Report on the improved models of melting temperature and thermal conductivity for MOX fuels and JOG

The current modelling of thermal properties (thermal conductivity, melting temperature) of mixed- oxide (MOX) nuclear fuels in Fuel Performance Codes (FPCs), e.g., GERMINAL, MACROS, TRANSURANUS, used in INSPYRE, is limited for various reasons. First, available experimental data concern mostly fresh MOX or Light Water Reactor (LWR) irradiation conditions, with only very few data about Fast Reactor (FR) MOX. Second, state-of-the-art correlations employed by FPCs mainly describe the temperature and porosity effects on the MOX thermal conductivity, without taking into account, e.g., the effect of the initial plutonium content. Task 6.3 of INSPYRE aims at overcoming these limitations by developing improved models for the thermal conductivity and the melting temperature of FR MOX, based on the most reliable and recent experimental data available in the open literature or accessible via the INSPYRE project. Additional experimental measurements performed in INSPYRE will also help to both further improve and validate the developed models herein presented. This document is the update of the version 1 published in March 2020. Since then, further analyses and developments (justified in Section 2) were performed, leading to two main modifications: ▪ An exponential degradation of the thermal conductivity with burnup is considered instead of a linear dependency ▪ A few spurious experimental points were eliminated from the fitting process of the melting (solidus) temperature because they correspond to fuel compositions outside the range of application of the correlation developed (i.e., Pu content > 50 wt.%, O/M ~ 1.90). This Deliverable first presents an exhaustive overview of the state of the art, both in terms of existing correlations (in literature and in FPCs) and available experimental data about MOX thermal conductivity and melting temperature. Then, the derivation and preliminary validation of new correlations oriented to FR MOX and dependent on all the significant parameters are described. Finally, the future model developments envisaged are outlined and the final assessment needed before the integration of these models in fuel performance codes is explained.