Correlation between simulations and cavitation-induced erosion damage in Spallation Neutron Source target modules after operation

Abstract

An explicit finite element (FE) technique developed for estimating dynamic strain in the Spallation Neutron Source (SNS) mercury target module vessel is now providing insight into cavitation-induced erosion patterns observed on interior surfaces of SNS targets during post-irradiation examination. The technique uses an empirically developed material model for the mercury that describes its volumetric stiffness combined with a tensile pressure cut-off limit to approximate the threshold and effect of cavitation. The longest period each point in the mercury is at the tensile cut-off threshold is denoted as “saturation time”. Patterns of saturation time can be obtained from the FE simulations and are being positively correlated with observed damage patterns as a qualitative measure of damage potential. Saturation time has been advocated by collaborators at the Japan Proton Accelerator Research Complex (J-PARC) as a factor in predicting bubble nuclei growth and collapse intensity. Larger ratios of maximum bubble-size-to-nucleus result in greater bubble collapse intensity; longer saturation times correlate to greater ratios. With the recent development of a user subroutine for the FE solver, saturation time is now provided over the entire mercury domain. Saturation time contour maps agree with patterns of damage seen on the SNS inner vessel beam window and elsewhere. The other simulation result which seems to correlate with observed damage patterns is the local mercury velocity. Related R&D has provided evidence that damage is mitigated by flow velocity. Surfaces which are near regions of low mercury velocity appear to be more vulnerable to damage than those where the mercury flow is strong and sustained. By combining the patterns of saturation time and velocity a viable explanation for observed damage patterns is presented.