is a diffusion process whose driving force is given by the BaO pressure of the kernels. Furthermore, it is shown by our investigations that BaO is reduced by the pyrocarbon of the coating and that there are indications of compounds which are subsequently formed between. Ba and the pyrocarbon. However, the formation of BaC, can be excluded by us. This is proven by an estimation of the measured Ba pressure over coated particles which is several orders of magnitude smaller than the dissociation pressure of BaC, given in the literature [ 131. Moreover, it follows from this observation that the concentration of the Ba present at any place in the coating must be less than about 10-l at % because according to ref. [ 141 the phase BaC, can only be generated if the Ba concentration is higher than this value. This conclusion is supported by y-spectrometric measurements. They yielded an average Ba concentration in the coating of about 7 X lop3 at %.

94

K. Hilpert et al., Yffporizotion of Ba from LrOz nuclear fuel particles

[ 71 M.G. Inghram, W.A. Chupka and R.F. Porter, J. Chem.

Acknowledgment The authors are grateful to Professor Dr. H.W. Ntirnberg for clarifying and valuable discussions and they wish to thank Mr. H. Gerads for skillful assistance in the measurements.

References Ii] R. Schulten, Atomwirtschaft

11 (1966) 218.

[ 2 ] H. Nickel, KFA Report, Germany, Jiii-687~RW (1970). [3] R. Farthmann, M. Hamesch and H. Nickel, KFA Report, Germany, Jill-846-RW (1972). [4] R. FGrthmann, E. Gyarmati, K. Hilpert and H. Nickel, Proc. IAEA Symposium on Principtes and Standards of Reactor Safety, Jiilich (Feb. 1973) IAEA-SM-169/40, p. 583. [S] K. Hilpert and H.W. Niirnberg, KFA-Report, Germany, Jiii-953-AC (1974). [6] R. FBrthmann, KFA-Report, Germany, Jiil-9SO-RW (1973).

Phys. 24 (1955) 2159. [8] J.B. Mann, J. Chem. Phvs. 46 (1967) 1646. ,n, 171 K.T. Grimley, in: The Characterization of HighTemperature Vapors, ed. J.L. Margrave (Wiley, New York, 1967). IlO1 N.M. Voronov and R.M. Sofronova, in: Physical Chemistry of AlIoys and Refractory Compounds of Thorium and Uranium, ed. 0.S. Ivanov (Israel Program for Scientific Translations, Jerusalem, 1972) p. 204. 1111 K. Hilpert, H. Beske, H.W. Niirnberg and A. Naoumidis, in: Advances in Mass Spec&ometry, Vol. 6, Roe. 6th Int. Mass Spectrometry Co&., Edinburgh (Sept. 19731, in press. [ 12j H.E. Schwiete, H. Miiller-Hesse and J.E. Planz, Study of Solid State Chemical Reactions in the System BaO-A12 03 -SiOz with Aid of Infrared Spectroscopy (in German) (Westdeutscher Verlag, K61n and Opiaden, 1961) p. 88. fl31 R.H. Flowers and E.G. Rauh, J. Inorg. Nucl. Chem. 28 (1966) 1355. [14] M. Laser, Contributions to the Transport Mechanism of Fission Products (in German) paper presented at the meeting on: Transport of Fission Products in High Temperature Reactors, Jiilich (March 1968).