Simulation of the NuScale SMR and Investigation of the Effect of Load-Following on Component Lifetimes

The NuScale small modular reactor (SMR) has been modeled using the Virtual Environment for Reactor Applications multiphysics environment and the results compared with the publicly reported data in the Design Certification Application. The results show an excellent agreement for the compared axial and radial power distributions, temperature coefficients of reactivity, boron and control rod worths, and fast neutron flux. This NuScale model is then used to investigate the effect of different operational modes on reactor components to determine how the flexible load-following operation may affect control rod and reactor pressure vessel (RPV) lifetimes. The control rod degradation is confirmed to primarily affect the silver-indium-cadmium rod tip. The degradation rate is observed to follow a nonlinear function of core power level where the increase in degradation decreases with insertion depth.

For the variation in core power levels expected with current load-following schemes, the total control rod degradation is found to be mild, at 5% to 10% of usable life per cycle for a reactor operating at <80% power. Nonetheless, this enables load-following strategies to be confirmed and/or modified to ensure that control rods do not need to be replaced during the 60+ year life of the reactor. The RPV degradation was found to be almost directly proportional to the core power level and was not overly sensitive to flux shape perturbations. Future work is planned using these damage functions to optimize operation over multiple NuScale SMR units and to develop strategies for prognostics and health management.