Topology optimization of the NaK78 inlet distributor for a liquid-cooled space nuclear reactor heat exchanger

Liquid-metal-cooled space nuclear reactors coupled with closed Brayton cycles are promising candidates for high-power deep-space missions. In these systems, the NaK78-to-He dual-fluid heat exchanger is critical. Yet, the concentrated liquid-metal inlet causes severe flow maldistribution, which can consume the limited pressure head (only 10–20% of that available from mechanical pumps) from electromagnetic pumps driving the NaK78 loop. Existing remedial measures rely on trial-and-error modifications to predefined geometric forms, offering no guarantee of optimality. In this work, we apply density-based topology optimization to autonomously determine the optimal solid baffle distribution within the NaK78 inlet plenum of a prototype space nuclear heat exchanger. The optimization is implemented in COMSOL Multiphysics 6.3 using Brinkman penalization and adjoint-based sensitivity analysis to minimize the integral of squared outlet velocity deviation. The inlet slot width is fixed at 8 mm ( Re ≈ 1980), ensuring the laminar flow assumption is strictly valid for this proof-of-concept study. Parametric investigations over solid volume fraction (φₛ = 0.3–0.7) and a uniformity-pressure-drop trade-off coefficient (C1 = 0–0.1 m −1 ) are conducted using β-continuation. The base-case optimized distributor achieves a coefficient of variation CV = 5.08% (uniformity index η = 0.949) and a pressure drop ΔP = 7.68 Pa, with φₛ = 0.5 identified as optimal. Notably, the uniformity-only objective (C1 = 0) is Pareto-dominant; increasing the pressure-drop penalty degrades both metrics simultaneously, a finding explained by the dual flow-organizing function of the curved baffles in the thin rectangular plenum. The optimizer autonomously discovers a curved bifurcating baffle topology physically analogous to wedge-shaped designs previously derived by empirical enumeration, yet obtained here rigorously and without geometric preconditioning. While the present 2D laminar model serves as a proof-of-concept, the discovered topology provides a physically meaningful baseline for future 3D turbulent validation under rated operating conditions. The present operating point corresponds to a thermal duty of approximately 6.8 kW; an order-of-magnitude assessment shows that remain laminar up to ∼500 kW(th) and only reaches transition at 1000 kW(th), while the inlet plenum becomes turbulent. This study establishes topology optimization as a systematic tool for inlet distributor design in liquid-metal nuclear heat exchangers, addressing the hydraulic root cause of maldistribution-induced thermal degradation.