A Preliminary Assessment of the T91 Behaviour for the Extension of the TRANSURANUS Code to the Fuel Rod Performance Analysis of LBE Cooled Nuclear Reactors

This work presents the status quo of the efforts carried out in the last years at the Politecnico di Milano in order to assess the information available in the open literature on the T91 steel, which is designed to be the cladding material in LBE (Lead–Bismuth Eutectic) accelerator-driven systems (ADS), like the Italian XADS and the Belgian MYRRHA, as well as in lead-cooled nuclear reactors (e.g., ELSY – the European Lead System). The work, which is oriented to embody this knowledge in the TRANSURANUS code database for cladding materials, is organized as follows. In the first part, a critical review of the empirical correlations for the heat transfer to liquid metals is presented, with the aim of identifying the most suitable Nusselt number correlation for the evaluation of the cladding temperature, which plays an important role in the fuel rod performance (e.g., in terms of corrosion, swelling and creep behaviour). Corrosion resistance being a key issue in the development of LBE/lead-cooled reactors, the well-known Tedmon’s model is discussed in the second part. This model has been employed as the starting point for the prediction of the T91 corrosion behaviour in a LBE or lead environment for the conditions of the so-called “in situ” oxide layer protection: on the basis of experimental data from the recent literature, a preliminary correlation for the T91 corrosion in flowing LBE under oxygen control has been set-up. In the last years, efforts have been spent by several research groups to find ways (e.g., via aluminization) in order to form and keep protective films on the T91 steel, but tests simulating core operational conditions are needed to verify the integrity and the adhesion of such coatings, which would require a specific modelling by means of a fuel performance code like TRANSURANUS. The third part is focused on the out-of-pile properties of the T91 steel, with the aim of choosing the most reliable correlations to be implemented in TRANSURANUS. Showing a critical comparison, a database is eventually proposed that includes the following thermo-mechanical properties: melting temperature, thermal expansion, specific heat, density, thermal conductivity, yield strength, Young’s modulus, Poisson’s ratio, rupture strain. In the last part, some relevant phenomena affecting the thermo-mechanical behaviour of the "fuel rod system" under irradiation (like swelling and creep of the cladding steel) are discussed, pointing out some open issues to be solved and the missing experimental data needed for a suitable modelling of the performance in reactor of the T91 steel, so that this new nuclear material can be properly treated by means of the TRANSURANUS code.