Convective Heat Transfer in CANDU Spent Fuel Racks After a Loss of Coolant

Abstract of the technical paper/presentation presented at:
International Conference on Nuclear Engineering (ICONE) 2018
July 22–26, 2018

Prepared by:
Derek Logtenberg and Wade Grant
Canadian Nuclear Safety Commission
Paul Chan and Emily Corcoran
Royal Military College of Canada

Abstract:

The near miss at the Fukushima Daiichi spent fuel pools (SFPs) has renewed interest in quantifying the safety margins related to loss of coolant accidents in irradiated fuel bays (IFBs). Thermal-hydraulic analyses of exposed spent CANDU fuel has been limited to a small number of bundles due to its complex bundle geometry and open rack design. This paper presents a process to predict the steady state temperature and velocity of air as it passes through a rack of spent fuel using analytical models and computational fluid dynamics (CFDs) techniques.

Two scenarios are examined which act as lower and upper estimates of the effectiveness of convection during a complete loss of coolant in a fuel bay: 1) the heat-up of a stand alone rack without flow resistance and open sides, and 2) the heat-up of a densely packed rack arrangement where side flows are negligible and air enters only through the bottom. For this natural convection problem, analytical models are derived from fundamental equations and correlations for flow resistance through tube banks. The results are compared to CFD models developed on the finite element solver COMSOL© Multiphysics, which uses porous media approximations to represent the fuel. The models can be used in conjunction with Cody Krasnaj’s work¹ to predict the maximum fuel sheath temperatures and serve as an important first step towards modeling a full-scale irradiated fuel bay.

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¹ C. Krasnaj, W. Grant, "Development of a 2-D Finite Element Model to Examine Both Natural Convection and Radiative Cooling of a Spent Bundle in Air," in The 19th Pacific Basin Nuclear Conference (PBNC 2014), Vancouver, 2014.