Functional renormalization group for nuclear structure theory

The atomic nucleus is the epitome of complexity : it is a strongly correlated system of nucleons (which are themselves composite degrees of freedom) coupled via the strong and electroweak interactions which features a wealth of emergent behaviors (deformation, supefluidity, clustering, ...). The long term endeavor of nuclear structure theory is to understand and predict how an arbitrary number of nucleons self-organize and become disorganized in nuclei. Among the various theoretical frames, the energy density functional (EDF) method, close, yet different from the density functional theory, provides the best compromise between the robustness of the description and its numerical complexity. However, the phenomenological ingredients entering the formulation of standard EDFs affect their predictive power.
The postdoctoral project aims at formulating the EDF approach from first principles, in order to benefit from a theoretical frame with both a maximal predictive power and a favorable numerical cost. The supervising team has identified the functional renormalization group (FRG) as the most relevant language for such a non empirical reformulation of the EDF method.
The present projet aims at formulating the EDF method from first principles via the FRG.

Evolution of ISAAC and Xpn codes for an extension of the QRPA method to the complete processing of odd nuclei; towards a database without interpolation for odd nuclei

The treatment of odd-isospin nuclei in microscopic approaches is currently limited to the so-called «blocking» approximation. In the Hartree-Fock Bogolyubov (HFB) approach, the ground state of an odd-mass nucleus is described as a one-particle excitation (qp) on its reference vacuum. Thus, in the QRPA approach, where the basic excitations are states «with 2 quasi-particles», the blocked qp is excluded from the valence space under the Pauli exclusion principle. As a result, the chosen qp is a spectator and is not involved in the QRPA collective states. If the single nucleon should have a significant contribution some levels will not be reproduced. The development in the QRPA codes (ISAAC and Xpn) of a procedure that allows all nucleons to participate in collective states is mandatory for a microscopic description of odd nuclei. Moreover, recent Xpn developments have allowed the description of forbidden ß- first decays improving the estimation of half-life time of fission fragments. This could be extended to address ß+ and electronic captures and could be adapted to large-scale calculations useful for nuclear astrophysics.

Innovative strategies for minor actinides using molten salt reactors

Within the framework of the ISAC (Innovative System for Actinides Conversion) project of the France Relance initiative, preliminary concepts of molten salt reactor capable of incinerating minor actinides have to be proposed in connection with prospective évolutions of the French nuclear fleet (stabilisation or reduction of the plutonium and americium inventory, minimization of the deep storage footprint, …) and contraints linked to the nuclear fuel cycle (plutonium and minor actinides inventories). The specificities of molten salt reactors will be exploited to design innovative transmutation strategies.
The postdoctoral fellow will be based in the reactor and fuel cycle physics unit of the IRESNE R&D institute at CEA Cadarache. He/she will develop expertise in neutronics, fuel physics, and in the design of Generation-IV reactors of the molten salt type.

Tensor factorization for the nuclear many-body problem

Numerical studies of laser plasma interaction in intermediate field on Laser Megajoule

In the Inertial Confinement Fusion experiments (ICF), intense laser beams cross a gas filled hohlraum. The gas is fully ionized and laser beams then propagate into a sub-critical plasma where laser plasma instabilites can develop. Optical smoothing techniques enable to break both spatial and temporal coherences so that both spatial and temporal scales of the beam become smaller than those required for the development of the instabilites. The breaking of spatial coherence is done thanks to the use of a phase plate which spreads the laser energy in a multitude of light grains called speckles. The breaking of temporal coherence is done by using a phase modulator which widens the spectrum and by dispersing each frequency with a grating. It is essential to know the statistical properties of speckles (width, lenght, contrast, coherence time, velocities ...) to be able to predict the instabilities levels which can depend on time and on the distance of propagation of the beam. .
For the sake of simplicity, the laser plasma instabilities are very often studied at the best focus of the beam. However, in the FCI experiments, laser beams are focused near the laser entrance hole of the hohlraum whose length is about 1 cm. The development of instabilities can then occur before the best focus (outside the hohlraum) and mainly beyond the best focus (far inside the hohlraum). The goal of this post-doctoral contract is to study the development of instabilities when it occurs in the intermediate field (far from the best focus of the beam) and to assess the efficiency of different smoothing options on Lase MagaJoule (LMJ) to limit these instabilities. We will especially study propagation instabilities (self-focusing, forward stimulated Brillouin scattering) and stimulated Brillouin backscattering. This work will be done thanks to numerous existing numerical codes and diagnostic tolls.

Minimizing the laser imprint through machine learning within the frameword of inertial confinement fusion

The postdoc will be based at the CELIA laboratory which develops studies on different patterns of inertial fusion by laser. In order to optimize the implosion of the target, the laser pulse is shaped spatially and temporally, in particular by a pre-pulse of a hundred picoseconds and intensity of a few hundred TW /cm2. However, the latter introduces spatial inhomogeneities to the surface and volume of the target, amplified by the initial solid behavior of matter. These fingerprints generated by the pre-pulse will degrade the symmetry of the target during its implosion, and therefore decrease the effectiveness of inertial confinement. At present, most models assume a plasma state from the beginning of the interaction, and are thus unable to account for certain experimental observations. To overcome this lack, we have just developed an original multi-physics simulation tool that includes the phase transition of a homogeneous material induced by the laser. In order to mitigate the laser imprint effect, a polystyrene foam (heterogeneous material) can be deposited on the surface of the target. The multiple optical reflections in the foam smooth the spatial profile of laser intensity, thus reducing absorption inhomogeneities. In order to reduce the influence of the laser fingerprint, the post-doctoral fellowship will aim to develop a microscopic model describing the evolution of the optical response of a foam during the solid-to-plasma transition. The first step of the work will be to couple the Helmholtz equation (describing laser propagation) to a solid transition model-plasma, and to study the influence of parameters. The second step will be to use an artificial intelligence algorithm (neural network) to optimize the optical response of the foam.

