Long life in Canada3 April 2002
Gentilly 2 and Point Lepreau have both initiated similar refurbishment projects, which are expected to extend the operating lifetimes of each unit by 25 years. By René Pageau, Paul Thompson and James H Nickerson
Gentilly 2 is a Candu 675MWe reactor (Candu-6) designed by Atomic Energy of Canada (AECL) in the early 1970s. The Gentilly station is located in the province of Québec and is the only nuclear plant owned by Hydro-Québec. A similar reactor, Point Lepreau in New Brunswick, is operated by New Brunswick Power Commission (NBPC). Both these reactors have a conceptual design life of 30 years.
Hydro-Québec and NBPC have initiated similar projects aimed at extending the reactors' operating lifetimes by 25 years. The process and proposed activities related to the preparation and implementation of the projects is ongoing, and some of the work is shared between the two utilities.
The case for refurbishment
Both reactors have been successfully operated since their first full-power operation in the early 1980s. Although the overall performance of those units continues to be very good, it is reasonable to expect that age-related degradation of key plant components will have an increasing impact on operations as the plants continue to age.
The Candu design houses the fuel bundles in horizontally-oriented fuel channels. A fuel channel is made up of the pressure-retaining pressure tube, which is surrounded by a calandria tube to minimise the heat loss to the heavy water moderator. An end fitting assembly is attached to each end of the fuel channel to allow for the ability of on-power refuelling of individual channels. The Candu-6 reactors have a total of 380 fuel channels.
The pressure tubes within the channels are exposed to high neutron flux and to high temperature pressurised heavy water. Under these harsh conditions they undergo metallurgical changes, mainly longitudinal and diametrical elongation (creep). This effect was taken into account during the reactor assembly design and was included in the safety analysis. After a period of approximately 30 years, depending on the integration of the neutron flux with time, the tubes have to be replaced. The elongation is not uniform for each channel and, while not all the channels have reached their design limit, it is preferable to replace all the tubes in a single outage. This replacement operation was performed at other plants for single tubes (Point Lepreau, Embalse) and for all the tubes at the four-unit Pickering A station.
The replacement of the reactor tubes is a major activity but it is necessary if the station is to operate safely and reliably for an extended period of 25 years. The challenges associated with extended life operation until at least 2030-33 are to maintain the margin of safety for plant operations by ensuring reliability of safety-related systems, to reduce plant unavailability due to age-related component failures, and to minimise the costs for repairs and replacement of ageing components.
The overall project is expected to spread over an eight-year period starting in 2001 for Gentilly 2 and 2000 for Point Lepreau. A project feasibility study (phase 1) of four years has been planned to assess the plant condition and to make an overall safety review. This project phase will confirm the economical viability of extended operation. A period of 39 months would be necessary for the procurement and manufacturing of all the reactor components, and for the engineering work related to safety and economic improvements. Reactor channel replacement is planned for an outage not exceeding 18 months.
The main objectives of phase 1 are: to give a reasonable assurance that operation can be economically viable and technically achievable after refurbishment; to identify the scope of the work required to perform the reactor refurbishment and to ensure compliance with regulatory requirements; to put a price tag on the extended life of the station; and to perform an environmental impact study and obtain governmental authorisation. To fulfil those objectives, it is necessary to:
• Perform in-depth assessment of the ageing of the critical structures, systems and components (SSCs).
• Review the major systems operation history and actual operating conditions.
• Define the strategy for fuel channel replacement.
• Review the current safety analysis requirements.
• Compare the current regulatory requirements to the original design requirements and, when reasonably feasible, some design changes will be made and implemented.
• Update the environmental impact studies.
• Obtain the required provincial and federal government authorisation through public hearings.
The plant condition assessment is an integrated plant assessment that demonstrates the plant's SSCs requiring ageing management have been identified, and that the effects of ageing on the functionality of such SSCs will be managed (through replacement and/or repair, as well as continued operation and maintenance programmes).
The plant condition assessment of Gentilly 2 and Point Lepreau is one of the major elements of the refurbishment project. It consists of assessments of plant equipment condition in order to determine which equipment requires replacement or repair during the maintenance outage scheduled to begin in early 2008, and which repairs may be made during other outages (either before or, more likely, after the refurbishment outage). The plant condition assessment will identify changes that are necessary and sufficient in order to deal with issues related to age effects and equipment obsolescence. The cost of the proposed design changes will be estimated, and this will serve as input to provide a business case decision to go forward.
In addition to the age-related degradation, there are three other equally important drivers affecting the disposition of component life extension:
• Obsolescence issues leading to component replacement.
• New regulatory requirements.
• Economic opportunities for improvement provided by the long duration
of the outage and/or component accessibility during the outage.
The plant condition assessment will determine the current condition of the SSCs and whether they will continue to operate for an additional 25 years. It will also identify which activities are needed to ensure component life extension of 25 years.
A typical approach to performing the plant condition assessment can be summarised as follows:
• First-level screening to withdraw SSCs that are not safety-related and/or important to power production. From this process 80 systems have been retained to be the subject of a more detailed assessment.
• Gather and review design, manufacturing, installation, operational history, and international experience data in order to define the design basis, and to identify the impact of plausible ageing mechanisms and related component/equipment degradation on system performance.
• Establish the physical and functional boundaries of the system and associated structures, components and equipment; prepare the list of structures, components and equipment for each system; identify any common or generic grouping of components (commodities).
