Postulated initiating events

A list of PIEs should be established for use in the safety analysis of the RCSASs. The likelihood of occurrence of the events and their potential consequences should be taken into account. For plants at which preventive maintenance at power is intended, the need for considering a PIE that is coincident with the maintenance of one safety system train should be evaluated. 

In establishing the list of PIEs, combinations of events relevant to the design of the RCSASs should also be considered, in accordance with Ref. [1]. 

Examples of PIEs that could significantly influence the design of the RCSASs include  

Structures, systems and components of the RCSASs should be designed on the basis of seismic ground motions appropriate to the site and the seismic category to which they are assigned as established in accordance with the procedures given in Ret [8]. Appropriate restraints, supports and snubbers should be provided so that the relevant limitations on stress and displacement and the no-loss-of-function criteria are met. 

The dynamic effect of flow instabilities and the dynamic loads (e.g. water hammer) induced by earthquakes should be taken into account in the design in accordance with the safety analysis. Some combinations of an earthquake and other PIEs likely to occur independently of an earthquake should be taken into account by the use of methods as stated in Ret [1], paras I.14-I.18, and appropriate provisions should be made for these combinations. 

This appendix elaborates on the definition and application of the concept of the postulated initiating event (PIE). 

A PIE is defined as an event identified in design as leading to anticipated operational occurrences or accident conditions. This means that a PIE is not an accident itself it is the event that initiates a sequence and that leads to an operational occurrence, a design basis accident or a severe accident depending on the additional failures that occur. Typical examples are equipment failures (including pipe breaks), human errors, human induced events and natural events. 

A PIE may be of a type that has minor consequences, such as the failure of a redundant component, or it may have serious consequences, such as the failure of a major pipe in the reactor coolant system. It is a main objective of the design to achieve plant characteristics that ensure that the majority of the PIEs have minor or even insignificant consequences and that if the remainder lead to design basis accidents, the consequences are acceptable or if they lead to severe accidents, the consequences are limited by design features and accident management. 

The number of PIEs to be used in the development of the performance requirements for the items important to safety and in the overall safely assessment of the plant should be limited to make the task practical, and this is done by restricting the detailed analysis to a number of representative event sequences. The representative event sequences identify bounding cases and provide the basis for numerical design limits for structures, systems and components important to safely. 

Initiating events can be individnal equipmerrt failrrres that conld directly or indirectly affect the safety of the plant. The list of these everrts adeqrrately represents all credible failures of plant systems and components. 

The starting point for the safety analysis is the set of PIEs that need to be addressed. A PIE is defined in Ref. [1] as an identified event that leads to anticipated operational occurrences or accident conditions . PIEs include events such as equipment failure, human errors and human induced or natural events. The deterministic safety analysis and the PSA should normally use a common set of PIEs. 

The set of PIEs developed for the safety analysis should be comprehensive and should be defined in such a way that they cover all credible failures of plant systems and components and human errors which could occur during any of the operating regimes of the plant (such as startup, shutdown and refuelling). This should include both internally and externally initiated events. 

The set of PIEs should be identified in a systematic way. This should include adopting a structured approach to the identification of the PIEs which could include 

The set of PIEs addressed should also include partial failures of equipment if these can make a significant contribntion to the risk. 

HAZOP is a systematic process which uses a set of key words to identify the failures which could occur and could lead to PIEs. 

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Anticipated operational occurrences are off-normal events, usually plant transients, which can be coped with by the plant protection systems and normal plant systems but which could have the potential to damage the reactor if some additional malfunction should happen. Their typical frequency of occurrence may be more than 10 year Some of the anticipated occurrences (PIEs - postulated initiating events) are due to the increase of reactor heat removal (as might occur for an inadvertent opening of a steam relief valve, malfunctions in control systems, etc.). Some are due to the decrease of reactor heat removal (such as for feed-water pumps tripping, loss of condenser vacuum and control systems malfunctions). Some are due to a decrease in reactor coolant system flow rate, as in the case of a trip of one or more coolant pumps. Some are connected with reactivity and power distribution anomalies, such as for an inadvertent control rod withdrawal or unwanted boron dilution due to a malfunction of the volume control system for a PWR. Events entailing the increase or decrease of the reactor coolant inventory may also happen, due to malfunctions of the volume control system or small leaks. Finally, releases of radioactive substances from components may occur. 

Systems to be considered available or not in the deterministic analysis of the Postulated Initiating Events (PIE) (EUR, Chapter 18). 

Categories of plant states Postulated initiating events... 

Components may be withdrawn from service for repair, periodic maintenance or testing. For the systems they belong to, the SFC is not applicable during this limited time period. During this period, the combined frequency of postulated initiating event and loss of safety function or the effect on the system s capability to perform its safety function shall be demonstrated to be insignificantly low. 

