Tritium Apparatus, Isolation Valves, and Purge Ports cont'd

DOE-HDBK-1129-99

The component is isolated with two valves on each side of the component, and a purge port has

been installed between each set of valves. The purge port allows evacuation of the volume

between the two valves to remove the tritium-contaminated gases from the isolation volume prior

to removal of the component.

In operation, the two sets of valves are closed to isolate the component from the rest of the tritium

manifold. A vacuum pump is connected to the two purge ports. The purge port valves are opened

and the gases trapped between the isolation valves are evacuated to remove the tritium-

contaminated gases. In most applications, air is allowed to enter the purged volume, and the

evacuation operation is repeated. Other gases, such as argon and nitrogen, may be used;

however, air is usually more effective at decontaminating the surfaces. Ambient air entering the

purged volume contains several thousand parts per million of normal water along with the nitrogen

and oxygen. Some of the tritium and HTO on the internal surfaces of the purged volume exchange

with the hydrogen and water in the ambient air and are pumped out during the purge cycle. This

purge/backfill cycle is repeated 3 to 6 times to remove as much of the tritium as possible before

disconnecting the component. Performance of three ambient air purge backfill cycles is typical,

and, in practice, more than 6 purge backfill cycles has not proven to be beneficial.

After the component has been removed and replaced, the new component is leak tested thorough

the purge port before the isolation valves are opened. The sequence of operations is as follows:

The four component isolation valves are closed.

The purge ports are uncapped and connected to a vacuum pumping system.

The port volume is evacuated and leak checked.

If no leaks are found, the purge port valves are opened, and the gases in the volume between

the isolation valves are evacuated to a tritium-contaminated waste gas collection and removal

system.

The evacuation valve is closed, and air is allowed to enter the isolation volume through a valve

mounted on the maintenance manifold.

The air inlet valve is closed. The ambient air is allowed to sit in the isolation volume for a few

minutes to allow the exchange of hydrogen and tritium to take place.

The evacuation valve is opened, and the volume is again pumped out to the cleanup system.

This evacuation and/or backfill sequence can be repeated several times.

The component is disconnected at the maintenance flanges and removed. The new

component is installed.

To control outgassing from the removed component, blind flanges with installed purge ports

may be installed over the open end of the removed component.

The new flange connections are leak tested through the purge ports. If the new flange

connections are not leak tight, they are repaired or replaced and retested.

When the component and flanges are leak tight, the purge valves are closed, disconnected

from the leak test system, and capped.

The component isolation valves are opened, and the new component is placed in service.

As is always the case when dealing with process quantities of tritium, adherence to procedures is

particularly important. If a large source (e.g., container) of tritium is connected via valving to the

piping used in evacuating, an inadvertent valve operation could result in a significant loss. One

such incident occurred at Mound, in which a valve was inadvertently opened, resulting in a loss of

approximately 10 grams of tritium to the cleanup system via the evacuation header.