Abstract
The description of intra-granular fission gas behavior during irradiation is a fundamental part of models used for the calculation of fission gas release and gaseous swelling in nuclear fuel performance codes. The relevant phenomena include diffusion of gas atoms towards the grain boundaries coupled to the evolution of intra-granular bubbles. While intra-granular bubbles during normal operating conditions are limited to sizes of a few nanometers, experimental evidence exists for the appearance of a second population of bubbles during transients, characterized by coarsening to sizes of tens to hundreds of nanometers and that can significantly contribute to gaseous fuel swelling. In this work, we present a model of intra-granular fission gas behavior in uranium dioxide fuel that includes both nanometric fission gas bubble evolution and bubble coarsening during transients. While retaining a physical basis, the developed model is relatively simple and is intended for application in engineering fuel performance codes. We assess the model through comparisons to a substantial number of experimental data from SEM observations of intra-granular bubbles in power ramp tested uranium dioxide samples. The results demonstrate that the model reproduces the coarsening of a fraction of the intra-granular bubbles and correspondingly, predicts gaseous swelling during power ramps with a significantly higher accuracy than is allowed by traditional models limited to the evolution of nanometric intra-granular bubbles.
Highlights
The behavior of the gaseous fission products xenon and krypton significantly affects the performance of nuclear fuel rods during irradiation [1,2]
Inter-granular swelling due to grainboundary bubbles is the dominant contribution to gaseous fuel swelling under normal operating conditions, intra-granular swelling becomes significant during transients to high temperatures and at high burnups [9e14]
We presented a model describing intra-granular fission gas behavior in UO2 that accounts for the bubble coarsening phenomenon under in-pile, high temperature transient conditions, and the corresponding contribution to gaseous fuel swelling
Summary
The behavior of the gaseous fission products xenon and krypton significantly affects the performance of nuclear fuel rods during irradiation [1,2]. Gas atoms are created in the fuel grains during fission events and due to their low solubility, tend to precipitate forming bubbles. Intra-granular bubble evolution is governed by gas atom trapping from the matrix into the bubbles and the counteracting mechanism of irradiation-induced re-solution of gas atoms from the bubbles back into the matrix. Inter-granular gaseous swelling and fission gas release to the rod free volume [2,8]. Inter-granular swelling due to grainboundary bubbles is the dominant contribution to gaseous fuel swelling under normal operating conditions, intra-granular swelling becomes significant during transients to high temperatures and at high burnups [9e14]. Intra-granular fission gas bubbles are generally limited to sizes of one to a few nanometers [15]. Sink strength of a dislocation bubble (/) Volume of a gas atom into bulk bubbles (m3 atom-1). Single gas atom diffusion coefficient close to the dislocation core (m2 s-1) Bulk diffusion coefficient of interstitials (m2 s-1)
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