Surrogate-based multi-objective optimization of reactor vessel radiative fin geometry for a PRISM-like RVACS configuration

Passive decay heat removal is a critical safety feature for sodium-cooled fast reactors (SFRs), where reactor vessel auxiliary cooling systems (RVACS) rely on natural circulation and radiative heat transfer to reject residual heat during off-normal conditions. The influence of fin geometry on enhancing RVACS performance while maintaining acceptable operational heat loss remains inadequately characterized, specifically in the inert gas annulus between the reactor vessel (RV) and guard vessel (GV). This study develops a surrogate-based multi-objective optimization framework to evaluate radiative fin configurations, in which a dataset generated from high-fidelity thermal simulations is used to train Gaussian Process Regression and support vector regression models for predicting thermal performance metrics within the design domain. The results demonstrate that increasing fin number is the dominant factor in enhancing RV shutdown heat flux, achieving values exceeding 57000 W / m 2 , while fin width introduces trade-offs by reducing packing density. Radial fin clearance improves RV shutdown heat flux enhancement while exhibiting minimal influence on the operational-to-shutdown heat flux ratio. Practical design limitations strongly govern achievable performance improvements, as demonstrated by imposing operational-to-shutdown heat flux ratio constraints. Enforcing ratio limitations less than 0.6 significantly reduces the feasible design space, while stricter limits (<0.5) yield even fewer viable solutions with reduced performance. Under the combined operational heat-loss and manufacturability constraints, the surrogate optimization identified a constrained screening candidate with a predicted RV shutdown heat flux enhancement of 10723 W / m 2 , an RV operational-to-shutdown heat flux ratio of 0.598, and a total fin surface area of 5.04 m 2 . These findings provide a computationally efficient framework to guide the design of enhanced passive heat removal systems, while highlighting the importance of balancing thermal performance with operational and manufacturability constraints.