Component functional safety nuclear power systems

Implementations of I C functions important to safety in nuclear power plants are increasingly realized with computer based systems, i.e. by its software. Often so called equipment families are used to develop these I C functions. Besides a hardware platform, these equipment families provide predeveloped software in the form of basic components from which I C functions can be composed by configuration and parameterization but also larger components which have been developed by conventional software engineering, as e.g. operating systems, I/O drivers, self-supervision software, etc. are included. Outside the equipment families, it may be desirable to introduce also other types of predeveloped software, e g. for simulation and for analysis purposes. 

Criterion 5 - Sharing of structures, systems, and components. Structures, systems, and components important to safety shall not be shared among nuclear power units unless it can be shown that such sharing will not significantly impair their ability to perform their safety functions, including, in the event of an accident in one unit, an orderly shutdown and cooldown of the remaining units. 

ABSTRACT Nuclear power plant includes multiple components and systems, which are maintained in order to limit or prevent failures resulting from the ageing and deterioration. These components and systems are imavailable during the maintenance activities. The unavailability of the safety systems results in increased risk of the nuclear power plant. A method for optimization of the maintenance activities in the nuclear power plant applying heuristics algorithms is presented. The maintenance optimization is modelled as a combinatorial problem. The minimal cut sets identified in the prohahiUstic safety assessment are used for assessment of the risk in the optimization function. The periodically tested component model is apphed for the modelling of the components included in the maintenance. The developed method is apphed on test models and the obtained results are presented. Results show that optimization of maintenance decreases the risk and thus improves the plant safety. 

The design of a nuclear power plant includes interdisciplinary reviews to assure the functional compatibility of the plant structures, systems, and components and compliance with licensing requirements. Safety reviews and accident analyses provide further assurance that system functional and licensing requirements will be met. Thus, the design and analyses of the plant take into account systems interactions. Nevertheless, the process may not consider all the interactions of various plant systems. Questions have been raised both as to the supporting roles such systems play and the effect one system can have on other systems, particularly with regard to the presumed redundancy and independence of safety systems. 

The technical aspects of the OLCs cover the limitations to be observed, as well as the operational requirements that stmctures, systems and components important to the safety of the nuclear power plant be able to perform their intended functions as assumed in the plant safety analysis report. Safe operation depends upon personnel as well as on equipment OLCs should therefore also cover actions to be taken and limitations to be observed by operating personnel. 

The "Guide for Classification of Safety Function Importance in Light Water Nuclear Power Reactor Facilities" defines the relative importance of various fimctions necessaiy to ensure the safely. The basis had been established for adequate requirement to be met by design of structures, systems and component that are expected to perform such function. 

Safety systems at nuclear power plants are designed with redundant trains such that a failure of a single component or train will not prevent the system from performing its safety function. Thus, a lack of breaker coordination does not necessarily pose a safety significant hazard. If a faulted condition in a safety related component causes an entire motor control center or switchgear to be lost, there is generally a completely electrically independent, redundant train to perform the same function and thus the function of the safety related system is not lost. 

Structure, systems and components with safety functions shall be so designed that the safety of the nuclear reactor facilities will not be impaired by postulated extenal man-induced events. (Guide 3.1) External postulated man-induced events are said to refer to airplane crashes, collapse of dams, explosions, etc. In Japan, nuclear power plants are not located in the vicinity of dams nor in areas where... 

The Safety Requirements publication Safety of Nuclear Power Plants Design [5] specifies the fundamental safety functions required to be performed to ensure safety as the control of reactivity the removal of heat from the core and the confinement of radioactive material and the control of operational discharges, as well as the limitation of accidental releases. This part of the SAR should identify and justify the fundamental safety functions to be fulfilled by the specific plant design. It should specify the corresponding structures, systems and components necessary to fulfil these safety functions at various times following a postulated initiating event (PIE). 

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. 

Stractnres, systems and components important to safety, except as described in para. 5. 44, shall be designed to be calibrated, tested, maintained, repaired or replaced, inspected and monitored with respect to their functional capability over the lifetime of the nuclear power plant to demonstrate that rehability targets are being met. The plant layout shall be such that these activities are facilitated and can be performed to standards commensurate with the importance of the safety functions to be performed, with no significant reduction in system availability and without undue exposure of the site personnel to radiation. 

Structures, systems, and components important to safety shall be designed, fabricated, erected, and tested to quality standards commensurate with the importance of the safety functions being performed.. .. A quality assurance program shall be established and implemented in order to provide adequate assurance that these structures, systems, and components will satisfactorily perform their safety functions. Appropriate records of design, fabrication, erection, and testing 6f structures, systems, and components important to safety shall be maintained by or under the control of the nuclear power unit licensee throughout the life of the plant. 

All the above aspects are being taken into account by nuclear power plant designers, and as a result, both novel and more or less proven passive systems and features are proposed in many new designs [2]. Some designs have only added a few passive components to the traditional systems. Some other designs make use of the passive systems/components and natural circulation phenomena for power production in normal operation, or to fulfil a number of safety functions, intended to prevent severe accidents and mitigate their consequences. 

As part of USNRC s ongoing regulatory activities, inspections such as Safety System Functional Inspections include the reliability of SOVs as well as other components required by safety related applications. The NRC also is providing technical advice to the Electric Power Research Institute s Nuclear Maintenance Application Center to assist in preparing an SOV maintenance guide. 

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 emergency power supply system (level D) supplies components performing functions indispensable to nuclear safety and needed immediately after LOSSP. Three emergency power supply batteries, one for each division, ensure an uninterruptable power supply for at least 32 h. The batteries are charged by the standby diesel generators. 

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