Linear stability studies of a natural circulation-based small modular reactor

A cooperative research and development agreement between Oak Ridge National Laboratory (ORNL) and Holtec International subsidiary SMR Inventec, LLC, was crafted to explore the primary flow stability of the SMR-160, a natural circulation-based pressurized-water small modular reactor. It is necessary to investigate the stability of the system at operational power levels when natural circulation is the driver of the primary flow to show that operation of the plant is stable, predictable, and controllable. The first phase of the analysis was a code-to-code benchmarking activity between RELAP5-3D and the Modelica-based, ORNL-developed TRANSFORM library. The benchmarking included both generic comparisons of heated channels and step-change transients of certain plant boundary conditions of interest. Following the benchmarking, a parametric series of linear stability tests was performed using discrete signals applied to one of several boundary conditions. These signals excite the natural harmonics of the system and produce small perturbations in the power of the reactor. Though some resonant behavior was observed, the results indicate stable operation of the SMR-160 at the conditions investigated. The frequency range covered included several octaves on either side of the loop transit frequency. Thus, a reasonable determination of the stability and controllability of the plant can be made over a large range of timescales.