Mechanism of fuel-cladding-coolant chemical interactions and high temperature thermomechanical response of uranium nitride fuels

Abstract

Uranium mononitride (UN) is a leading fuel candidate for lead-cooled fast reactors (LFRs) and a potential option for nuclear thermal propulsion due to its high thermal conductivity and uranium metal density. UN is also a potential option as a long-term solution for accident tolerant fuels in the current light water reactor (LWR) fleet. However, significant knowledge gaps still exist regarding qualification and deployment in advanced reactors and space applications. Particularly, the chemical compatibility of a UN fuel-coolant-cladding system under reactor relevant temperature conditions and the thermomechanical response of UN under high-temperature environments and extreme transients have yet to be investigated. To address these knowledge gaps, this work presents a systematic investigation of: (1) the chemical interactions of UN, alumina-forming austenitic (AFA) alloys (a promising cladding candidate for LFRs), and liquid Pb; (2) the thermomechanical response of UN under high-temperature creep and ultrafast thermal transients. Understanding these behaviors under operationally relevant conditions is essential for safety assessment and eventual licensing of UN fuels for advanced reactors and space nuclear systems. Chemical interaction studies between UN and as-cast AFA and preoxidized AFA have been conducted at 823 K (designed clad temperature) and 1023 K (beyond designed clad temperature) using the diffusion couple method to characterize potential chemical interactions between fuel and cladding candidate. No chemical interactions were observed between preoxidized AFA and UN. The formation of AlN was observed along the interface between as-cast AFA and UN samples not containing low weight percents of UO2. At lower test temperature (823 K) the observation of AlN was sparse determining that the temperature dependent diffusion of Al through AFA matrix to be the rate controlling mechanism. In UN samples with low weight percents of UO2 no AlN formation was observed. Chemical interaction studies between UN and liquid Pb were also conducted at the same temperatures. Good chemical compatibility between UN and liquid Pb was observed, confirming the potential application of UN as a leading fuel candidate for LFRs. During reactor operation, nuclear fuel experiences thermal creep. However, very limited data is currently available on thermal creep of UN, and the development and validation of the thermal creep correlations is necessary to decrease the safety margins currently needed for fuel performance modeling of UN. Previous historical creep tests on UN have been conducted on limited temperature and stress ranges with little overlap between experimental tests. In this work, compressive creep tests were conducted on both high-density UN and UN samples with a controlled microstructure on temperatures ranging from 1673 – 1973 K and stress range of 20 – 60 MPa. The activation energy and stress exponent were calculated for both sets of creep tests, and similar values to historical tests were found. Texture measurements were conducted on the high-density creep samples post testing with x-ray diffraction. A weak (1 0 0) fiber texture was observed resulting from the uniaxial compressive creep tests. Thermal shock properties of UN and a UN, Mo, W cermet composite fuel (potential fuel for space application) was investigated with temperature ramping rates above 100 K/s and two maximum temperatures of approximately 1700 K and 2000 K achieved. The cermet composite displays improved thermal properties with a 56% increase in thermal conductivity at 1273 K and 15-20% decrease in thermal expansion coefficient up to the maximum test temperature of 1573 K. No macroscopic cracking of either UN or the cermet fuel was observed, however loss of Mo spheres from the cermet fuel microstructure was observed after thermal shock testing up to a maximum temperature over 2000 K, displaying a failure mode for fuels with a heterogenous microstructure.