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

4.31
enough to prevent access of water to the interior of the composite. A barrier can be
formed, however, as is done in the PAMELA process, by suspending the waste-containing par-
ticles in a basket in the canister and filling the annulus between the basket and the can-
ister wall with pure matrix metal.
Coated Particles
Coated particle composite waste forms are being developed, partially based on the tech-
nology developed for the manufacture of high temperature gas-cooled reactor (HTGR) fuels
(Rusin et al 1978, 1979a and 1979b). These fuels consist of ceramic pellets that are coated
with pyrolytic graphite and silicon carbide, and embedded in a graphite matrix. The core
material that has been most studied for coated particle composite waste forms is the synthe-
tic mineral calcine described in Section 4.3.2.3; however, the concept can utilize other
core materials. Calcine pellets are formed in,a disk pelletizer and coated with pyrolytic
graphite and silicon carbide in a fluidized bed. Laboratory tests have shown that an outer
coating of durable A1 20 3 can be added. The coated particles would be surrounded by a
metal matrix in canisters before emplacement in a geologic repository.
Coated particles are a way of adapting the multiple barrier concept to the waste form
itself. Tests have shown that the particles can have very good chemical durability. How-
ever, the processing would be very complex and require a large amount of development before
it could be done remotely.
Cermet
This waste form concept, under development at Oak Ridge National Laboratory, produces
a uniform dispersion of waste oxide particles within a metal matrix (Quinby 1978). The
waste and specific additives required to form the desired ceramic oxide phases and metal
alloy matrix are dissolved together in molten urea. The urea solution is precipitated and
calcined and the fine powders produced in this step are compacted by extrusion or pressing
into desired shapes. In the final processing step the reducible metal oxides, such as
oxides of Cr, Ni, Fe, and Co, are reduced in a H 2 or CO atmosphere to form an alloy that
encapsulates the unreduced ceramic oxides. After the 800 0 C reduction the composite is mixed
with an organic binder, extruded to form rods and sintered in a nonoxidizing atmosphere at
1200 0 C to form a dense