FORM AND AGING OF PLUTONIUM IN SAVANNAH RIVER SITE WASTE TANK 18
This report provides a summary of the effects of aging on and the expected forms of plutonium in Tank 18 waste residues. The findings are based on available information on the operational history of Tank 18, reported analytical results for samples taken from Tank 18, and the available scientific literature for plutonium under alkaline conditions. These findings should apply in general to residues in other waste tanks. However, the operational history of other waste tanks should be evaluated for specific conditions and unique operations (e.g., acid cleaning with oxalic acid) that could alter the form of plutonium in heel residues. Based on the operational history of other tanks, characterization of samples from the heel residues in those tanks would be appropriate to confirm the form of plutonium. During the operational period and continuing with the residual heel removal periods, Pu(IV) is the dominant oxidation state of the plutonium. Small fractions of Pu(V) and Pu(VI) could be present as the result of the presence of water and the result of reactions with oxygen in air and products from the radiolysis of water. However, the presence of Pu(V) would be transitory as it is not stable at the dilute alkaline conditions that currently exists in Tank 18. Most of the plutonium that enters Savannah River Site (SRS) high-level waste (HLW) tanks is freshly precipitated as amorphous plutonium hydroxide, Pu(OH){sub 4(am)} or hydrous plutonium oxide, PuO{sub 2(am,hyd)} and coprecipitated within a mixture of hydrous metal oxide phases containing metals such as iron, aluminum, manganese and uranium. The coprecipitated plutonium would include Pu{sup 4+} that has been substituted for other metal ions in crystal lattice sites, Pu{sup 4+} occluded within hydrous metal oxide particles and Pu{sup 4+} adsorbed onto the surface of hydrous metal oxide particles. The adsorbed plutonium could include both inner sphere coordination and outer sphere coordination of the plutonium. PuO{sub 2(am,hyd)} is also likely to be present in deposits and scales that have formed on the steel surfaces of the tank. Over the operational period and after closure of Tank 18, Ostwald ripening has and will continue to transform PuO{sub 2(am,hyd)} to a more crystalline form of plutonium dioxide, PuO{sub 2(c)}. After bulk waste removal and heel retrieval operations, the free hydroxide concentration decreased and the carbonate concentration in the free liquid and solids increased. Consequently, a portion of the PuO{sub 2(am,hyd)} has likely been converted to a hydroxy-carbonate complex such as Pu(OH){sub 2}(CO{sub 3}){sub (s)}. or PuO(CO{sub 3}) {center_dot} xH{sub 2}O{sub (am)}. Like PuO{sub 2(am,hyd)}, Ostwald ripening of Pu(OH){sub 2}(CO{sub 3}){sub (s)} or PuO(CO{sub 3}) {center_dot} xH{sub 2}O{sub (am)} would be expected to occur to produce a more crystalline form of the plutonium carbonate complex. Due to the high alkalinity and low carbonate concentration in the grout formulation, it is expected that upon interaction with the grout, the plutonium carbonate complexes will transform back into plutonium hydroxide. Although crystalline plutonium dioxide is the more stable thermodynamic state of Pu(IV), the low temperature and high water content of the waste during the operating and heel removal periods in Tank 18 have limited the transformation of the plutonium into crystalline plutonium dioxide. During the tank closure period of thousands of years, transformation of the plutonium into a more crystalline plutonium dioxide form would be expected. However, the continuing presence of water, reaction with water radiolysis products, and low temperatures will limit the transformation, and will likely maintain an amorphous Pu(OH){sub 4} or PuO{sub 2(am,hyd)} form on the surface of any crystalline plutonium dioxide produced after tank closure. X-ray Absorption Spectroscopic (XAS) measurements of Tank 18 residues are recommended to confirm coordination environments of the plutonium. If the presence of PuO(CO{sub 3}){sub (am,hyd)} is confirmed by XAS, it is recommended that experiments be conducted to determine if plutonium carbonates are transformed back into PuO{sub 2(am,hyd)} upon contact with grout.
- Research Organization:
- Savannah River Site (SRS), Aiken, SC (United States)
- Sponsoring Organization:
- USDOE
- DOE Contract Number:
- DE-AC09-08SR22470
- OSTI ID:
- 1035777
- Report Number(s):
- SRNL-STI-2012-00106; TRN: US1201240
- Country of Publication:
- United States
- Language:
- English
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Related Subjects
ACID NEUTRALIZING CAPACITY
AGING
CRYSTAL LATTICES
HYDROXIDES
OXALIC ACID
PLUTONIUM
PLUTONIUM CARBONATES
PLUTONIUM DIOXIDE
PLUTONIUM HYDROXIDES
PLUTONIUM OXIDES
RADIOLYSIS
REMOVAL
RESIDUES
TANKS
THERMODYNAMICS
VALENCE
WASTES
WATER