Meso-scale multi-physics full coupling within porous CRUD deposits on nuclear fuel

Abstract

A 2D dimensional model was developed to simulate boron hideout within a porous Chalk River Unidentified Deposits (CRUD) on nuclear fuel in PWR. All thermal, fluid, transport, chemical, and radiolysis phenomena were fully coupled at meso-scale. A wick boiling structure with heat conduction was adopted, while Darcy's flow was solved for fluid dynamics in the porous medium. The diffusion and convection of soluble species were coupled with various chemical and radiolysis reactions. These reactions include water ionization, interaction of boric acid, adsorption of boron, precipitation of LiBO2, and distribution of volatile species at chimney surfaces. Results showed that boron hideout resulted from adsorption of boron in the lower side of CRUD and precipitation of LiBO2 in the upper side. Precipitation is a dominating mechanism. The precipitation of LiBO2 was shown to be dependent on the pH at the given temperature. Precipitated boron at CRUD close to bulk coolant might be an explanation why boron is easily re-dissolved into bulk coolant during shutdown chemistry. Also, this study compares thermal properties used in many models to compare differently predicted superheated liquid in the porous medium.