Dissolution of Fissile Materials Containing Plutonium and Beryllium Metals

Abstract

Scrap materials containing plutonium (Pu) metal were dissolved at the Savannah River Site (SRS) as part of a program to disposition nuclear materials during the deactivation of the FB‐Line facility. Some of these items contained both Pu and beryllium (Be) metal as a composite material. The Pu and Be metals were physically separated to minimize the amount of Be associated with the Pu; however, a dissolution flowsheet was required to dissolve small amounts of Be combined with the Pu metal using a dissolving solution containing nitric acid (HNO3) and potassium fluoride (KF). Since the dissolution of Pu metal in HNO3/fluoride (F−) solutions was well understood, the primary focus of the flowsheet development was the dissolution of Be metal. Initially, small‐scale experiments were used to measure the dissolution rate of Be metal foils using conditions effective for the dissolution of Pu metal. The experiments demonstrated that the dissolution rate was nearly independent of the HNO3 concentration over the limited range of investigation and only a moderate to weak function of the F− concentration. The effect of temperature was more pronounced, significantly increasing the dissolution rate between 40 and 105°C. The offgas analysis from three Be metal foil dissolutions demonstrated that the production of hydrogen (H2) was sensitive to the HNO3 concentration, decreasing by a factor of approximately two when the concentration was increased from 4 to 8 M. In subsequent experiments, complete dissolution of Be samples from a Pu/Be composite material was achieved in a 4 M HNO3 solution containing 0.1–0.2 M KF. Gas samples collected during each experiment showed that the maximum H2 generation rate occurred at temperatures below 70–80°C. A Pu metal dissolution experiment was performed using a 4 M HNO3/0.1 M KF solution at 80°C to demonstrate flowsheet conditions developed for the dissolution of Be metal. As the reaction progressed, the rate of dissolution slowed. The decrease in rate was attributed to the complexation of F− by the dissolved Pu. The F− became unavailable to catalyze the dissolution of plutonium oxide (PuO2) formed on the surface of the metal which inhibited the dissolution rate. To compensate for the complexation of F−, an increase in the concentration to 0.15–0.2 M was recommended. Dissolution of the PuO2 was addressed by recommending an 8–10 h dissolution time with an increase in the dissolving temperature (to near boiling) during the final 4–6 h to facilitate the digestion of the solids. Dilution of the H2 concentration below 25% of the lower flammability limit by purging the dissolver with air was also necessary to eliminate the flammability concern.