Structural Design - std60030117

DOE-STD-6003-96

b. Structural Design

See Section 6.1.3.4 for general guidance. The cryostat and its appendages should be

designed, fabricated, inspected, and tested in accordance with a recognized safety-related code

such as ASME 1992. Cryostat deflections should be calculated and analyzed to determine

potential interferences and to verify seal integrity.

Loads on the cryostat 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 release of magnet energy inside the cryostat as well as

energy from release of cryogenic fluid (liquid helium or nitrogen) to the interior of the cryostat.

Loadings from the response of adjacent systems to off-normal events should be evaluated. For

example, a plasma disruption or vertical displacement event may result in some loading of the

cryostat and attached supports. Load combinations should be developed conservatively based

on event sequences postulated in the safety analysis.

The cryostat is subject to cyclic loading during normal operations. Thermal cycling and

disruption loads (for tokamak devices) 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 conservative values for variables such as number of pulses, percentage of pulses that

have disruptions, and service life including expected changes in material properties with time.

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 bellows should

be qualified by analysis or testing in the anticipated operating and design-basis event environ-

ment to demonstrate required confinement integrity. As a minimum, bellows 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 component leakage.

c. Testing

See Section 6.1.3.6 for general guidance. The cryostat vessel and attached components

that provide a confinement barrier should be leak checked at design pressures before initial

operation to demonstrate that leakage requirements specified in the safety analysis are met by

the as-built design. Potential hazards of in-service leak testing at the design vessel pressure

after D-T operations have commenced may not justify such periodic leak testing. In its place, a

program of periodic vacuum leak testing and a formal configuration control program to ensure

cryostat repairs or modifications do not compromise the design pressure rating should be imple-

mented. Replacement structural components should be pressure tested before assembly in the

cryostat. Local repairs should be subject to rigorous NDE.

d. Instrument and Control Systems

I&C, where appropriate, should be provided to monitor system parameters important to

the safety function of the cryostat over their anticipated ranges for normal operation and