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

4.43
and Scheele 1978). The voloxidation process involves oxidation of UO 2 to U 3 0 8 at 4000 to
500°C in air. Essentially all of the tritium (plus portions of the other volatile radionu-
clides) is released to the gas stream by this process. The released tritium is removed from
the gas stream (as water) by a bed of adsorbent material.
Although the example process in this Statement includes the recovery of three gaseous
radionuclides, the study described in DOE/ET-0028 (Section 4.9) considered other possi-
bilities as well. These included 1) no gaseous radionuclide recovery, 2) recovery of 1291,
3) recovery of 129I plus 14 C, and 4) recovery of 1291 plus 8 5 Kr.
In the example process, iodine recovery is effected by adsorption on silver zeolite,
carbon recovery is accomplished by adsorption (as carbon dioxide) on zeolite molecular
sieves, and krypton is recovered by cryogenic (very low temperature) distillation. Silver
zeolite is a prepared by replacing sodium ions in a zeolite with silver ions. Zeolite mole-
cular sieves are crystalline aluminosilicates having pores of uniform size that completely
exclude molecules which are larger than the pore diameter, thus permitting selective adsorp-
tion of those molecules that are smaller than the pore diameter.
The example off-gas treatment system also includes filtration for removal of particu-
late material, absorption and catalytic destruction steps for the removal of the oxides of
nitrogen, NO and NO 2 , and ruthenium removal. A small portion of the ruthenium may be con-
verted to a volatile form during processing operations. The example system uses beds of
silica gel to remove this ruthenium before it reaches the processes used to recover the gas-
eous radionuclides.
The ruthenium-loaded silica gel and the iodine-loaded silver zeolite are ultimately
disposed of in those forms; the estimated generation rates are 0.046 55-gallon drums/GWe-yr
of the ruthenium waste (which requires remote handling) and 0.68 55-gallon drums/GWe-yr of
the iodine waste. The carbon dioxide is desorbed from the molecular sieve and converted to
solid calcium carbonate for disposal; 0.19 55-gallon drums/GWe-yr is the estimated quantity.
The krypton-rich.product (80% krypton and 20% xenon) from cryogenic distillation is col-
lected in pressurized gas cylinders for storage; 2.8 cylinders/GWe-yr is the estimated
quantity. These gas cylinders will require remote handling.
Alternatives exist for all of the processes employed in the example gaseous radionu-
clide recovery system. We do not mean to imply that the processes considered here are
necessarily the best, only that they are representative of currently available technology.
Krypton and carbon could be recovered by fluorocarbon absorption and iodine could be
recovered by different solid sorbents or by scrubbing with various aqueous solutions. These
alternatives have been discussed elsewhere (ERDA 1976).
4.3.5 Radionuclide Releases During Waste Treatment and Packaging
Estimates have been developed of radionuclide release during waste treatment and pack-
aging operations in both the once-through and the reprocessing cycles. These estimates are
summarized in Appendix 10A of DOE/ET-0028 for the packaging of intact spent fuel in a spent
fuel packaging facility (SFPF) in the once-through cycle and for a variety of waste