GB1253912A - Reactivity and power distribution control of a nuclear reactor core - Google Patents

Abstract

1,253,912. Controlling reactors. GENERAL ELECTRIC CO. 26 Feb., 1969 [26 Feb., 1968], No. 10385/69. Heading G6C. In order to provide an arrangement of burnable poison in a nuclear reactor core which will provide a substantially stationary power distribution in the core throughout the operating cycle, the reactor core is zoned to provide a step-function approximation to the ideal poison distribution. The density of the burnable poison is sufficient to provide effective self-shielding. The burnable poison is formed in a geometrical configuration such that as it is depleted with exposure, the surface area of the poison and hence the reactivity controlled by the poison decreases in a manner to match the decrease in reactivity of the nuclear fuel with exposure. The initial amount of the surface area of the burnable poison in each zone is selected to compensate for the initial excess reactivity to be controlled in the zone. The initial number of atoms of the burnable poison in each zone is selected to provide substantially complete burn up at the exposure to be experienced in the zone during the operating cycle. In the embodiment described, the core is formed of a plurality of fuel assemblies, each comprising a tubular flow channel containing an array of fuel elements supported between upper and lower tie plates. The burnable poison may be formed in separate rods or elements, cylinders of the poison material may be placed within annular fuel pallets or rods, rod-shaped particles of poison material may be dispersed in the fuel, or the poison material may be molecularly dispersed in the fuel. Suitable poison materials include B, Sm and Gd. In an example, 0À95 to 1À55 wt. per cent Gd 2 O 3 was mixed with UO 2 to form a solid solution and then prepared as pellets for loading into the fuel elements. The required distribution of burnable poison was obtained using pellets of only three different poison densities. The burnable poison was formed in a cylindrical configuration to provide a linear decrease in control strength which matches the typical linear decrease in reactivity of the fuel in large power reactors. Other configurations may be used, however, such as self-shielding burnable poison in a spherical configuration which provides reactivity control decreasing more rapidly with exposure during the early part of the fuel cycle than during the later part, and a relatively thin strip or slab of self-shielding burnable poison which provides reactivity control substantially constant with exposure with a rapid drop in control worth occurring as complete burn up of the poison is approached.