JOHN H. MOORE

PHWR FUEL CHANNEL LIFE MANAGEMENT ACTIVITIES BY THE CANDU OWNERS GROUPJOHN H. MOORECANDU Owners Group Inc.Toronto, CanadaEmail: john.moore@candu.org

Abstract

Fuel channel life management has been an extremely important subject for utilities that operate CANDU or other pressurized heavy water reactors (PHWRs). CANDU nuclear power plants (NPPs), for example, were originally designed assuming a life of 210,000 effective full-power hours (EFPH) of operation before they would need to be replaced during a refurbishment outage. This corresponds to about 30 years of operation at a conservative capacity factor of 80%. As NPPs approach this number of operating hours (210 kEFPH), operating organizations become increasingly interested in whether this limit can be safely exceeded and what the expected life of their asset can be assumed to be.

This paper describes collaborative work done through the CANDU Owners Group (COG) in addressing this important challenge of FC life management.

1. THE CANDU OWNERS GROUP

COG is a non-profit organization that was established in 1984 by its participating member utilities. It arranges for collaborative research, joint projects and information sharing among utilities that operate CANDU NPPs and among major CANDU suppliers. All utilities that operate CANDU and other types of pressurized heavy water reactors, some 47 units in total worldwide, are COG members. There are over 16 supplier participant companies and that number is rapidly growing. COG also runs, on behalf of its Members, the CANPAC vendor joint auditing programme, the CANIAC (CANDU Industry Assessment Committee) sub-supplier joint auditing programme and the non-destructive-evaluation-focussed CANDU Inspection Qualification Bureau (CIQB).

2. DEGRADATION MECHANISMS

A CANDU fuel channel/pressure tube schematic is shown in Figure 1. Individual fuel bundles and the primary heat transport coolant are contained within pressure tubes. Slightly larger calandria tubes surround each pressure tube, with the annular space between the two containing a gas and a number of spring-like spacers or ‘garter springs’. The spacers help to maintain the separation of the two tubes.

During operation, the pressure tubes are subjected to harsh conditions, namely high temperatures (312oC), pressures (11 MPa) and high radiation fields [1]. Installation practices, on-line fuelling operations, debris or fuel bundle vibration can introduce flaws such as scrape marks into the FC material. Such flaws or other degradation mechanisms, if left unchecked, could potentially degrade into cracks or ruptures, initiating a loss-of-coolant from the affected channel.

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FIG. 1. CANDU 6 Fuel Channel Assembly [2].

The main degradation mechanisms encountered over an FC’s life include FC deformation, corrosion and deuterium ingress, flaws and defects and material property changes associated with the neutron bombardments and deuterium ingress that reduce the material’s ductility and margin to failure.

Fuel channel deformation can manifest itself via channel elongation, diametric expansion, sag or wall thinning. This can cause issues with end fitting bearing travel (which has been addressed in more recent designs), fuel flow bypass, and blister formation. As FCs sag or change shape, the integrity of the FC spacers (item 7 in Fig. 1) becomes important, as they are designed to help maintain spacing between the hot FC pressure tube (item 9 in Fig. 1) and cooler calandria tube (item 10 in Fig. 1).

Corrosion and deuterium ingress can introduce wall thinning and an increased probability of delayed hydride cracking.

Since neutron bombardment and high temperatures are key degraders of FCs, operating organizations carefully track the number of hours that its PHWRs are at full power. Figure 2 provides some information on part of the world fleet of PHWRs and the comparative age of their FCs. Data is derived from IAEA PRIS sources (www.pris.iaea.org) from reported ‘time online’ rather than strictly from EFPH. Note that ‘time-online’ can include periods of low power operation that are less impactful to FCs.

JOHN H. MOORE

FIG. 2. Time online for selected CANDU PHWRs to end of 2016 [3].

3. BASE R&D PROGRAMME

The FC base R&D programme was established at COG in 1989 and has a strong record of developing sound solutions and providing assessment tools to minimize plant operational costs and improve operational service life. Over the years it has developed and supported models for FC crack initiation, deuterium ingress, FC material properties and FC deformation (FC to calandria tube gap). Working groups consisting of representatives from operating organizations, suppliers, scientists and academics have been established that deal with these particular areas of interest.

The base R&D programme has been instrumental in the development and application of CSA standard N285.8 [4] “Technical requirements for in-service evaluation of zirconium alloy pressure tubes in CANDU reactors”. This standard, whose last revision was in 2015, specifies mandatory technical requirements and non-mandatory evaluation procedures for fitness-for-service assessments for fuel channels. Non-mandatory annexes to the CSA standard cover procedures for the evaluation of pressure tube flaws, procedures for the evaluation of pressure tube to calandria tube contact, procedures for the assessment of a reactor core, material properties and derived quantities and a form for notification of in-service evaluations. CSA standard N285.4 [5], last revised in 2014, provides detailed inspection criteria for CANDU NPP components, including fuel channels.

Figure 3 shows the relationship between these two CSA standards related to fuel channels, fuel channel models, degradation mechanisms and operating and design data.

