Scaling-up assessment of natural circulation phenomena in integral Small Modular Reactor by TRACE code

Small Modular Reactors (SMRs) adopting passive mitigation strategies are currently the most promising technology for the near term deployment of nuclear power generation. Different SMRs designs are currently under development and are, in general, characterized by some common features with the current reactors and by other features typical of their designs. Therefore, though numerous code validation study against natural circulation (NC) have been performed for large scale reactors, further analyses are necessary to characterize the capability of codes against available experimental data representative of SMR phenomenology. Though different scaling methodologies have been developed, considering the complex geometry and phenomena of a NPP, in the design of scaled-down experimental facilities it is not possible to avoid distortions, which should be limited to nondominant phenomena. Even if dominant phenomena are preserved, due to the missing data at NPP scale, the code accuracy should be tested at different scales. Therefore, in a verification and validation process, the uncertainty related to the code scaling-up capability should be addressed, for example using counter-part tests. Since NC tests at different scales in integral test facilities devoted to SMR are currently not available, a numerical scaling methodology is here proposed. Based on previous activities, having as a reference the NC DOE tests developed in the OSU-MASLWR facility, the USNRC best estimate thermal hydraulic TRACE code has been validated for simulating NC in steady and transient conditions. Since the OSU-MASLWR is volume and height scaled, the target of this paper is to assess the scaling-up capability of the OSU-MASLWR Reactor Pressure Vessel nodalization in the prediction at different scales of NC and other phenomena typical of SMR, having as a base the OSU-MASLWR-002 single phase NC data. This, also, gives some first insights about the TRACE scaling-up capability against single-phase NC in integral type configurations.