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Prior laboratory testing identified sodium nitrate and nitrite to be the most promising agents to minimize hydrogen generation from uranium metal aqueous corrosion in Hanford Site K Basin sludge. Of the two, nitrate was determined to be better because of higher chemical capacity, lower toxicity, more reliable efficacy, and fewer side reactions than nitrite. The present lab tests were run to determine if nitrate’s beneficial effects to lower H2 generation in simulated and genuine sludge continued for simulated sludge mixed with agents to immobilize water to help meet the Waste Isolation Pilot Plant (WIPP) waste acceptance drainable liquid criterion. Tests were run at ~60°C, 80°C, and 95°C using near spherical high-purity uranium metal beads and simulated sludge to emulate uranium-rich KW containerized sludge currently residing in engineered containers KW-210 and KW-220. Immobilization agents tested were Portland cement (PC), a commercial blend of PC with sepiolite clay (Aquaset II H), granulated sepiolite clay (Aquaset II G), and sepiolite clay powder (Aquaset II). In all cases except tests with Aquaset II G, the simulated sludge was mixed intimately with the immobilization agent before testing commenced. For the granulated Aquaset II G clay was added to the top of the settled sludge/solution mixture according to manufacturer application directions. The gas volumes and compositions, uranium metal corrosion mass losses, and nitrite, ammonia, and hydroxide concentrations in the interstitial solutions were measured. Uranium metal corrosion rates were compared with rates forecast from the known uranium metal anoxic water corrosion rate law. The ratios of the forecast to the observed rates were calculated to find the corrosion rate attenuation factors. Hydrogen quantities also were measured and compared with quantities expected based on non-attenuated H2 generation at the full forecast anoxic corrosion rate to arrive at H2 attenuation factors. The uranium metal corrosion rates in water alone and in simulated sludge were near or slightly below the metal-in-water rate while nitrate-free sludge/Aquaset II decreased rates by about a factor of 3. Addition of 1 M nitrate to simulated sludge decreased the corrosion rate by a factor of ~5 while 1 M nitrate in sludge/Aquaset II mixtures decreased the corrosion rate by ~2.5 compared with the nitrate-free analogues. Mixtures of simulated sludge with Aquaset II treated with 1 M nitrate had uranium corrosion rates about a factor of 8 to 10 lower than the water-only rate law. Nitrate was found to provide substantial hydrogen mitigation for immobilized simulant sludge waste forms containing Aquaset II or Aquaset II G clay. Hydrogen attenuation factors of 1000 or greater were determined at 60°C for sludge-clay mixtures at 1 M nitrate. Hydrogen mitigation for tests with PC and Aquaset II H (which contains PC) were inconclusive because of suspected failure to overcome induction times and fully enter into anoxic corrosion. Lessening of hydrogen attenuation at ~80°C and ~95°C for simulated sludge and Aquaset II was observed with attenuation factors around 100 to 200 at 1 M nitrate. Valuable additional information has been obtained on the ability of nitrate to attenuate hydrogen gas generation from solution, simulant K Basin sludge, and simulant sludge with immobilization agents. Details on characteristics of the associated reactions were also obtained. The present testing confirms prior work which indicates that nitrate is an effective agent to attenuate hydrogen from uranium metal corrosion in water and simulated K Basin sludge to show that it is also effective in potential candidate solidified K Basin waste forms for WIPP disposal. The hydrogen mitigation afforded by nitrate appears to be sufficient to meet the hydrogen generation limits for shipping various sludge waste streams based on uranium metal concentrations and assumed waste form loadings

Mitigation of Hydrogen Gas Generation from the Reaction of Uranium Metal with Water in K Basin Sludge and Sludge Waste Forms