Evaluation of HWVP feed preparation chemistry for an NCAW simulant -- Fiscal year 1993: Effect of noble metals concentration on offgas generation and ammonia formation

Abstract

SummaryThe High-Level Waste Vitrification Program is developing technology for the Department of Energy to immobilize high-level and transuranic wastes as glass for permanent disposal. Pacific Northwest Laboratory (PNL) is conducting laboratory-scale melter feed preparation studies using a HWVP simulated waste slurry, Neutralized Current Acid Waste o. A FV 1993 laboratory-scale study focused on the effects of noble metals (Pd, Rh, and Ru) on feed preparation offgas generation and NH, production. The noble metals catalyze H, and NH, production, which leads to safety concerns. The information gained from this study is intended to be used for technology development in pilot scale testing and design of the Hanford High-Level Waste Vitrification Facility. The work performed during FY 1993 was initiated under the Hanford Waste Vitrification Program (HWVP).Six laboratory-scale feed preparation tests were performed as part of the FY 1993 testing activities using nonradioactive NCAW simulant. Tests were performed with lo%, 25%, 50% of nominal noble metals content and nominal noble metals content. Also tested were 25% of the nominal Rh and a repeat of 25% nominal noble metals.The following summarizes the results of the test activities:Offgas profiles exhibited three reaction stages: I, CO? decomposition, 11, NO, destruction, and 111, Hz plus NH, formation. Hydrogen is formed during reaction stage 111 as a product of noble metals catalyzed HCOOH decomposition. The peak Hz generation rate and total Hz measured increased with noble metal concentration until a maximum level was reached at 25%-50% nominal noble metals concentration. The maximum derivative of the Hz generation rate showed an almost linear increase as a function of noble metals concentration.The observed relationship between noble metals concentration and the H, generation rate may be coupled to a higher NO, concentration at the initiation of the H, peak in the 10% nominal noble metals test compared to the 25%, 50%, and nominal tests. Higher NO; would be expected to reduce the Hz generation rate (Smith 1993).A test performed with only 25 % Rh showed significantly different behavior than the 25 % nominal noble metals test; therefore, it was concluded that the other noble metals (Pd and Ru) contributed catalytic activity to Hz production. This conclusion does not agree with the work of King at the University of Georgia (King and Bhattacharyya 1993). A replicate of this test should be performed to verify the results.Ammonia is also formed during stage 111 as a product of a reaction between HCOOH and NO,. The amount of NH, measured in the slurry following stage 111 increased as the noble metals concentration increased. Ammonia production is speculated to be related to H, production because Hz is an intermediate reactant in the NH, reaction. Therefore, as the total Hz measured increases as a function of noble metals concentration, NH, generation increases as well.Formation of a black residue in the nominal noble metals concentration test indicated the presence of red...

Noble Metal Catalyzed Hydrogen Generation from Formic Acid in Nitrite-Containing Simulated Nuclear Waste Media

“…Thus, RhCl 3 was found to be the most active catalyst precursor for hydrogen evolution from the sludge simulant under the conditions of interest even though it was inactive as a catalyst precursor for hydrogen evolution from pure formic acid at the same temperature (17,18). Ammonia generation has also been reported (19) during the addition of formic acid to simulants containing noble metals.…”