CANDU Fuel Safety Criteria: A CNSC Perspective
Extended abstract of the technical presentation presented at:
13th International Conference on CANDU Fuel, Kingston, Ontario
August 15–18, 2016
Canadian Nuclear Safety Commission
The fuel safety criteria concept and its role in ensuring nuclear safety
The ultimate goal of nuclear power plant safety is prevention of radiation-related harm to the workers at the plant, the public and the environment. The application of defence in depth (DID), throughout the design process and operation of the plant, is a cornerstone of the Canadian regulatory philosophy.
DID is implemented first by means of successive physical barriers to the release of radioactive material. The nuclear fuel matrix and the nuclear fuel sheath constitute the first two physical barriers. The other two physical barriers in a reactor are the primary heat transport system (HTS) pressure boundary and the containment. The integrity of these physical barriers is protected by applying the concept of DID to them. This is achieved by ensuring effective performance of the fundamental safety functions, namely by controlling reactivity excursions, cooling the nuclear fuel and containing the radioactive material. Deployment of several independent and overlapping layers (levels) of provisions – such as design features; operational measures; and the introduction, for the whole spectrum of plant states, of various safety limits – assist in ensuring effective performance of safety functions and maintenance of adequate safety margins.
In this presentation, the term "fuel safety criteria" refers to safety limits established to prevent or mitigate, for any specific plant state, damage to the nuclear fuel, and damage that the fuel may cause to its interfacing systems and structures.
Fuel safety criteria must be met in order to achieve an adequate level of DID. They are, among other things, required input to:
- demonstrate fuel fitness-for-service for normal operation and anticipated operational occurrences
- determine effectiveness of special safety systems, through application of deterministic safety analysis, to protect the physical barriers that prevent the release of radioactive material for all design-basis events
- determine the plant safe operating envelope and adequacy of safety margins through application of deterministic safety analysis
For normal operation and anticipated operational occurrences, many CANDU fuel safety criteria have been defined to ensure that the fuel remains fit for service. Examples include limits on fuel element internal gas pressure, fuel sheath oxidation thickness, spacer pad and bearing pad wear, and axial loads on the fuel bundle.
The limit on total fuel enthalpy (the energy deposited in the fuel during normal operation plus the energy deposed due to the power pulse) in a large loss-of-coolant accident scenario, and the avoidance of fuel sheath dryout in a slow loss-of-regulation accident, are examples of CANDU fuel safety criteria for design-basis accidents.
Activities in Canada related to fuel safety criteria over the last 15 years
In the last 15 years or so, the Canadian nuclear industry has achieved a significant amount of developmental work on fuel safety criteria. Key drivers for these activities have been:
- the need for improved understanding of safety margins related to a large loss-of-coolant accident
- the decrease in safety margins for some relatively high-frequency postulated design-basis accidents due to the effect of HTS component aging on fuel bundle dryout power
- crud deposition on fuel bundles in Pickering B units
- the introduction of rod-based guaranteed shutdown state as a new means to both ensure reactor subcriticality and allow operational flexibility during an outage
- fuel bundle endplate cracking issues at Bruce B units
- the Fukushima event: fuel pool safety for beyond-design-basis accidents
- proposed new CANDU reactor designs, such as ACR-700/1000 and EC-6
- development of the low-void-reactivity fuel bundle
Although in Canada the prime responsibility for the safety of nuclear installations rests with the licensees, the regulator nevertheless plays an important role in ensuring that all nuclear activities are conducted safely and follow the best international practices. The Canadian industry’s activities related to fuel safety criteria are no exception.
Objectives of the presentation
The primary objective of this presentation is to provide a regulatory perspective on the work performed in Canada over the past 15 years related to fuel safety criteria. A brief overview of the CNSC’s regulatory oversight activities will also be provided. This overview will include a brief description and discussion of:
- the regulatory requirements, expectations and guidance pertinent to the topic of fuel safety criteria
- some of the above issues related to fuel safety criteria
- the licensing and compliance activities conducted by CNSC staff
- some of the ongoing R&D activities in Canada related to fuel safety criteria
- some of the challenges faced in performing those regulatory oversight activities and lessons learned
Given that I am currently the chair of an IAEA Working Group, whose mandate is to write a technical document (TECDOC) on fuel safety criteria used in countries where pressurized heavy water reactors are operated, I will conclude the presentation with a brief description of this project, covering the following topics:
- working group members and the participating countries
- purpose and applicability of the IAEA TECDOC
- project activity steps and timeline for completion
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