Flow distribution in the primary circuit of a fast reactor: Impact with reduced number of subassemblies in the core

Abstract

The primary circuit of a typical pool-type sodium-cooled fast reactor (SFR) is a complex flow network with multiple flow paths. The resistances of these flow paths vary significantly from very low to very high values. Estimation of flow fractions in these paths is essential for the design and analysis of various components in the primary circuit under various operating conditions. In the present work, the primary circuit of a typical medium-sized pool-type SFR has been modeled using the Flownex code, a commercial system dynamics code. The steady-state flow distribution in the primary circuit has been first studied and an overall flow balance has been established. Out of the total flow supplied by the primary pumps, ∼91 % flows through the core, and ∼ 93 % flows through the Intermediate Heat Exchanger (IHX). Notably, in the storage locations of the core, there is no leakage from the Grid Plate (GP, the structure on which the core subassemblies (SAs) are supported) to the bottom Core Support Structure (CSS) plenum. Instead, flow is in the reverse direction due to the high resistance offered by the sleeve holes in the storage locations.

Then, the effect of removing SAs from the fuel and storage locations of the core has been studied. The empty sleeves in the GP were modeled in 3D using Ansys® Fluent and have been coupled with the Flownex model of the primary circuit. Two different flow configurations were observed in the empty GP sleeves when the fuel SAs were removed and when the storage SAs were removed. When fuel SAs are removed, the sodium flows downwards in the empty sleeve bottom opening. However, when storage SAs are removed, the sodium flows upwards in the empty sleeve bottom opening. This happens because of the high hydraulic resistance offered by the storage SA sleeve holes. As a result, when fuel SAs are removed, the flow rates in the paths fed by the CSS plenum (main vessel cooling system path, shielding SA flow path, etc.) increase. When storage SAs are removed, the flows in these paths decrease. There is no significant change in the pump operating point when a single fuel or storage SA is removed from the core. However, when more fuel SAs are removed from the core, a change in the pump operating point is observed. For instance, the pump flow decreases by ∼ 4.1 % when seven fuel SAs are removed from the core.