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IAEA Hosts Webinar on Expanding NPP Risk Assessment to Multi-unit Context

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Yonggwang Nuclear Power Plant, South Korea. (Photo: IAEA)

Current energy demand and the growing difficulty in establishing new sites for nuclear power plants are powerful incentives for the nuclear industry to utilize existing sites to construct new reactor units. Most nuclear power plants contain two or more reactor units, and all nuclear power plants have other radiological sources, such as spent fuel storage and radioactive waste management facilities.

Safety of a nuclear installation can be enhanced if the risks associated with those installations are assessed and kept at an acceptable level. Probabilistic Safety Assessment is an effective tool for developing risk insights necessary for risk informed, and performance based, applications as key elements for a risk management process.

The IAEA hosted webinar — Expanding NPP Risk Assessment to Multi-Unit Context — held in July, addressed the sustainable efforts for enhancing nuclear safety addressing multi-unit interactions for multi-unit sites, with advanced technologies such as Small Modular Reactors (SMRs). Over 300 participants attended the webinar, which provided a platform to share experiences and facilitate discussions in the field of risk assessment and safety of nuclear power stations with multiple reactor units.

“The multi-unit risk assessment is a wide-angle lens which allows us to look at the safety of reactors at the site level,” said Greg Rzentkowski, Director of the IAEA Nuclear Installation Safety Division. “It is important to understand these new insights in the context of safety of both operating NPPs and those which are still on a drawing board.”

George Apostolakis, Head of Japan’s Nuclear Risk Research Centre and former Commissioner of US Nuclear Regulatory Commission, Marina Röwekamp,  chair of the OECD /NEA Working Group on Risk Assessment from 2014 until 2020, and Karl Fleming, member of the USA National Academy of Engineering, were the lead webinar speakers. They shared their experiences, challenges faced and best practices, as well as lessons learned so far and during the implementation of this IAEA three-year project from December 2016 to December 2019.

The webinar also provided a forum to summarize the results of recent IAEA project on Multi-unit (MU) Probabilistic Safety Assessment implemented by IAEA Division of Nuclear Installation Safety.  “The IAEA methodology for Multi-unit Probabilistic Safety Assessment is a significant step forward advancing the state-of-the-art and hopefully the safety assessment state of practice in this area of importance,” said Apostolakis.

Specifically highlighted was the methodology on Multi-unit Probabilistic Safety Assessment developed at IAEA, which  can be applied to multi-modules and/or multi-unit SMRs and IAEA is ready to support Member States in that matter. The IAEA methodology shows how to deal with the high complexity of the probabilistic safety assessment  in a multi-unit context, considering various combinations of inter-unit interactions including  selection of the initiating events, modelling of complex accident sequences, addressing inter-unit correlations and human errors, considering shared systems and resources among units. The IAEA methodology was tested by conducting a realistic case study that illustrated how it works for a site with four units and suggests that larger number of units can be handled as well.

“The IAEA provided a methodology as a tool that can be used in case MU probabilistic safety assessment is required by the national regulatory authority,” said Shahen Poghosyan, IAEA Nuclear Safety Officer. Explaining other aspects of this project, he said: “For a realistic safety assessment of multi-unit sites is necessary to consider both positive and negative aspects related to sharing systems and resources among the units.”

The speakers discussed the challenges and benefits of multi-unit risk assessment including risk-informed decision making. “As far as safety improvements are concerned, looking to multi-unit probabilistic safety assessment framework gives you a balanced view, especially with regard to external hazards,” Fleming said. “ It helps to maximize the benefits and minimize the risks associated with sharing systems and resources among the units.”

“There is a need for continuous developments and improvements of Site-Level probabilistic safety assessments and corresponding guidance at international level, sharing results, lessons learned and involving cooperation between IAEA and OECD/NEA,” said Röwekamp.

Why are probabilistic safety assessments (PSAs) important?

Probabilistic safety assessments have largely been conducted on a single unit basis, treating each unit as completely independent. Acceptance criteria and risk metrics are also applied to single unit evaluations. This assessment can also be applied to a multi-unit facility, for example, the probability of preventing and mitigating an accident at one unit cannot be assessed without considering the status of the other units and on-site facilities, including spent fuel storage facilities.

The reactors and collocated radiological sources typically share a common electrical grid and ultimate heat sink, and in some cases share structures and systems that provide vital safety functions. Each site can be characterized by a set of internal and external hazards that could initiate a sequence of events which go on to challenge vital safety functions and to cause accidents involving one or more units. Internal initiating events are caused by human error and hardware faults. Internal events include internal fires and floods. Examples of external hazards include natural events, such as seismic events, external flooding from tsunamis, river flooding, storm surges, high winds and wind generated missiles, and events caused by humans, such as industrial and transport accidents.

The IAEA supports Member States in strengthening their safety processes through methods and tools to enhance nuclear safety for those countries considering multi-unit context. These efforts will also contribute to increase feasibility of deployment of advanced power reactors contributing to reduction of the CO2 footprint.

The recording of the webinar can be found here.

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