5 - Design of passive safety systems for advanced reactors using CFD

Abstract

Soon after the discovery of fission, large number of nuclear reactors was deployed across the globe. These reactors utilize active components in their design during normal operations as well as accidental conditions. Even though these reactors were having provisions of redundancy and defense in depth, still the reliability of the active components cannot be assured. Further, after the accidents like Three Mile Island, Chernobyl, and most recently Fukushima, lot of emphasis is being given to employ passive components in reactor designs for safety purposes. In view of this, all the advanced reactors (Gen-III and III+) are being designed with multiple passive safety features such as passive residual heat removal system (PRHRS) and passive moderator cooling system (PMCS). All these passive safety systems are based on natural forces (Natural Convection) such as gravity. Before the incorporation of these new passive systems in reactor design, these need to be rigorously assessed in a scaled integral experimental test facility, which is having its own challenges and limitations. In this context, there has been a parallel development because of the increasing computational resources in the past three decades. It has now become possible to analyze the performance of these systems using computational fluid dynamics (CFD). Previously these systems were analyzed using one-dimensional computational tools such as RELAP and CATHARE. However, these tools were not able to provide detailed three-dimensional flow patterns and also utilize empirical correlations in their modeling. On the contrary, CFD gives detailed flow and temperature fields inside the components, which is not possible even to measure experimentally due to the limitations of the measurement techniques. However, most of the conventional CFD models were developed for the simulation of forced flows (high Re Pump driven). On contrast, flows in natural convection based systems; the flows are generated through the density difference caused by temperature gradients. Thus, the temperature and velocity fields in the natural convection systems are coupled and offer major challenges in CFD simulations. In this chapter, an effort has been made to assess the performance of different Passive safety systems using CFD, with emphasis lying on the modeling of turbulent natural convection and near-wall heat transfer. Also the CFD models were validated using experimental data generated in scaled test facilities. This chapter includes the simulation of the following four passive systems: (1) PRHRS, (2) PMCS, (3) air-cooled condenser, and (4) venturi scrubber. Finally, the effect of various geometrical parameters has been studied to optimize the design of these passive systems.