Prediction of release rates for a potential waste repository at Yucca Mountain

Nuclear waste may be placed in the potential repository at Yucca Mountain in waste packages. The waste will consist of spent fuel assemblies or consolidated fuel rods, as well as borosilicate glass in steel pour containers, each enclosed in sealed containers. Current design calls for the waste packages to be surrounded by an air gap. Although the waste package is generally not seen as the primary barrier for nuclear waste isolation, it must in fact meet specific regulatory requirements. The US Nuclear Regulatory Commission requires that the release rate of any radionuclide from the engineered barrier system following the containment period shall not exceed one part in 100,000 per year of the inventory of that radionuclide calculated to be present at 1000 years following permanent closure. For low-inventory radionuclides, those that constitute less than 0.1 percent of the calculated total curie inventory at 1000 years, the allowable annual release is a constant value, equal to 10{sup {minus}8} of the total curie inventory in the repository at 1000 years. Therefore it is necessary to calculate release rates for waste packages at Yucca Mountain. We calculate release rates for key radionuclides using analytic solutions presented in a companion report. We consider both wet-drip and moist- continuous water-contact modes. We consider the release three types of species: solubility-limited species, species released congruent with solid-solid alteration of spent-fuel matrix or borosilicate glass, and readily soluble species from the fuel-cladding gap, gas plenum, and readily accessible grain boundaries. In each case we give the release rates of the species as a function of time. 22 refs., 11 figs., 9 tabs