Page 437 - Fundamentals of Magnetic Thermonuclear Reactor Design

Safety of Fusion Reactors Chapter | 14 415

The effect of molecular tritium on human health is 4 orders of magnitude
less detrimental than the action of tritiated water [9]. However, molecular tri-
tium is quickly oxidised after its release into the atmosphere. Therefore, for
reactor design purposes, tritium oxide release is assumed to be only 10× more
detrimental to human health than molecular tritium.
Table 14.2 shows permissible releases of molecular tritium (HT, DT) and
tritium oxides (HTO, DTO), as well as activated materials and activated corro-
sion products from the cooling circuits, which have been calculated for typical
weather conditions.
Dose limits for the population depend on the reactor’s condition and are
as follows: 0.1 mSv/year for normal operation, 0.1 mSv for an incident, and
50 mSv for an accident. One of the design objectives is to avoid population evac-
uation even when a hypothetical low-probability severe accident is considered.
Design dose limits are much lower than the ICRP recommendations for nor-
mally operated nuclear power plants. Anticipated frequency of incidents is used
to differentiate the reliability assurance requirements (the so-called graded ap-
proach). For hypothetical events (accidents beyond the design basis) the design
−6
frequency is less than 10 /year.
ITER safety functions may be identified as follows. The principal safety
function is the confinement of radioactive and other harmful and hazardous sub-
stances within design limits and limitation of exposure to ionising radiation.
Auxiliary safety functions include

l monitoring and control of pressure inside the vessel and in other cavities
inside the vacuum boundary;
l control of energy released from chemical reactions;
l decay heat removal;
l in-service control of safety-related systems and their protection against ex-
ternal events, including fire and earthquake.
To sum it up, the ITER operation safety philosophy is based on a consistent
implementation of the optimisation design and the defence in depth protection
principles through the use of design, process-related and organisational strategies.
This includes the prevention of incidents, use of safety-related systems for the mit-
igation of accident consequences, and the allowance for low-probability and hy-
pothetical events by specifying ‘safety margins’ and taking additional precautions.

14.4.2 ITER Normal Operation
The optimisation principle is implemented through the use of the following
measures:
l control of detrimental and hazardous factors;
l the remote handling maintenance;
l dose control and personnel overdose prevention by restricting access to zones
with elevated radiation levels and regulating the time spent in those zones;