The Canadian Nuclear Safety Commission (CNSC) said on 11 August that it had completed its first collaborative activity under the Memorandum of Cooperation (MOC) on Advanced Reactor and Small Modular Reactor Technologies with the US Nuclear Regulatory Commission. The MOC, signed in August 2019, enhanced cooperation in areas of common interest, including aspects of reactor design, research, staff training, and the comparing of regulatory practices. A new joint report documents the results of collaboration concerning a request by US-based X Energy to obtain feedback about its Xe-100 reactor pressure vessel (RPV) construction code assessment.
During the official signing ceremony of the MOC in Ottawa, CNSC President Rumina Velshi had noted CNSC’s commitment to engaging in international cooperation activities that foster relationships, strengthen frameworks and share best practices to improve nuclear safety. “Globally, interest and advances in small modular and advanced reactors are growing rapidly. The CNSC and the US NRC are working together as regulatory leaders to ensure the development and deployment of these innovative technologies are done safely and efficiently,” she stated. USNRC Chair Kristine Svinicki said “Advanced technologies are emerging at a rapid pace, demanding that regulators keep in step with modernisation initiatives and the technologies of the future.”
In their joint conclusion to the report (dated June 2021) CNSC and USNRC said they had reviewed a white paper provided by X-Energy in June 2020 and had “concluded that X-Energy’s proposed approach for the design and fabrication of the Xe-100 RPV is viable, provided X-Energy includes additional technical justification and addresses the regulators’ observations.” The assessment concerned the construction codes the company would like to use for the RPV of its Xe-100 high temperature reactor design. X-energy hoped to meet the requirements of a certain code issued by the American Society of Mechanical Engineers (ASME) for the reactor design, but to fabricate it to the quality assurance and requirements of a different ASME code and stamp it as such.
CNSC emphasised that this is "informal" feedback which "does not result in any regulatory decision making”. The following key regulatory considerations and the main aspects identified in X-energy’s white paper, which provided the basis for the joint conclusions are:
- The RPV must be designed, fabricated, erected, and tested to quality standards commensurate with the safety functions it performs. The safety functions are fundamental elements of the RPV’s safety classification.
- The codes and standards proposed for use may be chosen and subsequently supplemented or modified as necessary to assure a quality product, provided that the level of safety is justified.
- The RPV must perform the safety function of maintaining core geometry, which ensures the following:
The decay heat removal flowpath is not disrupted;
Control rods can shut down the core and maintain the corein a stable shutdown state.
- The maintenance of the core geometry aids in the following aspects of the design, which also need to be satisfied:
The overall reactor design can meet acceptable radionuclide release criteria during the most challenging event sequences.
Normal operation release limits can be met.
Anticipated operational occurrences and design-basis accidents do not result in exceeding established dose acceptance criteria.
Quantitative and qualitative safety goals should be met.
- The deterministic and probabilistic safety analyses for the reactor design must support the decisions for the selected codes, safety functions, and classifications over the life of the reactor.
- Identified safety functions must be achieved in all operational states.
- The RPV design should include detail sufficient to demonstrate that all relevant design, manufacturing, and operational aspects have been adequately considered.
- The helium pressure boundary needs to be designed, manufactured, tested, and inspected to prevent brittle fracture and the rapid propagation of fractures.
- Air or water ingress into the RPV during pressure boundary failures that could lead to adverse chemical reactions needs to be considered.
The Xe-100 80 MWe high temperature gas-cooled reactor to be built in units of up to four. It is undergoing vendor design review with the CNSC, and is participating in the US Department of Energy's Advanced Reactor Demonstration Programme. Xe-100 is proposed for construction at both Energy Northwest's Columbia power plant in Washington state and at Ontario Power Generation's Darlington power plant.
