Alternative Chemical Cleaning Methods for High Level Waste Tanks: Simulant Studies

Abstract

We gratefully acknowledge the assistance of Holly Hall in the movement of equipment, supplies, samples, and chemicals between laboratories during testing. The efforts of Whitney Riley in analyzing most of the non-radioactive test samples are appreciated. SRNL-STI-2015-00517 Revision 0 vi SRNL-STI-2015-00517 Revision 0 vii oxalic/nitric acid reagent. Following treatment using the baseline oxalic acid reagent, detectable levels of actinide removal were achieved using the acidic permanganate cleaning method. Based on the results observed with non-radioactive and radioactive simulants, actual waste testing of both alternative cleaning technologies (optimized oxalic acid cleaning using supplementary nitric acid and permanganate-based cleaning under both acidic and basic conditions) is recommended.

Alternative Chemical Cleaning Methods for High Level Waste Tanks: Actual Waste Testing with SRS Tank 5F Sludge

“…The observation of nickel oxalate is consistent with simulant testing which indicated the formation of the same phase during contact of nickel hydroxide reagent with oxalic acid. 13 XRD results for the OOAC tests (both direct and pretreated) revealed the presence of the same crystalline phases in each sample (though a large amorphous peak was also observed for the P-OOAC sample). Both samples contained hematite and the previously unobserved trimetallic sludge phase trevorite, NiFe 2 O 4 .…”

“…However, within four days after contact of the reagents with Tank 5F sludge samples, the purple color of the permanganate in the more dilute reagent was observed to disappear, indicating a reaction between permanganate and reductants in the waste (such as oxalate anions). Subsequent analysis of the filtered solutions from these tests at approximately 4, 12, and 14 days (Table 3 -8) revealed that the plutonium concentrations observed for the dilute permanganate solution ranged from 3E-07 to 8E-07 M while the concentrations observed for the more concentrated permanganate reagent (0.05 M) ranged from 2E-06 to 3E-06 M. For comparison, plutonium concentrations observed by this method using radioactive simulants were near 7E-06 M. 13 As shown in Table 3-9, the plutonium concentrations observed for the more concentrated permanganate reagent correspond to only 3-4% removal of plutonium from the sample.…”

“…Analysis of filtered sub-samples after approximately 4, 12, and 14 days indicated that the plutonium concentration ranged from 3E-06 to 5E-06 M (Cycle 1; Table 3- 8). For comparison, the plutonium concentrations observed during radioactive simulant testing were near 2.5E-06 M. 13 As shown in Table 3-9, these plutonium concentrations correspond to the removal of 6 to 7.5% of the plutonium from the sample.…”

“…The smaller pore size filters were used for samples analyzed for actinide elements in order to be consistent with previous testing protocols. 13 The sub-samples were filtered directly into 15 mL shielded polyethylene bottles containing 5 M nitric acid for sample dilution and stabilization. The volume-based target acid dilution factor was 4.…”

“…Reagent volumes added during BOAC testing are provided in Caustic and acidic permanganate cleaning tests were conducted similarly to the oxalic acid tests but with a smaller reagent:sludge volume ratio (10-15 per cycle) that was based on tests conducted with radioactive simulants. 13 The acidic permanganate sludge sample was pre-acidified with two cycles of dilute nitric acid at a phase ratio of 20:1 prior to contact with the permanganate solution. Due to the small volumes involved in these tests, the settling characteristics of the material, and the challenges of seeing the settled solids in the shielded cells environment, decantation of the permanganate cleaning reagents at the conclusion of each test cycle was not possible with these samples.…”