Assessment of the Influence of the Reduction in the Caustic Addition During the Processing of Spent Nuclear Fuel - 21166

For nearly seventy years, the Savannah River Site (SRS) has produced and stored nuclear materials. The L-basin facility on-site currently stores and manages domestic and foreign research reactor spent nuclear fuel (SNF). Historically, highly enriched uranium (HEU) was recovered in the H-Canyon (HCAN) facility at SRS in order to optimize storage space in the L-basin facility. The SRS is considering alternative approaches for spent fuel management. One such possible approach would involve HCAN no longer recovering HEU that provides feedstock for commercial nuclear power, and instead processing the existing SNF inventories for direct disposal (DD) to the Concentrate, Storage, and Transfer Facility (CSTF) and ultimately to the Defense Waste Processing Facility for immobilization. This process change would result in a paradigm shift from current HCAN, CSTF, and DWPF operations. The Savannah River National Laboratory (SRNL) is conducting studies to investigate the influence of DD of the SNF stream on the service life of facility infrastructure and the efficiency of the waste immobilization process. The present HCAN process utilizes concentrated nitric acid to dissolve SNF in HCAN. The vessels and piping in HCAN were fabricated from 304L stainless steel and are ideally suited to handle the acidic waste stream. However, as the waste is transferred to DWPF, it will contact the carbon steel waste tanks in CSTF. In order to prevent corrosion of the carbon steel, the acidic waste is neutralized (i.e., adjusted to a pH greater than 11) by the addition of sodium hydroxide (NaOH). Further additions are made such that the stream that is transferred to CSTF contains a minimum of 1.2 M excess hydroxide (OH). In addition to corrosion protection, the neutralization step minimizes the potential for aluminum precipitation and gel formation in the transfer lines between HCAN and CSTF. When processing DD material, the NaOH solution volume is anticipated to be approximately equal to the volume of dissolver solution; reduction of the added NaOH volume would potentially allow more flexibility in HCAN operations. An analysis was performed to investigate the influence of reducing the excess caustic that is added to the HCAN neutralization tanks (i.e., reduced neutralization) on CSTF and DWPF infrastructure and operations. The investigation revealed that a two-fold reduction in the excess caustic added would not result in a significant volume reduction (i.e., tank storage space recovery) in HCAN. The reduction in the excess caustic added also did not influence the corrosion behavior of the stainless-steel infrastructure in HCAN. However, the resulting slurry would tend to be thicker rheologically and more reactive than the sludge that it will be combined with in the CSTF feed preparation tank. Corrosion prevention in the CSTF feed preparation tank can be managed without significant processing changes or increases in waste volume. The DD stream will increase the amount of sodium nitrite necessary to prevent corrosion during sludge batch (SB) washing in this tank. However, new corrosion control strategies are being developed to minimize this impact. From the processing standpoint, reducing the excess caustic would potentially require more sodium hydroxide addition to the CSTF feed preparation tank in order to ensure aluminum dissolution. Reduced neutralization is expected to have a very minor impact on the composition of sludge after washing and is not anticipated to impact rheology or settling. The projected supernate chemistry for CSTF feed tank to DWPF is not expected to change significantly. Although the DD stream would add uranium and aluminum to this tank, the uranium is a very small compared to the uranium in the SB and the added aluminum will be removed as part of the aluminum dissolution planned for each future SB. Therefore, the DD stream is not anticipated to have an influence on the corrosion behavior of DWPF infrastructure or processing To ensure waste will be processable in the CSTF and DWPF, the following recommendations were made to the facilities. - Perform testing with actual waste for each SB, which would ensure that the chemical and physical properties of the slurry are acceptable for transfer, mixing, and processing from HCAN through DWPF. - Measure the rheology and settling behavior of the freshly precipitated DD stream, DD combined with sludge, and then process the sludge through prototypic DWPF cycles. - Consider alternate corrosion control schemes that reduce the amount of nitrite inhibitor needed during SB washing in CSTF. (authors)