Effect of spacer on the dryout of BWR fuel rod assemblies

• Modeling of spacer for nuclear fuel assembly.
• A brief review of the existing approaches for spacer effect on dryout of BWR fuel assemblies.
• A methodology for BWR fuel spacer proposed.
• Dryout analysis for untested 54 rod bundle design of AHWR with the proposed spacer model.

Spacer is used in the fuel rod bundle of a nuclear reactor to maintain appropriate gaps among the fuel pins ensuring adequate heat transfer to the coolant. Hence, the design of such spacing devices is an important consideration to arrive at the acceptable configuration of the fuel bundle. The analysis of the flow behavior around the spacer is necessary to find its effect on the important phenomena like the pressure drop, dryout and heat transfer due to enhanced turbulence caused by the flow obstruction. Both experimental and analytical studies have been carried out in the past to investigate these phenomena. However, most of the experiments have been conducted under air–water conditions and the mechanism of the spacer effect has not been adequately modeled and validated due to the underlying complexity of the phenomena. In addition, the phenomenological based dryout modeling for the determination of thermal margins in BWR requires spacer model to be incorporated for evaluating the critical power of a fuel assembly. The present study briefly reviews the existing approaches and proposes a simple approach for incorporating the spacer effect on the dryout of the BWR rod bundle by analyzing the flow field near the spacer with the CFD application. In this study, a methodology for the formulation of a spacer model for BWR fuel assembly has been proposed and incorporated in the phenomenological dryout code (FIDOM-Rod). Thus, the paper briefly discusses the FIDOM-Rod approach, spacer modeling, its application to the for untested 54 rod bundle design for AHWR.