Page 396 - Fundamentals of Magnetic Thermonuclear Reactor Design

374 Fundamentals of Magnetic Thermonuclear Reactor Design

l Plasticisation of MS structural materials—to the extent that the structure’s
desired load-bearing capacity is proved by calculations that account for the
materials’ low-temperature degradation.
l Controlled fracturing—to the extent that a quantitative crack growth analy-
sis points to a more than a 10× margin on the critical number of loading
cycles.
l Local delamination of the winding insulation—to the extent that a numerical
analysis does not reveal any propagation of the loss of solidity. Insulation
wear near the delamination area due to microdislocations of adjacent insula-
tion layers relative to one another is virtually improbable due to the small
number of working cycles.
l Local plastic flow in the MS constructional elements, as it allows acceptable
strain level to be raised.
In addition, the welded joints strength criteria are less demanding for W7-X.
As we know, welding quality is measured by the joint strength factor, that is,
the ratio between the welded joint and the base metal strength metrics. The joint
strength factor is set as one of the design standards. The W7-X project specifies
the following:
l Each welded joint was treated as a load-bearing constructional element with
a strength factor of 0.85. An elastic–plastic approximation was used to make
necessary estimates.
l Welded structures have a two-level excessive pressure management system
that prevents pressure increases above 1.2 atm. In the regulatory context, this
allows the structure to be treated as a VV with ‘relaxed’ requirements for the
strength of welded joints.
l ITER criteria and ratios were used to assess the joints’ resistivity to cyclic
stresses.
l In the mathematical models of welded joints longer than 5 mm, the weld
curvature radius was taken to be 2 mm. For shorter joints, the curvature
radius was directly proportional to the joint length.
The ‘relaxed’ standards and the adequacy of these exceptions are justified in
the course of numerous tests and comparisons between theoretical and experi-
mental data.
In any case, the welder’s highest qualification is among the most important
‘design standards’ for any MFR.

APPENDIX A.12.1 MAGNETO-ELASTIC STABILITY OF ITER
POLOIDAL FIELD COIL SYSTEM

Let us consider the concept of magnetic stiffness in more detail. Let us focus
on the interaction of electric currents I and I in ring-shaped coils number i
i
j
and number j (Fig. A.12.1.1). Because of the small linear dimensions of the
coils’ cross-section compared with their radius and distance between them, the