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Ready for the market
                                                                     AECL has adapted the features of its Candu
                                                                     reactors in designing a Generation III+ plant –
                                                                     the Advanced Candu Reactor – to meet
                                                                     upcoming marketplace needs. By Ken Petrunik
                                                                     nuclear power plant with a 60-year         • Feedwater makeup, providing pas-
                                                                     design life. It is a light-water-cooled,     sive gravity driven makeup to the
                                                                     heavy-water-moderated pressure-tube          steam generators via the large vol-
                                                                     reactor, which has evolved from the          ume reserve water tank (RWT)

                              he basic design of AECL’ s             well-established Candu line. It retains      located at the top of the reactor
                              Advanced Candu Reactor,                basic, proven, Candu design features         building to prevent failure of fuel
                              the ACR-1000, has been                 while incorporating innovations and          and fuel channels and delay or pre-
                              chosen for generic design              state-of-the-art technologies to opti-       vent progression to severe accidents.
                     assessment in the UK, and there are             mise safety, operation, performance        • Heat transport system (HTS) make-
                     active ACR-1000 new-build initiatives           and economics (see Panel).                   up supplied from emergency core
                     in Canada – in Ontario, Alberta and                These technical improvements,             injection system water tanks and
                     New Brunswick.                                  along with system simplifications and        from the RWT to prevent progress
                       The ACR-1000 is a 1200MWe-class               advancements in project engineering,         to a severe accident state; cools the
                                                                     manufacturing, and construction,             fuel and keeps the HTS full.
ACR-1000 features                                                    result in a reduced capital cost and       • Moderator thermal capacity,
                                                                     construction schedule, while enhanc-         absorbing heat and delaying acci-
Key Candu strengths retained                                         ing the inherent safety and operating        dent from progressing beyond a
• Modular, horizontal fuel channel core.                             performance of the ACR-1000 design.          severe accident state; conducts heat
• Simple fuel bundle design.                                            All innovative features of the ACR-       directly to the moderator if the
• Separate low temperature and pressure heavy water moderator.
                                                                     1000 have been or will be fully tested       pressure tubes make contact with
• Passive safety features including reactor vault filled with light
  water surrounding the core and two independent, passively-         and proven before the first project.         the calandria tubes.
  driven, safety shutdown systems.                                   80% of plant features, equipment and       • Moderator makeup and thermosy-
• On-power refuelling.                                               specifications are based on the Candu         phoning, designed to prevent a
• Reactor building access for on-power maintenance.                  6 reference plant.                           severe accident from progressing, for
                                                                                                                  as long as there is water in the RWT.
ACR-1000 innovations                                                 ENHANCED SAFETY                            • Water-filled reactor vault, designed
• More compact core design.                                          The ACR-1000 design takes advantage          to mitigate a severe accident (if all
• Steel-lined, 1.8m-thick containment building, to withstand         of both passive and engineered safety        the above fail) by absorbing heat
  aircraft strike.
• Light water reactor coolant, reducing heavy water inventory
                                                                     characteristics, including distinctive       and preventing failure of the calan-
  and resulting in lower costs and reduced emissions.                features that arise from Candu design        dria vessel until reactor makeup or
• Thicker pressure tubes and thicker and larger calandria tubes.     principles. The core is designed for a       active cooling is re-established.
• Stainless steel feeders and headers.                               small negative reactivity coefficient.      • Reactor vault makeup and ther-
• Mechanical zone control rods; solid-rod guaranteed shut-           This feature provides inherent protec-       mosyphoning, designed to mitigate
  down state; no adjusters.                                          tion against transients with any inad-       the consequences of a severe acci-
• Use of low enriched uranium (LEU) fuel, in advanced Canflex-        vertent increase of reactor power, while     dent; this involves manually initiat-
  ACR fuel bundles, to help achieve negative void reactivity.
