CHAPTER 3: Essentials of NPP design and operation

Postulated initiating events

Definition

The design for the nuclear power plant shall apply a systematic approach to identifying a comprehensive set of postulated initiating events such that all foreseeable events with the potential for serious consequences and all foreseeable events with a significant frequency of occurrence are anticipated and are considered in the design (PIEs).

The postulated initiating events shall be identified on the basis of engineering judgement and a combination of deterministic analysis and probabilistic analysis.

A deterministic assessment focuses on accident types, consequences and possible releases, without considering probabilities. The approach is 'mechanistic', i.e. it considers what can go wrong, largely without the probability of such failure (very low probability accidents may be excluded from consideration, e.g. reactor pressure vessel rupture). A probabilistic approach focuses on the probability of accidents, and can assign more weight to accidents with a larger probability (e.g.resulting in more stringent requirements for mitigating such accidents). The concept is used to define the plant states (Chapter 2.4.3.1. Plant states considered for the design ). Further information is in Module 2, Chapter 2.4 Safety Assessment.

A justification of the extent of usage of deterministic safety analysis and probabilistic safety assessment shall be provided to show that all foreseeable events have been considered.

The postulated initiating events shall include all foreseeable failures of structures, systems and components of the plant, as well as operating errors and possible failures arising from internal and external hazards, whether in full power, low power or shutdown states.

An analysis of the postulated initiating events for the plant shall be made to establish the preventive measures and protective measures that are necessary to ensure that the required safety functions will be performed.

The expected behaviour of the plant in any postulated initiating event shall be such that the following conditions can be achieved, in order of priority:

1) A postulated initiating event would produce no safety significant effects or would produce only a change towards safe plant conditions by means of inherent characteristics of the plant.
2) Following a postulated initiating event, the plant would be rendered safe by means of passive safety features or by the action of systems that are operating continuously in the state necessary to control the postulated initiating event.
3) Following a postulated initiating event, the plant would be rendered safe by the actuation of safety systems that need to be brought into operation in response to the postulated initiating event.
4) Following a postulated initiating event, the plant would be rendered safe by following specified procedures.

More information can be found in SSR 2/1 (Rev. 1), Requirement 16 Read more → .

Examples of postulated initiating events from internal hazards

Indicative list of PIEs leading to Anticipated Operational Occurrences (AOO):

• Increase or decrease in the heat removal from the reactor coolant system;
• Increase or decrease in the flow rate of the reactor coolant system;
• Anomalies in reactivity and power distribution in the reactor core, or anomalies in reactivity in fresh or spent fuel in storage;
• Increase or decrease in the reactor coolant inventory;
• Leaks in the reactor coolant system with potential by-pass of the containment;
• Leaks outside the containment;
• Reduction in or loss of cooling of the fuel in the spent fuel storage pool;
• Loss of cooling of fuel during on-power refuelling;
• Release of radioactive material from a subsystem or component.

Indicative list of PIEs leading to Design basis Accidents (DBA):

• Increase in heat removal from the reactor;
• Decrease in heat removal from the reactor;
• Decrease in flow rate of the reactor coolant system;
• Anomalies in reactivity and power distribution;
• Decrease in reactor coolant inventory;
• Reduction in or loss of cooling of the fuel in the spent fuel storage pools;
• Loss of cooling of fuel during on-power refuelling;
• Loss of moderator circulation or decrease in or loss of moderator heat sink;
• Release of radioactive material due to leak in reactor coolant system;
• End-shield cooling failure.

Indicative list of PIEs leading to Design Extension Conditions (DEC):

• LOCA with a complete loss of the emergency core cooling;
• Complete loss of electrical power for an extended period;
• Complete loss of the residual heat removal from the reactor core.

Further details of categorisation of PIEs and examples of PIEs are in SSG-2 (Rev. 1), para 3.23-3.36 Read more →

Examples of initiating events from operating experience

Data for all unexpected reactor trips during power operation at US commercial nuclear power plants are yearly reviewed and categorized by the NRC. Other organisations (OECD, IAEA, etc) compile lists of nuclear incidents and accidents worldwide.