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in the facility authorization basis) in system piping that penetrates the vacuum vessel. For vac-
uum vessel penetrations, each line that is part of the vacuum vessel pressure boundary and that
penetrates the vacuum vessel should be provided with isolation valves, unless it can be demon-
strated 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 vacuum vessel should be located as
close to the vessel as practical and upon loss of actuating power, automatic 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).
b. Structural Design
See Section for general guidance. The vacuum vessel and its appendages should
be designed, fabricated, inspected, and tested in accordance with a recognized safety-related
code such as ASME 1992. Vacuum vessel deflections should be calculated and analyzed to
determine potential interferences and to verify seal integrity.
Loads on the vacuum vessel should be carefully determined using input from the safety
analysis process. Pressure loadings are typically inward for normal service and outward for off-
normal events. Off-normal events could include large disruptions or VDEs, release of plasma
energy (e.g., runaway electrons) or coolant energy inside the vacuum vessel. Loadings from the
response of adjacent systems to off-normal events should be evaluated. For example, a plasma
disruption or VDE may result in some loading of the vacuum vessel from attached penetrations
and supports. Load combinations should be developed conservatively based on event
sequences postulated in the safety analysis.
The vacuum vessel is subject to cyclic loading during normal operations. Thermal cycling
and unavoidable disruption loads are to be expected. The necessity of a fatigue analysis should
be evaluated based on the criteria of ASME 1992 or comparable safety-related code using con-
servative values for variables such as number of pulses, percentage of pulses that have disrup-
tions, and service life including expected changes in material properties with time.
Windows--The use of windows should be minimized to a level consistent with the need to
evaluate plasma properties with optical diagnostics. Where windows are used the area should
be minimized, and the windows should be qualified by analysis or testing in the anticipated
operating and design-basis event environment to demonstrate required confinement integrity.
Bellows--The use of bellows should be minimized to a level consistent with needs to
accommodate differential movement and alignment between fusion island components. Where
bellows are used the area experiencing differential pressure should be minimized, and the bel-
lows should be qualified by analysis or testing in the anticipated operating and design-basis off-
normal event environment to demonstrate required confinement integrity. As a minimum, bel-
lows should conform to relevant criteria in ASME 1992 or a comparable safety-related code. The
use of double-walled bellows should be considered as a design approach to minimize compo-
nent leakage.

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