129I is a component of spent nuclear fuel with a long half-life (>16 x 106 years) and the potential to be mobile in the environment. When spent fuel is reprocessed, the 129I can be isolated from the other radioactive components and then incorporated into a durable waste form designed especially for it to restrict its release into the environment over time.We have developed a waste form containing AgI-mordenite (AgI-MOR) that utilizes a low temperature sintering oxide glass that encapsulates the AgI-MOR particles to form a dense and durable composite. We have discovered a bismuth silicate based glass that can be sintered to full density at 550C and yet has excellent chemical durability. To fabricate the composite waste form, the glass powder is mixed with ground AgI-MOR, pressed into a compact and then sintered to form a dense and durable waste form.AgI-Mordenite encapsulated in Bi-Si-Zn-Al glass is an effective waste form for 129I because the glass is chemically durable, I release from AgI-Mordenite is relatively slow, and Bi in the weathered glass may sorb iodide.  Moreover, low temperature (< 600oC) Bi-Si-Zn-Al glass sintering minimizes iodide release from the AgI -Mordenite during processing.  Single-pass flow through (SPFT) tests show pH-dependent glass degradation at rates similar to high level waste glasses, reaching a minima near neutral pH at 25oC, increasing at higher and lower pHs.  Steady-state Si levels were less than 1 ppm and well below saturation with solid silica phases.  Measured non-stoichiometric release rates point to formation of a Bi leach layer at the glass surface. Ag and I release from AgI-Mordenite under oxidizing conditions are less affected by pH. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. DOE's NNSA under contract DE-AC04-94AL85000.

Chemistry and chemical durability of a new 129I glass composite material (GCM) waste form

247th National Meeting (2014)

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