IAEA Approach to Review the Applicability of the Safety Standards to Small Modular Reactors

Introduction

A continuously increasing interest in nuclear power, and particularly in small modular reactor (SMR) technologies, has been expressed over the past several years by numerous IAEA Member States particularly in contributing to the mitigation of climate change. Today there are over seventy (70) SMR designs under development according to the IAEA booklet Advances in Small Modular Reactor Technology Developments, a Supplement to IAEA Advanced Reactors Information System (ARIS) [5]. While SMRs are typically designed to generate electric power up to 300 MW, many new designs are also designed for other heat production applications, such as district heating, desalination and hydrogen production or a combination of the former.

SMRs present novel features and innovative technologies, including different types of coolant, nuclear fuel and neutron spectrums and inherent safety features. As for all reactors, SMRs shall meet the objective of ensuring the protection of people and the environment from the harmful effects of ionizing radiation. A key element of meeting this objective is demonstrating compliance with fundamental safety principles and safety requirements.

The IAEA safety standards reflect a widely accepted extensive approach to ensure nuclear safety, establishing safety principles, requirements and associated guidance. The IAEA safety standards are Member States consensus based documents that have been developed in an iterative fashion and capture wider aspects of legal, regulatory, siting, design, construction, commissioning, operation, decommissioning, and release from regulatory control of nuclear facilities and radioactive waste management, including disposal. Although the IAEA safety standards are considered to be technology neutral, they are influenced by specific issues pertaining to water cooled large reactors technology as most of the experience and knowledge in the Member States contributing to the development of safety standards is based   predominantly the existing technology.

In recent years, the IAEA has undertaken various initiatives to review the applicability of certain safety standards to particular types of SMRs. Until now, however, the IAEA has not systematically assessed the applicability of the IAEA safety standards to all types of SMRs throughout their entire lifecycle. Therefore, the IAEA has developed and is implementing an approach for the identification of areas of non-applicability of the IAEA safety standards, to suggest compensatory measures to close any identified gaps, in order to address the needs of the Member States.

The Early Work

The current approach adopted by the IAEA builds on the early work performed for the preparation of the IAEA in TecDoc-1936 [1] establishing the engineering judgement necessary to apply the design safety requirements contained in SSR-2/1 (Rev. 1) [2] to light water cooled and high temperature gas cooled SMRs (LW-SMRs and HTG-SMRs).

A team of international experts between 2016 and 2018 (see TecDoc-1936 [1], pg 144-145 ) was asked by the IAEA to perform a pilot study to assess, with comments, the applicability of SSR-2/1 (Rev. 1) [2] to the aforementioned SMR technologies on the basis of the following criteria:

• Applicable as is
• Applicable with modification
• Applicable with interpretation
• Not applicable

The participants were also encouraged to include any recommendations for new criteria not covered in SSR-2/1 (Rev. 1) [2] and to provide technical rationale for their recommendations. Applicable with modification inferred the need for the text to be updated for the requirement to be applicable to the specific design, whereas applicable with interpretation inferred the need to modify definitions of pre-existing terminology to encompass the new technologies. The Member States’ contributions were merged and refined into one comprehensive working document to provide an overarching list of recommendations. The creation of this working document highlighted the need for the two SMR designs assessed to initially be discussed independently in reference to their applicability to SSR-2/1 (Rev. 1) [2].

The working document, in its final form, evaluated the applicability of each of the 82 design safety requirements established in SSR-2/1 (Rev. 1) [2] to LW-SMRs and HTG-SMRs. The initial findings from the LW-SMR working group were presented and published at an international IAEA conference in 2017 [4]. The working material developed in this manner was channelled into an evolving project report that forms the basis of Appendixes I and II and Annex I in IAEA TecDoc-1936 [1]. The IAEA officers responsible for the development of this pilot study report were Kristine M.. Madden and Ramsey Arnold of the Division of Nuclear Installation Safety IAEA. The results of the study were subsequently extensively used to prepare an IAEA official publication. The work was completed by a team of international experts (see TecDoc-1936 [1], pg 143) with IAEA responsible officer Palmiro Villalibre Ares of the Division of Nuclear Installation Safety in the form of TecDoc-1936 [1].

Current Applicability Review Approach

The ongoing review of applicability of the safety standards builds on previous experiences and aims to consider in a holistic manner whether the current requirements and recommendations for SMRs cover the safety issues related to the new possibilities opened by the novel designs or, on the contrary, if there are gaps that need to be addressed to ensure that the level of safety established by the IAEA fundamental safety principles will be complied with.

The level of safety defined by the Safety Objective and the Safety Fundamentals [6] is considered as the departing point for the study. The review therefore focuses only on the applicability of the requirements and recommendations to meet the overall high level of safety defined in the Fundamental Safety Principles: Safety Fundamentals (SF-1) [6].

It is also expected that key elements of the safety design approach remain applicable to these technologies although additional guidance in the implementation may be necessary, such as 

• Defence in depth
• The elimination of high radiation doses to workers
• The practical elimination of early release

It is also important to clarify that the scope of some SSGs may be wider than the scope of the safety requirements. These SSGs reflect the best practices in Member States. The lack of linkage with safety requirements may lead to additional consideration on how to define the design objectives in terms of acceptability of the consequences of novel accident scenarios.

