CHAPTER 3: Essentials of NPP design and operation

Grouping of PIE

Postulated initiating events should be subdivided into representative groups of event sequences taking into account the physical evolution of the postulated initiating events.

Each group should include event sequences based on grouping by:
(a) Principal effect on potential degradation of fundamental safety functions,
(b) Principal cause of the initiating event,
(c) Frequency and potential consequences of the event,
(d) Relation of the event to the original NPP design (for existing plants).

These groups are also categorized in accordance with their frequency of occurrence.

This approach allows the selection of the same acceptance criteria and initial conditions, and the application of the same assumptions and methodologies to all postulated initiating events grouped under the same representative event sequence.

As an example, the postulated initiating events ‘stop of a main feedwater pump’, ‘stop of all main feedwater pumps’ and ‘isolable break on the main feedwater system’ are all typically grouped under a single representative event sequence such as ‘loss of main feedwater’. Representative event sequences can also be grouped by type of sequence, with a focus on aspects such as reduced core cooling and reactor coolant system pressurization, containment pressurization, radiological consequences or pressurized thermal shocks. In the example above, the representative event sequence ‘loss of main feedwater’ would belong to the type of event sequence ‘decrease in reactor heat removal’.

Grouping (a) by principal effect leading to potential degradation of fundamental safety functions leads to the following event categories, considered typically in the reactor design:

• Increase or decrease in the heat removal through 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 bypass 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 (pressurized heavy water reactor);
• Release of radioactive material from a subsystem or component (typically from treatment or storage systems for radioactive waste).

Grouping (b) by principal cause leads to e.g.

• Control rod malfunctions
• Interfacing System LOCA, ISLOCA
The ISLOCA is a leak from the primary coolant system caused by the failure of an isolation device to a low pressure system, upon which the low pressure system leaks or fails. Usual acronym: ISLOCA.
• Loss of power supply
This includes events where the external grid is lost, e.g. by a storm, but also loss of all alternative power 'AC' (which stands for Alternate Current, which comprises the grid plus the emergency generators, as distinct from Direct Current, DC, being the batteries), i.e. including loss of the on-site emergency power sources (usually diesel generators), which event is called 'Station Black-Out', usual acronym SBO. An SBO is usually considered to be a DEC.
• Anticipated transient without scram
In this event, scram should have been initiated by the Reactor Protection System, but has failed to be initiated or completed. This is usually considered to be a DEC (for CANDU plants this is a DBA, for which a second fast shutdown system is available). Usual acronym: ATWS.
• External events (seismic, flooding, etc.)
Grouping (c) by frequency/consequences is linked to the plant states discussed above. Read more →

Grouping (d) is mostly associated with safety upgrading /backfitting of older plants.