DISSOLUTION OF ORNL HLW SLUDGE AND PARTITIONING OF THE ACTINIDES USING THE TRUEX PROCESS

Abstract

ABSTRACT Experiments were conducted to evaluate acid dissolution of actual high-level radioactive waste sludge and to evaluate the transuranium extraction (TRUEX) process for partitioning actinides from the dissolved sludge solution. All tests were performed at a temperature of 24°C. Samples of sludge from Melton Valley Storage Tank W-25 were rinsed with mild caustic (0.2 M NaOH) to reduce the concentrations of nitrates and fission products associated with the interstitial liquid. In one campaign, the rinsed sludge was leached in nitric acid and about 50% of the dry mass of the sludge was dissolved. The resulting solution contained total metal concentrations of ∼1.8 M with a nitric acid concentration of 2.9 M. The solution began gelling immediately, and a visible gel layer was observed after 8 days. In the other campaign, the sludge was neutralized with nitric acid to destroy the carbonates and was subsequently leached with 2.6 M NaOH for ∼6 h before rinsing with the mild caustic. The sludge was then leached in nitric acid, and about 80% of the sludge dissolved. The resulting solution contained a total cation concentration (excluding H+) of ∼0.6 M and a nitric acid concentration of 1.7 M. This solution became hazy in ∼8 days, indicating gel formation, but did not display separated gel layers after aging for 20 days. Batch liquid-liquid equilibrium tests of the extraction and the stripping operations were conducted. Chemical analyses of both phases were used to evaluate the process. Evaluation was based on two metrics: the fraction of TRU elements removed from the dissolved sludge, and comparison of the results with predictions made with the Generic TRUEX Model (GTM). The fractions of Eu, Pu, Cm, Th, and U removed from aqueous solution in only one extraction stage were >95% and were close to the values predicted by the GTM. Mercury was also found to be strongly extracted, with a one-stage removal of >92%. In one test, vanadium appeared to be moderately extracted.