High Cr 2 O 3 containing Monofrax TM K-3 is a robust refractory that is used in the fiberglass industry and used in radioactive waste glass melters worldwide. Monofrax TM K-3 is tolerant of transition metal oxides but contains highly reduced solid solutions of spinels, that is, (Mg,Fe 2+ )(Al,Cr) 2 O 3 . Conversely, many of the waste feeds being processed are highly oxidizing. The K-3 refractory corrosion was tested in sealed crucibles starting with slurried melter feed instead of prereacted glass called for by ASTM C621. Testing the refractory coupon during the feed-to-glass conversion exposes the refractory to the oxidizing and reducing species being released during vitrification, for example, NO 3 À , NO 2 À , CO 2 , CO, O 2 . Corrosion rates measured in highly oxidizing (high nitrate) feeds § were~1.8-2.8 times higher than those determined using prereacted glass or reduced feeds. ¶ Confirmatory corrosion rates were measured on Monofrax TM K-3 coupons immersed in oxidizing feed in a 1/100th-scale HLW pilot-scale melter. Corrosion is heterogeneous or incongruent as Ni and Fe in the waste glass exchange with Mg and Al in the refractory. An insoluble NiFe 2 O 4 spinel corrosion product is formed that can build up a protective layer along the refractory walls or spall and settle to the melter floor depending on melt pool convection/agitation.
High Cr2O3 containing Monofrax™ K‐3 is a robust refractory that is used in radioactive waste glass melters worldwide. Monofrax™ K‐3 contains highly reduced phases. Conversely, many of the radioactive feeds being processed are highly oxidizing. The K‐3 refractory corrosion rates in oxidizing (high nitrate) feeds were ~1.8–2.8 times higher than the rates determined using reducing feeds. The corrosion product formed is a mixture of spinel and glass (slag) that can accumulate on the melter floor. A methodology to calculate the depth of slag deposits from refractory corrosion is presented and verified with slag measurements from the Defense Waste Processing Facility (DWPF) melter after it had processed oxidized feeds for 1.75 years. The calculations show that had the facility continued to process oxidized feeds the melter lifetime (based on when the deposits could have reached and blocked the pour spout riser) would have been ~4.5 years. The DWPF changed to a reducing flow sheet after ~3 years of operation. The lifetimes of Melter #1 and Melter #2, assuming a failure due to pour spout blockage, are calculated as 7.7–12 years based on corrosion rates measured with reducing feeds. Lifetimes of 9 and >11 years have actually been achieved.
Pitting corrosion, nitrite concentration fell outside that specified in the corrosion control program for HLW 30 A537SRNL-STI-2014-00281 Revision 0 design basis, but in the regions with cracking and cavitation the K-3 corrosion rate is at or greater than the DWPF design basis.5. The cavitation is likely due to the proximity of the refractory coupon to the bubbler orifice but the cracking is related to the Fe° depletion caused by excessive nitric acid used in the glycolic acid CEF campaign.The expected performance of the materials of construction within the CPC, specifically C276, Ultimet® and Stellite® at boiling temperatures, is questionable due to the susceptibility to localized corrosion identified during this testing. Since the glycolate anion concentration is at the highest in the CPC for the whole HLW processing system, determining operating limits where localized corrosion is not a concern is stressed. Additional testing for these materials is recommended to better understand the limits of these results and identify conditions for acceptable performance in service.These proposed tests would include performing electrochemical testing with formic acid based SRAT/SME supernates, which would provide a correlation between accelerated electrochemical test results and observed performance of components within the CPC and with glycolic acid based SRAT/SME supernates at levels of aggressive species (chloride, sulfate and mercury) where localized corrosion does not occur. Similar simulants would be used in hot-wall tests to verify that localized corrosion also does not occur under heat transfer conditions. Finally, a coupon immersion test is recommended to verify that the accelerated results of the electrochemical test occur with time during an extended exposure in the coupon test.Additional testing was also recommended for other materials where the presence of the glycolate anion impacted test results. These tests were recommended to better identify the temperature and glycolic acid limits for acceptable performance of the materials of construction, especially those susceptible to localized corrosion.1. Coupon immersion test with 304L in 70% glycolic acid at actual service temperature to determine if pitting occurs.2. Hot-wall testing for G30 and G3 in waste stream simulants without glycolic acid present for comparison to results with glycolate anion present, where pitting and crevice corrosion was observed.3. Metallurgical examination of the inner diameter of I690 pipe used in fabrication of melter bubbler to determine presence of cracks, which would reduce effective thickness and act as stress concentrators.4. Perform modified ASTM C621 tests in glycolic acid feeds to determine impact of glycolic acid vs. formic acid with and without the impact of Ar bubbling, i.e. all the formic acid flowsheet corrosion data has been derived in non-bubbled pilot scale melters except for the CEF glycolic acid campaign. This testing should suffice to get more precise comparative corrosion data for the K-3.viii Revision TABLE OF CO...
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