                                                                     limiting complexity in engineered sys-       ing reactor vault makeup from the
• Option to efficiently burn other fuel types such as mixed
  oxide (MOX), thorium and actinides.                                tems and operating procedures that           RWT to establish and sustain ther-
• Improved plant thermal efficiency through use of higher pres-       deal with large reactivity swings.           mosyphoning.
  sures and temperatures in the coolant and steam supply systems.       Central to ACR-1000 safety are two      • Containment spray, initiated auto-
• Enhanced accident resistance and core damage prevention            fast acting, fully capable, diverse and      matically to suppress the pressures
  features.                                                          separate shutdown systems, physically        and temperatures to within design
• Further enhanced passive safety.                                   and functionally independent of each         limits.
• Customer-driven improvements in operability and maintain-          other and also from the reactor regu-
  ability, with designed-in maintenance features.                                                               PLANT LAYOUT
• Distributed control system/plant display system; modern
                                                                     lating system. Based on proven Candu
  control centre incorporating human factors.                        technology, each shutdown system is        Designed for efficient operation,
• Improved plant performance through Smart Candu                     designed to cover the whole spectrum       increased safety and easier and faster
  advanced online diagnostic systems.                                of design basis events.                    maintenance, the ACR-1000 is laid
• Four-quadrant design: essential operating and safety systems          Enhanced defence in depth is            out to provide separation by distance,
  separated into four divisions; permits online maintenance,         derived from the inherent passive safe-    elevations and the use of barriers for
  flexibility during outages.                                         ty design features of the Candu fuel       safety-related structures, systems and
                                                                     channel core, including:                   components. Each corner of the reac-
                                                                                                                                                     POWER PLANT DESIGN

    ACR-1000 data                                                                            Reactor core
                                                                                                                               CANDU 6                 Darlington                     ACR-1000
                                                                                             Output (MWt)                        2064                      2657                          3187
     Containment structure
                                                                                             Coolant                      Pressurised D2O            Pressurised D2O             Pressurised H2O
                                              CANDU 6                    ACR-1000
                                                                                             Moderator                            D2O                      D2O                           D2O
     Type                                  Pre-stressed        Pre-stressed
                                       concrete/epoxy liner concrete/steel liner             Calandria diameter (m)                7.6                      8.5                           7.5
     Reactor building inside                                                                 Fuel channel                    Horizontal                 Horizontal                  Horizontal
     diameter (m)                                  41.4                     56.5                                         Zr 2.5wt%Nb alloy          Zr 2.5wt%Nb alloy           Zr 2.5wt%Nb alloy
                                                                                                                           pressure tubes             pressure tubes              pressure tubes
     Containment wall thickness (m)                1.07                     1.8                                             with modified               with modified                with modified
     Building height (base slab to                                                                                       403 SS end-fittings         403 SS end-fittings          403 SS end-fittings
     top of dome) (m)                              51.2                     74.0             Fuel channels                        380                      480                           520
                                                                                             Lattice pitch (mm)                   286                      286                           240

                               CANDU 6             Darlington             ACR-1000
                                                                                             Steam generators
     Material                  Natural UO2         Natural UO2         Low enriched UO2
                                                                                                                                   CANDU 6                 Darlington                   ACR-1000
     Burnup (MWd/kgU)                7.5                  7.8                 >20
                                                                          (reference)        Number                                      4                        4                          4
     Fuel bundle assembly      37-element          37-element            43-element          Type                               Vertical U-tube/        Vertical U-tube/              Vertical U-tube/
                                                                         Canflex-ACR                                            integral preheater      integral preheater            integral preheater
     Bundles per channel             12                   13                  12             Nominal tube diameter (mm)            15.9 (5/8”)             15.9 (5/8”)                17.5 (11/16”)
                                                                                             Steam temp (nominal) (°C)                   260                  265                          275.5
                                                                                             Steam quality                           0.9975                  0.9975                        0.999
     Heavy water inventory
                                                                                             Steam pressure (MPa)                        4.6                  5.0                           5.9
                                           CANDU 6         Darlington       ACR-1000
