Research regarding small modular reactors (SMRs) is increasingly receiving attention due to the advanced stage of development of several SMR concepts. Especially, integral pressurized water reactors (iPWRs) are a promising technology for nuclear new builds. Due to this fact, the applicability of system codes to simulate phenomena in SMRs should be investigated in more detail.

In order to ensure that AC2-ATHLET can adequately simulate SMRs and their most important phenomena, an input deck to simulate the International Collaborative Standard Problem test SP-3 at the OSU-MASLWR (Oregon State University–Multi Application Small Light Water Reactor) was further developed and analyzed within a national research project. Since numerous iPWRs use helically coiled steam generators (HCSGs) for heat removal from the primary side, the focus is on HCSG behavior. In comparison to straight tubes or U-tubes, centrifugal force affects the fluid in the pipes and forms a secondary flow normal to the main flow direction. This secondary flow influences heat transfer and is intended to increase it due to increased mixing.

Therefore, correlations for calculating the heat transfer coefficients in HCSGs were determined, implemented in an in-house version, and tested as part of a national project. Now, these are available as a first option in the current release of version AC2 2023.0. In addition to the new correlations, an option has also been provided to declare Reynolds-dependent pressure losses for specific components that are important for mass flow rate calculations. Comparative analyses are being carried out between AC2 2021.0 and 2023.0 in order to identify deviations and improvements.

Both AC2 versions show good agreement with the experimental data, and there are only slight deviations between the versions. In addition to the HCSG phenomena, there are also deviations in the calculation of the mass flow rate on the primary side. In principle, AC2 enables the adequate simulation of phenomena that are relevant for iPWRs.