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IC Concepts for an On-line Regional Overpower
Protection (ROP) System for CANDU Reactors
Presented by B. Sur

• Collaborators
• V. Mihaylov, H.W. Hinds,
• V. Anghel, G. Jonkmans,
• D. Wallace, V. Caxaj, F. Laratta

IAEA Technical Meeting on Increasing Power Output
and Performance of NPPs by Improved IC Systems,
29 - 31 May 2007, Prague, Czech Republic.
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Outline

• Background
• Instrumentation
• Concepts for improvements
• Issues and challenges

• CANDU Reactors
• Need and Requirements for ROP System
• In-core sensors, control and shutdown devices
• ROP Trip chain instrumentation
• Digital IC implementation
• On-Line flux shape monitoring
• Digital Safety System Qualification
• Risk

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CANDU (Canada Deuterium Uranium) Reactors

• Large core numerous critical flux shapes,
  possibility of localized flux shape distortions
• Regional Overpower Protection (ROP) needed

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Regional Overpower Protection (ROP) System

• ROP uses in-core flux detector signals to
  shutdown reactor before any fuel reaches dryout
  (Critical Channel Power or CCP)
• After 20 years service, for existing stations
• fuel channel diametrical creep
• flow partially bypasses fuel, lower CCP
• reactor power limited by ROP trip set point
• ROP system first implemented in Bruce A (4 x 800
  MWe) units early 1980s
• Darlington (4 x 935 MWe) is only CANDU station
  with computerized ROP system

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CANDU In-core Instrumentation
Shut-off Mechanical Control Absorber Rods
Vertical ROP Flux Detectors (SDS1)
HorizontalROP Flux Detectors (SDS2)
Liquid Zone Controllers
(Adjuster Rods)
Ion Chambers
Poison Injection Nozzles
Vanadium Flux Detectors
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ROP detector trip chain (One of 60)
Manually adjustable 2 time constants, gains
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Current ROP Methodology

• Determination of Trip set-points (TSP) performed
  off-line, every 2-4 years and requires
• 1000 standard case-sets of reactor flux shapes
  (channel powers, flux detector readings),
• Thermalhydraulics code to calculate Critical
  Channel Power (CCP)
• Probabilistic code to calculate TSP with
  stringent availability requirements for SDS1
  SDS2
• Detector gain adjustments for normal flux shape
  variations performed daily
• Reactor Physics code T/H code used offline
• Calculate Channel Power Peaking Factor (CPPF)

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Analog electronics-based ROP system
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Fully automated digital IC ROP concept
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Benefits Improved Performance

• Automated TSP download and tracking
• Automated detector gain adjustments and tracking
• Automated on-line trip testing
• On-line signal validation
• Improved dynamic response correction
• Automatic adjustments and tracking of dynamic
  response parameters
• Reduced operations and maintenance costs,
  improved reliability

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Benefit Increased Power Output

• Current trip set-points reflect the margin
  required to protect against any possible flux
  shape
• An on-line flux-mapping approach, on the other
  hand, calculates a trip set-point which is based
  on the instantaneous configuration of the reactor
• Still protected for events which have a low
  probability of occurrence, but not penalized
  during normal operations
• Increased margin-to-trip from an on-line
  (computer based) ROP trip system

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Assessment of set-point increase
D.J. Wallace, V. Caxaj, A.S. Seidu, W. Hartmann,
B. Sur and A. McDonald , Advanced On-Line
Regional Overpower Surveillance (AOL-ROS) System
Development 5th ITM on NPICHMI Albuquerque, NM,
Nov 12-16, 2006
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Digital Safety System Qualification
Usage of international standards IEC 61508 safety
critical systems IEC 61513 nuclear safety
systems IEC 62138 software lifecycle IEC 61131
programming languages
Safety Integrity Level 3 (SIL-3) certifiable
hardware (PLC, DCS, PSS) and certified software
libraries available ABB, Siemens, Triconex,
RTP, Pilz,
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Risks / Challenges

• Current Canadian regulatory standards are
  evolving from OH AECL Software Evaluation
  Standard (OASES)
• Stations must be convinced of cost / benefit
  ratio of digital retrofit
• history of safety system software projects
• risk of new system not licensable
• risk of new system not performing as expected
• phased approach? Initially install On-Line
  Regional Overpower Surveillance system

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Status of Digital ROP Development

• Alternatives for on-line flux-shape monitoring
  algorithms being evaluated
• Further analysis of real plant data underway to
  refine understanding of ROP margin gain by
  on-line flux-shape monitoring
• Generic requirements and program specifications
  being developed for retrofitting digital ROP to
  existing CANDU plants
• New CANDU reactors will have digital ROP systems

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