Nitric acid processing
Nitric acid is used both as a dissolution agent and a processing medium. Impure scrap plutonium or plutonium-contaminated items, such as glass, graphite casting molds, magnesium oxide crucibles, and incinerator ash, can be dissolved in or leached with nitric acid, while impure plutonium dioxide can be dissolved in the acid. (Dissolving plutonium metal in nitric acid produces an unstable residue that is pyrophoric and susceptible to shock-induced explosions. Thus for processing, if the plutonium is not already an oxide, it is converted to one by burning the metal in air.)
During processing, the plutonium containing nitric acid solution is passed through a column packed with thousands of beads of an anion-exchange resin. The resin is an organic polymer that contains cationic (positively charged) sites incorporated into the solid polymer with the charge balanced by mobile anions (negatively charged) such as nitrate. These mobile anions can be displaced by other anionic species in solution that bind to the fixed cationic sites on the resin depending on their relative affinity for the cationic site.
Plutonium in the tetravalent oxidation state is one of only a handful of elements that can form stable anionic complexes in nitric acid; it binds to the cationic sites on the resin as the plutonium hexanitrato complex, Pu(NO3)62–. This Pu(IV) complex has the highest sorption coefficient of any metal ion to certain cationic resins and has a peak value for sorption at nitric acid concentrations of about 7 molar (M). Pu(IV) is preferentially sorbed under these conditions, while most other elements simply flow through the column and are washed from the system using additional amounts of nitric acid, leaving the plutonium bound to the column.
Once the resin has been “loaded and washed,” the plutonium is recovered by sending an eluting solution (0.35-0.5 M nitric acid) through the column. The Pu(IV) has little affinity for the resin in the dilute nitric acid, so it desorbs and flows with the solution into holding tanks. The Pu(IV) in the dilute nitric acid solution is then reduced to Pu(III) by the addition of a solution of hydroxyl-amine. After the plutonium has been converted to the trivalent state, a small molar excess of oxalic acid is added to precipitate Pu(III) oxalate decahydrate, Pu2(C2O4)3·10H2O. The solid oxalate is collected by filtration, dried, and converted to pure plutonium dioxide by calcining at 600 degrees Celsius (°C). The PuO2 is then converted to metal or used directly for production of mixed oxide (MOX) nuclear fuels.
As with all plutonium processes, anion-exchange produces waste contaminated with low levels of plutonium. The resin does not remove all the plutonium from solution, and thus the nitric acid solutions always contain small amounts of plutonium after they have passed through the anion-exchange process. The contaminated effluents from the column and the holding tanks are therefore sent to an evaporator. The evaporator bottoms, which contain most of the residual plutonium, other actinides, and impurity elements are stabilized in cement and disposed of as TRU waste at the WIPP facility.
The condensed water and nitric acid vapor from the evaporator is sent to a second evaporator that recovers most of the nitric acid for recycle in plant operations. The condensate from the nitric acid recovery system is a very dilute nitric acid stream that has very low levels of alpha activity. This liquid is piped to holding tanks in the Radioactive Liquid Waste Treatment Facility at TA-50, where it is further treated with a series of processes including flocculation/precipitation, ultrafiltration, and reverse osmosis to produce water with very low levels of alpha activity that meets permit requirements, and then discharged to the environment following New Mexico Environment Department (NMED) regulations.
Over decades, Los Alamos has improved the nitric acid processing operations to reduce the hazardous components in the waste streams. For example, because some plutonium remains sorbed to the anion-exchange resin, ultimately the used resin must also be disposed of as a TRU waste to the WIPP. In the late 1980s, Los Alamos collaborated with Reilly Industries, Inc., to develop ReillexTM HPQ resin. Compared with earlier anion-exchange resins, HPQ has improved sorption properties for plutonium. It is also less prone to radiolytic or chemical degradation in the harsh conditions of solutions of nitric acid containing high concentrations of alpha-emitting plutonium isotopes. The enhanced stability of Reillex HPQ allows the resin to be used for approximately 50 plutonium recovery cycles before being replaced.
The photo above shows air-dried plutonium(III) oxalate from aqueous nitrate operations. The oxalate is converted into pure plutonium dioxide (below) by calcining at 600 degrees Celsius. While plutonium dioxide is normally olive green, samples can be various colors. It is generally believed that the color is a function of chemical purity, stoichiometry, particle size, and method of preparation, although the color resulting from a given preparation method is not always reproducible.