Numerical analysis of heat exchanger designs for passive spent fuel pool cooling to ambient air

Passive cooling of spent fuel pools via natural two-phase convection of a fluid with low boiling point is a promising alternative to active cooling circuits as such a passive heat transfer system would still work in safety-critical situations, such as a station blackout. For ambient air as the ultimate heat sink the heat exchanger design plays a crucial role as driving temperature differences may be low. This paper outlines a numerical investigation on a finned oval tube bundle heat exchanger operated under natural air convection in a chimney. We studied the role of chimney geometry and heat exchanger fin geometry. With respect to the chimney we found that velocity, Nusselt number and heat transfer are enhanced by 161.3%, 31.7% and 62.5% respectively, if chimney height increases from 2 m to 16 m. With respect to the fin design we determined an optimal fin configuration with a fin height of 17 mm, fin spacing of 3 mm and fin thickness of 1.5 mm, which improves the heat transfer performance by 28.7%, the Nusselt number by 28.9% and the fin efficiency by 19.2% at a given temperature difference of 40 K. The final optimized finned tube bundle heat exchanger design achieves a volumetric heat transfer density of q_vol=3.61 kW/m³K.