Piping

DOE-HDBK-6004-99

Materials capable of forming methane,

Elastomers, plastics or asbestos. An exception is the use of ultra-high molecular weight

polyethylene (UHMWPE) as a step tip in automatic valves. Valves will remain leak-tight longer

with an UHMWPE step tip than with a metal tip (e.g., stellite).

Carbon steel is significantly more susceptible to hydrogen embrittlement than Type 300-series

stainless steel and is not a recommended material for primary confinement. Additionally, at elevated

temperatures, carbon steel is vulnerable to hydrogen combining chemically with carbon,

decarbonizing the metal, forming methane and causing cracking and blistering.

Piping

The recommended piping construction method is welding in accordance with the applicable

ANSI/ASME standard (ASME 89d). Design should minimize the use of mechanical joints but, where

use is necessary, the recommended mechanical joint is a high vacuum connector. Mechanical joints

or welded joints are acceptable for piping enclosed within a secondary confinement glove box or

cabinet. Outside a glove box or cabinet, only welded joints are acceptable.

Metal-to-metal seals are preferable to elastomer seals. Where conditions necessitate the use of

elastomer sealing, design should provide a dual O-ring configuration with ease of replacement.

Pumps

Process requirements will dictate the selection of pumps. In addition to providing for maximum

design pressure, pumps should also have the capability of withstanding vacuum for cleaning purposes.

Therefore, the selection of pumps for normal operations should include compliance with the vacuum

specification. Pumps should not use organics, hydrocarbons or other volatiles on surfaces that can

contact the tritium process gases. Metal-to-metal pumping surfaces are satisfactory, and other

technologies may be also.

If the safety analysis indicates a pump is safety-class, it should meet the requirements of the applicable

ANSI/ASME standard (ASME 89c, ASME 93a). Pump motors should meet National Electrical

Code requirements for explosion-proof installation (NFPA 70).

Valves

Valves are components of the primary confinement, and as such, should be designed and tested to

the same standards of confinement/vacuum/leak tightness. When specifying leak rate for valves it is

necessary to understand that there are two modes of leakage; 1) across the seat, where the tritium

is still contained within primary confinement, and 2) bonnet/body leakage, where the tritium exits the

primary confinement.

Bonnet/body leakage creates a confinement problem and poses a personnel hazard potential. In

addition, recovery from the primary confinement breach requires processes to be shut down to repair

the leak. Welded, double metal bellows valve bonnets have proven to meet current leak tightness

criteria.