Further development of the AC² code ATHLET for evolutionary reactors and research reactors (phase A)

Abstract

The project RS1600, funded by BMUV, on the Further Development of the AC² code ATHLET for Evolutionary Reactors and Research Reactors aimed to develop and upgrade models and methods for the thermal-hydraulic system code ATHLET to support the numerous national and international code users with a powerful and reliable tool for carrying out safety analyses according to the current state of science and technology for existing reactors, advanced future reactor designs, research reactors, and SMR. The introduction of a central flow regime map module represents a significant milestone for the ATHLET development process. In addition to the consistent use of flow pattern information and associated closure equations in the models of interphase friction, mass and energy exchange, the calculation of additional variables such as the entrainment fraction, which was previously determined on a model-specific basis, was also standardized. The central module has been expanded to allow for the use of separate flow regime maps depending on the prevailing heat transfer conditions at the wall. This development enabled the introduction of specific flow patterns and transition criteria for conditions characterized by wall condensation, which differ from adiabatic flows or heating wall conditions. Additionally, a general interpolation routine was implemented to facilitate a smooth transition between two or more flow pattern maps. The new implementation could already be verified against several application cases with satisfactory results. By participating in the OECD/NEA project RBHT, GRS gained access to new and detailed experimental data for code validation. Evaluation of blind calculations performed in the frame of the benchmark focused on parameters relevant to quench front model validation, such as cladding temperatures at different core elevations, pressure drop along the test section, and quench front progression. Overall, the simulation results were satisfactory and generally reproduced the experimental trends. Uncertainty and sensitivity analyses were performed for two RBHT tests. Concerning the turbulence-induced precooling effect on the heater rods surface temperatures, which was observed in some tests and could not be adequately simulated by ATHLET, the forced convection to steam heat transfer and the onset of liquid entrainment were found to be the most influential parameters. The trend to underestimate pressure drops is most likely related to an overestimation of the void fraction below the swell level, which is affected by bundle interfacial shear modelling in non-dispersed flow below the quench front. In order to simulate pipe branchings, a T-junction model was implemented to simulate the complex flow and phase distribution as well as the pressure loss in the area of such a branch-off. The new model, developed as an extension to the 2-fluid 6-equation system, also incorporates a correlation for the pressure loss and recovery in the main line. In addition, ATHLET was extended by the possibility of accounting for the Reynolds number dependency of form losses. To this end, correlations for the determination of form losses in specific geometries, such as bends and orifices, were implemented, and the option to specify tabulated data with zeta values and according Reynolds numbers was established. Furthermore, the latest version of ATHLET includes a new, dedicated model for simulating compact heat exchangers. This model comprises one- and two-phase heat transfer correlations for plate and helical heat exchangers, enhancing the code’s predictive capability for passive safety systems in advanced light water reactors and SMRs. Concerning the thermo-mechanical modelling of the reactor core, the existing fuel rod model was updated and extended with a thermo-mechanical model that is capable to reproduce the burn-up-dependent densification and swelling of the pellets, as well as the radial relocation of the fuel material. Furthermore, a new gap conductance model was implemented, which enables more detailed simulations of open and closed gaps. The validation of the new fuel rod model against the Halden BWR experiment provided satisfactory results. The existing methods for quality assurance in program development were expanded by the introduction of new methods, and the code development process was adapted in alignment with current international standards. Two extended and quality assured, new code versions, ATHLET 3.3 and ATHLET 3.4, were released as part of the AC² program package and distributed to numerous code users. Datei-Upload durch TIB