Basis for the derivation of the safety goals and new challenges

An abstract of the technical presentation presented at:
19th Pacific Basin Nuclear Conference
Vancouver, British Columbia, Canada
August 24 – 28, 2014

Prepared by:
Smain Yalaoui
Canadian Nuclear Safety Commission (CNSC)
Ottawa, Ontario, Canada.

Abstract

In the last few years, the reference to the "safety goals" for nuclear power plants has become recurrent during the public hearings in Canada. On the international level, extensive work is being developed as this topic attracts the interest of the International Atomic Energy Agency (IAEA) as well as most of the regulatory bodies.

This presentation will shed some light on the definition of the safety goals, and will show how these are used to support the fundamental safety objectives which aim to protect people and the environment from harmful effects of ionizing radiation. The basis for the derivation of the safety goals on the Core Damage Frequency (CDF) and Large Early Release Frequency (LERF) (which are referred to as surrogate safety goals) will be included.

This presentation will also highlight the challenges related to the scope as well as the implementation of the safety goals in the regulatory framework for which, currently, there is no international consensus. Safety goals have been published in different ways spanning from legal documents to internal guides, and they can be applied as legal limits down to "orientation values".

Finally, this presentation will address the new challenge, as evidenced by Fukushima Daichi accident, regarding the development of acceptable representative risk metrics (goals) that include the integrated risks from multi-unit sites. In the current state-of-the-art of the PSA, it has been common practice in the application of the safety goals to consider the assessment of risk and the evaluation of the acceptability of the level of risk on an individual reactor basis. Such comparisons are now questionable for multi-unit sites, as the total risks from all the reactors on the site have not been taken into account. It is worth noting that the total risk of a multiunit CANDU reactor cannot be obtained simply by multiplying the CDF and LRF by the number of units as the contribution of some events will be double counted and the total risk overestimated.
The presentation will conclude with a high level discussion about the technical challenges in order to move from the current practice "PSA developed on an individual plant" to a "site integrated risk evaluation".