Title: SIMS Analysis of Xe and Kr in a High Burn-up UO2 Nuclear Fuel
Citation: MICROSCOPY AND MICROANALYSIS vol. 14 no. S2 p. 1092-1093
Publication Year: 2009
JRC N°: JRC56512
ISSN: 1431-9276
URI: http://publications.jrc.ec.europa.eu/repository/handle/JRC56512
DOI: 10.1017/S1431927608081257
Type: Articles in periodicals and books
Abstract: The inert gases Xe and ¿¿ constitute around 30% of the fission product inventory in UO2 nuclear fuel. At temperatures below about 1000oC die gas atoms are disposed in die fuel matrix and cause lew problems. Above this temperatare part of tbe gas forms bubbles and part is release to free volume of the fuel rod. The fonnation of gas bubbles causes die fuel tø swell, which can lead tø increased mechanical interaction between the fuel and its containment, whilst high levels of fission gas release can cause over-pressurisation of tbe fuel rod. Both diese effects can lead to failure of die fuel rod. The fission gases Xe and Kr in a section of UO2 fuel with an average bum-up of 65 MWd/kgHM have recently been analysed by secondary ion mass spectrometry (SIMS). The section exhibited a thin band of high bum-up structure (Rg.l) at the pellet rim [1]. SMS depth profiling carried out on the high bum-up structure revealed that the pores contained the fission gas lost from die fuel matrix (Fig.2). fa Fig.2 the intensity sinkes in tibe ¿¿¿ and ^Kr depth profiles mart the locations of bubbles in die high bum-up structure. Since the instrument measures the Xe and Kr ion signals consecutively, the number of gas bubbles encountered in the analysis is equal to die sum of die Xe and Kr intensity spikes in die two traces. The results for Kr in Figa are the first ever obtained for the behaviour of this fission gas in irradiated nuclear fuel. To confirm that Xe and Kr are together in die pores of the high bum-up structure, ion maps showing the distributions of mC& and ^b, which are die stable decay products of 133Xe (half-life, 5.3 days) and 85Kr (half-life, 10.8 yrs.), were produced. Unlike, die rare gases, which are ionized above die sample surface after release from die pores [2], Cs and Rb being solids at room temperature are ionized in-situ during the sputtering process, as is generally die case in SIMS. Ion maps for the distribution of mCs and ^Rb in the high bum-up structure are shown in Hg.3. Circles mark pores that unambiguously show evidence of having contained 133Xe and ^Kr. The concentration of retained Xe at the surface of the high bum-up fuel was analysed by SIMS and electron probe microanalysis (¿¿¿¿), fa Fig.4 the radial variation of the ' 2Xe ion intensity measured in the surface region of the fuel by SIMS depth profiling is superimposed on the ¿¿¿¿ Xe concentration profile. It can be seen that in die radial interval between r/r0= 0.95 and die fuel surface, where tbe high bum-up structure was present, ¿¿¿¿ measured a sharp fall in the concentration of retained Xe. Ibis is because ¿¿¿¿ measures «üy die gas concentration in the fuel matrix (it does not detect the gas in die pores), fa contrast SIMS records a steep increase in tbe 133Xe intensity in die same region since it detects all die gas present lbe rise in the gas concentration measured by SIMS is consistent with the increase in the local bum-up profile measured by ¿¿¿¿ at the pellet surface. Ibis agreement confirms that a large fraction (perhaps all) of (he Xe released from Ae UO2 matrix is contained in the pores of die high bum-up structure.
JRC Directorate:Nuclear Safety and Security

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