Evaluation of alternate extractants to tributyl phosphate. Phase I

Abstract

Preliminary evaluations have indicated that tri(n-hexyl) phosphate (THP) and tri(2-ethylhexyl) phosphate (TEHP) have some significant advantages over tri(n-butyl) phosphate (TBP) for fuel reprocessing although they also have some disadvantages. The longer alkyl chains in these new extractants decrease their aqueous phase solubility and increase the organic phase solubility of their metal complexes and the metal complexes of their degradation products. Both THP and TEHP extract uranium and plutonium more strongly than TBP; thorium extraction is in the order THP > TBP > TEHP. Tritium extraction is highest with TBP because of slightly higher water extraction. In extractions of thorium, a third liquid phase was formed using TBP at a solvent loading of about 40 g/L of thorium and above. Third-phase formation did not occur with THP or TEHP. The dialkyl phosphoric acid degradation products of THP and TEHP showed a markedly lower tendency to precipitate with thorium than did dibutyl phosphoric acid (HDBP). Chemical stability studies showed TEHP to have much greater stability to acid hydrolysis than TBP and THP, which were about equivalent. No differences were detected in the radiation stability of the three extractants. The phase separation properties of THP and TEHP are inferior to those of TBP in both the nitric acid and sodium carbonate (solvent wash) systems. Phase separation was improved appreciably by using a lower extractant concentration than 1.09 M (equivalent to 30 vol % TBP). Difficulties were encountered with TEHP, however, owing to rapid degradation of its phase separation properties with time of contact with HNO{sub 3}; this problem requires additional study.