Safety By Design

Beneath the stands of the University of Chicago’s athletic field, the first self-sustaining nuclear chain reaction transpired in 1942. In a wooden frame, graphite blocks interspersed with uranium comprised the ‘pile’ — the nuclear reactor. Above, a control rod hung on a rope, and a man wearing protective clothing stood by, ready to chop the rope with an axe if anything were to fail. The rods would fall into the reactor core, shutting down the chain reaction. The man personified the world’s first nuclear safety system.

In the succeeding decades, safety has influenced the evolution of reactors, from prototypes in the 1950s and commercialized power reactors in the 1960s to advanced designs that appeared in the 1990s. A far reach from that original axe man, today’s reactors feature designs and systems that ensure a high level of safety.

The new generation of nuclear reactors includes some reactors already in operation and reactor designs that have yet to be deployed. The IAEA distinguishes advanced nuclear reactors as evolutionary or innovative, both of which incorporate lessons learned from the 2011 Fukushima Daiichi nuclear accident. Evolutionary reactors improve existing designs, maintaining proven design features, while innovative reactors use new technology.

Most evolutionary reactors are available on the market and are already connected to the grid. These reactors’ underlying safety approach is based on applying an enhanced defence-in-depth strategy, compared to conventional reactors, supported by increasing emphasis on inherent safety characteristics and passive features and decreasing reliance on the operator’s intervention to minimize the risk of accidents.

Innovative reactors incorporate radical changes in the use of coolants, fuels, operating environments and system configurations. Some innovative concepts are being considered for deployment in the next 10 to 20 years.

“From the technology viewpoint, [innovative reactors] are very different because, typically, they do not use water as a coolant,” said Stefano Monti, Head of the Nuclear Power Technology Development Section at the IAEA. From the physical viewpoint, he added, the different coolant also changes the way in which heat is extracted and the way in which the nuclear fission reaction is produced and maintained.

Advanced fast neutron reactors that are sodium, lead and lead–bismuth or gas cooled, for example, use much higher-energy neutrons to cause fission. Fast neutron reactors are designed to improve fuel efficiency and therefore reduce high level radioactive waste. “From the safety perspective, the risks associated with their operation are very low, owing to the reduction in both the likelihood and the radiological consequences of accidents,” said Vesselina Ranguelova, Head of the Safety Assessment Section at the IAEA. The IAEA’s Advanced Reactors Information System provides detailed technical and safety information for all these types of advanced reactors.

The world’s first advanced small modular reactors (SMRs) were deployed last year in Russia, and many innovative SMRs are under development for near-term deployment. Globally, there are about 70 SMR concepts and designs, with two at advanced stages of construction in Argentina and China.

Lessons learned from the Fukushima accident led to significant strengthening of international safety requirements, which are to be reflected in the design of advanced reactors so that the likelihood of occurrence of an accident with serious radiological consequences is extremely low and the radiological consequences, should an accident occur, are practically eliminated. (To learn more about the Fukushima Daiichi accident, see Fukushima Daiichi: The accident.)

The proof of concept for SMRs requires the vendors to demonstrate the effectiveness of the fundamental safety functions — reactors control, core cooling and confinement of reactivity — based on the development and evaluation of the defence-in-depth strategies.

As an example, US-based NuScale Power has designed a modular light water reactor that integrates components for steam generation and heat exchange into a single unit, expected to be deployed in 2027. “The major safety challenge present in the existing nuclear fleet centres around the ability to remove residual (decay) heat and keep the reactor cool,” said Carrie Fosaaen, Director of Regulatory Affairs at NuScale Power. “The overall design of the NuScale power plant incorporates simpler systems, which preclude the need for the complex configurations currently required in existing nuclear facilities.”

Given the nature of innovation, the introduction of passive and other innovative safety features poses a regulatory challenge. Regulators are tasked with verifying designers’ claims of safety, which may require additional research and analysis to evaluate novel designs.

“To demonstrate design safety, a comprehensive assessment of all plant states — normal operation, anticipated operational occurrences and accident conditions — is required. On that basis, the capability of the design to withstand internal and external events can be established and the effectiveness of safety features can be demonstrated.” Ranguelova said. “While the innovative designs are promising, they must be complemented with a regulatory body’s sound safety assessment and licensing process that supports their utilization and deployment.”

The IAEA is assessing the level to which existing IAEA safety standards can be applied to innovative technologies. “Our safety standards are technology-neutral. However, they have mostly been developed using the operational experience of reactors, which are mostly water cooled reactors,” Ranguelova added. Though the standards are neutral in principle, implementation may differ for some or all types of SMRs.

“There are gaps where we will need to develop additional guidance or supporting documents to allow for the application of these standards to innovative technologies,” Ranguelova said. The IAEA expects to publish a Safety Report on the applicability of IAEA safety standards to SMR technologies in 2022.