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

4.18
ENCAPSULATION
SPENT FUEL WATER BASIN ASSEMBLY (WELD TOP, He CANI STERED
ASSEMBLIES STORAGE DRYING FILL, SEAL, FUEL ASSEMBLIES
LEAK TEST)
FIGURE 4.3.1. Flow Diagram for Encapsulation of Intact Spent Fuel Assemblies
solids (DOE/NE-0007, Section II.E.1). The waste package finally chosen will be tailored to
the geologic environment in which the package is to be disposed.
In the DOE/ET-0028 study, the cleaned and dried fuel assemblies are individually pack-
aged in square canisters(a) that are only slightly larger than the assemblies themselves. A
canister for a PWR assembly has dimensions of 0.24 x 0.24 x 4.88 m (9.5 x 9.5 x 192 in.) and
a canister for a BWR assembly has dimensions of 0.165 x 0.165 x 4.88 m (6.5 x 6.5 x 192 in.).
For the mixture of fuel used in this generic study (40% of the assemblies are from PWRs and
60% are from BWRs), 127 canisters are filled per GWe-yr.
The process concept described in ONWI-39 (Appendix C) is very similar except that
cylindrical canisters are used, and the BWR assemblies are packaged three to a canister. A
canister for a PWR assembly has dimensions of 0.36 x 4.72 m (14 x 186 in.) and a canister
for three BWR assemblies has dimensions of 0.41 x 4.72 m (16 x 186 in.). Seventy-eight can-
isters per GWe-yr are required in this instance for the mixture of fuel used in this generic
study.
The DOE/ET-0028 and the ONWI-39 studies present different estimates of TRU waste pro-
duced during the treatment operations. DOE/ET-0028 concluded that waste produced during the
treatment of the intact fuel assemblies could be considered to be non-TRU (as is waste pro-
duced during the irradiation and the subsequent storage of the assemblies). ONWI-39, how-
ever, lists appreciable quantities of TRU wastes resulting from packaging of the intact
assemblies (but does not say in which operations they arise). The actual amount remains to
be determined from operating experience; if a significant amount of TRU waste is indeed gen-
erated during the packaging of intact spent fuel, then the spent fuel capacity of the
repositories described in Chapter 5 may be somewhat overstated.
Consideration is also given in ONWI-39 (Section 10.3) to other canister design varia-
tions. Alternative canister void filler materials considered include gases other than
helium (e.g., air, nitrogen, or argon), monolithic solid fillers formed by pouring molten
materials (e.g., lead, aluminum, or glass) into the canister and then cooling, and granular
solid fillers (e.g., lead shot, sand, or glass frit). The use of thicker walls in the pri-
mary canisters, overpacks, and increasing the number of spent fuel assemblies per canister
were also considered.
(a) Square canisters allow a more close-packed array during interim storage but are not as
strong as cylindrical canisters with the same wall thickness.