Periodic Reporting for period 3 - INSPYRE (Investigations Supporting MOX Fuel Licensing in ESNII Prototype Reactors)
Reporting period: 2020-09-01 to 2022-02-28
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) use 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 when too little data is 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 used to describe nuclear fuel under irradiation
4) use the models developed to enhance the efficacy and reliability of operational fuel performance codes.
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) use 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 when too little data is 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 used to describe nuclear fuel under irradiation
4) use the models developed to enhance the efficacy and reliability of operational fuel performance codes.
Thanks to the combination of basic and applied research, INSPYRE brought significant advances on operational issues essential for the safety assessment and qualification of MOX fuels for future nuclear reactors, as well as on the simulation of their behaviour in reactor.
The new experimental and modelling results obtained constitute a significant and consistent contribution to the knowledge and understanding of the physical, thermal, chemical and mechanical properties of MOX fuels, the inert gas behaviour, the “Joint Oxyde-Gaine” (JOG) layer and the interaction between MOX and cladding at high temperature.
The thermodynamic modelling of the (U-Pu-Am-O) system was improved and the specific heat of MOX was evaluated at very high temperatures using atomic scale calculations. A new self-diffusion model for plutonium in MOX fuel and an associated mobility database was developed.
Concerning the inert gas behaviour, diffusion coefficients of Xe, Kr and He in fresh UO2 and MOX were determined using experimental techniques and modelling methods from the atomic to the grain scale. Various MOX fuel samples irradiated in past campaigns were characterised at the nanometre scale.
Complementary techniques and methods were combined to a precise control of materials and test conditions to determine mechanical properties and get further insight into the elementary processes governing their evolution under irradiation.
To improve the knowledge on the JOG layer formation, the transport properties of Cs and I were evaluated and the thermodynamic models of the Te-U, O-Te-U and Cs-Mo-Te-O systems were improved.
A significant part of these results were used to improve behaviour included in fuel performance codes. More physically justified correlations for were developed for the thermal and mechanical properties of MOX fuels including minor actinides as a function of all the parameters of interest in conditions relevant for GEN IV reactors. The modelling of fission gas and helium behaviour in MOX fuels were improved significantly thanks to the development of several physics-based models and their implementation in the SCIANTIX grain-scale module. An advanced micro-mechanical model suitable for both normal operation and off-normal conditions and including a description of the fuel rupture was also developed. Data obtained in the second half of the project are available to improve further the models and the codes.
The data obtained and models developed in the project were used to improve three European fuel performance codes: GERMINAL, MACROS and TRANSURANUS. The novel correlations for MOX thermal-mechanical properties obtained in the project were implemented in the codes and these were coupled with the SCIANTIX grain-scale module for inert gas behaviour.
The improved versions of the codes were then assessed on the simulation of the three past fast-neutron irradiation experiments SUPERFACT-1, RAPSODIE-I and NESTOR-3. The code validation against local and integral experimental data reveals harmonized predictions.
The new versions of the codes were used to simulate normal operation conditions in the ASTRID sodium fast reactor prototype, as well as transient conditions in the MYRRHA accelerator driven system. This enabled the evaluation of the safety margins of the fuel designs in the conditions considered.
These results were presented regularly to the project's User Group, which included designers of the ESNII prototypes, utility managers and fuel manufacturers.
Two schools dedicated to young researchers and three workshops were co-organized by INSPYRE partners, which contributed to present the results of the project to the nuclear fuel community. Exhibition material for general audience consisting of five posters on the energy mix, fission reactions, nuclear reactors, nuclear fuels and simulation codes and a video summarizing the objectives and results of INSPYRE were released.
The new experimental and modelling results obtained constitute a significant and consistent contribution to the knowledge and understanding of the physical, thermal, chemical and mechanical properties of MOX fuels, the inert gas behaviour, the “Joint Oxyde-Gaine” (JOG) layer and the interaction between MOX and cladding at high temperature.
The thermodynamic modelling of the (U-Pu-Am-O) system was improved and the specific heat of MOX was evaluated at very high temperatures using atomic scale calculations. A new self-diffusion model for plutonium in MOX fuel and an associated mobility database was developed.
Concerning the inert gas behaviour, diffusion coefficients of Xe, Kr and He in fresh UO2 and MOX were determined using experimental techniques and modelling methods from the atomic to the grain scale. Various MOX fuel samples irradiated in past campaigns were characterised at the nanometre scale.
