Post-Ballooning and Burst Steam Oxidation of Accident Tolerant Zirconium Alloy Cladding with Cracked Chromium Coating

Abstract

This study presents a mechanistic model for coating cracking and subsequent oxidation of burst Cr-coated Zircaloy cladding under Design Basis Accident (DBA) conditions. A series of sequential simulated Loss Of Coolant Accident (LOCA) experiments were conducted using the i-LOCA facility and the high-temperature oxidation facility at Seoul National University. 1.5m long Cr-coated claddings(15μm) with inserted ZrO2 pellets were internally pressurized and inductively heated in an inert environment using the i-LOCA facility to induce ballooning and burst under various internal pressures and pellet configurations (cylindrical pellet(<55 GWd/MTU) and spherical pellet(~94 GWd/MTU)). The resulting post-burst cladding geometries with various burst hoop strains were analyzed via 3D scanning. The burst region (±1.5 inches from the burst center) of the post-burst specimens was subsequently subjected to two-sided oxidation using the high-temperature oxidation facility.Mechanistic models for coating cracking and Equivalent Cladding Reacted (ECR) calculation were developed based on the conventional definition of ECR and a phenomenological understanding derived from microstructural characterization of post-burst specimens, thereby providing a general framework applicable irrespective of the specific coating. Under DBA conditions, the additional oxidation attributable to coating cracking was quantified to be no more than 25% relative to inner-sided oxidation. Accident coping time analyses indicated that, even when accounting for burst-induced deformation and coating cracking, the application of Cr coating maintains compliance with existing regulatory safety margins.

Keywords: Coating cracking, Ballooning and burst, Steam oxidation, Accident Tolerant Fuel (ATF), Loss of Coolant Accident (LOCA)