Analysis of the natural circulation flow map uncertainties in an integral small modular reactor

The understanding of the Natural Circulation (NC) mechanism and its flow regimes can be considered as a help to achieve a higher safety level and lower costs by reliance on this mechanism. NC Flow Map (NCFM) envelope diagram as an engineering tool for evaluating the NC Performance (NCP) has been attained from different test facilities database. Some Small Modular Reactors (SMRs) with their interesting safety and economic features have used the NC mechanism as their main primary flow drive mechanism. Uncertainty analysis of NCP of a SMR with NC flow drive mechanism, according to the NCFM envelope diagram, is the main objective of the current study. For this purpose, the NuScale reactor with helical coil steam generator at the normal operation NC mode has been modelled using RELAP5/Mod3.3 and the differences of the results in comparison to the designer’s data are in the acceptable range. Then an uncertainty quantification for several sensitive parameters of NuScale has been performed according to the Code Scaling, Applicability and Uncertainty (CSAU) method and NCFM uncertainties for this reactor have been derived. The NuScale NC uncertainties during the normal operation have been mapped to the NCFM envelope to evaluate the NCP of this reactor in high powers. The mass flow rate to power as an index parameter has been used in NCFM and the value of this parameter is evaluated in the range of 3–4 ( Kg MW.s). This range shows the better performance and good design of NuScale reactor for normal operation at the NC condition in comparison to the typical PWRs with high power NC.