UO2. Matrix Dissolution Rates and Grain Boundary Inventories of Cs, Sr, and Tc in Spent Lwr Fuel

ABSTRACTExperimental methods have been developed for measuring the grain-boundary inventories of radionuclides and for determining whether the UO2 matrix of spent light-water reactor fuel dissolves congruently. Both methods depend upon first separating the fuel into individual grains. With the grain boundaries thus exposed, the associated inventories of radionuclides can be completely dissolved and measured. To determine whether the UO2 matrix of spent fuel dissolves congruently, the fuel grains were placed in a flow-through column and water was pumped through the column at a rate sufficient to maintain the concentration of U in the column effluent far below saturation. Once the grain-boundary material has dissolved, the forward dissolution rate of the UO2 matrix can be measured and, by measuring the concentrations of other radionuclides in the column effluent, the degree of congruency of the dissolution process can be determined. Data obtained to date indicate that the grain-boundary inventories of Cs, Tc, and Sr are approximately equal to gap inventories and that the fractional dissolution rate of Cs from the U02 matrix is approximately equal to that of U, i.e., the Cs and U dissolved nearly congruently. In addition, the data appear to show a gradient in the concentrations of Cs and Sr across the individual UO2 grains.