Management of Commercially Generated Radioactive Waste - U.S. ...

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solids may be primarily Portland cement such as used in concrete, or may consist of cement
mixed with fly ash'and clays (grouts) and can be specially designed (usually high alumina)
cements (Stone 1977 and Lokken 1978).
Cements are intrinsically somewhat porous and due to the hydrated phases are poten-
tially sensitive to damage from radiation and long-term thermal exposure. They have been
considered for the treatment of defense HLW, and techniques that reduce the porosity and
water content may even make their use for commercial HLW feasible (Roy and Gouda 1978). One
such technique is the FUETAP process being investigated at Oak Ridge National Laboratory in
which the waste-containing cement mixture is processed at 250°C and 600 psi (Moore et al.
1979).
Calcine
Defense HLW has been calcined using a fluidized bed calcination process at the Idaho
Chemical Processing Plant (ICPP) since 1963. Over 1500 m 3 of granular calcined waste par-
ticles are now stored in stainless steel bins housed in underground concrete vaults. The
calcined waste is a good low-volume, noncorrosive form for storage.
The ICPP calcination process converts the HLW to dry salts and oxides. Consolidation
techniques that decrease the surface area of the solids, decrease the potential for airborne
fines, and increase the chemical durability are being investigated. The consolidation tech-
niques are either sintering processes that yield a type of glass-ceramic product or pro-
cesses that embed the pelletized calcine in an inert matrix (INEL 1978, Lamb et al. 1979,
see Section 4.3.2.4).
Synthetic Minerals
To create synthetic minerals, nuclear waste constituents are chemically incorporated
in crystalline mineral species. The long-term stability of synthetic mineral waste forms
can be deduced from the known behavior of analogous naturally occurring minerals. Of
course, unavoidable differences, such as radiation effects, must be studied. A review of
the stability of minerals that could contain radionuclides is given in Appendix P of
Volume 2.
Development of one synthetic mineral concept (called supercalcine) began at Pennsyl-
vania State University and the Pacific Northwest Laboratory (McCarthy 1977, 1979a; McCarthy
and Davidson 1975). The concept may be considered an evolution of the well-developed cal-
cination processes. Instead of calcining the liquid HLW as received, additions of calcu-
lated quantities of Ca, Al, Si, etc. are made to the HLW so that after calcination and a
heat treatment the waste constituents are chemically bound in predetermined mineral assem-
blages. However, because HLW contains so many different elements, the mineral assemblages
tend to be very complex and difficult to characterize. Recently the emphasis in some
investigations has switched to the development of stable synthetic minerals for only the
actinides in the waste. Fluorite and monazite structures appear to form very stable crys-
tals containing these long-lived waste constituents (McCarthy 1979b). Hot pressing tech-
niques are being investigated for consolidation of the synthetic mineral calcines.