U.S. Reactor Containment Technology: a Compilation of Current Practice in Analysis, Design, Construction, Test, and Operation, Volume 1 Page: 1-21
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1.2.3 Reliability of Reactor Systems
The reliability of a reactor system depends on instruments and control
systems to a degree that is largely determined by the skill of the reactor
designer in meeting the sometimes conflicting objectives of high perfor-
mance and safety of the plant. The degree of reliability attainable in
instrument systems and the degree of reliability needed in various reactor
applications have been widely discussed.26,27 It should also be recog-
nized that accident prevention and consequence-limiting systems, includ-
ing the containment system, should have a degree of reliability compar-
able with that of the reactor safety system.
Sidda126 defines reliability as the sum of safety and service-
ability and defines "The propensities to be free from unsafe failures and
safe failures...respectively...as 'safety' and 'serviceability'... ."
For the purposes of this report, reactor control systems are considered
to be those systems having serviceability as the main objective, and safety
(or protection, including containment) systems as those having safety as
the principal objective. Limited attempts are sometimes made to improve
the serviceability of safety systems, but excessive efforts in this direc-
tion may degrade reliability.
22.214.171.124 Control and Safety Systems Reliability
It is useful to consider reliability to be of two classes: (1) con-
trol-system reliability and (2) safety-system reliability. In a reliable
control system, the interlocks, annunciators, relays, controllers, etc.,
are for the purpose of causing the reactor to operate in the manner in-
tended by the designer and for minimizing damage to equipment from opera-
tor mistakes. The consequence of failure of these devices is a shutdown
of the reactor, either directly because of the failure or indirectly
through an increase of reactor power and subsequent action by the reactor
safety system. The reliability of the system will therefore be based on
economic considerations, with continuous operation of the plant a major
objective. Even when adopted for a reactor, the system usually has no
more than ordinary control-system reliability.
The reliability of the safety system, however, must be of an order
sufficiently high not only to protect the core from failure of the control
system but also to protect the operating personnel. Such reliability is
obtainable through the use of redundant independent channels completely
tested and monitored. This high degree of reliability is mandatory be-
cause the consequence of failure to operate correctly when required could
be damage or destruction of the core and exposure of individuals.
126.96.36.199 Reliability of Containment Systems
The pressure vessel and its related piping constitute the primary
barrier to the release of radioactivity for many reactors (after the
fuel element itself). Because of the structural requirements for pres-
surized operation, the integrity is considered to be high, and thus this
primary system constitutes an important barrier to the dispersion of
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Cottrell, William B. & Savolainen, A. W. U.S. Reactor Containment Technology: a Compilation of Current Practice in Analysis, Design, Construction, Test, and Operation, Volume 1, report, August 1965; Oak Ridge, Tennessee. (https://digital.library.unt.edu/ark:/67531/metadc101033/m1/49/: accessed March 21, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.