**2. Analysis of special operating conditions and limit states**

Experimental thermonuclear reactors of the tokamak type are fundamentally different from the nuclear power plants (NPPs) using tested thermal and fast neutron [4] technology that is currently in operation. These specific features of the experimental reactors include:

• a wide range of low operating temperatures, *t, f*rom room temperature of *t* = + 200 С to cryogenic temperature of −2690 С (liquid helium temperature) in superconducting systems of electromagnetic coils for the creation of magnetic fields;


The design features and loading conditions, listed by the parameters: *τ, Q*M, *Q*t, *Q*em(τ) *t*, *E*, σy, σ<sup>u</sup> σ<sup>u</sup> , α are combined with standard and emergency situations (for example, loss of superconductivity and contact of plasma with the chamber wall).

When justifying the strength of load-bearing structures, the two most important tasks are:


The limit states in the load-bearing structures of the tokamak will be related to:


The above are the basic parameters for determining the local stresses, σ, the strains *e*, strength and service life of a thermonuclear installation [6].

$$\{\sigma, e\} = F\{Q(\tau), E, \mu, \tau, t, S\} \tag{1}$$

$$
\sigma\_{\text{max}} \le \left\{ \frac{\sigma\_y}{n\_y}, \frac{\sigma\_u}{n\_u} \right\} \tag{2}
$$

*Theoretical and Experimental Analysis of Structural Properties of Load-Bearing Components… DOI: http://dx.doi.org/10.5772/intechopen.94531*

$$
\tau \le \frac{\tau\_{\epsilon}}{n\_{\epsilon}}, N \le \frac{N\_{\epsilon}}{n\_{N}} \tag{3}
$$

where *F*σ is functional, *S* is the characteristic of the dangerous section, σmax is the maximum stress at the critical point of the dangerous section, *n*y*, n*u are safety factors for yield stress and ultimate strength, *τ*с*, N*с is the critical time until fracture (service life) and *n*τ*, n*N safety factors for service life.

For thermonuclear installations *n*y < *n*u < *n*<sup>τ</sup> ≈ *n*N.

The creation of energy-efficient and safe thermonuclear tokamak installations is largely dependent on the successful solution of the problems of deformation and fracture mechanics.

A characteristic feature of large tokamaks as mechanical systems is the presence of significant ponderomotive loads Q em (τ), which act in combination with the special operating conditions in the main systems of the installation - the electromagnetic system (EMS) for plasma confinement and the discharge chamber.

The electromagnetic systems used in tokamaks give rise to high and ultrahigh forces *Q*(τ), which results in high mechanical stress on the structural elements. Moreover, in superconducting EMS, structural, current-carrying and insulating materials operate at cryogenic temperatures (down to 4.2 K), which affect the physical and mechanical properties of these materials. The discharge chambers of tokamaks are exposed to complex mechanical, thermal, and radiation loads.

One specific feature of tokamaks is that they operate in an alternating programmed stable and unstable mode, and so their power elements are subject to cyclic loads. The calculation and design of such elements needs to be carried out based on their actual operational characteristics, using an apparatus with static, dynamic and cyclic strength and taking into account plastic deformation of the materials.

The program of research into controlled thermonuclear fusion technology provides for an increase in the size and intensity of magnetic fields and more complex operating conditions of tokamak coils. Moreover, the standards in relation to the durability, rigidity, and reliability of load-bearing elements are also increasing, which will ensure that the required physical parameters are complied with in respect of the facilities. There is therefore a pressing need to consolidate the expertise in solving problems related to mechanics, durability, service life and safety issues which has been accumulated during the development of the tokamaks used in the largest thermonuclear installations. In Russia, these were the T-15 tokamak and the Strong Field Tokamak (SFT) installations.
