Reactivity worth of liquid poison jets in the moderator

Abstract

The safety requirement of two shutdown systems functioning on diverse principles has led to the introduction of a secondary shutdown system which functions by injecting liquid neutron poison jets at high pressure into the bulk moderator. The poison enters the moderator in the form of high-speed jets, in which the cross-section increases and the poison concentration falls as the jet develops. A formalism has been developed for estimating the reactivity worth of these jets. This formalism is different from currently used formalisms, in which the conical jet is approximated by equivalent cylinders whose radius changes from cell to cell. In the formalism presented in this paper, both the cross-section of the jet and the concentration of the poison are taken to be continuously varying from the origin of the jet to the end of the jet where it starts diffusing into the moderator. The formalism has been used to calculate the reactivity worth of the secondary shutdown system of the 500 MW(e) PHWR, making use of jet growth data that have been made available from experiments performed in our laboratory.