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6.1.3 General Design Guidance
Before providing system-specific design guidance, some general principles of design are
given below. These principles will assist in achieving facility safety requirements and goals and
also have broader value in meeting device performance specifications and providing a measure
of investment protection, which is a requirement for eventual electric utility acceptance of fusion
power plants. These principles apply specifically to safety-class SSCs but should be considered
using a graded approach for safety-significant SSCs. Design for Reliability
Unavailability limits for safety-class SSCs should be established to ensure the required
reliability for the performance of the key safety functions. The measures below should be used,
if necessary in combination, to achieve and maintain the required SSC reliability. The required
reliability should be developed in accordance with the importance of the safety function per-
formed by the SSC to protect on-site personnel and the public.
a. Simplicity. The principle of design simplicity should be applied, as appropriate, to
enhance the reliability of systems. Less complex systems are generally more reliable.
An example of simplicity may be choosing a burst disk over a relief valve for over-
pressure protection or designing the system for a greater pressure than all credible
design-basis events.
b. Diversity. The principle of diversity can enhance reliability and reduce the potential for
common cause failures. It should be adopted wherever feasible. Note that there is an
operational cost for diversity in terms of spare parts, operator training, and device
complexity. An example of diversity involves a relief valve and burst disk on a
mechanical system, each of which can relieve overpressure at the required rate.
c. Independence. The principle of independence should be applied, as appropriate, to
enhance the reliability of systems, in particular with respect to common cause failures.
Independence is accomplished in the design of systems by using functional isolation
and physical separation (e.g., separation by geometry and barriers). An example of
independence is a situation in which two relief valves on a mechanical system are at
opposite ends of the piping runs.
d. Redundancy. The principle of redundancy should be applied as an important design
principle for improving the reliability of safety-class SSCs and guarding against com-
mon cause failures. Multiple sets of equipment that cannot be tested individually
should not be considered redundant. The degree of redundancy should reflect the
potential for undetected failures that could degrade reliability of the safety function. An
example of redundancy is a situation in which each of two relief valves on a mechani-
cal system can relieve overpressure at the required rate.
e. Fail-safe and Fault-tolerant Design. The fail-safe principle should be applied to safety-
class and safety-significant SSCs; that is, if a system or component failed, the device

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