HIGH Cr_2O_3 REFRACTORY CORROSION IN OXIDIZING MELTER FEEDS: RELEVANCE TO NUCLEAR AND HAZARDOUS WASTE VITRIFICATION

摘要

Monofrax K-3 refractory is a robust refractory that has been selected to line both High Level Waste (HLW) and Low Level Mixed Waste (LLMW) melters throughout the DOE complex. Monofrax K-3 is tolerant of transition metal oxides but is highly reduced in nature. The corrosion of the reduced high Cr_2O_3 containing Monofrax K-3 refractory in highly oxidizing waste melter feeds was, therefore, studied. Testing of the corrosion resistance of the K-3 refractory was performed in sealed crucibles starting with slurried melter feed. The sealed crucible testing where the K-3 coupon was immersed in melter feed was used preferentially to the ASTM C621 refractory corrosion test which immerses the coupon in pre-reacted glass. The sealed crucible melter feed testing was performed so that the refractory coupon would be exposed to the oxidizing or reducing species being released during the feed to glass conversion, e.g. NO_3~-, NO_2~- ,CO_2, CO, O_2 which may impact refractory performance. The corrosion rates measured in highly oxidizing (high nitrate) feeds were approximately 1.2-1.6 times greater than those determined using the ASTM C621 test protocol. Confirmatory corrosion rates were measured on K-3 coupons immersed in oxidizing feed in a l/100th scale Defense Waste Processing Facility (DWPF) pilot scale melter. Calculations show that high nitrate oxidizing feeds, which produce highly oxidized melts, act as the driving force for attack of the highly reduced Monofrax K-3. Monofrax K-3 corrosion products are insoluble in the borosilicate waste glass tested. Hence, the corrosion products have the potential to settle to the melter floor and accumulate if they are not eliminated from the melt pool during pouring. For the DWPF melter, such accumulation would still allow the DWPF to operate well beyond (>5 years) its design basis of 2 years.