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Figure 8. High-quality secondary containment
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Design Considerations - index
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Figure 9. Intermediate-quality secondary containment


DOE-HDBK-1132-99
In each of the examples cited above, substantial amounts of tritium can be at
risk at any given time. Under such circumstances, the rule-of-thumb for the
design of high-quality, secondary containment systems is to assume that there
is no primary containment system, and that the secondary containment system
is the only barrier between the tritium and the outside environment. By default,
therefore, the secondary containment system design requirements become
identical to those used for primary containment systems; accordingly, the
materials and construction issues associated with these types of secondary
systems should also follow the guidance set forth in Section III of the ASME
Boiler and Pressure Vessel Code.
From a materials and construction perspective, there should be virtually no
difference between the requirements for the primary containment and
secondary containment designs. In most cases, the materials of choice will be
a 300-series stainless steel, preferably a low-carbon variety (e.g., 304L or
316L). Welding will be the preferred method of construction wherever
possible. Prior to being placed into service, individual portions of the system
will be certified to a maximum leakage rate of 10  -6 to 10-7 cm3/sec (helium), at a
minimum of 125 percent of the maximum expected pressure.
From an operational perspective, however, the design philosophy should be
that the void volume between the primary containment vessel outer wall and
the secondary containment jacket inner wall in such systems is generally
evacuated during service. Thus, if tritium is released into these secondary
containers, there are no dilution gases present, and any leakage from the
secondary container is in the same form as that contained in the primary
container. Following a release into a high-quality system, the tritium can be
recovered in almost the original purity without dilution by other gases by
pumping it into another primary container. Several days can elapse during the
recovery process without a significant release of tritium to the environment.
Intermediate-quality Secondary Containment Systems . It is not practical nor
possible to enclose all primary tritium systems inside high-quality, nondiluting,
evacuated, secondary containers. Therefore, intermediate-quality, secondary
I-115


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