The Hanford K Basins were used until the mid 1960s to store, under water, spent nuclear fuel from the site’s reactors before reprocessing to recover uranium and plutonium. A sludge consisting of cladding corrosion products, uranium, fission products and other debris accumulated on the floor of the K Basins while the Basins were in operation and during the period following cessation of reprocessing when some spent fuel was left in the Basins for an extended period. The spent fuel was recovered, dried and placed in dry storage early in the 2000s and, following that, the sludge was retrieved and placed into temporary containers to await treatment to stabilize and package it for permanent disposal at the Waste Isolation Pilot Plant (WIPP). One of the requirements for the treatment phase is to identify and demonstrate technologies that can be used to chemically treat the K Basin sludge. In particular, there is a requirement to oxidize the uranium metal particles contained in the sludge so that these cannot react with water and generate hydrogen gas during transport to WIPP. EnergySolutions was awarded a contract in mid 2010 by CH2M Hill Plateau Remediation Company to demonstrate a stabilization process based on oxidizing the uranium metal with hydrogen peroxide and subsequently converting the oxidized metal to uranyl carbonate with ammonium carbonate. The resulting slurry would then be straightforwardly immobilized in grout for disposal at WIPP. There are several advantages of this process over others. The reaction occurs at low temperature and does not require removal of any significant reaction heat. In addition, aside from the generation of oxygen arising from peroxide decomposition, no off-gas, including hydrogen, is generated. To assist with the chemistry development of this process EnergySolutions established a Commercial Work for Others Agreement with Oak Ridge National Laboratory to perform small-scale tests with simulated sludge containing uranium particles.
One of the main constituents of the K Basin sludge is iron oxide hydroxide, which is known to catalyze peroxide decomposition. Therefore, one of the early challenges was to identify process conditions that would minimize the volume increase arising from continuous addition of fresh peroxide to maintain its concentration. The addition of chelating agents or phosphate has been shown before to retard peroxide decomposition. However, neither of these approaches demonstrated any reduction in the decomposition rate, probably because the iron was in a solid form rather than in solution. Reducing the iron concentration relative to uranium also reduced the decomposition rate but this approach was discounted because of the difficulty in achieving this solid-solid separation on a production scale. Instead, the approach adopted was to reduce the reaction temperature from ambient to 10oC. Doing so also reduced the uranium dissolution rate but the reduction was significantly less than the reduction in the peroxide decomposition rate.
This presentation will describe the chemistry development work and indicate how this forms the basis of a production scale process for stabilizing the K-Basins sludge.
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