Page 45 - Fundamentals of Magnetic Thermonuclear Reactor Design

Facilities With Magnetic Plasma Confinement Chapter | 2 27

ket, is acceptable for future reactor generations. For example, the projected
tritium breeding ratio is ∼1.05 for the Russian OTR design (with a neutron
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fluence of ∼5 MW·year/m ) and in the range of 1.06–1.15 for the European
DEMO reactor.

2.5.5 Radiation Shielding
The MFR technological and biological radiation protection is performed by a
combination of the blanket and special structural components.
In addition to fusion energy utilisation and tritium breeding, the blanket
provides a considerable attenuation of the plasma radiation flux. Physical pro-
tection components that absorb neutrons and gamma radiation from nuclear
reactions in structural materials are located behind the blanket. Behind the ra-
diation shielding, the parameters of the irradiation effect (particularly onto the
magnet materials) must not exceed the following specified limits:
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l total absorbed dose for insulators: not higher than 5 × 10 Gy,
l nuclear reaction thermal power absorbed by a superconductor: within 10 kW,
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l fast neutron fluence on superconducting coil: within 10 n/cm .
The shielding material should contain light elements acting as neutron mod-
erators and elements with large atomic numbers absorbing the gamma radiation.
The well-reputed heterogeneous iron-and-water medium is generally used for
this purpose. Where a thin shielding is necessary, an advanced material based
on, for example, zirconium hydride can be used.
In the DEMO and FPP projects, the blanket + shielding thickness is close
to 1 m. It is the key component in the gap between the plasma and the TF coil.
Approximately 2-m-thick concrete bioshield is used to protect personnel.

2.6 STELLARATORS
2.6.1 Functional Layout and Key Characteristics

The stellarator is a ‘no-current’ version of a closed magnetic mirror. The name
refers to the Latin word ‘stella’, pointing to the similarity of physical processes
in the stellarator and inside stars. Unlike tokamaks, stellarators do not need to
excite and maintain the toroidal plasma current to achieve magnetic field rota-
tional transform. The use of external coils only for a magnetic field generation
predetermines the stellarator's stationary plasma operation. However, this impor-
tant advantage turns out to be a critical issue because sophisticated magnetic field
configuration needs to be provided with a high accuracy. The stellarator’s magnets
produce toroidal and poloidal fields. In some stellarators, the magnetic axis has
the form of a 3D curve. Stellarators are not azimuthally symmetric (Fig. 2.2A–B)
due to a variable plasma cross-section. The extremely tight requirements on the
topology of their magnetic field make the use of complex precision magnetic field
coils inevitable.