Page 427 - Fundamentals of Magnetic Thermonuclear Reactor Design

Safety of Fusion Reactors Chapter | 14 405

14.3 PHILOSOPHY OF THE MAGNETIC FUSION REACTOR
SAFE OPERATION

14.3.1 Basic Concepts
Economic competitiveness of future MFRs depends on their safety. To ensure
safety, it would be logical to make use of the safe operation philosophy that was
worked out in the course of the nuclear power engineering development and
outlined in IAEA documents and national regulatory guides [3,7].
Definition of fusion reactor safety. A fusion system is considered to be safe
when the impact of ionising radiation and other harmful products of its opera-
tion on the population, personnel and environment in all operational states and
in accidents is below the maximum permissible design limits and does not ne-
cessitate population evacuation.
An accident is an unintended event, in which the impact of harmful sub-
stances (radioactive or toxic) and/or ionising radiation is above the levels pre-
scribed by the design for normal operation. Any accident has an initiating event
(IE), an event sequence and consequences.
Regulatory documents distinguish the following operational states of a fu-
sion facility:
l Normal operation, that is, operation within specified conditions, including
overhaul and scheduled preventive maintenance activities, dismantling and
replacement of large components and systems.
l Incident (fault or off-normal) conditions, that is, unplanned operation of the
facility arising from an overrun of prescribed limits for normal operation,
which has not yet led to an accident.
l Accident within design basis (AWDB), that is, accident, for which the IE
and the finite conditions for all system are prescribed by the design, and in
respect of which safety systems (SS) and organisational mitigation measures
are devised based on the assumption that a single SS failure or single human
error (independent of the IE) may occur additionally.

Besides that, accidents beyond design basis (ABDB), which are unlikely
and may result from an IE not included in AWDB scenarios or be attended by
more than a single SS failure or human error should be considered. They may
be much more severe than ABDBs. The ABDB are analysed to determine if
population evacuation can be avoided and if consequences can be limited to just
loss of operability.
The problems of ensuring a fusion reactor’s safe operation begin to be ad-
dressed at the pre-design documentation preparation and site evaluation phases
and continue to be under scrutiny up to the end of the facility’s lifetime. The
starting point is the definition of purposes, criteria and principles of ensuring
safety. Then, approaches to achieve safety at every stage of the facility’s lifetime
are determined, and an analysis is carried out to understand if the facility and its
components meet the safety criteria.