Reduce the water content to less than 0.5 wt% and similarly reduce equivalent

DOE-STD-3013-2004

plutonium oxide after 950oC calcination, even with exposure to relative humidities

up to 50%, also is well below 0.5 wt% [Haschke/Ricketts 1995].

No new physical or chemical processes have been identified which result from

lowering the minimum plutonium content from 50 to 30 wt%. Based on process

knowledge, well-established thermal properties of likely initial impurity phases,

and MIS measurements of elemental composition and x-ray diffraction patterns,

the dominant impurity phases after calcination are expected to consist of binary

chloride salts of Na, K, Ca, and Mg and binary and compound metal oxides

involving Fe(III), Cr(III), Ni(II), Ga(III), Mg(II), etc. [Mason et al. 1999]. MIS

elemental analysis on 33 calcined site RFETS* and PFP* samples show iron,

nickel and chromium as common impurities at levels up to about 5 wt% (iron and

nickel) and 1.5 wt% (chromium). The other two most common elemental

impurities (other than Na, K, Ca and Mg associated predominantly with chlorides)

are gallium (up to about 2.5 wt%) and silicon (up to about 1 wt%). While

quantitative details will vary with the impurity, the conceptual model for

chemisorption and physisorption of moisture on trivalent oxides is expected to be

qualitatively similar to moisture interactions with plutonium oxide [Henrich/Cox

1996]. Therefore, moisture affinities and binding energies of oxide impurities are

anticipated to be qualitatively similar to those of plutonium oxide.

Uranium oxide is expected (and confirmed by MIS x-ray diffraction) to be present

predominantly as U3O8 after calcination at 950oC. Like other highly oxidized

impurity metal oxides expected after calcination, U3O8 represents a large

potential sink for any hydrogen gas that might be evolved [e.g., see the free

energy/temperature diagram for metal oxide/hydrogen reactions presented as

Figure 14-4 in Darker et al. 1953].

Measurements on impure oxides by the MIS project show that residual moisture

levels after 950oC calcination are reliably below the 0.5 wt% criterion, and well

below 0.2 wt% in most cases examined to date [Mason et al. 1999]. MIS

measurements also show that surface areas for these impure oxides are

* RFETS Rocky Flats Environmental Technology Site; PFP Plutonium Finishing Plant, a former

plutonium processing facility at Hanford currently being used for storage.