First Wall Design

DOE-HDBK-1129-99

polymers should not be used in tritium service. Polyvinyl chloride (PVC) and vinylidene chloride

(Saran) are among several polymers used in tritium protective clothing, but should not be used for

process equipment because they contain chlorine.

5.1.3.b Elastomers

Tritium readily permeates into and diffuses through elastomeric materials and, depending upon

thickness, begins appearing on the outside of the elastomeric seal within hours after exposure to

tritium. Elastomers are subject to radiation damage due to exposure to tritium and harden and

lose their sealing ability due to exposure to high concentrations of tritium.

Ethylene propylene diene monomer (EPDM) elastomers are employed for low-pressure process

flange gaskets because of EPDM is relatively good performance in tritium service. In some cases,

Buna-N process flange gaskets are being replaced by EPDM when the gaskets are changed;

however, in other applications Buna-N remains in use. VitonTM , a common O-ring material, is

used, but can embrittle in months in tritium service. Butyl rubber has low permeation for both

tritium and tritiated water, but is not as resistant to radiation damage as EPDM. Butyl rubber is

used for glovebox gloves. Water vapor in the air outside gloveboxes permeates gloves and can

lead to a significant portion of the residual tritiated water vapor in tritium gloveboxes.

Kel-FTM is a common chlorofluorocarbon polymer and is incompatible with tritium. It, like TeflonTM,

degrades in tritium gas and should not be used.

5.2 First Wall Design

5.2.1 High-Pressure Tritium

For high-pressure tritium, it is generally recommended that the first wall be of all metal construction

and hydrogen compatible materials including valves, valve seats, and tubing. The use of non-

hydrogen-compatible materials will result in material failure and the release of the contained

tritium. Elastomers are not tritium compatible, and, as a result, elastomeric seals and valve seats

are not recommended for use in the containment of high-purity tritium. There are exceptions to

this general case, and other criteria may be used when justified by analysis.

5.2.2 Low- and Medium-Pressure Tritium

For the containment of low- and medium-pressure, high-purity tritium, it is generally recommended

that the first containment wall be of all metal construction of hydrogen-compatible materials where

possible, including valves, valve seats, and tubing. Hydrogen and helium embrittlement of the

materials of construction is not usually significant at low and medium pressures. As a result, non-

hydrogen-compatible materials may be used if required by the design or if the required component

is not available in other materials. Again, elastomers are not tritium-compatible, and, as a result,

elastomeric seals and valve seats are not recommended for use in the containment of high purity

tritium. There are also exceptions to this general case, and other criteria may be used when

justified by analysis.

It is difficult to design a vacuum system, which in some cases is the first wall, without including

some non-hydrogen-compatible materials and elastomers. However, embrittlement of the

materials is not an issue because the tritium exposure is transient and the pressure is low. Under

these conditions, the elastomers are not exposed to tritium continuously, and most can be used in