Introduction

A nuclear fusion power station will be constructed out of materials that will be used to develop components that will perform multiple functions. These functions could be structural, magnetic, thermal, nuclear, fuel production, optical, or radiation shielding, for example. If the failure of any (or multiple) of these functions leads to a safety concern to the environment, workers, or members of the public, then a national regulator will impose assessments to determine that adequate mitigation schemes are in place to either reduce or remove the hazard.

A method used in the nuclear industry [1] to scrutinise a component’s safety function is the safety classification of Structures, Systems and Components (SSCs). The classification system has requirements proportionate to the manner in which the SSC is supposed to behave in the eyes of the owner. Safety classification of SSCs within fusion is important as it determines the quality of design and manufacturing requirements for each component within the system [2]. Whilst there are many approaches to achieve fusion (for example Magnetic-Confinement Fusion (MCF) and Inertial-Confinement Fusion (ICF)), all designs must be developed to reduce the risk of a radiological and/or chemical accident. The approach to classification is a top-down process starting with an understanding of the fusion reactor design, its safety analysis (such as normal, accident and unmitigated scenarios) and how the main functions will be substantiated. Once the classification of each SSC in the system has been conducted, a complete set of engineering rules for design should be specified. Without classification, the detailed design cannot be substantiated, and materials cannot be qualified. Classification leads to an understanding of the following:

  • the level of quality control needed on materials.

  • which “best practice” methods are appropriate (such as codes and standards).

  • The level of analysis required when considering the degradation caused by environmental factors, such as radiation damage, temperature, magnetic fields, and shock.

Private fusion companies will face the challenge of safety classification systems in the coming years because of the pace at which these companies wish to demonstrate commercial fusion power. The first step is to outline the high-level safety, fusion, and commercial goals of the first-of-a-kind (FOAK) fusion reactor. These safety goals will be used to begin a classification system breakdown, at a high-level, in order to understand how each SSC relates to one another before determining any safety classification system. The national regulator should set safety expectations (i.e., goals) that owners need to strive to achieve. Once these have been established, the beginnings of a safety classification system can begin with the evolving FOAK designs. It is important that the doctrine of design must be flexible in order to allow for the incorporation of a safety classification system.

This paper discusses the necessity of codes and standard in fusion energy and their critical role in enabling the commercialisation of fusion energy. Specifically, the paper will discuss the latest developments for fusion energy in the American Society of Mechanical Engineers (ASME)’s code and standard, as this has a focus on private fusion development.

Codes and Standards

A standard is a framework of agreements to which all relevant parties in an industry or organisation must adhere to, to ensure that all processes associated with the creation of a good or performance of a service are performed within set guidelines. Standards are not enforceable by law but are instead the socially and structurally accepted norm. Conversely, a code is a standard that has been adopted by one or more governmental bodies and could be enforceable by law. Structural integrity is concerned with designing and operating products that are safe, incorporating a thorough and complete understanding of the loading parameters and the environment they will encounter, underpinned by complete knowledge of the mechanisms by which the materials concerned will fail if their limits are exceeded.

The purpose of codes and standards is to establish national or international criteria based on state-of-the-art knowledge, experience, and experimental feedback from nuclear facilities to ensure structural integrity is maintained [3]. The design and construction of any nuclear fusion reactor should make use of appropriate codes and standards to provide reassurance and quality control for the structural integrity and safety of these plants. The codes provide the bridge between suppliers, participants, researchers, designers, manufacturers, and regulators. The documents containing codes and standards are considered live documents that are updated as improved operational experience, knowledge, and scientific advancement is made available [4].

Nuclear codes and standards provide rules for the design and manufacture of components within nuclear facilities such as pressure vessels, piping, pressure retaining portions of structures as well as pumps, valves, heat transfer systems, and support structures and confinement structures. These rules also contain requirements for quality assurance, materials specifications, design, fabrication, testing, examination, inspection, certification and stamping. Further, the rules provide provisions on how to accommodate the degradation of materials, such as fracture, high temperature and cyclic operations.

On the international scale, there are two main standards that are relevant to the mechanical components of a fusion reactor; these are the ASME Boiler & Pressure Vessel Code (BPVC) and French Association for Design, Construction and In-Service Inspection Rules for Nuclear Steam Supply System Components (AFCEN) RCC/RCC-MRx.

The ITER Structural Design Criteria for In-vessel Components (SDC-IC) [5] has been under development for the international project. SDC-IC is not a code but rather a design criterion for the in-vessel components and specifically sets out how to use ASME and RCC appropriately for design and construction [6]. SDC-IC sets out how to achieve the Essential Safety Requirements (ESR) set by the nuclear authority in France with the use of RCC-MRx in rules for the design, materials, fabrication, inspection, testing and marking amongst other aspects.

If the failure of a structural material can lead to the failure of a certain function being carried out by an SSC which could impact the environment or members of the public, then a regulator will require qualification of those structural materials. For a commercial fusion reactor, the frontrunning candidate materials are currently seen to be silicon carbide (and composites), reduced-activation ferritic-martensitic steels, oxide dispersion strengthened steels, vanadium-based alloys, and/or nickel-based alloys [7]. The purpose of materials qualification is to ensure the material properties are well understood in both a virgin and degraded state (due to the environment, such as radiation, temperature, chemical, and/or cyclic effects) in order to design for mitigation of failure.

