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

5.22
release rate of the nuclides. Preliminary results indicate that, although some radionu-
clides are released more rapidly than others as a function of experimental conditions, spent
fuel is a durable waste form that exhibits low release of radionuclides when subjected to
ground water under normal repository conditions.
Historically, glass, particularly borosilicate glass, has been the major focus of
alternate waste form work, and in 1977 it was selected as the reference material for immo-
bilization of the Savannah River Plant high-level waste (Stone et al. 1979). Small-scale
operating facilities have demonstrated practicality of the vitrification process (EPRI
1979). In addition to U.S. work, studies and pilot plants involving glass are under way in
France, Germany, Belgium, and England. Recently, however, more attention has been devoted
to other waste forms, and studies are being conducted to evaluate their characteristics
(DOE 1979).
A number of other waste forms are being studied (ERDA 1976, DOE 1979). Prior to the
decision to defer reprocessing, significant progress had been made in the development and
testing of waste forms, such as glass, for wastes generated by commercial reactors. Subsequent
to that decision, the emphasis of work on alternate waste forms has shifted to defense
related wastes. DOE is continuing to sponsor work on alternate forms, and it is fully
expected that the results and technology developed would be transferable, in large part, to
the commercial waste program and existing liquid wastes (EPRI 1979).
5.2.2.2 Materials
For filler materials as stabilizers in spent fuel canisters, candidate materials
include lead, glass, clay, sand, inert gases (e.g., helium) and castable solids (e.g.,
glass, lead and lead alloys, zinc and zinc alloys) (Jardine 1979 and Morgan 1974). Basic
physical and chemical properties of candidate stabilizer materials are well known. Some of
these candidate materials have been evaluated (under expected repository conditions) for use
as barrier materials other than as stabilizers (e.g., as canister, overpack, and/or backfill
barriers). Since the overall waste package functions are similar (e.g., corrosion resis-
tance, nuclide sorptive properties, protection of the waste form), the same materials test-
ing can, in many cases, be applied to several system components.
Canister, Overpack, and Sleeve. Candidate material selection for canister and overpack
will be based largely on the results of corrosion tests as a function of temperature, radia-
tion, and ground-water chemistry (e.g., pH, Eh, composition, and ionic strength) that are
typical of the water in various media of interest (i.e., basalt, granite, salt, and shale).
Applicable materials studies to date include consideration of general corrosion rates, pit-
ting and crevice corrosion susceptibilities, stress corrosion cracking, effects of oxygen
concentration, solution volume to solid surface area ratio, and possible effects from
radiolysis products (Braithwaite 1979 and Magnani 1979). Filler material may also be used
between the canister, overpack and sleeve.
Emplacement Shaft Backfill. Closure of the loaded repository will require backfilling
the waste emplacement shaft; backfill materials are being tested for selective nuclide