Xe gas bubble re-solution in U-10Mo nuclear fuel

Abstract

The U.S. High-Performance Research Reactor program aims to convert high-power research reactors from highly enriched uranium to low-enriched uranium using a monolithic U-10Mo fuel design. A critical aspect of U-10Mo fuel performance is fission gas bubble behavior. These bubbles grow by trapping gas atoms (particularly Xe) but can disintegrate via irradiation-induced "re-solution". The interplay between the trapping and re-solution rates governs bubble evolution, impacting fuel performance and safety. In this study, binary collision approximation (BCA) and molecular dynamics (MD) simulations were performed to quantify the Xe gas bubble re-solution rate in U-10Mo fuel. First, the energy loss of fission fragments (FFs) through electronic and nuclear stopping was evaluated. The effect of electronic stopping on re-solution was then analyzed using MD simulations coupled with the two-temperature model. Results indicate that thermal spikes generated by electronic stopping do not contribute to gas bubble re-solution in U-10Mo. To quantify re-solution due to nuclear stopping, BCA simulations of FFs in U-10Mo were performed to obtain the average FF incidence probability, energy, and angle as a function of distance from the FF origin. Subsequent simulations assessed FF--bubble interactions in U-10Mo for different FF energies and bubble radii. From these analyses, an overall re-solution rate $b$ was calculated at equilibrium bubble pressure per unit fission rate density, yielding values ranging from $4.4 \times 10^{-26}$ m$^3$/fission for the largest bubbles to $8.8 \times 10^{-25}$ m$^3$/fission for the smallest. The effect of bubble pressure on the re-solution rate was also evaluated, revealing an inverse relationship between the two.

Figures (15)

• Figure 1: Nuclear and electronic stopping powers of FFs. Nuclear and electronic stopping powers of (a) light FF _39^97Y with an initial energy of $101.3$ MeV, and (b) heavy FF _53^136I with an initial energy of $74.6$ MeV as a function of distance in U-10Mo. Data were obtained from $2,000$ independent BCA simulations for each FF using RustBCA.
• Figure 2: Temporal evolution of a thermal spike simulation in U-10Mo. Snapshots of a $30$ keV/nm thermal spike simulation at (a) $0$ ps, (b) $2.4$ ps, (c) $10.1$ ps, and (d) $29.1$ ps. Xe gas atoms are represented in black, U in red, and Mo in blue. The local ionic temperature rises rapidly for $1.5$ ps before beginning to cool. No Xe atom was observed to escape the gas bubble. The images were rendered using OVITO.
• Figure 3: Coordinate system for the calculation of the re-solution rate.a Rotation of fission event positions and velocities around the origin such that all velocities point to the $-x$ direction. Red dots represent positions and gray arrows represent velocities. b A coordinate system in which a Xe gas bubble is at the origin and FFs are all pointing toward the $-x$ direction. The radial coordinate $w = \sqrt{y^2 + z^2}$ represents the perpendicular distance between the initial FF trajectory and the bubble center.
• Figure 4: FF trajectory discretization scheme.a Trajectories of $100$ simulated _39^97Y ions in U-10Mo, starting from the origin and directed along the $x$ axis. b Schematic of the annular surface discretization scheme used to collect data across the volume. The discretization allows for the extraction of three key properties at each $(x, w)$ coordinate: the probability of a FF passing through a unit surface area centered at $(x, w)$, its average incidence energy, and its average incidence angle relative to the $x$ axis. The simulation volume is discretized using a two-dimensional grid with uniform spatial intervals of $\Delta x = \Delta w = 50$ nm.
• Figure 5: Spatial distribution of FF incidence properties in U-10Mo. Calculated FF properties in U-10Mo as a function of coordinates $(x, w)$ for FFs initiated at the origin and directed along the $x$ axis. Data represent the converged results from $30,000$ simulations of _39^97Y and $40,000$ simulations of _53^136I. a, b Ion incidence probability per unit surface area centered at $(x, w)$ for (a) _39^97Y and (b) _53^136I. The probability profiles show a plume-like broadening as FFs travel further from the origin. c, d Average incidence energy for (c) _39^97Y and (d) _53^136I, showing a predominantly linear energy loss with distance. e, f Average incidence angle relative to the $x$ axis for (e) _39^97Y and (f) _53^136I. Discrete paths visible in the low-probability regions of the energy and angle plots represent rare scattering events.