IntroductionThe events at the Fukushima-Daiichi nuclear power plants (hereafter Fukushima) which were caused by the March 2011 earthquake and tsunami off the coast of Japan have been the subject of review by nuclear and regulatory organizations in Canada and around the world.
One of the cornerstones of the NWMO’s Adaptive Phased Management, Canada’s plan for the long-term management of used nuclear fuel, is a commitment to continuous learning from new developments, and adapting and refining its plans. The NWMO has considered the events in Japan and has examined the potential implications of Fukushima on the implementation of Adaptive Phased Management.
The overall findings from the NWMO’s review are summarized below:
What are the “lessons learned” based on the NWMO’s review of the events at Fukushima that are applicable to the implementation of Adaptive Phased Management?The safety of the Adaptive Phased Management deep geological repository and used fuel transportation system is built upon a system of natural and engineered barriers which are designed to contain and isolate used nuclear fuel from people and the environment. The robustness of the transportation casks, used fuel containers and repository sealing systems, plus the stability of the geosphere over time, protects against natural events such as earthquakes, climate change and extreme events like glaciation.
The events at Fukushima were a strong reminder that we need to ensure that we consider a wide range of extreme events in our designs and safety analyses, and that we have an obligation to deal with uncertainty in our approach to long-term management of used nuclear fuel. Our path forward will be refined as we gain new knowledge and understanding.
The lessons learned from events at Fukushima for the implementation of Adaptive Phased Management include:
- Beyond-design-basis accidents need to be carefully assessed by the NWMO and properly reflected in the engineering design, construction and operation of a deep geological repository and used fuel transportation system.
- Safety analyses and emergency response planning need to consider a full range of low-probability, high-consequence events.
- The potential impact of extreme natural events is more relevant during operation of the facility rather than during the postclosure period since major events are unlikely to affect conditions at repository depth.
- The multi-barrier, defence-in-depth approach to used fuel management is a key component of the design to mitigate against the propagation and impact of low-probability, high-consequence events.
How can Canadians be assured that a used nuclear fuel repository will withstand an earthquake?The principal objective of the Adaptive Phased Management deep geological repository is to safely contain and isolate used nuclear fuel in a stable rock formation. The repository has been designed to protect people and the environment against a wide range of natural events, like earthquakes, in a passive manner without relying on active maintenance or active safety features such as backup power or cooling systems.
It is well-known that deep underground structures are more resilient to earthquakes than surface structures. Long-lived used fuel containers will be sealed in a deep geological formation using a multi-barrier system at a nominal depth of about 500 metres.
As well, the selection of a safe site for the repository is based on a comprehensive set of geoscientific criteria and a thorough site evaluation process which is expected to take seven to 10 years. The potential impact of earthquakes is a key component of the site evaluation process.
Current and future seismic activity is a critical technical evaluation criterion for selecting a site to safely host Canada’s deep geological repository for used nuclear fuel. To be selected, a site will have to satisfy a comprehensive set of geological siting criteria which include looking at active faults and seismicity. Any candidate site will be excluded if there are geological conditions that would make it unsafe.
Experience in Canada and internationally tells us that a good predictor of future performance is what has happened in the past. There are many deep geological formations in Canada that have remained stable for hundreds of millions of years despite numerous past events such as earthquakes and glaciation.
The Adaptive Phased Management deep geological repository will be designed and built in a manner that will ensure the integrity of the multi-barrier system for containing and isolating used nuclear fuel.
The deep geological repository and associated surface handling system will be subject to an environmental assessment and regulatory process by the Canadian Nuclear Safety Commission to ensure that it can be implemented safely.