• Second-level screening to cull specific structures, components and commodities (SCC) to other ongoing plant programmes (if any), thereby not requiring a SCC condition assessment.
• Undertake the condition assessment of the selected structures, equipment, components and commodities in each candidate system.
• Provide recommendations for each component with a poor health prognosis in terms of replacement, refurbishment, life cycle management (LCM) and/or maintenance activities to assure the maintenance of plant safety and production goals over the life of the plant.
Before the decision to do specific work regarding the Canadian Candu-6 refurbishment, AECL and NBPC jointly began work early in 1993 to assess the "ageing" of the main plant components. These so-called "ageing studies" are necessary to implement a good life cycle management (LCM) programme to manage the effects of ageing degradation and to ensure continuing safe, reliable and cost-effective operation of existing Candu-6 units.
The objectives of these early LCM studies were to ensure the long-term reliability and safety of Gentilly-2 and Point Lepreau during the nominal design life of 30 years (life assurance); maintain the long-term availability and capacity factor with controlled and reasonable generating costs during the nominal design life of 30 years (life assurance); preserve the option of extending the life of Gentilly 2 and Point Lepreau with good safety and availability at reasonable costs, beyond the nominal design life of 30 years (life extension).
These ageing studies are either ongoing or completed and they cover the following critical SSCs: containment and reactor structure; large motors; steam generators; large pumps, heat exchangers, and pressure vessels; nuclear and conventional piping; cables, including electrical penetrations and motor terminations; special safety systems I&C; turbine-generator; important civil structures.
One of the main tasks of the refurbishment outage is to replace the pressure tubes and calandria tubes. AECL has been chosen to perform this activity, which will involve:
• Improvement of retubing technology to incorporate removal of all feeders from the reactor face for fuel channel replacement. A "feeder replacement study report" detailing the scope, sequence of feeder removal and replacement, cut and weld locations, and materials and inspection requirements
will be prepared.
• Assessment of the benefits of performing a decontamination of the primary heat transport system (PHTS) prior to commencement of reactor face retubing activities, and assessment of reactor face radia-tion sources. If a PHTS decontamination is necessary prior to commencement of retubing activities, the scope of the study will be expanded to determine whether a single or multi-step decontamination process is preferred for the station, and the decontamination process equipment requirements and tie-ins to existing process equipment will be defined.
• Defining a retubing process that will describe the sequence of operations to be performed during the replacement of fuel channels, calandria tubes and feeders. These operations will be simulated on a full scale 3-D model to optimise replacement of the reactor components and to reduce worker dose.
• Preparing a conceptual layout of the retubing waste facility, which will store highly radioactive waste materials such as pressure tubes, calandria tubes, end fittings, feeders and related components resulting from the retubing process.
• Reviewing the fuel channel design and identifying possible improvements to improve reactor viability and safety. The proposed basis for the fuel channel is the layout used
at Qinshan - the most recent Candu-6 reactor.
Another important element of the refurbishment project phase 1 is the safety evaluation. This evaluation will review the safety requirements for the most recent plants, define the station risk baseline, and develop a safety analysis programme.
Review of requirements
While the reactors were always operating safely, some design changes were implemented since initial start-up to improve overall safety. However, it is important for a 25-year life extension to define a process by which safety related design changes could be identified to improve the safety margins and the reliability of the safety systems. This process includes a formal review of the actual safety requirements by comparison to the current design codes and standards, an assessment of all the design changes incorporated in the latest Candu-6 reactor (Qinshan, Wolsong) and a review of the reliability improvements for the special safety systems.
Revision of the risk baseline
A probabilistic safety assessment (PSA) was not a requirement for the Gentilly 2 and Point Lepreau initial design, however some probabilistic assessments were done in the late 70s/early 80s to improve overall safety. A generic PSA (level 1 and level 2) was also developed by AECL for the Candu-6 reactor. The risk baseline for Gentilly 2 and Point Lepreau will be revised by performing an adaptation of this generic PSA to each specific station design. This risk baseline will be used to assess the safety benefit of each design modification in comparison to cost. Also, a programme to develop a specific PSA for each station will be prepared.
A safety analysis plan will be developed to ensure a complete update of the safety report. Many of the safety analyses were revised since initial commissioning based on new findings or new improvements of the safety analysis tools. However these efforts, even though considerable, did not cover the complete set of the safety analysis. In addition, further requirements for safety analyses have been introduced in Canada. The programme for safety analysis review will consider the requirements of the regulatory authorities, events that have to be analysed for the refurbishment activity, and the start-up of the station with new fuel and new design changes. These analyses will also be done with the latest tools developed for the Candu-6.
In Canada, the owner of a nuclear power plant has to obtain a renewal of its operating licence every two years. This process does not cover the life extension of the station from original design life. Gentilly 2 and Point Lepreau are the first Canadian nuclear reactors to request some direction of the necessary licensing requirements to ensure extended life to the station without undue regulatory risk.
The Canadian Nuclear Safety Commission (CNSC) has already indicated that the development of a PSA specific to each station will be expected and that a review to the actual code and standards will be necessary. Also the CNSC indicates that a periodic safety review (PSR) based on IAEA guidelines will be advisable.
The safety factors by a PSR, as per IAEA guidelines, were reviewed and compared with the actual ongoing work of the refurbishment project phase 1. Plant condition assessment, safety evaluation and some of the work at the station were found in concurrence with the IAEA standards. However, some of the safety factors are not part of the refurbishment project and a process to assess those factors has to be defined and implemented. A summary of the findings are presented in the Table.
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