GRS Code System AC2

The best-estimate and multi-physics system code package AC is developed by the Gesellschaft für Anlagenund Reaktorsicherheit (GRS) gGmbH for the simulation of all relevant phenomena during normal operation, anticipated operational occurrences, design basis accidents, and severe accidents in nuclear power plants (NPPs). AC consists of the thermal-hydraulic code ATHLET, its extension for severe accidents in the coolant circuit ATHLET-CD and the containment code COCOSYS. Development for these codes was started more than 40 years ago and, until 2016, they were developed separately. Recognizing the increasing need for multi-physics simulations of the integral reactor system behaviour, like the interaction between the cooling circuit and the new containment, the demands posed by passive safety systems in advanced (Gen III/III+) reactors and small modular reactors (SMRs), and new models required for innovative (Gen IV) reactor designs, GRS decided to combine its system codes into the integral code package AC. This requires enhanced coupling strategies for the codes, which is a current focus activity of GRS. In addition, the codes need further improvements for new innovative components (e.g. bayonet, plate or helical coil steam generators), new working fluids (e.g. supercritical water, liquid metals, gases, molten salts), and heat transfer models needed for containment concepts with an infinite passive containment cooling to an ultimate heat sink, which could be either air or water or also large water pools. Further development priorities are coupled multi-physics calculations (e.g. considering 3D fluid dynamics or 3D neutronics and/or structural mechanics) and the qualification of the code system for passive (safety) systems with small driving forces. The development of AC was and is mainly funded by the German Federal Ministry of Economic Affairs and Energy (BMWi) to maintain and enhance nuclear safety competencies in Germany, to provide an independent evidence tool chain, and to collaborate internationally on nuclear safety issues. AC is part of GRS’ nuclear simulation chain and is used not only for safety research but also for evidence in nuclear licensing and supervisory procedures, e.g. for independent confirmatory calculations. Numerous national as well international organisations (e.g. universities and research centres) support GRS in the development and validation of the code package. AC can be provided to interested parties like universities, research centres, TSO and regulators upon request and free of license fees. In this way, AC contributes to the worldwide improvement of the nuclear safety standards and is especially important for developing and emerging countries, which do not have the financial means and/or the necessary know how for this purpose. The current release is AC 2019, which is available since June 2019 and includes numerous new features. Besides extensive model improvements, there are an improved coupling interface to CFD codes, and a new numerical tool kit (NuT) to speed up the calculation of complex ATHLET/CD models. The new release also includes effective and powerful software tools for preand postprocessing (e.g. ATLAS). The new release and its features were presented to the AC user community at an international user meeting in Garching from November 19–21, 2018. During the meeting, the users reported on their experiences with the AC code system. Additionally, there were several round table discussions on further user needs for both the individual programs ATHLET, ATHLET-CD and COCOSYS as well as the coupled code system AC. Outstanding contributions of this user meeting are presented in this edition of the independent Journal for Nuclear Engineering Kerntechnik and thus made available to the wider scientific community. It should be noted that Kerntechnik is well established inter alia in the Chinese research environment, e.g. special issues on the Sino-German Symposium Series on Fundamentals of Advanced Nuclear Safety Technology (SG-FANS). China is currently a hot spot of reactor development and construction. For this reason, a scientific exchange especially with Chinese researchers is of particular interest for the AC development and validation team. EDITORIAL