operated in the continuous flow mode, these types of cleanup systems can
easily reduce the tritium concentration in the gas stream to the part-per-billion
level (i.e., 2.6 mCi/m3), or lower, as long as they are operated for a sufficiently
long period of time.
Although the operation of a typical catalyst/molecular sieve tritium removal
system, like that described above, can be very effective, the operation of such
systems can also have drawbacks. In the standard mode of operation, for
example, little or no gas flows through either the glovebox or the cleanup
system for relatively long periods of time because both sides of this type of
containment scheme tend to be activated on an " n demand"basis only. A
substantial drop in the catalyst bed temperature should be expected when the
system is turned on (i.e., the flow on the inside of the catalyst bed goes from a
static situation to several hundred cubic feet per minute in a matter of a few
seconds). Under such circumstances, the temperature in the bed can easily
drop below that which is required to break down tritiated methanes, and the
cleanup system will have to be run long enough to allow the system
temperatures to re-equilibrate. When cleanup systems have to run for
relatively long periods of time, leakage rates into and out of the glovebox in
question, and leakage rates into and out of the cleanup system tend to become
additional factors that must be considered in the overall design of the facility.
When the parameters are appropriately balanced, calculations indicate that the
flow rates for these types of cleanup systems (i.e., high-quality primary
containment system environments in combination with intermediate-quality
secondary system environments) should be high enough to remove the bulk of
the tritium within a few hours to a few tens of hours. Calculations further
indicate that the overall release rate to the environment will be 1 Ci for every 3
to 30 hours of operation.
When similar calculations are performed on the use of a high-quality, primary
containment system in combination with a low-quality secondary system, the
results indicate that the flow rates should be high enough to remove the bulk of
the tritium within a few minutes to a few hours.