Page 392 - Fundamentals of Magnetic Thermonuclear Reactor Design

370 Fundamentals of Magnetic Thermonuclear Reactor Design

The main thermomechanical loads on the chamber are due to the plasma
neutron radiation and plasma heating as part of wall conditioning. The eddy cur-
rent–induced heating is usually insignificant (within 20°C for the ITER vacuum
vessel).
Coolant pressure in the cooling channels is at its highest during the vessel
baking and hydraulic tests. For example, the hydraulic pressure between the
walls of the ITER VV is 2.4 MPa during the baking. Seismic activities are a
major concern in the design of the vessel supports.

12.6.2 Strength and Life-Time

A fusion reactor VV has a very important role in that it acts as a first safety
containment barrier. This places rigorous demands on its reliability and requires
that an adequate computational support be provided for its design.
The VV complex 3D-structure and the variety of mechanical load com-
binations require a wealth of mathematical and experimental modelling to
enhance the VV strength and durability. In the absence of a statistically
proven regulatory base for MFR strength calculations, the latter are gov-
erned by nuclear reactor design standards. For instance, the European RCC-
MR code (Appendix A.12.3) is a reference to guide the ITER VV structural
assessment. However, fusion and fission reactors differ in many respects,
including the mechanical system. The fusion reactor VV is mostly exposed
to mechanical loads of electromagnetic origin, while in the fission reactor,
the main stress is due to the weight and the coolant pressure. A number
of uncertainties are inherent in the calculation of tokamak electromagnetic
forces, which in itself is a difficult engineering task (see Chapter 4). One has
to introduce higher safety factors to make for the ‘lack of knowledge’ about
allowable stresses.
Plasma displacements and plasma current disruptions giving rise to inten-
sive EMLs may have considerable dynamical effects on the structural compo-
nents as discussed earlier. Because many vessel elements are asymmetric rela-
tive to the tokamak axis, analytical estimates and 3D numerical modelling need
to be used in the stress analysis. Excessive pressure in the chamber may only be
the result of accidents, such as in-vessel leakage.
These issues make the adoption of a regulatory framework for the MFR
structural assessments one of the key engineering challenges. Static strength
calculation standards for the fission reactor require that apparent stresses be dif-
ferentiated as membrane and bending stresses. This approach is hardly suitable
for the vacuum chamber due to the 3D stress field. Instead, the load-bearing
capacity is widely used as a measure of the VV static strength. With the elas-
tic–plastic approximation in mind, we increase the theoretical load to a maxi-
mum, at which point residual deformations appear in the material. A structure’s
strength is deemed to be ensured if the limit to theoretical load is less than
prescribed standard.