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Temperature Considerations
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Design Considerations - index
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Figure 3. Relative radiation resistance of elastomers.


DOE-HDBK-1132-99
perturbation in their own internal systems, and Le Chatelier' Principle indicates
s
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.
I-104


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