To achieve this objective, the review approach follows four steps that are presented in detail in this publication to identify:

• the relevant safety standards to be considered in the review
• areas of novelty when compared to operating land-based water-cooled large reactors
• gaps in the applicability of the safety standards to SMRs, based on the areas of novelty identified.
• areas of where the safety standards may not be applicable or could be adapted for a better application to novel design to some SMRs, based on the areas of novelty identified.

Identification of the relevant safety areas to be considered in the review

The safety areas and topics considered in the applicability review are presented below.

Identification of areas of novelty

The first step of the applicability review is the identification of areas of novelty in the lifecycle of the SMR when compared with land based water-cooled large reactors.

The technologies considered as part of the review include transportable SMR, micro-sized reactors water cooled SMRs, and non-water SMRs (sodium fast reactors, lead fast reactors, high temperature gas cooled reactors, molten salt reactors).

The identification of areas of novelty is based on a systematic comparison of the characteristics of SMRs with a Light-water Cooled Reactor Reference defined as part of the project.

The characterization of these technologies in terms of areas of novelty is developed at the onset based on expert knowledge, literature review and detailed questionnaires responses by technology developers, reflecting designers’ current practices and claims. This information is then reviewed by regulatory authorities, technical support organisations and other organisations from member states participating in the project.

Identification of gaps in the applicability of the safety standards to identified areas of novelty

The areas of novelty identified are compared to the contents of the IAEA safety standards in terms of requirements and recommendations. Gaps in the safety standards may be associated to one or several of the following areas of novelty:

• New barriers, new safety functions or new provisions to deliver the safety functions.
• New failure mechanisms, faults, phenomena that could lead to the failure of a barrier or a provision to deliver a safety function.
• Eliminated failure modes, faults, phenomena by design which may imply the need for additional design requirements and/or recommendations.
• The use of new technologies, including new fuels, new coolants, novel safety provisions, novel materials and construction/ manufacturing techniques that could lead to the need for additional requirements and/or recommendations (e.g. recommendations for fuel qualification, qualification of materials to higher temperatures, use of more corrosive coolants, etc.)
• New operating, maintenance, testing, refuelling and/or wates management strategies.
• New facilities and/or activities on site and/or off site needed to support the construction, operation and/or post-operation management of the nuclear power plant.

To ensure exhaustivity, for each of the above areas, the potential gaps are closely examined to judge if existing requirements and recommendations are sufficiently overarching to address the specific differences. In some cases, additional and more detailed review may be necessary at a later stage to characterise and confirm the identified potential gaps.

Identification of areas where the safety standards may not be applicable to (some) areas of novelty

The areas of novelty identified are then compared to the requirements and recommendation in the safety standards. Areas of non-applicability in the safety standards may be associated to one or several of the following novelties:

• Failure modes, faults, phenomena typically considered in light-water cooled reactor reference that are not relevant to the design, which may imply some requirements and/or recommendations may not be applicable
• Safety functions and provisions to deliver these functions that are no longer needed which may imply some requirements and/or recommendations may not be applicable
• New operating, maintenance, testing, refuelling and/or post-operation management strategies which may imply some requirements and/or recommendations may not be applicable

When areas of non-applicability are identified, the review also considers:

• If there are associated gaps (not identified in the gap review)
• If there is sufficient experience to confirm that the relevant areas of the safety standards are not applicable. In some cases, claims may have not been confirmed by sufficient operating experience or there are still considerable uncertainties related to the areas of novelty. For these cases additional features may be needed in the design until sufficient experience is available. The potential non-applicability and uncertainty will be captured in the review.

Expected Outcomes

The IAEA expects to publish this work as a Safety Report providing a roadmap for the applicability of the IAEA safety standards to novel advanced reactors and particularly SMRs throughout their entire lifecycle. The large team of IAEA experts and international experts from member states supporting the development and implementation of the presented approach will be acknowledged in the Safety Report publication.

As a secondary outcome, it may be possible to further analyse gaps and areas of non-applicability to identify potential pathways for resolution and help to build a further programme of work to address areas of non-applicability and potential gaps.  

References

[1]       INTERNATIONAL ATOMIC ENERGY AGENCY, Applicability of Design Safety Requirements to Small Modular Reactor Technologies Intended for Near Term Deployment: Light Water Reactors and High Temperature Gas Cooled Reactors, IAEA Technical Document No. 1936, IAEA, Vienna (2021).

[2]       INTERNATIONAL ATOMIC ENERGY AGENCY, Safety of Nuclear Power Plants: Design, IAEA Safety Standards Series No. SSR-2/1 (Rev. 1), IAEA, Vienna (2016).

[3]       INTERNATIONAL ATOMIC ENERGY AGENCY, IAEA Safety Glossary: Terminology Used in Nuclear Safety and Radiation Protection (2007 Edition), IAEA, Vienna (2007).

[4]       K. Madden et al. “Applicability of IAEA Safety Standard SSR-2/1 to Water Cooled Small Modular Reactors”. Proceedings of an International Conference organized by the International Atomic Energy Agency held in Vienna, 6 – 9 June 2017: Topical Issues in Nuclear Installation Safety: Safety Demonstration of Advanced Water Cooled Nuclear Power Plants, V2. IAEA, Vienna (2018).

[5]       INTERNATIONAL ATOMIC ENERGY AGENCY, Advances in Small Modular Reactor Technology Developments, a Supplement to IAEA Advanced Reactors Information System (ARIS), IAEA Booklet, 2020 Edition, IAEA, Vienna (2020).

[6]       INTERNATIONAL ATOMIC ENERGY AGENCY, Fundamental Safety Principles, IAEA, Vienna (2006).