Probabilistic Risk Analysis

A Probabilistic Risk Analysis (PRA) is a technique used to estimate the probability of an adverse event in a plant. This estimate is quantified by summing up all combinations of deficient systems that lead to this undesirable event.

A PRA provides a structured understanding of the risk and consequences of incidents and is an ideal way to quantify the risk of installations. Using this insight, the weakest safety or reliability links of a plant can be identified and addressed. PRA in the Netherlands has its origin in the nuclear sector. Due to the far-reaching radiological consequences of incidents at a nuclear power plant, there is a great need to understand the probability of the failure of (redundant) systems. PRA methodology is also used to calculate the probability of the failure of flood defences to close, like the Oosterschelde and the Maeslant surge barriers in the Netherlands.

How to perform a PRA?
A PRA model is established in three (main) steps:
• mapping adverse events in and around a vital installation
• modelling the (combinations of) failures that can lead to the undesirable events.
• bringing together and quantifying these models, resulting in new insights to improve control of undesirable events

Conducting a PRA requires the application of a range of risk analysis techniques, including the drafting of event trees and fault trees, Failure Mode Effect Analysis (FMEA), Data Analysis, Human Reliability Analysis (HRA) and Hazard Analysis. To perform a PRA also requires in-depth understanding of the interaction of systems and the influences of common or systematic failures.

How is NRG involved in PRA?
Team Risk Management Consultancy (RMC) NRG is involved in the PRAs of all major nuclear installations in the Netherlands. These are the EPZ Borssele nuclear power plant (NPP), the High Flux Reactor (HFR) in Petten, and in the past the nuclear plant Dodewaard. RMC also provides expertise in the infrastructure sector and civil installations and vital structures such as the Oosterschelde and the Maeslant surge barriers.

The NRG risk specialists focus on:

• Providing on-site risk expertise for specific risk questions about PRA
• Conducting PRA projects relating to surge barriers, locks, bridges and tunnel sections
• Continuously improving the PRAs of Borssele NPP and HFR
• Conducting research on the failure mechanisms of digital I&C
• Compiling and maintaining of databases of failure probabilities of components
• Providing communication between authorities (KFD, RWS, Province) and operators