This work will be done in the frame of studies on the thermo-mechanical and physico-chemical behaviour behaviour of the « uranium and plutonium mixed oxide fuel » during irradiation currently considered for the future reactors of 4th generation. Because of its particularly hight thermal level during irradiation this kind of fuel is subject to several physical and chemical phenomena duringf its stay in reactor. Those one can have a strong impact on the behaviour of the whole fuel element (pellet and clad), but we can focus on two specific phenomena that take place at middle and high burnup :
- the formation by evaporation-condensation of a fission products layer between the external surface of the fuel pellet and the inner surface of the cladding material at middle burnup, designed as JOG for Joint Oxyde-Gaine;
- the formation of a corrosion layer on the internal surface of the clad, containing fission products and elements constituting the cladding material at high burnup, and resulting from the FCCI (Fuel-Cladding Chemical Interaction),
The occurence of this two phenomena is a limiting factor for increasing the burnup. Thus it is important de be able to estimate quite precisely the chemical composition of the fuel pellet and of the fuel-to-clad gap during irradiation. Previous experimental work had shown that the JOG consisted mainly of caesium, molybdenum and oxygen, with the presence of other elements such as tellurium and barium. Observations have also shown the presence of chromium, iron and nickel, along with other volatile fission products (VFP), in areas of ROG. These observations were backed up by thermodynamic calculations, which led to the assumption that the JOG consisted mainly of caesium molybdate Cs2MoO4. However, until recently, there had been no direct evidence of the presence of this compound. Recently, more detailed characterisation methods carried out as part of a current thesis on (U,Pu)O2 fuel samples confirmed quantitatively that the JOG was mainly made up of Cs, Mo and O, but also of I and Ba distributed in several phases. Other elements were detected and measured in localised areas, namely Te, Zr as well as U and Pu. With regard to corrosion, phases based on Fe, Te and Pd were observed, as well as the joint presence of Cr and O.
At the same time, work was started on modelling the axial redistribution of caesium, with a view to improving the description currently used in GERMINAL. The chemical element inventory at a given axial dimension has a first-order effect on the calculated JOG thickness and ROG thickness.
The aim of this thesis is to improve the description and modelling of JOG and ROG formation in the GERMINAL scientific calculation tool (OCS), which is dedicated to calculating the thermo-mechanical and physico-chemical behaviour of 4th generation reactor fuel irradiated under nominal and/or incidental conditions.
To this end, research will be developed in three areas:
- Further development of the radial migration methodology adopted in the GERMINAL code through comparison with existing and recently obtained experimental results. This is based on a coupling with a thermochemistry module in which several hypotheses for the release of volatile fission products created in the pellet towards the pellet-cladding gap can be considered.
The aim of this PhD subject consists in improving the JOG and FCCI modeling into the fuel performance code (FPC) GERMINAL, dedicated to the calculation of the thermo-mechanical and physico-chemical behaviour of the 4th generation reactors’ fuel irradiated in normal and off-normal conditions. For that purpose, an acurrate experimental caractherization of some irradiated fuel samples, to which the PhD student will contribute, will be elaborated and coupled to a thermodynamic approach. The research will be based on the two items :
- Determination and experimental identification of the chemical elements and phases located into the fuel pellet, into the gap and at the fuel-to-clad interfaces at the end of the irradiation using the implementation of microprobe-SIMS-SEM/FIB techniques, by combining elemental and isotopic analysis results with microscopic observations.
- Comparison of the results with thermodynamic calculations: type and local quantities of the chemical phases formed in the fuel pellet as well as the phases constituting the JOG and those resulting from the FCCI.
Thus, based on those results, it will be possible to evaluate precisely the chemical composition of the irradiated fuel, of the JOG and of the corrosion compounds by using the FPC GERMINAL, from which the input inventory in chemical elements will be estimated in function of burnup at the different radial and axial localisations.
The PhD student will be attached both to a multi-scale modeling group and to an experimental team having sophisticated tools. Furthermore, academic or international collaborations are possible, in particular in the frame of the OECD/NEA with the development of the TAFID database. The student will have the opportunity to enhance the skills learned in the field of nuclear materials characterisation as well as in the field of thermodynamic calculations and irradiated fuel behaviour simulation.
To this end, the research will be developed along three lines:
- Further development of the radial migration methodology adopted in the GERMINAL code through comparison with existing and recently obtained experimental results. This is based on a coupling with a thermochemistry module in which several hypotheses for the release of volatile fission products created in the pellet towards the pellet-cladding gap can be considered.
- Further development of a [simplified] model for the axial redistribution of caesium and, by extension, of volatile fission products, leading to an initial implementation in the GERMINAL code for testing and preliminary validation of the axial inventories estimated by calculation by comparison with experimental results,
- Finally, thermodynamic calculations to determine the nature and local quantity of the chemical phases formed in the fuel pellet and the constituent phases of the JOG and ROG will be carried out on the basis of the axial inventories estimated by the GERMINAL code.
This will enable a more accurate assessment of the chemical composition of the irradiated fuel, the JOG and the ROG products as a function of the burn-up rate using the GERMINAL OCS as a function of time at the various radial and axial locations.
The PhD student will be integrated into the fuel behaviour study and simulation group(IRESNE Institute, CEA CAdarache) which has or is developing various simulation tools, and will also be able to interact with a characterisation laboratory with cutting-edge experimental tools. Academic and international collaborations are also possible, particularly within the OECD/NEA framework with the development of TAFID database. These will enable the PhD student to make the most of the skills he or she has acquired in the field of characterisation of nuclear materials, as well as in thermodynamic calculations and simulation of the physico-chemical behaviour of irradiated nuclear fuel.