Steam Line Break Analysis in NuScale SMR with the Coupled System Code ATHLET-DYN3D in the Framework of the EU H2020 McSAFER

The small modular reactor (SMR) NuScale is modelled by Helmholtz-Zentrum Dresden-Rossendorf (HZDR) in the framework of the EU H2020 McSAFER project. NuScale is a SMR of integral pressurized water reactor (iPWR) type, operated by light water driven by natural circulation in all operation modes. This work summarizes the modelling approach of NuScale SMR with the coupled thermalhydraulic/neutronic code ATHLET-DYN3D. The 3-D neutronic calculation is performed with a XS-library developed with Serpent based on 4-neutron group homogenized nuclear constants for fuel and heavy reflector. The paper presents results and discussion from a non-isolable double-ended steam line break (SLB) sequence, based on the Design Certification Application (DCA) report. The simulation results at steady-state show agreement with the reference values from the DCA report. The transient calculation shows that both steam generators (SGs) boil-off and the reactor is tripped upon “low main steam pressure” function. The rapid depressurization and high steam rates towards the break lead to enhanced primary-tosecondary heat removal. However, the reactor symmetry imposed by the arrangement of the two compact SGs enhances flow mixing and limits coolant temperature reduction at the core inlet, thereby preventing a
power excursion and highlighting the inherent safety of this reactor design. Acceptance criterion is met
regarding pressure increase below acceptable limits inside the intact SG after steam isolation valve closure.