Bison as a PCI screening tool - PCI failure model development

BISON is a multi-dimensional finite element-based nuclear fuel performance code, developed at Idaho National Laboratory (INL) under DOE's NEAMS and CASL Programs, with capabilities that are uniquely suited for the analysis of pellet-cladding interaction (PCI) problems. PCI-related failures are caused by fission-products-activated stress-corrosion-cracking (SCC) of the cladding at pellet radial cracks. This condition is aggravated by a manufacturing defect in the form of chipped pellet, referred to as missing pellet surface (MPS). The result is cladding failure by stress corrosion cracking induced by high concentrations of localized pellet-clad mechanical interaction (PCMI) stresses. BISON's intended application most affected by PCI include start-up power ramps, flexible-power/load-follow operations and operational transients. In the multi-physics application domain, BISON as part of CASL's VERA-CS coupled system of codes, which include neutronics, thermal hydraulics, coolant chemistry and fuel performance, is used for core wide PCI screening analysis. In such analysis BISON would calculate the cladding stress concentrations at the expected points of failure but cannot predict cladding failure because it lacks a failure model that relates calculated stresses to actual failure. This paper describes the development of a PCI failure model and outlines a procedure for PCI-screening failure-analysis utilizing the developed failure model.