Organics cont'd - hdbk1132990120

DOE-HDBK-1132-99

perturbation in their own internal systems, and Le Chatelier' Principle indicates

that the system will adjust to the perturbation with the establishment of a new

equilibrium. Under such circumstances, exchange reactions can be expected

to dominate over simple solubility reactions, and the available tritium can be

expected to replace the available protium in any-- and all-- available sites.

Once the tritium has been incorporated into the structure of the organic

material, the structure begins to break down from the inside out, primarily as a

result of the tritium decay energy.

The specific activity of tritium gas at atmospheric pressure and 25 EC is 2.372

Ci/cm3. The expected range of the average energy tritium beta particle in unit

density material is only 0.42 m. This means that all energy from the decay of

the dissolved tritium is deposited directly into the surrounding material. At

2.732 Ci/cm3, this becomes equivalent to 2.88 10 4 rads/hr.

The general rule for elastomers used for sealing is that total radiation levels of

107 rads represent the warning point that elastomers may be losing their ability

to maintain a seal. At 108 rads, virtually all elastomers used for sealing lose

their ability to maintain a seal. Typical failures occur as a result of compression

set (i.e., the elastomer becomes brittle and loses its ability to spring back). At

106 rads, on the other hand, the total damage is relatively minor, and most

elastomers maintain their ability to maintain a seal. At 10 7 rads, the ability of

an elastomer to maintain a seal becomes totally dependent on the chemical

compounding of the elastomer in question. It only takes about 2 weeks for an

elastomer to receive 107 rads at a dose rate of 2.88 10 4 rads/hr. Elastomers

cannot be used for sealing where they might be exposed to high

concentrations of tritium.

Similar analogies can be drawn for all organic materials. The preferred rule of

thumb is the use of all organic materials should be discouraged wherever they

might be exposed to tritium. Since this is neither possible nor practical, the

relative radiation resistance for several elastomers, thermoplastic resins,

thermosetting resins, and base oils is shown graphically in Figures 3 through 6,

respectively.

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