As the existing nuclear codes and standards for construction do not adequately cover the design, manufacture or construction of fusion energy devices that are currently being considered for future constructions, the necessity for the development of such codes and standards is being discovered.

ASME BPVC

Section III Division 4

The ASME BPVC Section III Division 4 committee is considered to be leading the development of codes and standards for commercial fusion energy. The goal of ASME BPVC Section III Division 4’s committee is to develop a recognised fusion construction code and standard to be issued by ASME [8]. Division 4 was issued in November 2018 as a Draft Standard for Trial Use of proposed code rules entitled “Rules for Construction of Fusion Energy Devices” ASME FE.1-2018 for 3 years [9]. A roadmap for Division 4 has been published [10]. This new construction code would be used in the United States of America and/or globally as an acceptable basis for nuclear regulators for the construction, licensing and operation of new fusion power stations.

ASME BPVC Section III covers the design of pressure boundaries that contain nuclear material, whereas ASME BPVC Section VIII covers the design of non-nuclear pressure boundaries. ASME BPVC Section III Division 4 is therefore currently concerned with the design of pressure boundaries that contain nuclear material, such as a vacuum-vessel or in-vessel components of a tokamak fusion reactor [11].

The fusion SSCs that will require design rules to be developed under Division 4 include:

  • Vacuum vessels.

  • Cryostats.

  • Resistive / superconductor magnet structures.

  • In-vessel Components (Divertors, Breeders, First-wall tiles) and their interaction with each other.

  • Other related support structures, including metallic and non-metallic materials, containment or confinement structures, piping, vessels, valves, pumps, and supports will also be covered.

The latest updates on ASME BPVC Section III Division 4 are that the code recognises that there have been significant advancements within fusion over the last decade and > 4.8 billion USD in capital has been invested into private fusion companies so that Q > 1 can be demonstrated by 2025 [12]. As multiple fusion concepts have received investment, a fusion code and standard is being designed to provide common rules and good practice across the industry. The latest changes in the draft standard are as follows:

  • Acknowledges the range of approaches to fusion which include MCF and ICF, as well as MTF (magnetized target fusion).

  • Provides the pathway for future edits to develop the code over the decades.

  • Preparation for ASME acceptance as a new Division within Section III.

The organisation of the ASME BPVC Section III Division 4 is found in Fig. 1. ASME BPVC during the November 2022 “Code Week Meeting” voted to publish Division 4 Draft Standard as a full Division 4 under Section III within the ASME BPVC 2023 edition. This strong endorsement from ASME provides the foundation to develop the fusion code and standard to deliver for the fusion industry.

Fig. 1
figure 1

The organisation chart of the ASME BPVC Section III Division 4 committees, its working groups (WP), and special WPs.

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ASME Fusion Stakeholders

The Special Working Group for Fusion Stakeholders (SWGFS) subcommittee was formed in November 2021 by the author. The aim of the subcommittee is to provide a venue for stakeholders to voice their needs and development direction, provide comments and suggest input on the development of rules for the construction of fusion energy devices within ASME Section III, Division 4 ‘Fusion Energy Devices’ code. SWGFS will identify the research and development efforts required to support the technical development of the code rules within other subcommittees. In addition, the subcommittee interfaces with BPVC XI Division 2 on Inservice Inspection applications.

The stakeholders of the committee membership seek to have input from private fusion companies (potential owners of fusion power stations), operators, the fusion supply chain, national regulators, national laboratories, governmental bodies, and universities to understand their needs with regards to codes and standards.

Path Forward and Conclusion

The predicted fusion operational environments reveal that the neutron fluxes (and fluences) are orders of magnitude higher than experimental fusion devices, thus placing greater emphasis on radiation resistant materials and high temperatures. In addition, if fusion power stations seek to operate continuously (either “always on” or under high frequency pulsed operation), this will drive up the demands on SSCs in such a harsh environment. It is these conditions that existing nuclear codes and standards for construction do not adequately cover for the design, manufacturing or construction of future fusion power stations currently being considered.

Therefore, on the pathway towards fusion commercialisation, there must be a clear plan from private fusion companies on incorporating the relevant code and standards into their designs. This process will identify the gaps in these codes; the needs are what fills these gaps. However, the development of these codes will not happen in isolation. The fusion community will need to work collectively and collaborate on non-IP critical routes to enable the quickest path towards meeting the expectations of any future fusion regulator on safety critical components.

The classification process of SSCs in any fusion power station is the first step in the path forward in understanding the needs for codes and standards. The classification process underpins the entire understanding of the fusion reactor and how each SSC operates relies on each other. Consequently, the failure pathways can be drawn up to determine which components need the greatest level of quality control and assurance in order to achieve the As Low As Reasonably Achievable/Practicable (ALARP/ALARA) approach to safety around the world. With this, an owner, designer, or licensor of fusion power stations will understand the criteria for materials, developing appropriate and proportionate design rules, and quantifying the radiological inventory and risk to the public.

The necessary codes and standards may not be obvious today with the experimental fusion reactors (as these needs are met), but for tomorrow’s fusion power stations, it is a need that must be filled.