4.14. In the long term, progressive degradation of the engineered barrier system cannot be ruled out and, consequently, radionuclides may be released into the geological environment where they may eventually migrate to the biosphere. The disposal system should provide a combination of natural and engineered characteristics to support efficient containment and isolation of the waste by maintaining package integrity, limiting the solubility of radionuclides and the waste form, minimizing where possible groundwater inflow and/or providing a long travel time for radionuclide transport from the disposal facility to the biosphere. Factors limiting inflows and contributing to long travel times include low permeability formations, low hydraulic gradients and dispersion characteristics of the geosphere. Any potential concentrations of radionuclides in the biosphere would be further reduced by the retardation and precipitation capability of the engineered barriers and the host rock. In addition, radioactive decay progressively reduces the activities of radionuclides present in the disposal system. Materials used for backfilling or sealing should have properties that do not degrade unduly the safety functions of the geological barriers.