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DOE-STD-3013-2000
Measurements on impure oxides by the MIS project show that residual moisture
o
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
o
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
o
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
24


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