A model that describes creep behavior is essential in the design or life assessment of components and systems that operate at high temperatures. Using the RCC-MRx data and the LCSP (logistic creep strain prediction) model, processed design data were generated over the whole creep regime of 316L(N) steel—i.e., primary, secondary, and tertiary creep. The processed design data were used to develop three models with different approaches for the creep rate: a phenomenological approach; an artificial neural network; and an artificial intelligence method based on symbolic regression and genetic programming. It was shown that all three models are capable of describing the true creep rate as a function of true creep strain and true stress over a wide range of engineering stresses and temperatures without the need of additional micro-structural information. Furthermore, the results of finite element simulations reproduce the trends of experimental data from the literature.

This paper presents a finite element analysis of the small punch creep test for 316L(N), which is compared with experimental data for 650 and 700 °C. Special emphasis is placed on (i) assessing the influence of friction and (ii) comparing two different creep models: the simple Norton creep and the more general creep model. The computed normalized deflection rate versus time is almost identical for all cases, which allows for scaling of the results. The computed time to rupture increases linearly with the friction coefficient due to a reduction in the mean stress. There is a good overall agreement between the experimental values and the computed deflection rate for a friction coefficient of around 0.3. It is shown that the initial reduction in deflection rate is due to stress relaxation and homogenization, and is only marginally affected by primary creep hardening. The computed results are compared with the equivalent stress and strain rates in the recently published small punch standard (EN 10371). The computed von Mises stresses at minimum deflection decrease linearly with the friction coefficient but are consistently slightly higher than the equivalent stress in the standard. For the strain rates, the computed values are significantly higher than the equivalent values in the standard. The presented simulations give a deeper insight of the small punch creep and impact of key parameters such the friction coefficient and in general as a guidance to refinement and improvement of the empirically based formulae in the standard.

The two running EURATOM-funded projects EU-QUALIFY and EU-CONVERSION, coordinated by the HERACLES consortium, are developing and further qualifying advanced LEU fuel systems for conversion purposes of high-power research reactors (HPRRs). Three fuel systems have been identified as potential candidates to replace the use of highly-enriched uranium fuel, namely the uranium silicide dispersion fuel (U₃Si₂/Al), the U-10Mo monolithic fuel and the dispersed U-7Mo/Al fuel system. The U-Mo monolithic and the U₃Si₂/Al dispersion fuel systems are currently being advanced in technology readiness level, towards more representative generic test assemblies, mimicking more closely target reactor specific conditions. A number of irradiation experiments have so far been conducted on the two dispersion fuel systems and the first monolithic irradiation experiment at the Belgian BR2 reactor is targeted to commence during 2025. The post-irradiation examinations of the historical and ongoing tests will be concluded within the projects and the complementary HERACLES work programme. These projects are also important contributors in establishing industrial-level fabrication processes of high-loaded and high-density uranium silicide fuels as well as U-Mo monolithic fuels. The operational times of the current HPRRs are foreseen to be extended by their conversions and, thus, have an essential impact on the future European supply of reactor-produced medical radioisotopes.

Working group WG59 CEN/TC54 has drafted a CEN technical report on how to determine no- and negligible creep temperature limits to support EN 13445-3 (unfired pressure vessels). Presently the standard assumes “rule-of-thumb” values for no-creep temperatures of ferritic and austenitic steels, 375°C and 425°C correspondingly. It is though well known that the lower ferritic grades could be creeping below the limit temperature and creep strength enhanced ferritic steels may show significant creep at temperatures well beyond it. Similarly, austenitic steels; nickel and other alloys are likely to have an even wider range of limit temperatures. This paper describes the data assessments and test methods used to determine limit temperatures for a specific set of steels and comparisons are made to other standards and nuclear design codes. The new approaches are promoting iso-stress testing in the low temperature range and applying Wilshire equation assessments on historical data.