Modeling of nuclear charge polarization as part of fission yield evaluation: applications to actinides of interest to the nuclear fuel cycle

Nuclear data is crucial for civil nuclear energy applications, being the bridge between the micoscopic properties of nuclei and the “macroscopic good values” needed for cycle and reactor physics studies. The laboratory of physics studies at CEA/IRESNE Cadarache is involved in the evaluation of these nuclear physics observables, in the framework of the JEFF Group and the Coordinated Research Project (CRP) of IAEA. The recent development of a new methodology for thermal neutrons induced fission product yield evaluation (fission product yields after prompt neutron emission) has improved the accuracy of the evaluations proposed for the JEFF-4.0 Library, together with their covariance matrix. To extend the assessments of fission yields induced by thermal neutrons to the fast neutron spectrum, it is necessary to develop a coupling of current evaluation tools with fission fragment yield models (before prompt neutron emission). This coupling is essential to extrapolate the actual studies on thermal fission of 235U and 239Pu to less experimentally known nuclei (241Pu, 241Am, 245Cm) or to study the incident neutron energy dependence of fission yields. One of the essential missing components is the description of the nuclear charge distribution (Z) as a function of the mass of the fission fragments and the incident neutron energy. These distributions are characterized by a key parameter: the charge polarization. This polarization reflects an excess (respectively deficiency) of proton in light (respectively heavy) fission fragments compared to the average charge density of the fissioning nucleus. If this quantity has been measured for the 235U(nth,f) reaction, it is incomplete for other neutron energies or other fissioning systems. The perspectives of this subject concern as much the impact of these new evaluations on the key quantities for electronuclear applications as well as the validation of the fission mechanisms described by microscopic fission models.

Study of the dynamics of molten salt fast reactors under natural convection conditions

Molten Salt Reactors (MSRs) are presented as inherently stable systems with respect to reactivity perturbations, due to the strong coupling between salt temperature and nuclear power, leading to a homeostatic behavior of the reactor. However, although MSRs offer interesting safety characteristics, the limited operational experience available restricts our knowledge of their dynamic behavior.
This research work aims to contribute to the development of a methodology for analyzing the dynamics of MSRs, with the goal of characterizing complex neutron-thermohydraulic coupling phenomena in an MSR operating in natural convection, identifying potentially unstable transient sequences, prioritizing the physical phenomena that cause these instabilities, and proposing simple physical models of these phenomena.

This work will contribute to the development of a safety-oriented methodology that will help MSR designers better understand and model the reactor dynamic behavior during transients, through dimensional analysis and the study of the flow stability. This methodology aims to define simple and robust criteria to ensure the intrinsic safety of a fast-spectrum MSR, depending on its design and operational parameters allowing compliance with the operating domain limits.

This PhD lies at the crossroad of theoretical analysis of the physical phenomena governing the MSR’s behavior, particularly the study of unstable regimes (oscillatory or divergent in nature) due to neutron-thermohydraulic coupling under natural convection conditions, and the development of analytical and numerical tools for conducting calculations to characterize these phenomena.

The PhD student will be based within a research unit dedicated to innovative nuclear systems. He/she will develop skills in MSR modelling and safety analysis, and will have the opportunity to present his/her work to the international MSR research community.

Mapping the tower of nuclear Effective Field Theory

The ability of nuclear models to accurately predict the rich phenomenology emerging in nuclei (whether for fundamental purposes or nuclear data applications) is conditioned by the possibility to construct a systematically improvable theoretical framework, i.e. with controlled approximations and estimation of associated uncertainties and biases. This is the goal of so called ab initio methods, which rely on two steps:
1 - The construction of an inter-nucleon interaction in adequation with the underlying theory (quantum chromodynamics) and adjusted in small systems, following effective field theory paradigm.
2 - The resolution of nuclear many-body problem to a given accuracy (for structure or reactions observables). This provides predictions in all nuclei of interest and includes the uncertainty propagation stemming from the interaction model up to nuclear data predictions.

This PhD thesis mostly deals with Step 1. The goal of the thesis is to construct a family of ab initio interactions by developing a new adjustment procedure of the low energy constants (including the evaluation of covariances for sensitivity analysis). The adjustment will include structure data but also reaction observables in light systems. This will open the door to a new evaluation of p+n->d+gamma cross sections (which have large uncertainties despite their importance for neutronics applications) in the context of state-of-the-art effective fields theories.

The thesis will be done in collaboration between CEA/IRESNE (Cadarache) and IJCLab (Orsay), the PhD student will spend 18 months in each laboratories. Professional perspectives are academic research and R&D labs in nuclear physics.

Magnetic fusion turbulence: where do reduced models fail, how to enrich them?

One of the key challenges facing the field of fusion plasma modeling is the nonlinear nature of the plasma response. This means that factors such as temperature and density gradients, flows, and velocity gradients all have an impact on the transport of heat, particles, and momentum in complex ways. Modeling such a system requires a range of approaches, from the highly detailed flux-driven gyrokinetics method to simpler quasilinear models within an integrated framework. These have proven effective in interpreting experimental data and predicting plasma behaviour. However, there are two significant challenges to this approach. Firstly, modeling the peripheral region of the plasma edge, at the transition between open and closed field lines, is challenging due to the confluence of significantly different underlying physics. Recent research indicates that current quasilinear transport models may have significant shortcomings in this region. Secondly, modeling the 'near marginality' regime is challenging due to the fact that it involves a state of dynamic equilibrium where the system's behaviour is self-regulated by slow, large-scale modes. Computing this state is challenging and requires a flux-driven gyrokinetic approach to move away from the typical assumption of time scale separation between turbulence and transport. Recent work from within our team indicates that current quasilinear transport models may also be facing significant shortcomings in this regime. It is crucial to understand this regime in depth as it is relevant for future machine operation. We are now in a position to address these two issues, as we have access to cutting-edge in-house tools relevant to both ends of the spectrum.
We plan to compare transport predictions in the edge and near marginality regimes from the advanced flux-driven gyrokinetic code GYSELA with those from the integrated framework using the reduced quasilinear QuaLiKiz model. The research will contribute to the development of robust reduced models for transport, crucial for the interpretation of current experimental data and for future burning plasma operation.

Top