Nuclear plant safety protection functions

Protection functions These are the most critical of the safety functions. These relate directly to nuclear safety and provide a line of defense against serious failure in other plant systems. The following are major issues ... 

SISs are most often used in process facilities, such as refineries, chemical plants, and nuclear power plants, to provide safety protection. Example protection functions include ... 

Control functions provide assurance that the plant is controlled and kept within its operating envelope under normal and abnormal conditions. Control functions can also mitigate the effects of plant transients or PIEs, thereby contributing to nuclear safety by minimizing the demand on protection functions. 

The objective of the event-independent part of the Emergency Operating Procedures (EOPs) is to provide means to evaluate and restore the plant nuclear safety. The concept is based on the premise that radiation release to the environment can be minimised if the barriers to activity release are protected (barriers of defence in depth). In order to accomplish this goal, a set of functions has been defined which are critical from the plant nuclear safety point of view. These are the Critical Safety Functions. To be able to evaluate the status of these functions. Status Trees have been designed, one per CSF. Once the state of the CSF is evaluated, based on their state and the rules of priority one can designate a Function Restoration Guideline to be implemented for restoring CSF (see Appendix 3). 

Nuclear power plants are provided with a series of automatic systems, in particular in the field of reactor protection and other safety systems. Many of these systems were designed to mitigate design basis events and may not be optimized for lesser events. Therefore, it may be necessary to override or modify the automatic safety functions as part of the EOPs. If it does become necessary to override or modify safety functions as part of the EOF actions the following administrative rules are recommended ... 

Computer based systems are of increasing importance to safety in nuclear power plants as their use in both new and older plants is rapidly increasing. They are used both in safety related applications, such as some functions of the process control and monitoring systems, as well as in safety critical applications, such as reactor protection or actuation of safety features. The dependability of computer based systems important to safety is therefore of prime interest and should be ensured. 

In order to ensure safety, an adequate degree of fire protection should be provided in nuclear power plants. A fire hazards analysis including consequences of fire fighting should be performed before initial fuel loading and be updated during operation to verify that the main safety functions to shut down the reactor, to remove residual heat, and to contain radioactive material are maintained against the consequences of a fire. 

The fire protection system (FPS) and the fire hazard analysis (FHA) are presented in Chapter 9.5.1 and Appendix 9A of Reference 4.4 respectively. The FPS has been designed to provide appropriate fire detection and suppression in line with the nuclear safety implications of fires in specific areas of the plant it has been designed to take consideration of the fire hazard analysis, to make sure that safety-significant SSCs required for safe plant shutdown or to prevent significant releases of radioactive material can maintain functionality. It is not however required to ensure nuclear safety in the event of a fire. A full suite of safety functions associated with the FPS is specified in Chapter 6 of this PCSR. 

Nuclear safety for the APIOOO is less dependent than current plants on ihe duty systems (IAEA Level 1) and the systems that are deployed to control abnormal operation and detect failures (IAEA Level 2), because of the presence of robust safety measures (IAEA Level 3) that stop ihe loss of nuclear safety function these Level 3 safety measures are robust because they do not require support systems such as ac power, component cooling water and service water. The five IAEA levels of protection are discussed in more detail in Section 8.2.1 of this PCSR. 

The regulations for nuclear power plants in the US require that the fire protection water delivery system for fires affecting safety-significant equipment must be classified as seismic Category 1. The APIOOO has the additional requirement that safety functions must be performed without ac power. 

In nuclear facility design, safety class structures shall be protected against loss of functions due to postulated plant generated and extreme environmental missiles depending on aircraft crash should be considered. 

The major functions and architecture of a Nuclear Power Plant Protection System are designed to meet the safety issues. Both the analogic and the digital designs are able to satisfy the requirements. 

The design management for a nuclear power plant shall ensure that the struc-trrres, systems and components important to safety have the appropriate characteristics, specifications and material composition so that the safety functions can be performed and the plant can operate safely with the necessary reliability for the full duration of its design life, with accident prevention and protection of site personnel, the public and the envirorrment as prime objectives. 

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