Since backfill is just one part of a waste package that might contain five or more components, its performance has been considered as part of the performance of the whole package. A simple method has been developed for estimating the required behavior of this complete system. The method is based on:

  1. federal regulations concerning permissible concentrations in solution at the point of discharge to the accessible environment
  2. a simple and conservative transport model
  3. baseline and potential “worst-case” release scenarios.
Use of the transport model enables calculation of maximum permissible release rates within a repository for each of the scenarios. The maximum permissible release rates correspond to performance requirements for the engineered barrier system. It has been determined from the model that the most important nuclides in spent fuel located in a repository in basalt at Hanford are: 129I, 79Se, 99Tc, 107Pd, 237Np, 239,240,242Pu, 241,243Am, and members of the 238U decay chain. Backfill can operate as a diffusion-controlled, retardation barrier for most of these key radionuclides under conditions of relatively rapid water flow. Under expected low-flow conditions (<1 m·yr−1 velocity), however, the concentration gradients across the backfill would be so small that they could not retard radionuclide migration. The maximum permissible release rates for the waste package have been translated into concentrations at the inner and outer surfaces of the backfill under low- and high-flow conditions. It appears that solubility constraints should control the concentrations of uranium and possibly of plutonium, americium, neptunium, and selenium below the required levels.