Silicon nitride protective coatings for Accident Tolerant Fuels

Accident tolerant fuels are proposed to extend the coping time of a core during a loss of coolant accident, in addition to providing enhanced resistance to other common reactor transients. Nearterm and long-term improvements in fuel and cladding combinations are proposed by fuel vendors to not only enhance the performance of the core during off-normal events but also to increase fuel efficiency while allowing more time between refueling, extending the current 18 month cycle to 24 months. Silicon nitride presents a refractory ceramic alternative to more reactive metallic coatings, (e.g. chromium) for current Zircaloy cladding. Silicon nitride forms a protective barrier to reduce oxidation kinetics (Zr + 2H2O(g) ? ZrO2 + 2H2(g)) in order to prevent hydrogen production during a loss of coolant accident. Phase I will present an opportunity to measure oxidation rates with high temperature steam cycles (T>1200°C) on a Zircaloy proxy material with and without silicon nitride (SixNy) coatings of varying thickness and stoichiometry. Material degradation and delamination studies will be performed with TEM, SEM, EDS, and XPS measurements. Silicon nitride will also be studied as an oxidation barrier to high density fuels such as uranium silicide (U3Si2) and uranium nitride (UN) through similar high temperature steam tests on a surrogate compound. These materials are considered long term accident tolerant fuel candidates due to their increased uranium density, thermal conductivity, and enhanced irradiation performance (hypothesized improved high burn up structure) compared to traditional UO2. This project represents a collaboration between Cerium Labs and Nuclear Materials Consulting, LLC in Phase 1. Cerium holds patents for silicon nitride coatings as well as ongoing DoDsupported work applying silicon nitride to the thermal barrier coatings of gas turbine engines. Nuclear Materials Consulting, LLC leverages the experience and expertise of two nuclear fuels leaders, Dr Elizabeth Sooby and Dr Brian Jaques. NMC will perform limited testing under Phase 1 in The Extreme Environments Materials Laboratory (PI: Prof. Elizabeth Sooby) at the University of Texas at San Antonio, a soon-to-be external service organization at UTSA. In Phase 2, if awarded, subawards to UTSA and Boise State University will facilitate extensive testing including oxidation rate determination in high temperature steam and pressurized water, advanced characterization, and graduate student engagement and training on the project.