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Release of Fission Products During and After Oxidation of Trace-Irradiated Uranium Dioxide at 3Oo-9Oo°C

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Release of Fission Products During and After Oxidation of Trace-Irradiated Uranium Dioxide at 3Oo-9Oo°C XA0056017 RELEASE OF FISSION PRODUCTS DURING AND AFTER OXIDATION OF TRACE-IRRADIATED URANIUM DIOXIDE AT 3OO-9OO°C P. Wood, G.H. Bannister Central Electricity Generating Board Berkeley Nuclear Laboratories UK Abstract Should defected U02 fuel pins come into contact with air then oxidation of the fuel may occur, the rate and consequences of which are dependent upon temperature and oxygen partial pressure. At CEGB-BNL an experimental programme is underway investigating the kinetics, and extent, of release of fission products during and after oxidation of trace-irradiated U02 to U3O3, and reduction of U3O3 to UO 2« This paper presents preliminary results and analysis of experiments performed at 300-900°C. Dense sintered U02 has been oxidised at 300-500°C using a thermobalance with simultaneous counting of released 85Kr. The kinetics of the 35Kr release are shown to correlate with the kinetics of oxidation, and the extent of release has been determined as 3-8% of that in the UO 2 converted to U3O3. The release of 106Ru and 137Cs during this oxidation has been estimated by Y~c°unting of the fuel sample, before and after oxidation, and of glassware in the vicinity of the sample. This indicates slight release of ruthenium and caesium. Greater fission product release is caused by oxidation at higher temperatures or by heating of the oxidation product. U 308 produced at 400°C has been heated at 800 and 900°C in air for 20 hours. This results in near total release of 85Kr and 106Ru, but still only slight release of 137Cs. The kinetics of the 85Kr release have been analysed and found to follow the Booth diffusion equation at 900°C, but not at 800°C. The fuel burn-up level may also have an effect. Some results of fission product release during reduction of the oxidation product U^Og are presented, and the influence of chemical effects upon the release of individual fission products is discussed. The future programme is outlined. - 139 - 1. INTRODUCTION The product of oxidation of uranium dioxide in oxygen containing atmospheres depends upon oxygen pressure and temperature. In air the observed final oxidation products are i) T > ~ 1550°C - UO ; formed by incorporation of interstitial oxygen atoms into the cubic fluorite lattice of U02» ii) ~ 1550 > T > ~ 1150°C - U308 via intermediate UO2+x; the crystal structure of U30g is orthorhombic, and it is of lower density 3 than U02 (8.36 vs. 10.96 g cm" , room temperature values), iii) ~ 1150 > T > ~ 600°C - U 308 via intermediate u\09; the crystal structure of U^Og is similar to UO 2, containing additional ordered interstitial oxygen atoms. iv) ~ 600°C > T - U308 via intermediate U 307 ; the crystal structure of U3O7 is again similar to UO 2, containing additional ordered interstitial oxygen atoms; this results in distortion of the cubic lattice to a tetragonal form. UO3 is thermodynamically most stable but is rarely observed. These temperatures are in broad agreement with those indicated on the phase diagram presented by Naito and Kamegashira (1976); their diagram also indicates the effect of reduced oxygen pressures. When irradiated uranium dioxide is oxidised then it is expected that fission product release may occur during the conversion and, further, that the transport properties of the new oxide phase will differ from those of unoxidised UO-* The release by diffusion of Kr and Xe has been studied by Miekely and Felix (1972) using U0_ , and by Lindner and Matzke (1959) using UO and U 308 , both studies being in the temperature range 600-1200°C. Shiba (1975) has studied release of I and Xe from oxides in the composition range UO2 - U30g with emphasis on the annealing of fission damage during temperature ramping (5 K min~1) to 1000°C. Parker et al (1967) have reported fission product release percentages from irradiated U02 after heating for 90 minutes in air at temperatures in the range 500-1200°C. This paper considers fission product release (85Kr, 106Ru, 137Cs) during formation of U3O8 by oxidation of small trace irradiated UO2 discs at low temperatures (300 - 500°C) in air. The progress of the oxidation was followed using a thermobalance. Some measurements of fission product release from U3O8 on annealing at 800 and 900°C in air are also reported, together with the results of some reduction/reoxidation experiments. - 140 - 2. MATERIALS AND METHODS The uranium dioxide samples were small discs ~ 3 mm diameter, 0.5 mm thick, ~ 45 mg mass, density 10.8 g cm"3. They were cold-irradiated in PLUTO in a flux of 6 x 10 12 n cm"2s~1 to 29 MWd te~1 and allowed to cool for 100 - 