The results of analyses, performed during the first stage of work showed that the units have large reserves and in some defined conditions also leakages, larger that the considered by DBA, can be controlled. At the same time, the necessity for realization of some technical measures was defined. All necessary measures were implemented and now the units are in a status, which allows them to overcome all the postulated initial events (design deviations, abnormal conditions and accidents). 

This Safety Guide discusses postulated initiating events (PIEs) that may occur in the different operational states of the plant as stipulated in Ref [1], and supplements the relevant paragraphs of Ref [1]. It introduces the probabilistic and deterministic approaches for reviewing the following ... 

A postulated initiating event is an event identified during design as capable of leading to anticipated operational occurrences or accident conditions. The primary causes of PIEs may be credible equipment failures or operator errors (both within and external to the facility), and human induced or natural events. 

The sensing of one or more variables indicative of a specific postulated initiating event, the signal processing, the initiation and completion of the safety actions required to prevent the limits specified in the design basis from being exceeded, and the initiation and completion of certain services from the safety system support features. 

A deterministic safety analysis should be conducted for each nuclear power plant, should confirm the design basis for items important to safety and should describe the plant behaviour for postulated initiating events [6]. Therefore, a documented safety design basis (original and updated) should be... 

Postulated initiating events (for the existing safety analyses and a comparable list for a modern nuclear power plant) [6]. 

Safety group is defined as The assembly of equipment designated to perform all actions required for a particular postulated initiating event to ensure that the limits specified in the design basis for anticipated operational occurrences and design basis accidents are not exceeded. ... 

Postulated initialing events leading to anticipated operational occurrences... 

Where the LBB approach cannot be applied effectively, a determination of pipe break locations and dynamic effects is made. These are identified in CESSAR-DC, Section 3.6.2. The criteria used to define pipe break and/or crack locations and configurations are given in CESSAR-DC, Section 3.6.2.1. Postulated ruptures are classified as circumferential breaks, longitudinal breaks, leakage cracks, or through wall cracks. Each postulated rupture is considered separately as a single postulated initiating event. 

Acceptance criteria are specified bounds on the value of a functional or condition indicator used to assess the ability of a structure, system or component to perform its design function. Acceptance criteria as used here means specified bounds on the value of a functional or condition indicator for a structure, system or component in a defined postulated initiating event (e.g. an indicator relating to functionality, leaktightness or non-interaction). 

Postulated initiating event. An event identified during design as capable of leading to anticipated operational occurrences or accident conditions. 

When necessary to meet the reliability requirement, the form of diversity to be introduced into the system s design. Diverse features can be specified in terms of diverse detection, measurement, voting and actuation methods in terms of different functions to react to the same postulated initiating event and in terms of independent and diverse system components. The appropriate types of... 

The dynamic tests should be based on an analysis of the plant transients induced by postulated initiating events. The test profiles should be representative of the expected plant parameter variations that would place demands on the computer system. The number of tests executed should be sufficient to provide confidence in the system s dependability. 

An analysis of the plant transients induced by postulated initiating events for use in the statistical tests ... 

The effects of flooding on a nuclear power plant site may have a major bearing on the safety of the plant and may lead to a postulated initiating event (PIE) that is to be included in the plant safety analysis. The presence of water in many areas of the plant may be a common cause of failure for safety related systems, such as the emergency power supply systems or the electric switchyard, with the associated possibility of losing the external connection to the electrical power grid, the decay heat removal system and other vital systems. Details are provided in Ref [6]. 

At the Ulchin NPP Units 3 and 4, each postulated initiating event has been assigned to one of the following categories ... 

Full scale simulators began at Bugey in 1977 to simulate normal reactor operation and postulated initiating events (primary coolant in a monophasic state), break < 2 inches. 7 other simulators were installed and used to simulate some hundreds of potential breakdowns. 

The DAS is included wilhin the instrumentation and control architecture in order to support the risk goals in the APIOOO PRA for analyzed events. The DAS reduces the probability of a severe accident resulting from Ihe imlikely coincidence of a Postulated Initiating Event (PIE) and postulated common-mode failures in Ihe protection and control systems. Common-mode failure between the Protection and Safety Monitoring System and Ihe DAS is unlikely because each runs on a different operating system from the other, and there are no sensors shared between the two systems (see Section 8.4.4.4 of this chapter for a further enhancement to the DAS to reduce common-cause failure even more). 

Towards this objective, a theoretical model has been developed and based on this model, the response of the system has been estimated using the indigenously developed computer code CAPCIS (Code for Analysis of Passive Containment Isolation System). The numerical analysis performed takes into account the pressure transient because of the postulated initiating event (PIE) and the solution includes determination of levels in the two compartments of the isolation tank and the cumulative volumetric spill into the ventilation duct at any point of time during the transient. A case of a 200% break in inlet header is considered for the analysis and the pressure transient for the considered PIE is numerically simulated as shown in Fig. 10. Figure 11 shows the volumetric spillover during the entire transient. 

The appendix provides a list of the selected postulated initiating events to be considered in the safety analysis for a research reactor. 

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