Radical assays used as a screening tool for determining the radiolytic stability of monoamide complexants

The PUREX (Pu and U Reduction Extraction) process commonly uses tributyl phosphate (TBP) as the extractant to separate out uranium and plutonium from nuclear waste. Using TBP results in third-phase formation and makes waste disposal more difficult because the presence of phosphorus in TBP complicates incineration. To avoid these drawbacks, N,N-dialkyl amides (thereafter, monoamides) have been studied as alternatives to TBP. Monoamides are more readily incinerated than TBP and their amine and carboxylic acid degradation products are water-soluble, preventing third-phase formation. Although monoamides are promising complexants, they must also be radiolytically stable to allow scale-up of the separation process. γ-Radiolysis is the gold standard technique for predicting radiolytic degradation products and kinetics, but it is also a low-throughput, high-cost process, so studies of monoamide radiolytic stability are limited. To help eliminate the bottleneck, we are developing non-radioactive radical assays as a potential screening tool for use in conjunction with radiolysis. Upon heating in toluene, azohydroperoxide forms hydroxyl and tert-butyl radicals that degrade monoamides, including process-relevant DEHBA and DEHiBA. We have identified correspondence of the same degradation products from the radical assay and γ-radiolysis of the monoamides in toluene, including amine, secondary amide, amide - H₂, and solvent-adduct products. We are working to extend the qualitative agreement of products to quantitative predictions of degradation product ratios and dose-dependence. Based on these correlations, these radical assays show promise as a screening tool to quickly examine the potential radiolytic stability of monoamide complexants before γ-radiolysis studies.