1000 days before being used. The air used for the oxidations was from cylinders with a typical moisture content of 2 vpm. High purity argon was used during preheating of samples, and argon/2% hydrogen was used for the reductions. The oxidations, anneals and reductions were performed in a thermobalance which was constructed using a CI Electronics Ltd. MK II Microforce 1 g capacity weighing head. Glassware was designed to enable rapid changeover of atmosphere and rapid transport of all reacted gas through a Millipore filter to a flow through (3-counter (Nuclear Enterprises Ltd. NE 802/DM 1-2/5289). All UO2 samples were counted by y-;spectroscopy before oxidation to determine 85Kr, 106Ru and 137Cs content. Samples were brought to temperature in high purity argon before switching to air (50 seem). During oxidation to completion continuous measurements were made of weight gain and amount of 85Kr released. This latter quantity was obtained after calibration of the counting equipment by injection of known amounts of a 85Kr standard. In two experiments the product of oxidation at 400°C was heated to 800°C (ITB 11) and 900°C (ITB 12) in flowing air and left at this temperature overnight during continuous 85Kr counting. All oxidation products were recounted by y-spectroscopy and have been retained for possible particle size analysis. The thermobalance glassware and crucible were dismantled and counted after experiments ITB 2 and ITB 12. Some samples were reloaded into the thermobalance, reduced to UO 2 and recounted by y-spectroscopy. One of them (ITB 5) was then reoxidised with continuous counting of 85Kr release and final recounting by y-spectroscopy. 3. RESULTS AND DISCUSSION 3.1 Oxidation of Trace-irradiated U0 0 Pellet Fragments to U 3OR In experiments ITB 1-9 UO2 samples were completely oxidised in air at 300 - 500°C. In all experiments the observed weight gain (4% of sample weight) corresponded to formation of U3Og. Figure 1 is a plot of the weight gain measurements made during ITB 1 at 500°C. Rate curves of this general form have been recorded with trace-irradiated samples at 300 - 500°C in air, and with unirradiated - 141 - pellet fragments at 200 - 550°C in air (Simpson and Wood, 1983). U3O8 is produced during the rapid weight gain phase (after ~ 200s in Figure 1). This is preceded by an initial period of low weight gain. The formation of a thin layer of U3O7 during this initial period at low temperatures (225 - 275°C) has been detected by surface X-ray studies (Taylor et al, 1980) and indicated by more sensitive weight gain measurements (Simpson and Wood, 1983). With irradiated U02 (spent fuel) at low temperatures (200 - 250°C) the kinetics differ in that there is a much reduced or no initial period of low weight gain (Gilbert et al, 1984, Wood, 1985). This has been interpreted (Gilbert et al, 1984) as due to an enhanced rate of U3O7 formation. The kinetics of release of ^^Kr during formation of UgOg are also illustrated in Figure 1. The total amount released corresponds to 8% of that present in the sample; in other experiments the amount was 3-8% (Tables 1,2). The shift between the two curves in Figure 1 is due to the time taken for 85Kr, after being released from the sample, to be transported to the counting cell. The similarity of the two curves (evident in all experiments) indicates that the release occurs on transformation of the lattice; no further release was observed at 300 - 500°C after completion of oxidation. The incomplete release indicates that the lattice transformation occurs by local readjustment of the parent U0 2 lattice. Estimation of the extent of release of 106Ru and 137Cs during oxidation was attempted by counting of the samples before and after oxidation (Table 1). Interpretation of these figures is difficult but it is evident that there is only slight release of 106Ru and 137Cs during oxidation. A more meaningful estimate of ^06Ru and ^37Cs release was attempted by counting of the thermobalance glassware after experiment ITB 2. Assuming all the Ru and Cs released during experiments ITB 1 and 2 were deposited on the glassware, then the counting indicates a total Cs release of (2.6 ± 1) x 10"3% and a total 106Ru release of 0.5 ± 0.02% from the two experiments. The glassware was reassembled without decontamination. 3.2 Fission Product Release during High Temperature Annealing of U ,0g Oxidation Product The oxidation product, I^Og, is thermodynamically stable in air at temperatures from ~600°C to ~ 1550°C, and in atmospheres containing 6 10~ atm 02 at temperatures up to ~ 900°C (Naito and Kamegashira, 1976). - 142 - In experiments ITB 11 and 12 UO 2 samples were completely oxidised in air at 400°C and then heated overnight in air at 800 and 900°C respectively. Figure 2 is a plot of the 8^Kr release measurements made during experiment ITB 12.
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