Oxides - Continued - std301320000032

DOE-STD-3013-2000

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

levels after 950 C 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

comparable to those of pure oxides after calcination.

MIS elemental analysis on the 33 site materials studied to date shows only one

of these items has as-received chloride content above 20 wt%. In all cases, the

chloride ion content after 950 C calcination was reduced to less than 8 wt%. In

general, residual chloride salt is expected (and is observed) to be strongly

reduced by calcination, a factor which will restrict moisture uptake by chlorides

before packaging. Sodium and potassium chloride melt below the 950 C

calcination temperature and therefore relatively large particle sizes and low

surfaces areas are likely to result for these phases. A recent literature survey

indicates that these chlorides will not resorb water to an appreciable extent after

calcination unless quite high relative humidities are encountered [Smith et al.

1999].

Residual magnesium and calcium chlorides, on the other hand, can adsorb and

chemically bind as stoichiometric hydrates substantial amounts of water at much

lower relative humidities [Smith et al. 1999]. However, these chlorides also are

known to partially or completely convert to their respective oxides when heated

in the presence of moisture, as confirmed by MIS elemental composition and x-

ray diffraction results on calcined impure oxides. [Mason et al. 1999]

Unfortunately, magnesium and calcium chlorides also readily release water with

modest heating. At elevated temperature in sealed containers, this moisture

potentially could migrate to the plutonium oxide phase and physisorb as weakly

bound multiple layers, theoretically exceeding 0.5 wt% equivalent for the oxide

phase. This process, if it happens, likely would affect the equilibrium vapor

pressure of moisture over plutonium oxide, and possibly affect the radiolytic

behavior of the adsorbed water. Therefore, control of time and atmosphere

between calcination and packaging is strongly advised when handling plutonium

oxides containing magnesium and calcium chloride impurities to avoid the

potential for concentrating unacceptable levels of moistures on other phases,

and potential initiation of additional gas generation mechanisms. A survey of