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Probabilistic Safety Assessments Help Strengthen Safety at Nuclear Power Plants


The nuclear power plant in Flamanville, France (Photo: A. Morin/EDF)

One of the key factors contributing to continuous safety improvement at nuclear power plants is a comprehensive safety assessment, factoring in insights from the probabilistic safety assessment to ensure that accident sequences leading to large or early radioactive releases are practically eliminated.

“The risk perspective, gained from a probabilistic safety assessment, is used to evaluate and optimize the overall reactor defence-in-depth strategy, including design extension conditions, to prevent any such radioactive releases,” said Greg Rzentkowski, Director of the IAEA’s Division of Nuclear Installation Safety, at a recently held IAEA technical meeting on this specific topic.

90 participants from 30 countries and two international organizations shared experiences and applications related to regulatory practices, industry developments and supporting research and development studies on level 2 probabilistic safety for nuclear power plants.

“The establishment of the probabilistic criteria and safety goals lies under the responsibility of national regulatory bodies,” said Smain Yalaoui, a senior regulator from the Canadian Nuclear Safety Commission.

Safety from the start- what is a probabilistic safety assessment?

A probabilistic safety assessment is a systematic and comprehensive approach to evaluate complex technological infrastructures at nuclear power plants. The objective of such assessments is to understand and evaluate the risks, examining the design and operation of components at nuclear power plants. The key goal is to ensure and demonstrate the safety at nuclear power plants.

A nuclear power plant in Novovoronezh II, Russian Federation (Photo: A. Sukhonin/ASE)

There are three levels of a probabilistic safety assessment. Level 1 evaluates what failures can result in severe damage of reactor core or the stored fuel in the spent fuel pool, and this includes internal events such as equipment failures, human errors and internal and external hazards such as an internal fire or an earthquake.

Level 2, which was the focus of this recent IAEA meeting, covers the assessment of the potential failures of a reactor’s containment building leading to radiation risks such as contamination due to radioactive releases that can impact people and the environment. At this level, the assessment aims at determining the frequency, the nature and the magnitude of release of radioactive material from a severe accident. It involves the modelling of a great number of complex phenomena, such as molten core cooling both inside and outside of the reactor pressure vessel, steam explosion, hydrogen combustion and radionuclides transport, and the assessment of uncertainties from such situations.

Level 3 aims at assessing off-site consequences, such as radiation doses to the public and economic impact, due to the radioactive releases.

Participants learned that almost all nuclear power plants currently in operation and those under construction have already developed a probabilistic safety assessment up to the level 2.

“Although, probabilistic safety assessment, is often subject to considerable uncertainty, the results can effectively be used to compare different options or different design approaches to inform decisionmakers about appropriate defence-in-depth measures,” said Rzentkowski. “This methodology provides sufficient guidance to improve existing facilities and to implement safety improvements to reduce potential risks.”

Factors of importance

Participants also discussed the subjects considered for the revision to the current IAEA safety standard guide SSG-4  for development and application level 2 probabilistic safety for nuclear power plants.

Among the considered topics was the importance to address all sources of radioactive material on site, including the spent fuel pool; the definition of safety goals; the impact of multi-unit onsite; the use of portable equipment; the updated data from the research conducted on severe accident phenomena and human factors.

General objectives to use probabilistic studies contributes to ensuring that the severe accident management provisions and guidance, used to cope with potential accidents, are robust enough to adequately protect the population,” highlighted Emmanuel Raimond, the Head of Probabilistic Safety Assessment and Accidental Procedures Department at Institute for Radiological Protection and Nuclear Safety, (IRSN), France.

The IAEA support to Member States in this vital area of safety assessments for nuclear power plants was also reaffirmed during the deliberations. “We will continue to support Member States to excel in probabilistic safety assessment models and techniques, to ensure that results of this type of evaluations are used in adequate manner to support the safety improvement of nuclear power plants,” said Vesselina Ranguelova, Head of the IAEA Safety Assessment Section.

“The evaluations will help also the design process for new generation of such facilities, in particular innovative technologies were the lack of operational experience, is obvious – and for which we rely  a lot on such techniques  and computer simulations to confirm that the new designs are prepared in a way that will ensure long safe operation of those reactors,” Ranguelova added.

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