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This study explores the feasibility of applying the Serpent-DYN3D sequence to the analysis of Sodium-cooled Fast Reactors (SFRs) with complex core geometries, such as the ASTRIDlike design. The core is characterised by a highly heterogeneous configuration and was likely to challenge the accuracy of the Serpent-DYN3D sequence. It includes axially heterogeneous fuel assemblies, non-uniform fuel assembly heights and large sodium plena. Consequently, the influence of generation and correction methods of various homogenised, few-group crosssections (XS) on the accuracy of the full-core nodal diffusion DYN3D calculations is presented. An attempt to compare the approximate time effort spent on models preparation against the accuracy of the result is made. Results are compared to reference full-core Serpent MC (Monte Carlo) solutions. Initially, XS data was generated in Serpent using traditional methods (2D single assemblies and 2D super-cells). Full core calculations and MC simulations offered a moderate agreement. Therefore, XS generation with 2D fuel-reflector models and 3D single assembly models was verified. Super-homogenisation (SPH) factors for XS correction were applied. In conclusion, the performed work suggests that Serpent-DYN3D sequence could be used for the analysis of highly heterogeneous SFR designs similar to the studied ASTRID-like, with an only small penalty on the accuracy of the core reactivity and radial power distribution prediction. However, the XS generation route would need to include the correction with SPH factors and generation of XS with various MC models, for different core regions. At a certain point, there are diminishing returns to using more complex XS generation methods, as the accuracy of full-core deterministic calculations improves only slightly, while the time effort required increases significantly.

This study presents a benchmark analysis of an unprotected loss of flow transient in a sodium-cooled fast reactor at the Fast Flux Test Facility (FFTF), carried out as part of an IAEA coordinated research project. Three codes, namely Serpent (Monte Carlo), DYN3D (3D nodal diffusion) and ATHLET (system thermal hydraulics), were employed in the benchmark exercise. Two distinct modeling approaches were utilized: 1) stand-alone ATHLET with point kinetics (PK) and system thermal hydraulics (TH); and 2) coupled DYN3D/ATHLET with spatial kinetics (SK) and system TH. Neutronics data essential for both approaches were generated using Serpent. The study is organized into three parts. Part I presents a summary of the preparation of neutronics data for PK or coupled SK/TH simulations and includes the outcomes of the static neutronics stage of the benchmark. The main focus lies on verifying the cross-section generation method for DYN3D by comparing its results against the reference Monte Carlo solutions obtained with Serpent. Part II will provide a detailed description of the ATHLET TH model of the system. This model will be thoroughly evaluated by comparing the results with the ANL benchmark solution and experimental data for the transient. Furthermore, a sensitivity analysis will be conducted to explore various modeling options and assess their impact on the simulation results. Part III will showcase the transient calculation results using the two modeling approaches. Additionally, an adaptive decay heat model for nodal codes will be introduced. The performance of both modeling approaches will be assessed by comparing their results to the available experimental data. • Methodology to generate neutronics data for point kinetics and spatial kinetics simulations. • Based on the use of the continuous-energy Monte Carlo code Serpent. • Verification of the homogenized cross-section generation method for nodal code DYN3D. • Results of the neutronics stage of the FFTF LOFWOS benchmark. [ABSTRACT FROM AUTHOR]

• Development of new coupled DYN3D/ATHLET code systems. • Dedicated to transient 3D neutron kinetics/TH analysis of SFRs. • Modeling of thermal expansions of major in-core and out-of-core structures. • Validation against unprotected stage of Phenix EOL natural convection experiment. DYN3D is a 3D reactor dynamics code originally developed for Light Water Reactors (LWRs) analyses. In recent years, the applicability range of DYN3D was extended to coupled 3D neutronics/thermal-hydraulics (TH) simulations of steady-states and transients in Sodium cooled Fast Reactors (SFRs) at reactor core level. The main objective of this study is to scale up the simulation capabilities of DYN3D from SFR core to system level by coupling DYN3D with a TH system code ATHLET capable of sodium flow modeling. The paper describes the adaption and extension of the existing LWR-oriented DYN3D/ATHLET coupling to SFR analysis. This includes a description of the coupling techniques used to integrate DYN3D with ATHLET and a summary of modifications required to enable the modeling of SFR specific phenomena. The paper also presents the approach to the modeling of reactivity effects caused by the thermal expansions of the reactor components located outside the reactor core, such as reactor vessel, core support structure (strongback), control rod drive lines (CRDLs), etc. The paper also includes the description of the initial verification and validation activities of the extended DYN3D/ATHLET code system. The unprotected stage of the Phenix End-Of-Life natural convection test is used as a test case for this purpose. [ABSTRACT FROM AUTHOR]

• The paper describes the application of the DYN3D/ATHLET coupled code system for analysis of the Superphenix (SPX) reactor transient benchmark. The benchmark comprises six operational transients initiated during the SPX reactor start-up tests covering a wide range of reactor operating states. The peculiarity of these transients is the necessity to consider thermal expansions of major structural elements of the primary system, which significantly influence the transient position of control rods in the core. • This is the first application of the coupled spatial kinetics and thermal hydraulics (TH) code system to the analysis of the six transients using one model. The paper includes the benchmark specification overview, description of the neutron kinetics and thermal hydraulics models, analysis of the simulation results, and comparison to the experimental data. • The study contributes to further validation of the DYN3D/ATHLET code system for safety analysis of SFRs. The capabilities of the code system allow considering all SFR specific reactivity feedback mechanisms, including reactivity effects associated with the thermal expansion of out-of-core primary system components such as the reactor vessel wall, strongback, and control rod drivelines. In the coupled spatial kinetics and TH simulation, these transient thermal expansions predicted by ATHLET are converted into the dynamic control rod position in DYN3D. Another peculiarity of the modelling is that for every transient the initial control rods position needed to be adjusted in a realistic manner to ensure the pre-specified differential worth in accordance with the control rods worth curve. A reasonable agreement between the simulation results and the available experimental data was demonstrated, confirming generally good performance and consistency of the developed models. The phenomenology of each transient was reproduced by the coupled solution. Potential reasons for the differences and model limitations were indicated. The capabilities of the DYN3D/ATHLET coupled code system were recently extended to allow for 3D neutron kinetics/thermal hydraulics analyses of Sodium cooled Fast Reactors at reactor system level. In this paper, the DYN3D/ATHLET coupled code system was applied to analyze the Superphenix reactor transient benchmark. The benchmark comprises six operational transients initiated during the SPX reactor start-up tests, covering a wide range of reactor operating states. The peculiarity of these transients is the necessity to consider thermal expansions of major structural elements of the primary system, which significantly influence the transient position of control rods in the core. The paper includes a brief summary of the benchmark specifications, description of the neutron kinetics and thermal hydraulics models, an analysis of the simulation results, and a comparison to the experimental data. [ABSTRACT FROM AUTHOR]