     Moderator system (Mg D2O)               265                312               250
     Heat transport system (Mg D2O)          192                280                0         Heat transport system
     Total (Mg D2O)                          457                592               250                                                            CANDU 6              Darlington           ACR-1000
                                                                                             Reactor outlet header
                                                                                             pressure (MPa)                                          9.9                 9.9                     11.1
     Heat transport pumps
                                                                                             Reactor outlet header temp (°C)                         310                 310                     319
                                          CANDU 6         Darlington        ACR-1000
                                                                                             Reactor inlet header pressure (MPa)                    11.2                 11.3                    12.6
     Number                                  4                   4                 4
                                                                                             Reactor inlet header temp (°C)                          260                 267                     275
     Rated flow (l/s)                        2228                3240           4320
                                                                                             Single channel flow (max) (kg/s)                         28                  27.4                     28
     Motor rating (MWe)                      6.7                9.6            11.5

     Turbine generator
                                                                           CANDU 6                                   Darlington                                          ACR-1000
     Type                                                  Hitachi impulse-type, tandem-compound                  Tandem-compound                          Impulse-type, tandem compound
     Composition                                           One double-flow high pressure cylinder       One double-flow high pressure cylinder           One double-flow high pressure cylinder
     Net to turbine (MWt)                                                     2060                                     2650                                                3180
     Gross/net electrical output (nominal) (MWe)                            728/666                                   935/881                                            1165/1085
     Efficiency                                                               35.3%                                     35.3%                                              ~36.6%
     Steam temp at main stop valve (°C)                                        258                                      263                                                    273
     Final feedwater temp (°C)                                                 187                                      177                                                    217
     Condenser vacuum (kPa)                                                    4.9                                      4.2                                                    4.9

tor auxiliary building houses redun-                            designed to withstand external events                 struction, with allowance for onsite
dant safety equipment in a four-quad-                           such as earthquakes, tornadoes, floods,                fabrication of module assemblies.
rant design.                                                    aircraft crashes and malevolent acts.                    The footprint of the two-unit plant is
  Security and physical protection                                 The plant layout is also designed to               minimised with the adoption of com-
have been addressed to ensure that the                          achieve the shortest practical construc-              mon areas for the main control room
response to potential common and                                tion schedule while facilitating mainte-              and service and maintenance build-
abnormal events meets latest criteria.                          nance. Buildings are arranged to                      ings. The plant is designed for an
The ACR-1000 containment is                                     minimise interferences during con-                    exclusion zone of 500m.

Section through                                               • Station blackout capability, ensur-        activities, and comprising more than
ACR-1000                                                        ing a rapid return to full power on        10,000 activities – has been issued and is
showing four-                                                   restoration of electrical grid.            being carefully tracked.
quadrant design
                                                              Other important aspects of the ACR-          LICENSING BASIS
                                                              1000 operations and maintenance-             The design of the ACR-1000 systems,
                                                              based design include:                        structures and components is based on
                                                                                                           the successful Candu 6 and Darlington
                                                              • Improved, long-life materials and          nuclear plants. Minimal manufactur-
                                                                experience-based plant chemistry           ing and supply changes are anticipated
                                                                specifications.                             due to the similarities of major plant
                                                              • Improved plant performance                 equipment and components for the
                                                                through integrated Smart Candu             ACR-1000 and Candu 6. Major
                                                                modules, which provide online              equipment and components have been
                                                                health monitoring and diagnostics          proven through many years of continu-
                   IMPROVED O&M                                 for plant chemistry, predict future        ous operation of ten Candu 6 plants. A
                   The design basis lifetime capacity fac-      performance of components, deter-          proven licensing and safety basis builds
                   tor for the ACR-1000, over the operat-       mine maintenance requirements              on 40 years of Candu licensing experi-
                   ing life of 60 years, is greater than        and optimal operating conditions           ence in Canada and around the world.
