Pompes primaires sodium à grandes dimensions

Design of large sodium pumps

¹ CEN/Cadarache-Chargé d'affaires des pompes des réacteurs rapides
² BERGERON S. A.

Abstract

The primary pumps of high-speed reactors operate under a free sodium surface level and deliver the f1uid to the reactor core. This report describes a design study of a sodium pump from the following angles : a) Dimensions: Decreasing the diameter of a pump also decreases its weight (and therefore cost) and the diameter of the basic reactor unit. b) Technology : The aim is to achieve a simple design whilst ensuring technological continuity of the pumps in the system. The first design step was to determine reactor parameters to be considered in designing the pump. The most important of these are the following : a) Rate of flow b) Pressure drop across the reactor core, c) Available NPSH (here : pressure at the top of the reactor + head of sodium between a rotor and the free sodium surface). Design assumptions were 1 200 MW, four pumps, and all corresponding external parameters. The second step was to consider all potentially suitable hydraulic jet geometries associated with combinations of various rotor types (Fig. 1) and diffuser designs. The following were eliminated on initial analysis : a) Volute designs, as too bulky for a high-speed reactor. b) Crossf1ow designs, as involving excessively difficult design and manufacturing problems. This left the following designs for further consideration : a) Two-stage pumps with bottom intakes and non-dismantleable manifolds. b) Two-stage pumps with top intakes and non-dismantleable manifolds. c) Two-stage pumps with top intakes and dismantleable manifolds. d) Twin-rotor pumps with non-dismantleable manifolds. e) High-level intake pumps with dismantleable manifolds. f) High-level intake pumps with non-dismantleable manifolds. g) Low-level intake pumps with non-dismantleable manifolds. These designs were analysed, with the following results : a) Two-stage designs are very complicated and do not result in any substantial improvement as regards geometry. Hence, they were eliminated. b) Though twin-rotor designs are attractive as regards geometry, their bearings, overhung rotors and critical shaft speeds are major problems. They were therefore eliminated. c) High-level intake pumps require the largest amount of space and, in addition, have to be provided with an intake skirt. This resulted in their elimination. d) Axial low-level intake pumps (Fig. 9) combine simple design with optimal diameter. It was concluded that such pumps offered both optimal dimensions and a suitably simple overall design for the considered reactor parameters. The report ends with a brief review of specific hydraulic problems associated with these pumps, e.g. choice of diffuser design, inflow to the hydrostatic bearing, etc.