Innovative Control Strategy for the Fast-Runback Transient in a Sodium-Cooled Small Modular Reactor

The recent interest in the Small Modular Reactors for their potential increased economic competitiveness has focused attention in part on reducing operational costs to offset those plant costs that do not benefit from the economies of scale of large traditional units. Plant economics is significantly driven by plant availability, which can be enhanced by means of innovative operating strategies that act to avert plant or unit trips. A strategy that involves fast runback of the power plant to avert a turbine trip following a disconnection from the grid has been developed and assessed for a typical sodium-cooled SMR (adopted for proof-of-principle demonstration). A classic control approach based on PID feedback controllers has been adopted for establishing a baseline for performance improvement. Simulations outcomes have shown that although the reactor power level can be appropriately reduced, the net result is a global cooling of the system circuits. Such system conditions do not facilitate the objective of a rapid reconnect to the grid once the issue has been resolved. Moreover, the classic approach requires relatively large control rod motion. An alternative MIMO (Multiple Input Multiple Output) control scheme on the other hand provides additional regulation flexibility by allowing for coordinated control actions on the mass flow rates and on the control rods. A Model-based Predictive Control implementation of the MIMO approach has ensured prompt power level reduction with reduced overcooling and control rod motion, facilitating a quicker reconnect to the grid. Such flexibility could lead to improved system availability. An investigation of the metal-fuel form as an alternate to the oxide fuel form and its wider experience base has been performed for the same operational transient. By comparing the plant response for these two different fuel forms, it has been possible to evaluate the favorable properties in terms of inherent safety and plant operation of the metal-fuelled core configuration.