Pressure Relief

DOE-HDBK-6004-99

Metal-to-metal valve seats are preferred. An exception is the use of ultra-high molecular weight

polyethylene (UHMWPE) as a step tip in automatic valves that are not subject to high temperature

environments. Valves will remain leak tight longer with an UHMWPE step tip than with a metal tip

(e.g., stellite). During normal operation, seat leakage does not present a personnel or confinement

hazard, but it does have a major impact on process operations, accountability, operability and

volumetric segmentation. Consideration should be given to the use of double valve isolation to

mitigate the potential for valve seat leakage consequences during maintenance breaching of primary

confinement and to increase the confidence level for product purity.

Pressure Relief

Where the potential exists for the over, or under, pressurization of primary or secondary confinement

barriers, pressure relief should be provided. Pressure protection may serve as process equipment

protection without providing a safety function. For the primary confinement system, stringent leak

tightness specifications necessitate that pressure protection be through the use of rupture disks instead

of pressure relief valves, seal pots, etc. For the secondary confinement system, pressure differentials

and leak tightness requirements are not as stringent, therefore pressure protection may use seal pots,

bubblers, surge volumes, etc.

Relief valves should not be used for tritium service. Their performance is inadequate for leak tight

resealing after relief and reliable relief at low differential pressures.

Heating and Ventilation - Personnel Zones

Heating and ventilation systems should promote tritium confinement for zones occupied continuously

or intermittently by operations or maintenance personnel.

The design should provide heating and ventilation systems with pressure differentials to cause air flow

from least contaminated areas to most contaminated in tritium process areas. Ventilation pressure

in personnel zones should be greater than pressure in secondary confinement.

Primary System Cleaning

The design should include provisions to accommodate the cleaning of all tritium systems for initial

installation, particularly vacuum cleaning of the primary confinement system. After tritium has

contaminated the primary confinement' interior, limited cleaning is permissible and this limited

cleaning must avoid use of waters or organics that could oxidize tritium.

Past Design Practice

Tritium Confinement

Some tritium facilities have used primary confinement, secondary confinement and tertiary

confinement. The minimum essential design, however, uses a primary confinement with a secondary

confinement as a cost effective and safe design for tritium operations. The following sketch illustrates

this minimum essential design.