Microscopic nuclear structure models to study de-excitation process in nuclear fission
The FIFRELIN code is being developed at CEA/IRESNE Cadarache in order to provide a detailed description of the fission process and to calculate all relevant fission observables accurately. The code heavily resides on the detailed knowledge of the underlying structure of the nuclei involved in the post-fission de-excitation process. When possible, the code relies on nuclear structure databases such as RIPL-3 that provide valuable information on nuclear level schemes, branching ratios and other critical nuclear properties. Unfortunately, not all these quantities have been measured, nuclear models are therefore used instead.
The development of state-of-the-art nuclear models is the task of the newly-formed nuclear theory group at Cadarache, whose main expertise is the implementation of nuclear many-body solvers based on effective nucleon-nucleon interactions.
The goal of this thesis is to quantify the impact of the E1/M1 and E2/M2 strength functions on fission observables. Currently, this quantity is estimated using simple models such as the generalized Lorentzian. The doctoral student will be tasked with replacing these models by fully microscopic ones based on effective nucleon-nucleon interaction via QRPA-type techniques. A preliminary study shows that the use of macroscopic (generalized Lorentzian) or microscopic (QRPA) has a non-negligible impact on fission observables.
Professional perspectives for the student include academic research as well as theoretical and applied nuclear R&D.
Measurement and evaluation of the energy dependence of delayed neutron data from 239Pu
This PhD proposal aims to measure and characterize the delayed neutron emissions from the fission of 239Pu. This actinide is involved in various reactor concepts, and the nuclear data available remains insufficient, particularly with fast neutrons. The project has a strong experimental focus, with multiple measurement campaigns at the MONNET electrostatic accelerator from JRC Geel, in which the candidate will actively participate.
The first phase focuses on the intercomparison of the neutron flux measurement methods (dosimetry, fission chamber, long-counter detector and recoil proton scintillator) which will be confronted to Monte-Carlo simulations of neutron emission from charged particle interactions (D+T, D+D, p+T). This work will ensure proper neutron flux characterization, a crucial step for the project.
Next, the candidate will replicate the delayed neutron measurements for ²³8U using an existing target in order to verify the results from a 2023 experimental campaign.
Finally, the candidate will measure the delayed neutron yields and group abundances for ²³?Pu in a neutron energy range from 1 to 8 MeV. The objective is to produce an energy-dependent evaluation, integrated into an ENDF file, to be tested on reactor calculations (beta-eff, power transients, absorber efficiency calibration, etc.). These measurements will complement a thermal spectrum study conducted at ILL in 2022, forming a coherent model for ²³?Pu from 0 to 8 MeV.
This project will contribute to the OECD/NEA's JEFF-4 nuclear data file, addressing a strong demand from the nuclear industry (highlighted by the IAEA) to improve the precision of multiplicity measurements and delayed neutron kinetic parameters, thus enhancing reactor safety and reducing safety margins.
Investigation and use of uranium glasses for optical neutron detection
The Dosimetry, Sensors and Instrumentation Laboratory of the CEA/IRESNE Cadarache develops, manufactures and operates neutron flux detectors used in the vicinity of and inside nuclear reactor cores. In addition to conventional detectors (fission chambers, collectrons, etc.), the laboratory is working on innovative measurement methods such as optical detectors, semiconductors, fiber scintillators, etc. As part of this PhD thesis, the laboratory wants to explore the potential of Uranium-doped glasses. These glasses are known to show bright fluorescence under various types of radiations. The main idea of this thesis is to try to exploit this fluorescence to detect the fission reactions induced when the glass is exposed to a neutron flux. This could enable the development of a new generation of optical neutron detectors halfway between a fission chamber and a scintillator.
The thesis will focus on two main topics:
- firstly, a detailed understanding of fluorescence mechanisms, and the synthesis of uranium glass with properties optimized for our needs (sensitivity, emission spectrum, isotopic vector, etc.). Synthesis will be carried out in partner laboratories;
- secondly, the development of a dedicated instrumentation, probably in the form of optical fibers, to test these prototypes in a reactor.
Kinetics of the Melting Front in a Phase Change Material Used for Decay Heat Removal in an Innovative Nuclear Reactor
In the context of developing innovative sodium-cooled fast reactors (SFR), this PhD aims to explore the use of a phase change material (PCM) to remove residual power. The PCM studied in this project is Zamak, a metallic alloy that presents advantageous properties for such thermal applications. Some SFR designs incorporate passive safety systems intended to ensure the removal of residual power, which refers to the heat generated by delayed fission and radioactive decay of fuel isotopes after reactor shutdown. The use of PCM is a promising option, as they can absorb and store heat through a melting process and subsequently release it gradually during a solidification process.
The core of this PhD focuses on Computational Fluid Dynamics (CFD) modeling of the Zamak melting process and the scaling of this model for use in a system-size calculation tool. The main challenge lies in predicting the behavior of the melting front, its stability, and its impact on the kinetics of residual power removal. This melting front is influenced by numerous factors such as the wetting angle and the physico-chemical properties of the PCM-wall or PCM-surrounding gas interface, which will be examined throughout the thesis. The research will thus involve developing a CFD model that integrates these aspects, using a porous enthalpy approach, allowing predictive simulations of the PCM's behavior in the residual power removal system. A scaling analysis will then be conducted.
