This Standard is primarily concerned with the nuclear safety of Shutdown Systems, i.e., their ability to terminate the nuclear reaction during a potentially hazardous situation. However, it recognizes that the repeated unnecessary operation of a Shutdown System will needlessly stress the nuclear plant and, possibly, lead to earlier system and component failures.
Therefore, this Standard addresses itself to two objectives:
(a) Ensuring that the Shutdown System operates as intended when required;
(b) Minimizing Shutdown System operation when no potentially hazardous situation exists.
This Standard pertains to all the components of a Shutdown System, covering all the engineering disciplines involved, including mechanical, process, electrical, and instrumentation and control design.
All systems in a CANDU nuclear power plant fall into one of the two categories, as follows:
(a) Process Systems. These are all the nuclear and conventional systems in the plant required for operation in any defined state expected during the life of the plant;
(b) Special Safety Systems. These are systems specifically incorporated in the plant design to prevent the failures of plant process systems, or to limit or mitigate the consequences of such failures, thereby ensuring that any resultant release of radioactivity to the environment and the public is kept within acceptable limits.
The Special Safety Systems comprise the following: one or two Shutdown Systems, the Emergency Coolant Injection System, and the Containment System. Shutdown Systems are those Special Safety Systems which monitor plant conditions and terminate the nuclear reaction (reactor trip) when required in order that the remaining plant systems can ensure the safety of the public.
Note: The philosophy relative to the incorporation of these systems in the plant design is given in the paper Reactor Licensing and Safety Requirements, D.G. Hurst and F.C. Boyd, 72-CNA-102.
This Standard is applicable only to Shutdown Systems. Where the plant design incorporates two such systems, it shall be applied equally to each of them.It is noted that, in accordance with AECB Licensing Document #13, The Use of Two Shutdown Systems in Reactor, Jan. 11, 1977, all nuclear power reactors licensed for construction in Canada after January 1, 1977 shall incorporate two independent shutdown systems unless otherwise approved by the Atomic Energy Control Board.
The design basis for the Shutdown Systems shall be established in the Safety Report and the associated accident analysis reports for a particular nuclear power plant and shall be documented in the systems' Design Manuals (see Clause 5.2.1). This Standard does not discuss what the roles of the Shutdown Systems should be nor does it provide the means of assessing the performance capability of the systems in meeting these roles; it indicates rather those detailed requirements which the systems should meet in order that they may carry out their defined roles with acceptable assurance.