SUSTAINABLE DEVELOPMENT

Modularity Of SMRs Might Have Safety And Security Implications

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KARINE HERVIOU tells Elodie Broussard at  the SMR Regulators’ Forum new safety recommendations for SMRs are now available online, following a meeting of experts working on the development of national standards specific to SMRs.

ELODIE BROUSSARD: What kind of SMR design specificities may impact safety?

KARINE HERVIOU: The modularity and compactness inherent to SMRs may have safety implications and introduce new challenges to the SMR lifecycle, associated with design, construction, commissioning, operation and decommissioning stages.

ED : What does the term “modular” refer to?

KH: It refers to the way SMRs are built (“modular construction approaches”) or to the capability to allow additional power units on the same site or in the same infrastructure (so-called “modular design approaches”). The SMR Regulators’ forum is considering both definitions of modularity as both may have safety implications.

ED: What are the safety implications that could arise from the use of modularity in SMRs?

KH: Multi-module SMR designs may have certain operational and safety benefits, such as interconnections between modules to strengthen the availability and reliability of support services (electric power, compressed air, water) or qualified personnel. On the other hand, the current operational experience with multi-unit nuclear power plants indicates that they may require specific consideration for nuclear safety, emphasized by the lessons learned from the multi-unit Fukushima-Daiichi nuclear power plant accident. The use of shared systems may for instance introduce risk of vulnerabilities in the design, along with dependencies among the facilities. In other words, when one module faces a safety problem, it should not have any bearing on the other modules.

ED: How can SMR designers improve safety?

KH: The SMR Regulators’ Forum considers that it would be beneficial for both designers and regulators to think beyond the single unit mindset. This might involve extending their considerations to whole site risk including developing methods of aggregating risk from differing on site sources. For instance, for maintenance of modules, consideration should be given to ensure that a hazard in a module under construction, in maintenance or in operation would not have any safety consequences for neighbouring operating unit or the safety consequences are properly considered by designing provisions to mitigate such hazard (for example a protective shell or constraints on the operation of neighbouring modules during introduction of a new module or during sensitive activities).

ED: What are the safety implications related to the compactness of SMRs?

Ka: Most SMRs are designed to be compact in order to enable their manufacture at a factory and to facilitate their transportation to the plant site. However, the compact nature of SMRs may prove challenging when it comes to performing the necessary inspections, operations and maintenance, not only during the manufacturing stage, but for the entire life cycle of the SMR. For instance, inspection and non-destructive examination of the welds of components are necessary to check the quality of the manufactured items.

ED: Who could contribute to address this situation?

KH: Manufacturing processes would need to be properly coordinated to allow accessibility for the necessary inspections and examinations. Vendors should also need to consider up-front, how inspections would be performed during the operation of the reactor and discuss accessibility with potential licensees and regulators prior to finalizing such designs. Moreover, the development of specific control devices may be requested.

ED: What are the safety challenges faced within the SMR lifecycle licensing framework?

KH: New challenges to the SMR lifecycle regulatory framework are mainly associated with the construction, commissioning and decommissioning stages, especially for multi-module SMRs. In addition to novel design features and approaches, SMR projects may indeed introduce several differences to a new-build projects, ranging from factory manufacturing and testing, to new construction and commissioning methods. These may impact potential stages for SMR licensing and pose challenges to the traditional view of the licensing approach that considers subsequently the following high-level stages of activities: siting and site evaluation, design, construction, commissioning, operation, decommissioning and release from regulatory control. For instance, all activities associated with the project, including the impact of construction and operation of multiple modules (or units) on a single site, should be considered in the licence application.

ED: What are the implications of shifting more of the manufacturing and construction from site to factories, as it is the case for SMRs?

KH: This may change how and where initial plant tests are conducted, as compared to conventional nuclear power plants. Many tests that were previously initial plant tests conducted at the site may now be factory tests, conducted by the supplier. Since the Licensee remains responsible for the design and construction of the SMR, the Licensee would have to provide enough oversight of any factory tests performed as part of initial plant testing.

  • Karine Herviou is the Director at the Institute for Radiological Protection and Nuclear Safety (IRSN, France) and member of the Forum, to present these latest recommendations.

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