The PhD candidate will be part of a research team on innovative reactors at the IRESNE institute located at the CEA Cadarache site. Career opportunities after the thesis include academic research, R&D, and the nuclear industry, as well as sectors utilizing PCM technologies.
Deterministic neutron calculation of soluble-boron-free PWR-SMR reactors based on Artificial Intelligence
In response to climate challenges, the quest for clean and reliable energy focuses on the development of small modular reactors using pressurized water (PW-SMR), with a power range of 50 to 1000 MWth. These reactors aimed at decarbonizing electricity and heat production in the coming decade. Compared to currently operating reactors, their smaller size can simplify design by eliminating the need for soluble boron in the primary circuit water. Consequently, control primarily relies on the level of insertion of control rods, which disturb the spatial power distribution when control rods are inserted, implying that power peaks and reactivity are more difficult to manage than in a standard PWR piloted with soluble boron. Accurately estimating these parameters poses significant challenges in neutron modeling, particularly regarding the effects of the history of control rod insertion on the isotopic evolution of the fuel. A thesis completed in 2022 explored these effects using an analytical neutron model, but limitations persist as neutron absorbers movements are not the only phenomena influencing the neutron spectrum. The proposed thesis seeks to develop an alternative method that enhances robustness and further reduces the calculation biases. A sensitivity analysis will be conducted to identify key parameters, enabling the creation of a meta-model using artificial intelligence to correct biases in existing models. This work, conducted in collaboration with IRSN and CEA, will provide expertise in reactor physics, numerical simulations, and machine learning.
Systematic study of the neutron scattering reactions on structural materials of interest for nuclear reactor applications
Elastic and inelastic scattering reactions on structural materials have a significant impact on the simulation of neutron transport. The nuclear data of structural materials of interest for nuclear reactors and criticality studies must be known with good precision over a wide incident neutron energy range, from a few tens of meV to several MeV. The thesis proposal aims to carry out a systematic study of the scattering reactions above the resolved resonance range up to 5 MeV. In this energy range, neither the R-Matrix formalism nor the statistical Hauser-Feshbach model are valid for structural materials. A new formalism will be developed by using high-resolution measurements of the scattering angular distributions. This work will focus more precisely on measurements already done at the JRC-Geel facility (sodium [1], iron [2]) and will be extended to other elements studied within the framework of the IAEA/INDEN project, such as copper, chromium and nickel. As part of this thesis, the experimental database will be complemented by new measurements on the copper isotopes (Cu63 and Cu65). The measurements will be carried out at JRC Geel GELINA facility with the ELISA detector. Concerning the copper isotopes, integral benchmarks from the ICSBEP database revealed several issues in the nuclear data libraries, which provide contradictory integral feedbacks on the nuclear data of U235. For example, the ZEUS benchmarks, which is routinely used to study the capture cross section of U235 in the fast neutron energy range, are very sensitive to the nuclear data of copper. This type of benchmark will provide an ideal framework for quantifying the impact of any new formalism developed to evaluate the nuclear data of structural materials.
This study will allow the PhD student to develop skills in experimental and theoretical nuclear physics, as well as in neutron physics. The results will be communicated to the JEFF working group of the Nuclear Energy Agency (OCDE/AEN).
[1] P. Archier, Contribution à l’amélioration des données nucléaires neutroniques du sodium pour le calcul des réacteurs de génération IV, Thèse, Université de Grenoble, 2011.
[2] G. Gkatis, Study of neutron induced reaction cross sections on Fe isotopes at the GELINA facility relevant to reactor applications, Thèse, Université Aix-Marseille, 2024.
Integral measurement of fission products capture cross-section using a combination of oscillation and activation techniques
This thesis is proposed as part of the POSEIDON (Fission Product Oscillation Experiments for Improving Depletion Calculations) project that deals with the integral measurement of the neutron capture and scattering cross-sections of the main fission products contributing to the reactivity loss in irradiated fuel. It consists of measuring the reactivity effect of separated isotope samples using a pile oscillation device, coupled with neutron activation measurements, in three different core spectral configurations : thermal, PWR and epithermal.
Part of the work will be done at CEA IRESNE in Cadarache and part at the Research Center of the Czech Republic, CV Rez. The PhD student will be involved in testing and optimizating the oscillation device that is currently being designed, as well as performing the measurements in the LR-0 Czech experimental reactor. The work at Cadarache will be on the analysis of the measurements with MC simulation tools. Functionalities needed for data analysis will require additional developments of the codes by the student.
The expected impact is a better prediction of the reactivity loss in reactor cores as a function of burn-up. Indeed, even with the most recent international nuclear data libraries, there is an important bias in the estimation of this reactivity loss.
The PhD student will develop competences in experimental and theoretical neutronics. Following job opportunities include R&D laboratories and nuclear industry.
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.