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

4.61
4.4.4 Krypton Storage
The 85Kr removed from the off-gas stream as described in Section 4.3.4.2 must also be
stored. This gaseous radionuclide can be encapsulated and stored in pressurized gas cylin-
ders. Alternative krypton encapsulation techniques being investigated include 1) zeolite
encapsulation, where krypton is diffused into "crystalline cages" at high temperatures and
pressures, and where escape of the krypton is slow at low temperatures; 2) dissolution in a
glass matrix, where krypton is trapped within a glass when it solidifies; and 3) entrapment of
krypton in metal solids during high-rate sputtering.
The krypton storage facility chosen for this Statement stores gas cylinders containing
about 80% krypton and 20% xenon. The radionuclide heat generation rate from such cylinders
is appreciable and refrigerated air cooling is provided. The surface dose rates of the cyl-
inders are such that remote handling is required; this is provided by special transfer con-
tainers and cranes.
The storage plan for krypton differs from those for the other wastes in an important
respect. Since the half-life of 85 Kr is relatively short (10.7 yr), it is assumed that
after storage for 50 years or so the 85 Kr can be released. In 50 years the amount of 85Kr
remaining will be only 4% of the initial amount; after 60 years only 2% will remain.
The krypton storage facility (see DOE/ET-0028, Section 5.6) is located adjacent to a
fuel reprocessing plant and is sized to handle the output of the plant during its lifetime.
Separate storage cells, each holding 104 cylinders, are provided. The number of cells is
increased every ten years to provide the necessary storage capacity; 14 cells are required
for each ten years' output. The facility also includes hot cells for use in cylinder
inspection and gas transfer (e.g., from a leaking cylinder to a sound cylinder) operations.
The gas cylinders are passed into the storage cell through ball valves and rest hori-
zontally on shelves within the cell. Each storage cell contains five shelves and is pro-
vided with a self-contained air circulation and heat removal system. These air circulation
systems are monitored to provide detection of leaks. If a minor leak is detected, the cyl-
inder is sent to the hot cell and the contents are transferred to a new cylinder. If a cyl-
inder suddenly ruptures, the cell atmosphere will be pumped to a holding tank where it will
be sampled and then either returned to the fuel reprocessing plant or sent to the storage
facility stack for release.
The normal release of 85 Kr from the storage facility occurs in two ways: 1) the small
leakages from a number of cylinders, and 2) the planned discharge of the krypton at the com-
pletion of the storage period. The former release is estimated to amount each year to no
more than 0.1% of the amount of 85 Kr present during the year. The latter release does not
begin until completion of the planned storage period. For a 50-yr storage period, this
release amounts to 4% of the amount initially placed into storage. The planned storage
period (and, thereby, the planned release) can be changed after storage has begun.