Polymers

DOE-HDBK-1129-99

susceptible to hydrogen embrittlement than the austenitic stainless steels. Additionally, free-

machining grades of austenitic stainless steel (such as Type 303) should not be used.

Other materials that must not be used for tritium service are any material that forms hydride near

room temperature and atmospheric pressure. Examples include zirconium, tantalum, niobium, and

many alloys of these materials.

5.1.3 Polymers

All polymers degrade when exposed to radiation. Both tritium and tritiated water permeate all

polymers, and permeated tritium deposits the beta decay energy throughout the polymer bulk.

(Although the tritium beta energy is very low and has a small penetration depth in matter,

permeation allows tritium atoms to be near enough to polymer chains throughout the bulk to cause

changes in the polymer by radiation.) Types of radiation-induced changes in polymer properties

include either softening (degradation) or hardening, ductility loss, color change, dimensional

change, and gas evolution. Because of these effects, polymers should only be used in tritium

systems where no metal alternatives exist. Normally, only polymers that harden during radiation

exposure are employed, and are replaced before they begin to deteriorate. In addition to polymer

breakdown itself, products of degradation can form corrosive liquids or acids such as HF and HCl.

Polymer parts must be easily replaceable as a part of normal operations, and a program of regular

inspection and replacement should be established. The system should be designed to expose any

polymers to as little tritium as possible. Typical uses of polymers in gas-handling systems include

gaskets, O-rings, electrical cable insulation and valve parts, including seats, stem tips, and

packing.

Polymers relatively resistant to radiation can typically withstand up to about 1 million rad (1 rad =

100 erg energy deposited per gram of material). By knowing the solubility of tritium in a polymer at

a given temperature and tritium partial pressure and the decay rate of tritium, the approximate

dose can be calculated, assuming the tritium concentration has reached equilibrium.

Many effects of radiation on polymers are accentuated by oxygen. Protecting polymers from

oxygen or air will likely lengthen the lifetime of polymers exposed to tritium. Also, temperatures

above about 120C accelerate radiation effects in polymers, so the temperature of any polymer

parts should be kept as low as possible. Inert additives such as glass or graphite generally

enhance the resistance of polymers to radiation. Addition of antioxidants may also enhance

radiation resistance.

5.1.3.a Plastics

VespelTM, a polyamide, has been successfully used for valve stem tips in some tritium laboratories.

Ultra-High-Molecular-Weight polyethylene (UHMWPE) and High Density Polyethylene (HDPE)

have been used for valve stem tips in automatic valves. Quantitative data still needs to be

accumulated on the effectiveness of all of these materials. Success as a stem tip material,

particularly UHMWPE and HDPE, have been overstated in the past as described in EH Technical

Notice 94-01, "Guidelines for Valves in Tritium Service."

Low-Density polyethylene (LDPE) is very permeable by tritium and tritiated water and should not

be considered for use in tritium systems. Polytetrafluoroethylene (PTFE, a trade name is TeflonTM)

degrades and decomposes in tritium, thus resulting in HF and HCl. In humid air, hydrochloric and

hydrofluoric acid are then formed, which are highly corrosive. Generally, chlorofluorocarbon