                   90%, and the design basis year-to-year       and ensure maximum power output.           The balance of plant (BOP), compris-
                   capacity factor is greater than 95%.       • On-power maintenance strategy,             ing 40% of total plant equipment, is a
                   The high average annual output is            maximising component life and              scale-up of the proven Candu 6 BOP.
                   achieved by a low forced loss rate of        minimising component replace-                 The ACR-1000 is designed to meet
                   less than 1.5%. AECL designed and            ment time, thus reducing radiation         regulatory requirements in Canada and
                   built Candu 6 units are already achiev-      exposure, maintenance costs and            other countries. It is an evolutionary
                   ing a lifetime capacity factor of 88.1%,     staff requirements.                        design and enhancements have resulted
                   and the combined average for 2006          • Improved plant layout, with a per-         from extensive AECL efforts in code
                   was 92.4%. Also, the newer Ontario           manent elevator, walkways and              validation and R&D on reactor and fuel
                   Power Generation and Bruce Power             platforms and provision for electri-       design and materials properties, as well
                   multi-unit Candu stations are operat-        cal, water and air supplies built in       as operations and maintenance input
                   ing well, with annual capacity factors       for on-power and normal shutdown           provided through COG. Comprehen-
                   in excess of 90%.                            maintenance.                               sive R&D facilities at AECL’s Chalk
                      Direct feedback provided by Candu       • Computerised testing of major safe-        River Laboratories remain instrumen-
                   plant operators on how to enhance            ty systems and automatic calibration       tal to the success to the new Generation
                   operability and facilitate maintenance       of in-core detector control systems.       III+ ACR-1000 and continue to assure
                   has allowed new features to be             • Increased shielding in radiological-       ongoing support to operating Candu
                   designed into the plant to reduce oper-      ly controlled areas, reducing worker       reactors worldwide.
                   ating risk. Use of data on Candu oper-       exposure and occupational dose.
                   ating experience provided by the                                                        PROJECT IMPLEMENTATION
                   Candu Owners’ Group (COG) net-             DESIGN STATUS                                Maximum use of modularisation and
                   work has also been applied.                The successful Candu 6 fleet is the          ‘open-top’, parallel, construction –
                      Designed-in online inspection and       foundation for the ACR-1000. The             which have already been demonstrat-
                   maintenance has improved the tradi-        ACR-1000 programme focus is to plan          ed at the Qinshan Phase III units, both
                   tional outage of up to one month to        and execute work based on risk analy-        delivered under budget and ahead of
                   one planned outage every three years,      sis, assessment and mitigation, ensur-       schedule – are key to AECL’s ACR-
                   with a standard duration of 21 days.       ing licensability and addressing             1000 project model.
                   For plant life management purposes,        customer input, to achieve an in-ser-           Advanced integrated project man-
                   the reactor design supports a planned,     vice date of 2016. The programme will        agement tools have also contributed
                   mid-life, extended outage for retubing.    have all design documentation com-           significantly to AECL’s successful pro-
                      Operability-enhancing features in       pleted prior to construction.                ject performance. These include:
                   the ACR-1000 include:                         The ACR-1000 programme is being
                                                              managed as a full-scale project, under       • Intergraph 3-D plant modelling
                   • Advanced computer control and            AECL’s Commercial Operations group.            and design.
                     interaction systems based on             New technology input has been con-           • TRAK electronic document man-
                     human factors engineering.               firmed and the licensing basis has been         agement systems.
                   • Enhanced power manoeuvring               established. All elements of the detailed    • Candu Materials Management
                     capability to simplify reactor opera-    engineering programme are in progress          System (CMMS) supply chain
                     tion and make the ACR-1000               and project risk management processes          management system.
                     inherently more responsive; this         and procedures are in place. The pre-        • IntEC wiring design and manage-
                     includes load following and daily        liminary safety case package (PSCP) will       ment system.
                     load cycling capability and lower        be submitted in 2008 and the prelimi-
                     xenon load following reactor power       nary safety analysis report (PSAR) for a     AECL has assessed and qualified inter-
                     reduction.                               site construction licence is scheduled for   national manufacturers and suppliers
                                                              2010. The Level 3 production schedule        of nuclear and conventional equip-
 Dr Ken Petrunik, Chief Operating Officer, Atomic Energy of    – covering the detailed engineering pro-     ment and materials and these form the
 Canada Limited, 2251 Speakman Drive, Mississauga,            gramme together with completion of           foundation of the AECL supply base
 Ontario, L5K 1B2, Canada                                     the remaining R&D work and licensing         for international projects.