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FIG. 3. Relationship of CSA standards to fuel channel models, degradation mechanisms and operating and design data.

4. FUEL CHANNEL LIFE MANAGEMENT PROJECT

As operating organizations in Canada came to approach the 210kEFPH licensing limit, the companies involved decided to accelerate the R&D efforts to support license extensions beyond 210kEFPH. A dedicated fuel channel life management project was established to coordinate these efforts in 2009.

The FCLM project has been divided into a number of phases. Each phase has had a defined scope of investigation and each has provided increased detail and knowledge surrounding the degradation of FCs and their associated components. The specific phases and timeframes are shown in Table 1 below. Over the programme’s life, specific working groups have been established covering the technical areas of hydriding and fracture toughness, crack initiation, spacer material and modelling and on probabilistic core assessments.

TABLE 1. COG FC RELATED PROJECTS AND BASE PROGRAMME.

Joint Project #

Title Timeframe Status

Not applicable

Fuel Channel R&D (base programme) 1989-XXXX Ongoing (annual number of smaller projects directed by a technical committee)

4299 Burst test programme 2009-2015 Completed4363 FCLM Phase 1 2009-2015 Completed4452 FCLM Phase 2 2013-2016 Completed4491 FCLM Phase 3 2015-2018 In progress4583 FCLM Phase 4 2017-2020 Started 20174584 Spacer Life Management 2018-2022 To start 2018

JOHN H. MOORE

Major supplier participants to the programme include the Canadian Nuclear Laboratories (formerly AECL), Kinectrics, AMEC Foster Wheeler and Candu Energy Inc. (a Member of the SNC-Lavalin Group).

Current project efforts are focussing on the following:• Improving the FC fracture toughness model;• Investigating various effects on fatigue crack initiation (e.g. operating conditions, materials, flaw

root radius, etc.), delayed hydride cracking initiation and hydride overload. These investigations work with both non-irradiated and irradiated material and include burst tests of pressure tubes (Fig. 4);

• Probabilistic fracture evaluation computer models (developing acceptance criteria for the probability of pressure tube rupture supporting fitness for service evaluations)

• Improving spacer integrity modelling and analysis (e.g. fatigue, loading impacts) to develop fitness for service guidelines. This includes a programme of elevated irradiation of new spacer material using the High Flux Isotope Reactor (HFIR) research reactor at the Oak Ridge National Laboratory in the USA and examination of removed spacers from operating reactors. Experiments done include crush testing and endurance testing of spacers (Fig. 5)

• Third party reviews in support of operating organization regulatory submissions.

FIG. 4. Pressure tube material undergoing burst testing.

FIG. 5. Typical spacer material undergoing testing.

5. PROGRAMME BENEFITS

The COG FCLM programme has provided substantial benefits to participating Members. Ontario Power Generation and Bruce Power have received extended operating licenses for the Pickering and Bruce NPPs to

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247 kEFPH [6, 7]. OPG’s Darlington station has been extended to 235k EFPH [8], which is sufficient to operate those units until their planned refurbishments, one of which is already in progress. The Pickering and Bruce license extensions represent an 18% increase in facility life over that originally planned or over 5 years of additional generation. Current and future efforts are planned to increase knowledge even further and may lead to lives of 300 kEFPH or over 40 years before mid-life refurbishments. International Members of COG, whose NPPs tend to be slightly younger that those identified above, are increasingly interested in joining this vital life management programme.

6. REFERENCES

[1] UNENE, The Essential CANDU, A Textbook on the CANDU Nuclear Power Plant Technology, GARLAND, W.J., Ed, UNENE, Hamilton, ON (2014).[2] CANTEACH, CANDU 6 Fuel Channel Assembly, https://canteach.candu.org/Image%20Library1/C6%20FC%20Section.pdf.[3] TOWNSEND, D., “Role of COG Fuel Channel R&D Program in Addressing Operational Issues”, presented at the 12th CANDU Owners Group Fuel Channel Seminar, Ajax, ON, Canada, 2017.[4] CANADIAN STANDARDS ASSOCIATION, Technical Requirements for In-Service Evaluation of Zirconium Alloy Pressure Tubes in CANDU Reactors, N285.8-15, 3rd ed., CSA, Toronto (2015).[5] CANADIAN STANDARDS ASSOCIATION, Periodic Inspection of CANDU Nuclear Power Plant Components, N285.4-14, 6th ed., CSA, Mississauga (2014).[6] CANADIAN NUCLEAR SAFETY COMMISSION (CNSC), Record of Decision: Application to Amend Nuclear Power Reactor Operating Licence for the Pickering Nuclear Generating Station, CNSC (2016).[7] CANADIAN NUCLEAR SAFETY COMMISSION (CNSC), Summary Record of Proceedings and Decision: Application to Renew the Power Reactor Operating Licences for Bruce A and Bruce B Nuclear Generating Stations, CNSC (2015).[8] CANADIAN NUCLEAR SAFETY COMMISSION (CNSC), Summary Record of Proceedings and Decision: Application to Renew the Power Reactor Operating Licence for the Darlington Nuclear Generating Station, CNSC (2015).