In performance assessment of geological disposal systems for High Level
Radioactive Waste (HLW), the change of environment over the long-term must
be considered. Therefore, it is necessary to consider a wide range of
parameters concerned with radionuclides migration, especially the dependence
of solubility on geochemical environment. In this study, assuming that the
release rate of the nuclides from buffer material is limited by inventory
ultimately, the relationship between the initial inventory and the
solubility that produces a solubility-invariant maximum release rate from
the buffer is examined by using a simple steady-state analytical solution
without decay. As the result, the threshold of “effective” solubility in the
performance assessment of the geological disposal systems for HLW is
obtained as a function of initial inventory, distribution coefficient (Kd),
diffusion coefficient, and thickness, porosity and density of the buffer.
Also, the threshold of “effective” steady dissolution rate corresponding to
the threshold of “effective” solubility is obtained.