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Numerical optimization of a finned tube bundle heat exchanger arrangement for passive spent fuel pool cooling to ambient air

Unger, S.; Krepper, E.; Beyer, M.; Hampel, U.ORC
The passive cooling of nuclear spent fuel pools is a promising alternative to active cooling. Since such systems work even in safety-critical situations, e.g. station blackout, the reliability of nuclear power plants would be enhanced. As in such systems heat needs to be transfer to the environment, the heat exchanger to air has a crucial influence on the system performance. This paper describes investigations of the Nusselt number, the achievable efficiency and the volumetric heat flux density of the tube bundle heat exchangers for a passive cooling system located at the bottom of a chimney. The effect of tube bundle configuration, tube shape, longitudinal tube pitch, transversal tube pitch and tube row number on natural convection heat transfer was numerical studied. These parameters were varied to optimize the heat transfer performance of the heat exchanger. It was found, that the staggered configuration performs better than the inline arrangement, since the flow mixing is higher. Furthermore circular tube shape and an oval tube shape with the aspect ratio of 1:2.1 were optimum for the inline and staggered configuration respectively. The longitudinal and transversal tube pitches of 63 mm and 65 mm performed best, since higher values reduced heat transfer. A tube row number greater than 5 did not improve the heat transfer and therefore a tube row number of 5 is recommended. The Nusselt number and volumetric heat flux density of the optimized tube bundle arrangement enhanced by 15.4 % and 47.8 % respectively at a temperature difference of 40 K compared to the initial design.
Keywords: passive cooling, spent fuel pool, natural convection, tube bundle heat exchanger, air cooling, design optimization
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