Three dimensional multiphysics modeling and validation of CRUD-induced localized corrosion (CILC) in PWRs

Fuel rod integrity is of the utmost importance in pressurized water reactors (PWRs). One of the main threats is corrosion, specifically CRUD-induced localized corrosion (CILC) accelerated by the buildup of porous corrosion deposits (CRUD), which impede heat transfer and exaggerate water chemistry effects. Accurately predicting corrosion, particularly CILC, would mark a significant improvement in PWR plant operation and safety. The authors are developing a more science based, mechanistic, multiscale finite-element model of Zircaloy corrosion by investigating the corrosion of various Zircaloys under a range of applicable conditions and accidents. This new model, known as HOGNOSE, links together detailed simulations of CRUD's effects on cladding and Zircaloy corrosion, thereby simulating CRUD-cladding behavior under both operating and accident conditions. HOGNOSE incorporates oxygen transport through the metal and different phases of ZrO{sub 2}, metal/oxide phase change, cracking, and resultant effects of compressive stress, oxide off-stoichiometry, and phase change on oxygen diffusivity. It also simulates the microstructural evolution of the oxide by differentiating between both protective and transparent oxides, including the transition from one to the other. In contrast with other corrosion models, HOGNOSE does not assume any form of oxide thickness growth over time, but rather uses science based, mechanistic parameters to tune the simulation. HOGNOSE utilizes data from atomistic simulations and molecular dynamics in its treatment of oxygen transport in order to predict corrosion on the CRUD-cladding length scale. An accurate simulation of CILC, an inherently complex and coupled phenomenon, requires a combination of fundamental and engineering scale experiments for validation. As such, the authors rely on both controlled experimental studies of Zircaloy corrosion, as well as available measurements of oxide and CRUD thickness from operating commercial PWRs. Three dimensional simulations were used to predict the effects of a CRUD flake on the corrosion of the cladding beneath it. (author)