Solvent Performance in THTR Nuclear Fuel Reprocessing. Part II: On the Formation of Dibutyl Phosphoric Acid by Radiolytic and Hydrolytic Degradation of the TBP-n-Paraffin Extractant

Abstract

Radiolysis and hydrolysis of tributylphosphate (TBP) in n-paraffin diluent were investigated under conditions simulating several parameters of the THOREX flowsheet for reprocessing thorium containing spent nuclear fuel. The solvent (30 vol% TBP-n-paraffin) was equilibrated with 0.25 to 3 molar nitric acid, and the organic phase was inspected by gas chromatography. Radiolysis in pure solvents (30 and 5 vol% TBP-n-paraffin) with doses from 0.5 to 13 Wh . l/sup -1/ produces HDBP with G values (TBP-based) of approx. 2 and approx. 6, slightly decreasing with higher doses. No H/sub 2/MBP was found. Radiolysis in solvent (30 vol% TBP-n-paraffin), equilibrated with 0.25 to 3 molar nitric acid at a constant dose of 7 Wh . l/sup -1/, leads to a considerably reduced radiolytic fraction of the total HDBP yield with a minimum of approx. 0.3 molar nitric acid in the organic phase (2 molar in the aqueous phase). Irradiating the solvent (30 vol% TBP-n-paraffin), equilibrated with 0.75 molar nitric acid (0.12 molar in the organic phase) with doses from 0.5 to 13 Wh . l/sup -1/, leads to a mixture of hydrolytically and radiolytically formed HDBP, the hydrolytic fraction of which raises the total HDBP yield over the radiolytic HDBP yield frommore » acid-free solvents up to doses of approx. 3 Wh . l/sup -1/. It is proposed to denominate TBP degradation product yields according to their origin, hydrolysis or radiolysis (G value), or just by a concentration value in case of mixed origin, valid for the experimental conditions.« less