Oxides - Continued - std3028

DOE-STD-3028-2000

This ensures that the stabilization requirements of the Standard will accomplish the

above objectives.

a) Reduce the moisture/volatiles content, which might produce pressurizing

gases during long-term storage via radiolysis, to less than 0.5 wt % of

contained solids content.

The three stable forms of uranium oxide are UO2, UO3, and U3O8. Heating the

material above 750C (1382 F) is sufficient to eliminate free and hydrated

waters.

Water also might be present in 233U oxides as chemisorbed water. However,

since chemisorbtion is a surface phenomenon it will have little impact on the

0.5 wt% limit for 233U oxides. A simple model for H2O suggests that 0.2 mg

(4.4E-7 lbm) H2O forms a monolayer on a square meter surface [Haschke and

Ricketts 1995]. Thus, to be able to chemisorb more than 0.5 wt% would require

specific surface areas in excess of 25 m2/g (122,000 ft2/lbm). To achieve such

large surface areas would require special conditions not commonly found in 233U

oxide production environments. As an example, UO2 produced at ORNL was

made to a surface area specification of less than 6 m2/g (29,300 ft2/lbm) [Parrott,

Sr. et al. 1979]. Surface areas of 233U material found in ORNL records are

generally well below 10 m2/g (48,800 ft2/lbm) with the largest area found to be

12.5 m2/g (61,000 ft2/lbm).

b) Reduce potential for moisture/volatiles readsorption above the 0.5 wt%

threshold;

Moisture does not tend to readsorb in significant quantities on stabilized U3O8.

Calcining the material at 750C (1382F) will convert the oxides to U3O8 thus

eliminating the concern of readsorption and will ensure that the material will have

gotten to the stable U3O8 phase.