Safety functional requirements

Definition of safety functions required to achieve and maintain a controlled or safe shutdown state. 

According to lEC 61508 and lEC 61511 the safety validation should be performed in terms of the overall safety functions requirements and the overall safety integrity requirements, taking into account the safety requirements allocation for the E/E/PE safety-related system in designing. In particular PFDavg value has to be verified in the probabilistic modeling process for the architectures considered of the E/E/PE safety-related systems taking into accoimt the probabihstic interval criterion for selected SIL. 

Certain critical safety functions require systems more reliable than a DCS, and they must be handled separately. These functions are associated, for example, with rectifiers, hydrogen compressors, and brine purging of membrane cells. In the past, redundant programmable logic controllers (PLCs) often provided the necessary reliability, but newer systems with the robustness of a standard PLC also have internal redundant logic and input checking and may also have redundant outputs. 

Criterion [7], ensures that the safety functions required after a PIE considered within the design basis are performed and the limits specified in the design basis for that event are not exceeded, assuming a single failure in any one component of the safety group . 

Hazard and risk assessment To determine the hazards and hazardous events of the process and associated equipment, the sequence of events leading to the hazardous event, the process risks associated with the hazardous event, the requirements for risk reduction and the safety functions required to achieve the necessary risk reduction 8 Process design, layout, manning arrangements, safety targets A description of the hazards, of the required safety function(s) and of the associated risk reduction... 

Safety functions requirements snecification Safety integrity 1 requirements 1 specification ... 

UK relevant good practice for the classification of SSCs is to categorise safety functions required to maintain safety in the event of specific fault sequences occurring, identify which SSCs deliver these safety functions and classify them accordingly. This process makes sure that all SSCs associated with the dehvery of safety functions are identified, so a consistent measure of their importance to safety is obtained. 

The safety features for the APIOOO will function within their design requirements across the full range of extreme ambient temperatures and humidity. Each of the APIOOO principal buildings has been reviewed to identify their specific safety functional requirements and specify the supporting evidence. These objectives are achieved by claims of the following types ... 

The Internal Hazards Topic Report (Reference 7.1) identifies safety functional requirements that represent the design requirements on systems, structures and components (SSCs) that enable the key safety functions to be maintained or their loss to be protected against when challenged by internal hazards. 

This is necessary to ensure that such equipment is maintained to appropriate standards, thereby providing confidence that it would fulfil its safety functional requirements if called upon. 

At the start, a detailed schedule of functional requirements - especially safety functional requirements - must be drawn up from the specifications. These safety functional requirements (SFRs) must then be traced through the various intermediate stages to ensure proper implementation. 

The diverse reactor protection system (RPS) design should be developed by a different team, using independently derived safety functional requirements ... 

In broad terms, the process for producing a specification for a safe design involves hazard identification (HAZID), which asks what sort of accidents do we need to worry about , followed by detailed analysis to identify the magnitude of potential accidents. From a safety perspective, a most important step is the clear and robust definition of the safety functional requirements, i.e., the requirements for the control and protection systems on the completed plant. The history of accidents involving design failures shows a frequent root cause to be inaccurate or inadequate definition of the safety functional requirements (see Fig. 2.3). 

Thereafter the designers have to identify the necessary barriers and controls for the identified hazards and safety functional requirements. (A barrier or a control, which prevents a hazard is called a safety critical element (SCE).) The barriers and controls will typically consist of a mixture of mechanical barriers, instrumentation and control (I C) systems, and fire-fighting systems, combined with administrative controls. Functional requirement specifications for all the necessary SCEs are then included in the overall design specification. 

Safety function requirements, definition of the safe state and how it is achieved System documents (e.g., P EDS, cause and effect matrices, logic diagrams, process data sheets, equipment layouts)... 

The design of the SRP/CS and safety function can then be undertaken based on the required level of the PL and the PL Verification of the safety function requires assessment of ... 

To perform the safety functions required for DBEEs the designer should use either systems specific to external events or the safety systems already present in the plant for internal events. In both cases, the design of the plant for safety should show due regard for the single failure criterion this may be achieved by means of the redundancy of safety systems. 

There are, however, many definitions of V V. In one of the most commonly used safety standards, lEC 61508, the objective of safety verification is ""...to demonstrate, for each phase of the overall, E/E/PE and software safety lifecycles (by review, analysis and/or tests), that the outputs meet in all respects the objectives and requirements specified for the phase [7]. The objective of safety validation is to. . validate that the E/E/PE safety-related systems meet the specification for the overall safety requirements in terms of the overall safety functions requirements and overall safety integrity requirements... [7]. These definitions differ in level of detail, but they can be summarized as verification is to answer the question are we building the system right While validation is to answer the question are we building the right system ... 

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 basis should be derived from an accurate evaluation of the external hazard, with strong reference to the safety functions required of the affected structures, systems and components and the potential for radiological consequences from interactions with non-safety related items. 

In both cases, lEC 61508 is quite clear that the safety functional requirements (specifying functionality and performance of the safety functions) must be com-... 

During an accident, the principal focus of the control room staff is on maintaining critical safety functions required to prevent core damage. Instrumentation, information-display, and operating procedures assist in maintaining critical safety functions and provide sufficient information to permit the threat or actual occurrence of core damage to be assessed. 

Overall safety requirements pedfication To develop the overall safety requirements spedfication (in terms safety functions requirements and safety integrity requirements) for all saifety-related systems and external risk reducticm facilities to adiieve functional safety. 

Realization of safety-related control systems To create safety-related control systems (SRCS) conforming to the safety requirements specification (safety functions requirements specification and safety integrity requirements specification). 

Overall safety requirements To develop the specification for the overall safety requirements, in terms of the safety functions requirements and safety integrity requirements. EUC, the EUC control system and human factors. Description of, and information relating to, the hazard and risk analysis Specification for the overall safety requirements in terms of the functions and safety integrity. Includes SIS, non-SIS and external risk reduction measures. 

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