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Investigations Supporting MOX Fuel Licensing in ESNII Prototype Reactors

Periodic Reporting for period 2 - INSPYRE (Investigations Supporting MOX Fuel Licensing in ESNII Prototype Reactors)

Periodo di rendicontazione: 2019-03-01 al 2020-08-31

Fuel is at the heart of all nuclear reactor systems. Mastering the understanding of its behaviour is challenging due to the complex coupled phenomena (physical, chemical, radiation, thermal and mechanical) induced by fission. All occur in steep temperature gradients and have consequences at a multitude of length and time scales. Fuel performance predictions for licensing under normal operation and accidental conditions have relied traditionally upon extensive integral irradiation testing to generate empirical laws. Though eminently successfully deployed for the four fast reactors operated in Europe thus far, they are not easily extrapolated to other conditions prevalent for the licensing of first uranium-plutonium mixed oxides (MOX) cores for the ESNII reactor systems.
Leveraging the knowledge from past integral irradiation testing programmes is essential to overcome the challenges of timely cost effective licensing of ESNII first cores. The solution lies in a basic science approach to develop the intricate models underpinning the empirically derived performance laws, so that they can be extended into other operational regimes.
INSPYRE will bring a thorough understanding of fuel performance issues in normal and off-normal conditions through harnessing of basic and applied science. The goals of INSPYRE, focused almost exclusively on MOX fuel, are to:
1) utilise out of pile separate effect investigations to underpin basic phenomena governing fuel behaviour with soundly based physical models
2) perform additional examinations on selected irradiated samples to yield data when too little is currently available
3) combine the results of the separate effect experiments, physical modelling and simulation, and integral neutron irradiation tests to extend the reliability regime of traditionally deduced empirical laws governing various aspects of nuclear fuel under irradiation
4) use the models developed to enhance the efficacy and reliability of operational fuel performance codes.
The first two periods of INSPYRE brought significant advances in the understanding and simulation of nuclear fuels.

Three important properties affecting the margin to fuel melting were evaluated for (U,Pu)O2 as a function of the composition using experimental characterizations, atomic scale calculations and/or CALPHAD modelling: melting temperatures, thermal conductivity and specific heat.
Concerning self-diffusion properties of (U,Pu)O2, elementary mechanisms were investigated using electronic structure calculations. A complete thermo-kinetic model also was developed for Pu diffusion using available data and a semi-empirical model to estimate missing parameters. Furthermore, the feasibility of fabrication of diffusion couples was shown.
The analysis and interpretation of the experiments carried out on UO2 using ion irradiation and the development of an improved cluster dynamics model yielded significant new insight into the mechanisms governing gas behaviour and the interplay between gas and defects.
Set-ups aimed at measuring mechanical properties of UO2 and (U,Pu)O2 were developed. Measurements of fuel swelling under ion and neutron irradiation, which will enable us to estimate creep properties, have also started. Then, multiscale modelling from the atomic to the microscale yielded results on the underlying mechanisms of the evolution of mechanical properties under irradiation.
Concerning the properties of the Joint Oxyde-Gaine, numerous thermal properties were yielded by the experimental characterisations and modelling of Cs-Mo-Te-O compounds suspected to form in the JOG layer.

To capitalize the results of the basic research investigations, numerous physics-based models were developed to describe the fuel behaviour under irradiation at the grain scale. A modelling framework coupling gas release and swelling was developed and implemented in the SCIANTIX code. Concerning the mechanical modelling, novel correlations for the elastic constants, thermal expansion, and creep behaviour of MOX fuels were derived using recent experimental and modelling data and assessed. A methodology was developed to describe the creep behaviour of a polycrystalline microstructure. New correlations describing the melting temperature and the thermal conductivity as a function of the important parameters were also developed.
The implementation of the models developed in the three fuel performance codes considered, GERMINAL, MACROS and TRANSURANUS, was also completed, directly or via a coupling with the SCIANTIX grain-scale module. The detailed assessment of the code’s improved predictive capabilities has started against a selection of irradiation experiments. The codes are also applied to the simulation of ASTRID and MYRRHA cores.

Two education and training events were organised. The first INSPYRE school, co-organised with the GENIORS H2020 project, took place in May 2019. The third edition of the Nufuel-MMSNF workshop, coorganised by two INSPYRE partners, was held in November 2019.

Finally, exhibition material for general audience consisting of five posters on the energy mix, fission reactions, nuclear reactors, nuclear fuels and simulation codes was also designed.
The activities conducted during the first two periods and the results obtained show that INSPYRE is first and foremost a project beyond the state of the art in its approach and goals.
A novel approach is used for the development of models for fuel performance codes traditionally derived from fitting available pot-irradiation experiments. Then, the integration of the whole community working on fuels from the scientists performing basic research on nuclear fuels to fuel designers, which enables the coupling between materials engineering and physics/chemistry, is highly innovative.
Major advances are also made in techniques and methods, as shown by the large number of new experimental set-ups developed. This permits very detailed characterization and yields results of unprecedented quality. First-of-a-kind atomic scale calculations of complex properties on complex compositions are also performed.
This will bring the breakthrough needed on important operational issues for the licencing of MOX fuels. Significant progress will be made regarding the predictive capability of changes in material properties and behaviour and subsequent refinement of Generation-IV reactor design codes. The transfer of information and codes to manufacturers and operators will guarantee crucial benefits in overcoming the bottlenecks in the certification of materials safety of GEN IV systems.
The research conducted in INSPYRE will also have a scientific and technical impact: to solve the very complex issue of fuel behaviour under irradiation, INSPYRE needs excellent science and thus drives the need for improved experimental techniques and modelling methods at all scales. It will also answer a large number of basic and applied questions, which will bring progress in the fields of materials science and chemistry.
The significant number of PhD students and post-docs involved in INSPYRE, as well as the summer schools planned and the exchange scheme will bring a significant contribution to the training of the next generation of researchers on fuels and fuel performance codes.
Finally, the outreach activities will contribute to convincing young Europeans to become scientists to answer the challenges faced by Europe concerning energy needs and sustainability.
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