Proof-of-principle of high-fidelity coupled CRUD deposition and cycle depletion simulation

Abstract

A multiphysics framework for the high-fidelity simulation of CRUD deposition is developed to better understand the coupled physics and their respective feedback mechanisms. This framework includes the primary physics of lattice depletion, computational fluid dynamics, and CRUD chemistry. The three physics are coupled together via the operator-splitting technique, where predictor–corrector and fixed-point iteration schemes are utilized to converge the nonlinear solution. High-fidelity simulations may provide a means to predict and assess potential operating issues, including CRUD induced power shift and CRUD induced localized corrosion, known as CIPS and CILC, respectively. As a proof-of-principle, a coupled 500-day cycle depletion simulation of a pressurized water reactor fuel pin cell was performed using the coupled code suite; a burnup of 31MWd/kgHM was reached. The simulation recreated the classic striped CRUD pattern often seen on pulled fuel rods containing CRUD. It is concluded that the striping is caused by the flow swirl induced by spacer grid mixing vanes. Two anti-correlated effects contribute to the striping: (1) the flow swirl yields significant azimuthal temperature variations, which impact the locations where CRUD deposits, and (2) the flow swirl is correlated to increased shear stress along the cladding surface and subsequent erosion of the CRUD layer. The CIPS condition of the core is concluded to be primarily controlled by lithium tetraborate precipitation, referred to as boron hideout, which occurs in regions experiencing subcooled nucleate boiling as soluble boron and lithium species reach their solubility limit within the CRUD layer. Subsequently, a localized reduction in power occurs due to the high neutron absorption cross section of boron-10.