Measurement of Soluble Nuclide Dissolution Rates from Spent Fuel
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MEASUREMENT OF SOLUBLE NUCLIDE DISSOLUTION RATES FROM SPENT FUEL
C. N. WILSON AND W. J. GRAY Pacific Northwest Laboratory, P. 0. Box 999, Richland, WA99352. ABSTRACT Gaining a better understanding of the potential release behavior of water-soluble radionuclides is the focus of new laboratory spent fuel dissolution studies being planned in support of the Yucca Mountain Project. Previous studies have suggested that maximum release rates for actinide nuclides, which account for most of the long-term radioactivity in spent fuel, should be solubility-limited and should not depend on the characteristics or durability of the spent fuel waste form. Maximum actinide concentrations should be sufficiently low to meet the NRC annual release limits. Potential release rates for soluble nuclides such as 99Tc, 135Cs, 14 C and 19I, which account for about 1-2% of the activity in spent fuel at 1000 years, are less certain and may depend on processes such as oxidation of the fuel in the repository air environment. Dissolution rates for several soluble nuclides have been measured from spent fuel specimens u~sing static and semi-static methods. However, such tests do not provide a direct measurement of fuel matrix dissolution rates that may ultimately control soluble-nuclide release rates. Flow-through tests are being developed as a potential supplemental method for determining the matrix component of soluble-nuclide dissolution. Advantages and disadvantages of both semi-static and flow-through methods are discussed. Tests with fuel specimens representing a range of potential fuel states that may occur in the repository, including oxidized fuel, are proposed. Preliminary results from flow-through tests with unirradiated U02 suggesting that matrix dissolution rates are very sensitive to water composition are also presented.
INTRODUCTION The Waste Package Task of the Yucca Mountain Project' (YMP) is studying the dissolution and radionuclide release behavior of spent nuclear fuel. The candidate YMP repository horizon is above the water table in the unsaturated zone and contact of the spent fuel by water is not expected during the first several hundred years while waste package temperatures exceed the boiling point of water. During the post-thermal period, dissolution and transport by a limited amount of water that may infiltrate the rock is considered to be the most probable mechanism for release of most radionuclides. A "semi-static" testing method has been developed and three laboratory test series completed. Results obtained from these tests and from geochemical modeling have provided a clearer perspective on the potential for radionuclide releases from spent fuel under proposed Yucca Mountain disposal conditions[l]. Radionuclides in spent fuel appear to fall into three groups according to potential release mechanisms. The first group is the nuclides of elements with very low solubility for which release will be limited by solubility limits and water flux through the repository, and will not depend on the 1. YMP is the former Nevada
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