Candu reactors: ageing well

The CANDU Owners Group (COG), a member organisation for CANDU nuclear plant operators worldwide, invests more than $65 million in R&D and joint projects on behalf of its members, annually.

Much of the investment comes from its Canadian member plants, where the youngest reactors are now reaching middle age and the oldest units have provided close to 50 years of service. Although the last new-build in Canada was in the early 1990s, the industry is thriving. In Ontario, two of the three nuclear plant sites are undergoing massive multi-billion dollar, mid-life refurbishment projects while the one-unit New Brunswick station is hitting its stride post-refurbishment.

In Ontario, success came with a challenge. Nuclear accounts for about 60 percent of the electricity used in the province. The task at hand is to refurbish 10 units at two nuclear plants for continued operation into the mid-2050s to 2060. Both plants came due for refurbishment at roughly the same time, while a third plant in the same jurisdiction was approaching permanent shutdown. The consequence could have been losing almost half of the province’s electricity generation within a few years.
Instead, Ontario Power Generation (OPG) obtained regulatory approval to extend the life of the province’s oldest nuclear plant, Pickering. OPG was aided by industry research conducted through COG, collaboration between members and research suppliers and advances in operating and maintenance methods. Parts of the Pickering plant were originally conceived to run to 2014, but evidence from research on fuel channel life helped achieve regulatory approval to continue operations until 2020. More research is underway and its goal is to result in approval of a further extension to Pickering’s operations to 2024.

The same research has validated safety margins at the two plants in refurbishment. OPG’s Darlington station and Bruce Power’s Bruce station received approval for lifetime operating extensions to 235,000 and 247,000 equivalent full power hours (EFPH), respectively. Operators can now stagger the shutdown of units for refurbishment or major component replacement, while other units continue to produce electricity. Research continues to confirm a further extension for the Bruce units to 300,000 EFPH.

The extensions are possible, in part, because of the conservatism built into the original lifespan of the units. Nevertheless, regulatory requirements are stringent and approvals have only been granted after rigorous defence of the safety margins. That is where the collaborative research conducted over many years comes in.

“Significant conservatism was built into the design basis for these units at the beginning because the designers did not have the operational experience, the analytical capability and the sophisticated inspection tooling we have today,” says COG president and CEO Fred Dermarkar. “We now have the benefit of decades of research, thousands of reactor-years of experience as well as a far greater capability for inspections. All of this informs our understanding of the safety margins. It also contributes to knowledge for development of effective preventative maintenance and operating methods. Together these factors have not only validated safety margins but also increased the safe operation and capability factors of the plants for much longer periods.”

Today, says Dermarkar, ‘age’ is relative. It depends on how well the plant was maintained, the knowledge the operator has of the condition of installed components and, through research, an understanding of how the components will perform in the future.

Fuel channel life extension

One major research initiative contributing to extended operation at the Ontario plants is COG’s fuel channel life management (FCLM) programme. Research to validate the safe operating lifetime for pressure tubes has helped COG member stations extend their operating cycle prior to refurbishment by  four to five years and potentially longer.

CANDU reactors were originally targeted for a 30-year design life at an 80 percent capacity factor. As nuclear fleets aged, operators needed more definitive predictions of when individual components could no longer be kept in service and when major replacements or refurbishments would be necessary.

The FCLM work has improved the understanding of degradation of fuel channel components. By uncovering the factors that contribute to ageing – primarily hydrogen uptake and irradiation – plans, tools and methodologies could be developed to acquire and analyse data from inspections, operating history and surveillance for more accurate calculations of life expectancy. Testing and analysis was conducted on both components removed from in-service power reactors and similar materials that have undergone accelerated ageing in research reactors.

Now in Phase 3 of 4, the FCLM project has already: delivered predictive models for fracture toughness to assess pressure tube fitness-for-service; provided sufficient evidence to confirm the integrity of the spacers that are used to maintain a gap between pressure tubes and calandria tubes; and improved knowledge of associated ageing mechanisms.

The combination of the FCLM deliverables, condition assessments, and extensive engineering analysis has helped confirm fitness for service and extended licensing of all the Ontario plants.

Where pressure tubes were once expected to provide up to 30 years of service, today with improved knowledge of ageing, in-service inspection and confirmation of safety margin, some units may serve for up to 40 years before a mid-life changeout.
Bruce Power has a contract with the Ontario government to provide power until 2060. At end of life, the Bruce units, which went into service in the mid-80s, will have operated for almost 80 years with one pressure tube changeout at the midpoint.  

New frontiers

Research such as the fuel channel life management programme and the fuel bundle modification research (see sidebar story), as well as dozens of other research and joint projects underway through COG, are making reactors safer and more efficient.

Because CANDU technology includes more than one design configuration, some work is specific to the CANDU 6 reactor design, which is used by international operators in countries including Romania, China, Argentina and Korea as well as at New Brunswick. In other cases, lessons from research conducted for the Ontario reactors are relevant and have lessons for the C6 design. It is all part of a commitment to use collaboration as a means to create operational excellence that prompted creation of COG back in 1984.

At Pickering, some of the units are enjoying their highest lifetime performance results; something attributed to operator knowledge from research and experience gained over almost 50 years.

“The work completed by COG to better understand the ageing of life-limiting reactor components has given us confidence that these components can operate safely well beyond what we originally anticipated. It has also saved ratepayers billions of dollars for premature replacement electricity generation that would have been required without this work,” says Dermarkar.

“In my mind, it confirms the idea that the safest and most efficient plants rely not only on excellence in operations, maintenance and engineering, but also on excellence in the R&D that provides a platform for innovation.” 

Managing ageing through modification

With the benefit of decades of operational experience, the CANDU heat transport system (HTS) has had many modifications since its original design to improve its operability and emergency response. Research continues today on specific aspects of the HTS to further validate safety margins and support risk assessment, as well as define predictable outcomes. 

One of the effects of ageing in the HTS is a non-uniform change in the dimension of reactor pressure tubes through diametral creep. This causes a crescent-shaped gap to develop between the top of the fuel bundle and the pressure tube, allowing some of the coolant to take the path of least resistance and bypass the fuel bundle. Coolant flows through the centre of the bundle are reduced, reducing critical heat flux (CHF), and the corresponding margin to fuel element dry-out under postulated accident conditions. Reduced CHF decreases the operating margin for a nuclear reactor, and, if not mitigated, leads to the derating of reactor power (up to 15 percent of full power by the time the reactor is ready for fuel channel replacement) to maintain safety margins.

A COG programme aimed to recover some of the safety margin through a modification to the 37-element bundles that increased flow through its centre, compared a reference element bundle. Testing has been completed and has confirmed the modified bundle has higher and more consistent dryout power than the reference bundle, validating a higher safety margin.

The initial research supported the Canadian regulator’s risk-informed decision-making and determination of the maximum predicted fuel sheath temperature under postulated accident conditions. Further research on the modified 37-element bundle CHF experiments with new axial flux distribution includes:

• A full scale 37M-bundle simulation with a new symmetric near-cosine peak style axial flux distribution; and
• CHF and post dryout experiments in water at representative reactor conditions at 0%, 3.3% and 5.1% creep, to expand the previous data obtained.
• The analysis is currently underway for these experiments and further study is planned.