Measurements and Calculations of Reactivity Effects of Material Rearrangements in a Plutonium-Fueled Fast Reactor Rod Lattice

In the critical assembly SNEAK 12B, the reactivity effects of material movements occurring in a postulated fast reactor accident were investigated in a test zone composed of PuO₂UO₂ rod bundles. The experimental program included addition and removal of fuel, addition of steel between the fuel rods, and vertical and horizontal relocation of fuel. The measured reactivity changes could be well reproduced by eigenvalue calculations in S_N transport theory, while first-order perturbation transport theory and diffusion theory in most cases yielded substantial deviations.