Instrumentation and Control (I&C)

I&C Systems important to Safety
In each NPP, instrumentation is provided to determine the values of the main variables which indicate the status of the fission process, the reactor core, the reactor coolant system and the containment, in order to obtain all information needed for the plant's safe and reliable operation, to determine its status in operational occurrences and accident conditions and to make decisions for the purposes of accident management.
Appropriate and reliable control systems are available to maintain and limit the relevant process variables within the specified operational ranges.
Instrumentation and recording equipment is provided to ensure that the status of essential equipment to mitigate incidents and accidents is known, and to monitor the location and magnitude of possible releases.
The primary functions of instrumentation and control systems that are important to safety are.
• Control functions;
• Protection functions;
• Monitoring and display functions; and
• Testing functions.
Control functions provide assurance that the plant is controlled and kept within its operating envelope under normal and abnormal conditions. Protection functions provide a line of defence against failures in other plant systems.
Control functions can also mitigate the effects of plant transients or other PIEs, thereby contributing to nuclear safety by minimizing the demand on protection functions.
In addition, there are service functions, also important to safety, which are carried out in support of the previously mentioned functions. Examples are electric, pneumatic and hydraulic power, and data communication systems.
An example of the instrumentation and control systems important to safety is presented in the Figure 1-5.

Figure 1-5: Examples of I&C systems important to safety. (Examples are given for illustration. Some systems are listed in one column although they may belong in another column, e.g. control room I&C),
SSG-39 .
I&C design basis
The reactor protection system is designed such that it detects unsafe plant conditions and initiates safety actions to achieve and maintain safe plant conditions. Where limited time is available for the operators to respond, the actuation of the safety systems is automatic. It is often also designed that it overrides unsafe actions by plant personnel.
For example, it is not possible to switch off ECCS injection as long as the initiation criteria for ECCS are present.
For all instrumentation and control systems important to safety the design includes the environmental conditions which the system will be required to withstand, and the expected duration of operation under such conditions, for operational states and for design basis accident conditions.
Read more → SSG-39
Use of I&C during severe accidents
The environmental parameters anticipated during accident conditions depend on the plant design, the type of the initiating event and the resultant level of core degradation. When accident conditions proceed to a severe accident, the equipment located in the containment may be exposed to conditions that significantly exceed the values for extended time periods for which the equipment has been qualified.
Therefore, the plant parameters needed for severe accident management measures should be identified and it should be checked whether these parameters are available from the instrumentation in the plant, notably under severe accident and environmental conditions for the instruments.
Hence, for the qualification of all relevant instruments should be recognized that, in order to obtain the required information, the equipment may need to operate well beyond its qualified range. Alternative instrumentation should be identified where the primary instrumentation is not available or not reliable. For instruments still available, it should be checked how much the measured parameters may deviate from their values under nominal conditions.
Recommendations on instrumentation and control for severe accident management in new NPPs are presented in SSG-54 paras 3.91-3.98. Read more → SSG-54
Technical bases to consider when assessing the capability of electrical and I&C equipment to perform reliably during a severe accident are presented in TECDOC-1818. This document also provides examples of calculation tools to determine the environmental parameters as well as examples and methods that may be adopted by Member States in order to assess equipment reliability.
Read more → TECDOC-1818
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