Fission-enhanced diffusion in dispersion fuels

Irradiation-induced diffusion in finite, two-phase systems is analyzed and applied to the geometry of dispersion fuels of the type used in research and test reactors. A fissioning sphere irradiates the surrounding medium in which fission also takes place. In place of the fission rate used in conventional irradiation-induced diffusion coefficient (D∗) correlations, the energy deposition rates due to electronic and nuclear stopping of the fission fragments are separated. The former is used to drive the point-defect contribution to D∗ and the latter is the source of the thermal-spike component. This separation accounts for the preponderance of electronic energy loss early in the track of a fission-fragment and the dominance of nuclear stopping near the end of the range. This distinction accounts for the difference in the relative intensities of these two energy loss modes in a fission-fragment that exits one phase and deposits energy in an adjacent medium in which D∗ is to be determined. Fission-fragment stopping powers and projected ranges are obtained from SRIM software, thereby permitting extraction of the two types of energy deposition rates from the fission rate. As expected, the ratio of nuclear stopping to electronic stopping in the medium surrounding a fissioning inclusion increases with distance from the interface. The effective irradiation-enhanced diffusivity for use in the diffusion equation depends upon two parameters: the fraction of D∗ in an infinite, homogeneous solid attributable to nuclear stopping and the ratio of the volumetric fission rates in the dispersed and continuous phases.