Building Ventilation System cont'd

DOE-HDBK-1129-99

To ensure good mixing and dilution by outside air, the exhaust gases should be released to the

environment through an elevated stack at high velocity.

The ventilation system should be designed to use pressure zone control to minimize cross-

contamination. The ventilation control system is designed to hold the spaces occupied by the

tritium operations at a negative pressure relative to the spaces surrounding the facility. If the

whole building is a tritium facility, then the building is at a slightly negative pressure relative to

the environment. If the tritium is in a single room in a building, the room is at a negative

pressure relative to the adjacent rooms. Air continuously leaks into the tritium operating areas

from the surrounding environment. The ventilation zones of tritium processing areas should be

maintained under controlled temperature and humidity conditions at all times to reduce the in-

leakage of moisture to inert atmosphere gloveboxes. (Further reductions in inleakage of

moisture into inert gloveboxes can be achieved by selecting a glove material that has a low

permeation rate for moisture and by reducing the exposure time (e.g., glove port covers) and/or

total glove surface area).

The walls separating adjacent rooms must be reasonably sealed to minimize tritium released

from contaminating an adjacent room. Administrative controls need to be in place to require

caulking and sealing around wall penetrations such as conduit and piping.

For the ventilation system to work as designed, the inside and outside doors and airlocks need

to be used properly. Propping an outside or inside door open will upset the pressure zone

control system. An outside door bypasses the stack and provides a path for a ground level

tritium release. The doors should be equipped with automatic door closures, and all personnel

should be instructed on the proper use of doors.

For facilities handling gram quantities of tritium, a rule of thumb is 6 to 10 air changes per hour

as the standard of performance. Depending upon the operating conditions associated with the

ventilated area, ventilation rates of 3 to 20 air changes per hour have been used.

The ventilation rate should be based on analysis of the hazards of the operations. A facility

that has the potential to release only a few curies of gaseous tritium into the breathing space

does not need to operate at the same ventilation rate as a facility that has the potential to

release tritium into the breathing space.

Single-pass ventilation systems are expensive to operate because all air must be conditioned

to make its single pass through the facility, and this conditioned air is not reused. Additionally,

a high flow rate is desired in order to remove any released tritium from the facility as soon as

possible to protect the workers.

Special precautions such as Personnel Protective Equipment, including respiratory protection

and passing exhaust through particulate filters, are needed when working with SMTs. D&D

work with SMTs should occur within a confined airspace, if possible.

It is feasible to design future ventilation systems to operate at a variable flow rate that is a

function of the time of day and the measured tritium concentration in the rooms. This would

entail a higher initial cost, but would decrease the long-term operating costs without significant

impact on the facility safety.