Verification and validation are basic quality assurance elements in code development and essential for code release. Therefore, the codes of AC(2) (ATHLET - ATHLET-CD - COCOSYS) are tested on separate effect tests, integral tests as well as plant scenarios to verify and validate the models after new implementation or updates. The verification assures that the models are implemented and working correctly while the validation checks if the models predict the right phenomena and combined with other models and modules. The selected experiments are summarized in GRS's validation matrices, which in turn are based on the CSNI validation matrices derived from OECD/WGAMA task groups as well as current activities on experimental test campaigns. For ATHLET several test series are used to cover a wide range of phenomena which can occur in PWR, BWR and VVER. Additionally, plant transients are considered for German LWR. The ATHLET-CD validation matrix contains experiments covering most phenomena which can occur during a severe accident. But due to the interaction of several effects even in small scale experiments mainly integral experimental campaigns are used for the validation. Over the last decades the validation of the AC(2) codeds ATHLET and ATHLET-CD has reached a high degree of fulfilment of GRS's validation matrices over all code versions. Innovative and advanced reactor concepts come with new or newly relevant phenomena, which AC(2) needs to provide models for. Extending the validation base of AC(2) for these models is one challenge for further code validation efforts besides the on-going update of the validation basis to recent code versions.

The transition from Generation 2 to Generation 3/3+ and 4 reactors, as well as the development of small modular reactors (SMR), place new demands on computational programs designed to simulate conditions of normal operation, operational occurrences, design basis accidents and severe accidents. On the one hand, most passive safety systems of advanced and innovative plants operate at low pressures even down to vacuum conditions and the driving forces are low compared to active systems. On the other hand, the containment is no longer just a barrier to retain radioactive material in the event of leakage of the cooling system, but it is an important link in the passive cooling chain. This requires an expansion and improvement of the existing simulation programs for the cooling circuit and containment, as well as the realization of a coupling between these simulation programs. The new AC(2) program package combines the proven simulation codes ATHLET/ATHLETCD and COCOSYS in one software suite to hit this target. The individual components of the suite are continuously extended and validated for their application to novel safety systems. This makes it possible to simulate the entire spectrum of accidents for Generation 3/3+, 4 and light water cooled SMR systems with just one program package. This publication gives an overview of the current state of development of AC(2) and its individual modules.

Abstract Small modular reactors (SMRs) are one interesting option for new builds in almost all countries worldwide continuing to use nuclear energy for commercial power generation. For asserting of legitimate nuclear safety and/or security interests German authorities require in this context own and independent expertise for the safety assessments. The Gesellschaft für Anlagen und Reaktorsicherheit (GRS) mbH, which is the main technical support organization in nuclear safety for the German federal government, has among others performed a study on Safety and International development of Small Modular Reactors (GRS-376) to identify essential issues of SMR safety as well as needs for adaption, improvement and validation of nuclear evidence tools developed and applied by GRS. For the light-water-cooled SMRs, which have (at least according to GRS) the best prospects for realization, selected results of this study will be presented.

GRS has been developing the system code ATHLET over many years. Because ATHLET, among other codes, is widely used in nuclear licensing and supervisory procedures, it has to represent the current state of science and technology. New reactor concepts such as Generation III+ and IV reactors and SMR are using passive safety systems intensively. The simulation of passive safety systems with the GRS system code ATHLET is still a big challenge, because of non-defined operation points and self-setting operation conditions. Additionally, the driving forces of passive safety systems are smaller and uncertainties of parameters have a larger impact than for active systems. This paper addresses the code validation and qualification work of ATHLET on the example of slightly inclined horizontal heat exchangers, which are e.g. used as emergency condensers (e.g. in the KERENA and the CAREM) or as heat exchanger in the passive auxiliary feed water systems (PAFS) of the APR+.