System safety functions

Using this definition, a block diagram can be drawn to illustrate the system safety function (see figure 9-2). 

Malasky (1982) discusses some of the difficulties sometimes encountered during implementation. Problem formulation is cited as one issue. This may be caused by the conflicting demands of the vari- 

Organizational interfaces may pose other problems. When management does not take responsibility for decision making in the process, functional and system safety groups may be at odds with each other. Management perception is another difficulty. System-safety concepts are more abstract than those in many other disciplines. Therefore, it is important for the safety professional to structure the program so its impact is clear. 

Elements of a System-Safety Program Plan (SSPP) 

According to MIL-STD-882B (Department of Defense, 1984) the SSPP must specify the four elements of an effective system-safety program  

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Suitable redundancy in components and features, and suitable interconnections, leak detection, and isolation capabilities shall be provided to ensure that for on-site electric power system operation (assuming off-site power is not available) and for off-site electric power system operation (assuming on-site power is not available) the system safety function can be accomplished, assuming a single failure. 

Consideration of redundancy and diversity requirements for fluid systems important to safety. A system could consist of a number of subsystems each of which is separately capable of performing the specified system safety function. The minimum acceptable redundancy and diversity of subsystems and components within... 

Criterion 35 - Emergency core cooling. A system to provide abundant emergency core cooling shall be provided. The system safety function shall be... 

Criterion 38 - Containment heat removal. A system to remove heat from the reactor containment shall be provided. The system safety function shall be to reduce rapidly, consistent with the functioning of other associated systems, the containment pressure and temperature following any loss-of-coolant accident and maintain them at acceptably low levels. 

Zone 1 and Zone 2A Ventilation Exhaust Systems Safety Function... 

Periodic, scheduled facility safety inspections are essential in any operational area, especially where hazardous tasks are performed on a regular basis. Compliance with safety inspection requirements should not be difficult to accomplish since similar requirements should already exist in an established occupational safety program. The facility inspection encompasses all facets of daily operation and considers the human-machine interface a primary candidate area for potential mishaps. Frequent facility inspections are an excellent method of maintaining current awareness of facility conditions and how those conditions affect, or might affect, the safe operation of that facility. A system should be in place to ensure implementation of corrective actions and to track repetitive items. Results of inspections should be properly documented and accountability for discrepant items appropriately determined and assigned in order for the inspection process to be effective. If properly performed, the facility safety inspection is an excellent tool in the overall success of the system safety function. 

Through the use of basic probability theory and statistical analysis, the system safety function can actually assign expected values to certain hazards and/or failures to determine the likelihood of their occurrence. The availability of such quantifiable information further enhances the management decisionmaking process and justifies the existence of the system safety effort within the organization. 

Part II of this Basic Guide to System Safety presents and briefly discusses some of the more common system safety analytical tools used in the performance of the system safety function. Through example analyses of hypothetical mechanical and/or electrical systems, the reader should become familiar with each type of system safety analysis method or technique discussed. However, it must be understood that it is not within the limited scope of this volume to provide a detailed explanation of each of these methods and/or techniques. The intention is to merely introduce the reader to the various tools associated with the system safety process. The value of each concept in the analysis of hazard risk will vary according to the individual requirements of a given organization or company. 

What is the importance of the system safety function to an organization ... 

HEAT TRANSPORT SYSTEM CSA Class 1 DBE The system safety functions are to facilitate heat removal from the friel under both normal and accident condiuons. and to maintain the mtegniy of the reactor coolant pressure boundary... 

PRESSURE AND INVETORY CONTROL SYSTEM CSA Class 1,3 DBE The system safety functions are to provide overpressure protection of the heat transport system, to mamtain sufficient reactor coolant mvenlory during and after operananal stales The system consists of the pressunzer, degasser-condenser, heat transport system safelyfrehef valves, and feed/bleed Sow paths with D,0 storage tank and feed pump... 

Component Cooling, and Service Water fluid systems shall, in order to assure that the system safety functions can be accomplished, be designed to withstand the adverse dynamic loads imposed by condensation-induced and other water hammer events, and include features to minimize the probability of water hammer occurrences. BTP ASB 10-2 requires pre-operational tests to be performed on the feedwater system to demonstrate the effectiveness of the design and operating procedures to inhibit steam generator water hammer. 

Because safety is a system property (and not a component property), there must be a focused and coordinated system safety function that spans the entire organization, with direct links to decision-makers and influence on decision-making ... 

HA is the systematic examination of a system, item, or product within its life cycle, to identify hazardous conditions including those associated with human, product, and environmental interfaces, and to assess their consequences to the functional and safety characteristics of the system or product. In order to design-in safety, hazards must be designed-out (eliminated) or mitigated (reduced in risk), which can only be accomplished through HA. Hazard identification is a critical system safety function and is one of the basic required elements of an SSP. 

All products have to go through the various phases of the safety process shown in Figure 2. One outcome of the FHA-phase is the definition of system safety functions and related failure modes. The failure modes are categorised according to the severity of their worst case end-effect. Hazards are then defined based on each safety related failure mode and assigned the worst of the related failure mode severities. Any mitigation means defined by the development team, composed of experts of all relevant groups at Frequentis, form the basis for the definition of safety requirements imposed on the product and its development. 

The moderator recovery system (MRS) is intended to supply reactor coolant makeup for small breaks in the PWS pressure boundary. The system safety function is to ensure that specified acceptable fuel design limits are not exceeded as a result of primary coolant loss due to leakages from the PWS pressure boundary and rupture of small piping or other small components which are part of this boundary. If light water systems are leaking concurrent with a PWS small break/leak,- the MRS may pump degraded moderator into the PWS. 

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