COMPENSATION OPTIMIZATION OF BURNUP REACTIVITY EXCESS BY BORON ABSORBERS IN PROPULSION REACTORS

摘要

Most of the reactivity excess on fuel burnup (～30%) in propulsion reactors is compensated by consumable absorber rods. Gadolinium serves as the absorber. Shaping by variation of the gadolinium density and height and the number of rods in the fuel assemblies in different shaping zones makes it possible to attain matched rates of fuel and absorber burnup in the core with minimum reactivity unbalance as well as to level and stabilize the power distribution over the core volume during a run. The increase in reactivity at the final stage of gadolinium burnup due to unscreening does not exceed 2%. Such a scheme was developed at the beginning of the 1970s as more promising than a previous scheme based on the use of a boron absorber only. The adoption of a gadolinium absorber made it possible to increase the reactivity excess on burnup, to significantly reduce the under-burning of the absorber and, which is also important, to minimize intervention by means of control rods for reactivity compensation. A reduction of the reactivity excess compensated by control rods reduced the nonuniformity of the power distribution and burnup over the core volume and improved compliance with nuclear safety requirements.