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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - INSPYRE (Investigations Supporting MOX Fuel Licensing in ESNII Prototype Reactors)

Teaser

Summary

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 (full length pins and assemblies) 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 (high Pu content, low temperature operation, coolant interactions, etc.) prevalent for the licensing of first uranium-plutonium mixed oxides (MOX) cores for the reactor systems of the European Sustainable Nuclear Industrial Initiative (ESNII).
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. A first proof of principle of this approach was made on uranium dioxide in the F-BRIDGE project (2008-2012).
INSPYRE is the unique path forward to cost effective nuclear fuel licensing. It 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, focussed almost exclusively on MOX fuel, are:
1) To utilise out of pile separate effect investigations to underpin basic phenomena governing fuel behaviour with soundly based physical models. This approach is applied to four important operational issues: Margin to fuel melting; atom transport and fission product behaviour; evolution of mechanical properties under irradiation; fuel thermochemistry and interaction with the cladding.
2) To perform additional examinations on selected irradiated samples to yield data when too little is currently available
3) To 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) To use the models developed to enhance the efficacy of operational fuel performance codes and to improve their reliability in normal and off-normal situations.

Work performed

The main objective of the first period was to lay solid foundations for the work planned in INSPYRE.
First, available data and models were analysed and gaps were identified. In particular, a comprehensive review of the results available in the literature and of models for MOX fuel in fast reactor conditions currently implemented in codes was published in the first technical deliverable of the project.
Then, a large number of new experimental set-ups were developed and commissioned in hot labs of several partners to enable the study of UO2 and MOX fuels, in particular an electrical conductivity device, a positron annihilation lifetime spectrometer, a compression test equipment with control of the oxygen content, a Raman spectrometer and laser heating devices. These are now available for the activities of INSPYRE or will be very soon. In addition, feasibility studies were completed and first detailed characterizations of uranium-plutonium oxide samples were carried out.
There was also significant progress in the preparation of the two experiments planned in large research infrastructures, the cyclotron of the CNRS/CEMHTI and the High Flux Reactor in Petten, for the measurement of creep under irradiation and the experiments will start in 2019.
Then, the modelling activities from the atomic to macroscopic scale have started to yield significant results. At the atomic scale, first-of-a-kind electronic structure calculations were performed on defect behaviour and fission gas incorporation in MOX. Then, to extend the simulation to mesoscale in time, the capability of the Adaptive Kinetic Monte Carlo method for the investigation of fuel under irradiation has been assessed.
At the microscale, physics-based models describing inert gas behaviour and mechanical evolution were developed using recent mathematical advances. New conductivity and melting temperature correlations including the effect of temperature, plutonium content, stoichiometry, porosity and burn-up were also obtained from the most recent data. These new models are available for implementation in fuel performance codes.
Finally, the preparation of the education and training activities has been an important focus of this first period: the exchange scheme fostering mobility of researchers between the partners was implemented. Then, two events were prepared: the first INSPYRE school, which will take place in May 2019 in Delft (the Netherlands) and the third edition of the Nufuel-MMSNF workshop, which will be held in November at Paul Scherrer Institut (Switzerland).

Final results

The activities conducted in the first period 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 will permit very detailed characterization and yield 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.