Depletion calculation of nuclear reactor fuel using Monte Carlo method: moving towards a reference solution

Modern computers offer the possibility to use Monte-Carlo codes to get reference solutions to neutron transport problems. Nevertheless, such reference solutions are only accessible in stationary conditions for practical problems.
The proposed research work aims at exploring and testing methods to obtain a reference Monte-Carlo solution for fuel cycle quantities in depletion problems using present-day computing resources. Such a reference, obtained at a reasonable computational cost, would provide a better control over calculation biases and uncertainties in deterministic solutions typically used in the industry.
Studies will be performed using the Monte Carlo code TRIPOLI-4® coupled with the MENDEL deterministic depletion module. The post-doctoral fellow will perform extensive work on neutron leakage consideration in order to ensure criticality of the model, neutron flux and reaction rates normalization, control of the energy deposition in the different model regions, fine descriptions of the irradiation history, cross section stochastic temperature interpolation, as well as the impact of considering only a limited number of isotopes. Comparisons will be made with the results published by other groups using different approaches and Monte-Carlo codes .
The post-doctoral fellow will be positioned in a team of researchers/engineers in nuclear reactor physics. He/she will improve and deepen his/her knowledge of applied Monte-Carlo simulations as well as the code validation process.

Development of a modular multi-detector instrumentation for the measurement of atomic and nuclear parameters

The LNE PLATINUM project (PLATFORM OF MODULAR NUMERICAL INSTRUMENTATION) aims to develop a modular platform, in order to test new instrumentation using two or more detectors in coincidence. The principle implemented in this project is based on the simultaneous detection of interactions taking place in two different detectors, by collecting information on the type of particle and its energy (spectroscopy). This principle is the basis for absolute measurements of activity or active continuous background reduction systems to improve detection limits. But it also allows the measurement of parameters characterizing the decay scheme, such as internal conversion coefficients, X-ray fluorescence yields or angular correlations between photons emitted in cascade.

Thanks to its expertise in atomic and nuclear data, the LNHB has noted for many years the incompleteness of decay schemes for certain radionuclides. These schemes, established at the time of evaluation from existing measured data, sometimes present inconsistencies or poorly known transitions, in particular in the presence of highly converted gamma transitions or very low intensity (for example, recent studies on 103Pa, 129I and 147Nd have revealed such inconsistencies). It therefore appears important for LNHB to better master the technique of coincidence measurement, taking advantage of the new possibilities in terms of data acquisition and time stamping to provide additional information on decay scheme and contribute to their improvement.

Analysis of the SEFOR experiments for the multi-physics validation of fast reactor simulation tools :

In the Verification Validation and Uncertainty Quantification process of modern simulation tools, the validation phase relies mainly on the comparison between calculation and experimental results for the major quantities of interest. For neutronics, the experiment database focuses on measurements coming from zero power reactors for which the reference states does not require complex multi-physics modeling: isothermal state (very low power such as few hundreds of watts) and fresh fuel (un-irradiated).
However, the VVUQ of power reactor needs to go beyond zero power experiments and thus arises the necessity to apply a multi-physics VVUQ approach. This new frame requires the integration of phenomena from other disciplines outside of pure neutronics: temperature and density dependence of the main quantities of interest (keff, power distribution, and feedback coefficients), temperature field inside the pins as function of core power and irradiation.
Regarding Doppler Effect, the set of experiments held at the SEFOR facility in the 70’s is of major interest for the VVUQ process. This sodium cooled fast reactor fed with mixed oxide fuel was built in support of the US R&D program for indigenous code validation at the time.
Based on the available data, the proposed work focuses on core characterization using a fully neutronic/thermo-mechanic/thermalhydraulic process for both nominal and transient states based in high-fidelity modeling. In order to quantify the benefit of such approach, a step-by-step comparison will be done with the same results obtained by the traditional “chained approach” which assumes a weak dependence between the three mentioned disciplines.
The work will be performed using the last generation of simulation tools available at CEA.

Development of multiphysics tools dedicated to the modeling of FSR and associated studies.

The sodium group of DM2S (department of CEA Saclay) develops numerical coupling tools in order to realize accidental case studies (fast transient). The physical domains concerned are neutronics, thermo-hydraulics and mechanics. The subject of this post-doc deals within this framework.
The aim is to carry out several studies: the integration of a coupling within the CORPUS platform, to carry out studies in order to test (and introduce) in the coupling the impact of the deformation of the assemblies by the Temperature on the flow of liquid sodium, the use of the neutronic cross sections generated by the code APOLLO3, the study of other accidental cases, and extend the modeling to the subchannel and pin scales.