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shield. The chamber is usually cylindrical with a top and bottom. There are usually large pene-
trations in the top, bottom, and sides of the cryostat, primarily for access to the vacuum vessel
and magnets for maintenance and inspection. The cryostat may be double-walled with an evac-
uated or filled annulus. It may be lined with cryogenic panels or superinsulating material.
If the safety analysis indicates that the cryostat is a confinement barrier to meet the eval-
uation guidelines, the required robustness of the barrier will be defined in the safety analysis
and implemented in the design. In performing this public safety function, the cryostat should be
classified as a safety-class system. Hardware internal or adjacent to the cryostat whose credible
failure could result in evaluation guidelines being exceeded should be classified as safety-class.
If the cryostat is not considered a confinement barrier in the safety analysis, those cryostat
components whose single failure results in loss of capability of another safety-class system to
perform its safety function should be designated as safety-class components.
If the safety analysis requires that the cryostat be a confinement barrier, the following
safety functions should be considered:
The cryostat may serve as a barrier (normally a secondary barrier but this is design
specific) for tritium and tritiated compounds, radioactive impurities and activated dust during nor-
mal operation, anticipated operational occurrences, maintenance external to the cryostat, and
off-normal events including design-basis events. The cryostat may serve as part of the structure
of the biological shielding; if this is the case see Section for shielding guidance.
During maintenance inside the vacuum vessel, the cryostat may serve as a partial con-
finement barrier as defined in the safety analysis. That is, the vacuum vessel may be breached
for specific repairs, but the confinement function should be provided by the cryostat, to ensure
evaluation guidelines are not exceeded from residual in-vessel radioactive and toxic materials;
this confinement barrier may include temporary features to allow maintenance access.
In addition to the general design guidance in Section 6.1, the following system-specific
design guidance is provided:
a. Confinement Boundary
The cryostat system confinement boundary should be defined as the cryostat proper,
including all penetrations up to and including the first or second isolation valve as appropriate
(depending on system pressure and as defined in the facility authorization basis) in system pip-
ing that penetrates the cryostat. For cryostat penetrations, each line that is part of the cryostat
system boundary and that penetrates the cryostat should be provided with isolation valves,
unless it can be demonstrated that the confinement isolation provisions for a specific class of
lines, such as instrument lines, are acceptable on some other defined basis. A simple check
valve should not be used as the automatic isolation valve. Isolation valves outside the cryostat
should be located as close to the cryostat as practical and upon loss of actuating power, auto-
matic isolation valves should be designed to take the position on failure that provides greater
safety. The power to operate isolation valves whose function is required to meet the evaluation
guidelines should meet the requirements of Class 1E Electric Power Systems (IEEE 1980).

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