Public summary – Containment for Novel Reactor Designs

Background

One of the fundamental safety functions for nuclear power plants is the confinement function. In the IAEA Nuclear Safety and Security Glossary 2022 (Reference 1), confinement is defined as:

Prevention or control of releases of radioactive material to the environment in operation or in accidents. Confinement is closely related in meaning to containment, but confinement is typically used to refer to the safety function of preventing the “escape” of radioactive material, whereas containment refers to the means for achieving that function.

Containment is defined as “methods or physical structures designed to prevent or control the release and the dispersion of radioactive substances” (Reference 1) and is a significant contributor to meeting safety objectives and keeping radiation exposures below prescribed dose limits and as low as reasonably achievable. In addition to the confinement of radioactive substances in operational states and in accident conditions, containment systems traditionally contributed to the protection of the reactor against natural external events and human-induced events and to radiation shielding. A traditional reactor containment system includes a structure that sits on a thick concrete and rebar foundation with steel-reinforced concrete walls and an interior steel liner plate. In Canada, large pressure-retaining, steel-lined, steel-reinforced concrete structures have been used to meet the confinement function of existing CANDU reactors.

This is a public summary of a CNSC report on Containment for Novel Reactor Designs.

To obtain a copy of the full report, please contact us at cnsc.info.ccsn@cnsc-ccsn.gc.ca.

Issue

There are a number of areas in which proposed small modular reactor (SMR) designs may challenge the CNSC’s current regulatory expectations. One of those areas is the how the confinement function is achieved. All reactor designs need to ensure that all safety functions, including the confinement function, are adequately implemented. These functions may be performed by barriers other than what would normally be referred to as a “traditional” containment system. In some novel designs, the expected contribution of the different barriers of the containment system to the fulfilment of the safety functions is different than for traditional reactors. In these new designs, fuel may be considered the dominant contributor to the confinement function, and less importance may be placed on the containment structure. Multiple barriers are put in place to control the release of radioactivity to the environment and to ensure that the related design conditions are not exceeded.

The CNSC conducts regulatory assessments of the confinement function of proposed SMR designs based on applicable regulatory requirements and expectations. The CNSC applies technical requirements in a risk-informed manner to ensure that safety objectives are met and carries out the review of a project commensurate with its risk, complexity and novelty.

CNSC staff have completed a review of existing regulatory requirements for the confinement function to make recommendations on how they will assess novel means of containment approaches from vendors and applicants as novel designs are introduced.

Current CNSC guidance

Regulatory document REGDOC-2.5.2, Design of Reactor Facilities, sets out requirements and guidance for the containment design of new nuclear power plants.

Some of the high-level objectives of containment are to:

• minimize the release of radioactive materials to the environment during operational states, design-basis accidents, and to the extent practicable, beyond-design-basis accidents
• ensure that pressure and temperature are managed

What vendors are proposing

The CNSC has been approached by vendors of novel designs who are proposing different ways of achieving confinement functions, using non-traditional means of containment (for example, they are not designed to be leak-tight or for pressure/energy management).

Guidance to meet high-level containment objectives

As a risk-informed and performance-based regulator, the CNSC performs objective interpretations of current containment requirements as they would apply to novel designs being proposed by vendors and future applicants. If a reactor design other than a water-cooled reactor is to be considered for licensing in Canada, the design would be subject to the safety objectives, high-level safety concepts and safety management requirements associated with REGDOC-2.5.2. This is consistent with section 9 of REGDOC-2.5.2, which recognizes that alternative approaches to meeting requirements can be proposed. In such a case, the CNSC would be required to make regulatory interpretations to ensure that the novel designs result in an equivalent or superior level of safety while meeting the high-level intentions of existing containment requirements.

In performing their analysis of and making recommendations on containment for novel reactor designs, CNSC staff also considered IAEA and international (U.S. Nuclear Regulatory Commission and U.K. Office for Nuclear Regulation) regulatory experience with the challenges of containment requirements for novel designs.

To help achieve technology-neutral requirements, the following conclusions were noted:

• The CNSC definition of containment needs to be clarified to avoid the implicit assumption that all containment means must be structures made of leak-tight concrete.
• Most REGDOC-2.5.2 clauses are already technology-neutral and can be used for most types of reactor designs.
• A limited number of REGDOC-2.5.2 clauses apply exclusively to water-cooled reactor designs.
• Some of the existing containment requirements may be addressed by structures, systems and components (SSCs) other than a containment structure. As an example, section 6.6.2 of REGDOC-2.5.2 states: “The containment structure shall protect systems and equipment important to safety in order to preserve the safety functions of the plant.” While this is an important requirement, it can be addressed by SSCs other than the traditional containment structure.
• Some of the technology-neutral interpretations of REGDOC-2.5.2 can be addressed through the safety case presented by a vendor.
• One of the main challenges requiring the consideration of higher-level objectives has been means of containment that deviate from the traditional containment structures. Relevant clauses include those related to:
  • containment structure
  • containment penetration
  • containment isolation
  • leakage
• Some requirements in REGDOC-2.5.2 were written specifically for water-cooled reactor designs and do not lend themselves to being rewritten to be technologically neutral. These would not be applicable to many new reactor designs.
  • As an example, many requirements for traditional water-cooled containment structures relate to containment penetrations for SSCs such as electrical wiring and piping. These requirements are important for traditional containment structures but may not be applicable to some new designs that rely more heavily on fuel, for example, to achieve their confinement goals.
• Containment is a large part of the defence in depth concept as defined in REGDOC-2.5.2. It is one of the barriers credited in Level 3 and Level 4 defence in depth. Defence in depth sets expectations for the presence of a series of barriers that, depending on the objective, can prevent or minimize the consequences of radioactive releases. As part of defence in depth, barriers must be independent to the extent practicable. For some novel designs, justification will be needed from prospective applicants to support their safety case, especially for defence in depth levels 3 and 4.

Summary

This summary is meant to support SMR readiness by informing and providing guidance to proponents and the public about containment systems. CNSC staff’s position is that many of the overall high-level requirements and intentions of the CNSC regulatory requirements on containment are applicable to all reactor designs. Containment requirements will continue to be further refined in a future version of REGDOC-2.5.2, Design of Reactor Facilities: Nuclear Power Plants.

The CNSC will continue its efforts to provide additional internal and external guidance, updating the CNSC glossary terms for containment and revising regulatory documents.