A numerical study on parameters affecting boric acid transport and chemistry within fuel corrosion deposits during crud induced power shift

Crud buildup within Pressurized Water Reactors with or without power uprates has led to axially downward shift in power production. This shift in power has been defined as Axial Offset Anomaly (AOA). Boric acid, which is used as a soluble neutron absorber in PWRs, has been identified as the chief contributor to AOA through accumulation within porous crud. Previously, one-dimensional transport and chemistry models for solute in porous structure have been studied. This study aims to advance previously developed thermal hydraulics model to study boric acid transport within crud; and develop a 2-D coupled crud-chemistry and thermal hydraulics model to serve as a framework for development of advanced crud deposition and growth model. Various parameters affecting boric acid transport in crud; along with a model for aqueous chemistry of lithium hydroxide, LiOH, and boric acid, H3BO3, within crud were simulated. A two-dimensional wick-boiling model has been utilized to simulate thermal hydraulic conditions in the crud. Evaluations shows elevated cladding surface temperature for simulated operational conditions. Using past data obtained from Callaway plant, estimation of boron leading to AOA in the reactor core was done and compared to published results from neutronic simulations. A crud-chemistry model is proposed to provide a basis for evaluation of precipitate formation and pH changes within the crud. Results from this thermal hydraulic model indicate porosity, crud-thickness and heat flux as the key factors in H3BO3 transport in crud. Plant data evaluations show that boron estimations using current model agrees with results from other neutronic simulations. The crud-chemistry model suggests precipitation of Lithium Metaborate LiBO2 is significant when crud-thickness is greater than 40 um at normal power levels. Precipitation of LiBO2 leads to significant variation of pH in the crud and suppression of corrosion mechanisms of Zircaloy tubes. Results from this model were found to be consistent with published plant observations and comparative modeling studies elsewhere