Boron Concentration Gradient for Improved Thermal Reactor Performance of Boron-Stainless-Steel Control Rods

An experimental plate-type neutron absorber assembly containing ¹⁰B dispersed in Type 200 austenitic stainless steel was irradiated in the active lattice of the 10 MW-SM-1 Reactor for 1.2 full power years. The ¹⁰B was distributed in a concentration gradient, increasing from 1 wt% in the surface layer to a maximum of 3 wt% 0.024 in. below the surface, to ensure a uniform burnup of ¹⁰B atoms in each volume increment through an exposure to thermal neutrons resulting in an average ¹⁰B burnup of 20 at.%. Postirradiation evaluation did not reveal any significant dimensional changes or structural damage to the dispersions at this burnup, which is a demonstration that the use of the boron concentration gradient results in at least a fourfold increase in the reactor performance capability of plate-type neutron absorbers containing dispersions of ¹⁰B in stainless steel.