General Design Guidance

DOE-STD-6003-96

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.

6.1.3.1 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