Page 424 - Fundamentals of Magnetic Thermonuclear Reactor Design

402 Fundamentals of Magnetic Thermonuclear Reactor Design

The reduction of such risks is an imperative for any innovation project—includ-
ing the development of fusion reactor technology.

14.2 FUSION REACTOR AS A RADIATION SAFETY OBJECT
An important impetus for developing controlled thermonuclear fusion is the
conviction of the majority of the international engineering community that the
fusion power engineering is less harmful for the environment than power engi-
neering based on fission reactors or on combustion of hydrocarbons.
Unlike fission, fusion poses no risks associated with the large amounts of high-
ly radioactive daughter radionuclides or potential uncontrolled reactor runaway
leading to severe accidents. A tokamak runaway is improbable for natural reasons:
in normal operating conditions, the reactor’s power is controlled by the rate of the
DT fuel injection using the feedback mechanism. Any loss of control will lead to
a current disruption (fractions of a second) or power excursions over times com-
mensurate with the particles/plasma energy confinement time (on the order of a
few seconds). This time is enough to take preventive measures, such as fuel cut
off. In case of failure of a discharge suppression system, an uncontrolled power
excursion may lead to a plasma disruption and the natural reaction termination due
to the growth of plasma density and the parameter β above their critical values.
Another, passive, way of discharge suppression is the release of impurities
from the vacuum chamber walls into the plasma as a result of material evapora-
tion or loss of leaktightness of the cooling channels.
The fusion reaction has a much lower specific energy release than the fis-
sion reaction. The fusion reactor’s large heat exchange area and mass enables
a passive residual heat removal after reactor shutdown owing to heat capacity,
thermal conductivity, natural convection and radiation—even in the event of a
cooling system failure.
Potential risks for the population and operating staff (Table 14.1) are associ-
ated with the following:
l tritium (source of beta-particles);
l thermonuclear neutrons, activating the materials of in-vessel components
and adjacent structures;
l gamma-radiation;
l activated corrosion products in the cooling systems and dust from plasma
erosion of the first wall (FW);
l quasi-stationary magnetic and high-frequency electromagnetic fields;
l toxic and cryogenic substances, and inert gases;
l energy related to the magnetic field, residual heat release, high-pressure
coolants, and so on;
l energy releasing in off-normal situations as a result of chemical reactions
taking place in the vacuum chamber and other systems; and
l flammable and combustible substances, including molten alkali metals and
diesel fuel.