Complementary techniques and methods were combined to a precise control of materials and test conditions to determine mechanical properties and get further insight into the elementary processes governing their evolution under irradiation.
To improve the knowledge on the JOG layer formation, the transport properties of Cs and I were evaluated and the thermodynamic models of the Te-U, O-Te-U and Cs-Mo-Te-O systems were improved.
A significant part of these results were used to improve behaviour included in fuel performance codes. More physically justified correlations for were developed for the thermal and mechanical properties of MOX fuels including minor actinides as a function of all the parameters of interest in conditions relevant for GEN IV reactors. The modelling of fission gas and helium behaviour in MOX fuels were improved significantly thanks to the development of several physics-based models and their implementation in the SCIANTIX grain-scale module. An advanced micro-mechanical model suitable for both normal operation and off-normal conditions and including a description of the fuel rupture was also developed. Data obtained in the second half of the project are available to improve further the models and the codes.
The data obtained and models developed in the project were used to improve three European fuel performance codes: GERMINAL, MACROS and TRANSURANUS. The novel correlations for MOX thermal-mechanical properties obtained in the project were implemented in the codes and these were coupled with the SCIANTIX grain-scale module for inert gas behaviour.
The improved versions of the codes were then assessed on the simulation of the three past fast-neutron irradiation experiments SUPERFACT-1, RAPSODIE-I and NESTOR-3. The code validation against local and integral experimental data reveals harmonized predictions.
The new versions of the codes were used to simulate normal operation conditions in the ASTRID sodium fast reactor prototype, as well as transient conditions in the MYRRHA accelerator driven system. This enabled the evaluation of the safety margins of the fuel designs in the conditions considered.
These results were presented regularly to the project's User Group, which included designers of the ESNII prototypes, utility managers and fuel manufacturers.
Two schools dedicated to young researchers and three workshops were co-organized by INSPYRE partners, which contributed to present the results of the project to the nuclear fuel community. Exhibition material for general audience consisting of five posters on the energy mix, fission reactions, nuclear reactors, nuclear fuels and simulation codes and a video summarizing the objectives and results of INSPYRE were released.
A novel approach was used for the development of models for fuel performance codes traditionally derived from fitting available pot-irradiation experiments.
Major advances were also made in techniques and methods, as shown by the first-of-a-kind atomic scale calculations of complex properties on complex compositions and the large number of new experimental set-ups developed, which yield results of unprecedented quality.
Significant progress was made regarding the predictive capability of changes in material properties and behaviour in operational conditions, which has brought refinement of Generation-IV reactor design codes. The transfer of information and codes to manufacturers and operators guarantees crucial benefits in overcoming the bottlenecks in the certification of materials safety of GEN IV systems.
The research conducted in INSPYRE also has a scientific and technical impact: to solve the very complex issue of fuel behaviour under irradiation, INSPYRE needed excellent science and has thus driven the need for improved experimental techniques and modelling methods at all scales.
The significant number of PhD students and post-docs involved in INSPYRE, the summer schools and the exchange scheme contributed significantly to the training of the next generation of researchers on fuels and fuel performance codes.
Finally, outreach activities contributed to inform the public on the research conducted on nuclear energy, which is part of the answer to the challenges faced by Europe concerning energy needs and sustainability.
Major advances were also made in techniques and methods, as shown by the first-of-a-kind atomic scale calculations of complex properties on complex compositions and the large number of new experimental set-ups developed, which yield results of unprecedented quality.
Significant progress was made regarding the predictive capability of changes in material properties and behaviour in operational conditions, which has brought refinement of Generation-IV reactor design codes. The transfer of information and codes to manufacturers and operators guarantees crucial benefits in overcoming the bottlenecks in the certification of materials safety of GEN IV systems.
The research conducted in INSPYRE also has a scientific and technical impact: to solve the very complex issue of fuel behaviour under irradiation, INSPYRE needed excellent science and has thus driven the need for improved experimental techniques and modelling methods at all scales.
The significant number of PhD students and post-docs involved in INSPYRE, the summer schools and the exchange scheme contributed significantly to the training of the next generation of researchers on fuels and fuel performance codes.
Finally, outreach activities contributed to inform the public on the research conducted on nuclear energy, which is part of the answer to the challenges faced by Europe concerning energy needs and sustainability.