Safety Improvements at Cernavoda NPP Dumitru Dina, Nuclearelectrica S.A. Sorin Ghita, Nuclearelectrica S.A. The Eighth Meeting of the EMRAS Working Group on Modeling of Tritium and Carbon-14 Transfer to Biota and Man, Bucharest, May 30th

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Safety Improvements at Cernavoda NPPDumitru
Dina, Nuclearelectrica S.A. Sorin Ghita,
Nuclearelectrica S.A. The Eighth Meeting of the
EMRAS Working Group on Modeling of Tritium and
Carbon-14 Transfer to Biota and Man, Bucharest,
May 30th June 1st, 2007
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Safety Improvements at Cernavoda NPP Cernavoda
Nuclear Power Plant - General Overview -

• Situated at 160 km east from Bucharest, in
  Dobrogea area, near the Cernavoda town, Cernavoda
  Nuclear Power Plant is a CANDU 6 type NPP (700
  MW).
• Having an annual gross output of 5.2-5.4 mil.
  MWh, the Cernavoda Unit 1 NPP provides 10 of the
  Romanian electric power production.
• Starting with the first connection on July 11,
  1996, it supplied into the grid 54.468.227 MWh
  electric power and the gross capacity achieved
  since coming into commercial operation was 87.55.

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Safety Improvements at Cernavoda NPP
Nuclear steam supply system Reactor

• The reactor comprises a cylindrical stainless
  steel assembly (the calandria) housed within a
  steel lined concrete structure (the calandria
  vault) filled with light water, which provides
  thermal shielding and cooling.
• The calandria contains heavy water (D2O)
  moderator, reactivity control mechanisms and 380
  fuel channels that contain fuel bundles over
  which pressurised D2O coolant from the heat
  transport system is directed.

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Nuclear steam supply system
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Safety Improvements at Cernavoda NPP
Nuclear steam supply system Fuel Handling System

• Refuels the reactor with new fuel bundles
  without interruption of normal reactor operation
• It is designed to operate at all reactor power
  levels
• Provides facilities for the storage and handling
  of new fuel
• Transfers the irradiated fuel remotely from the
  reactor to the storage bay.

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Fuel Handling System
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Safety Improvements at Cernavoda NPP
Nuclear steam supply system Heat Transport
System

• The heat transport system circulates pressurised
  coolant (D2O) through the reactor fuel channels
  to remove heat produced by fission in the fuel.
• The heat is carried by the reactor coolant to the
  steam generators.

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Heat Transport System
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Safety Improvements at Cernavoda NPP
Nuclear steam supply system Moderator System

• The heavy water moderator in the calandria is
  used to thermalize fast neutrons produced by
  fission.
• The moderator is circulated through the
  calandria and moderator heat exchangers to remove
  the heat generated in the moderator during
  reactor operation.
• The heavy water moderator functions as a heat
  sink in the unlikely event of a loss of coolant
  accident coincident with failure of emergency
  core cooling. The capability of this heat sink is
  assured by controlling the heavy water
  temperature in the calandria within specified
  limits.

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Moderator System
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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 1 Major Improvements after 10
  years of operation
• Replacement of refrigerating units chillers
• Improvement of Pumphouse strainer system
• Replacement of ECCS inlet filters
• Finalization of Spent Fuel Intermediate Storage
  Facility (Modules 1, 2 3)
• Replacement of defected Vertical Flux Detectors
• Replacement of Gaseous Fission Products
  Monitoring system
• Replacement of D2O in H2O Leak Detection system
• Replacement of Area Gamma Monitors system
• Replacement of Thermal Cycle Bellows Assemblies
• Refurbishment of Failed Fuel Location System
• Finalization of New Alternative Emergency
  Control Centre

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 1 Major Improvements after 10
  years of operation (contd)
• Replacement of Fire Protection system pipes
• New system SERGI type fire extinction system
  for Plant Power Transformers
• Finalization of integrated Probabilistic Safety
  Assessments (PSA Level 1) for Internal and
  External Events
• Implementation of Risc Based Decision Making
  Process (EOOS)
• Developed capacity of performing
  Thermal-Hydraulic Analyses for all CANDU 600
  Design Basis Accidents
• Member of COG RD Program for CANDU 600 Plants
• Member of WANO Level 3 Member INPO from 2006
• CONCLUSION Annual investments of. 20 25 MIL.
  EURO in development, additional to current costs
  for OM

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 2 Design Changes
• In the years since Cernavoda 1 design was
  finalized, there have been many developments in
  the nuclear industry in Romania, Canada and the
  world
• CANDU 6 plants similar to Cernavoda 1 and 2 have
  been built and placed in service in South Korea
  (3 units at Wolsong) and in China (2 units at
  Qinshan)
• Additional experience has been gained from
  operation of CANDU plants, including Cernavoda 1,
  around the world
• During the design and construction of new plants
  and operation of existing plants, improved ways
  of doing things are continually developed
• As a prudent owner, Societatea Nationala
  Nuclearelectrica (SNN) SA decided to consider all
  of the known improvements to identify those which
  were appropriate for installation in Cernavoda 2,
  considering the progress of construction.

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 2 Design Changes (contd)
• After much review, 156 design changes were
  selected for implementation on
• Cernavoda Unit 2
• Design changes to meet revised licensing
  requirements.
• in response to revision of codes, standards or
  regulatory requirement documents
• provide increases in the margin of safety
• Changes due to development of CANDU technology.
• In general, these changes result in improved
  performance or reliability of operation
• Other design improvements that improve system or
  station performance.
• Replacement of Obsolete Equipment, and
  modernization
• Result in improved availability of spare parts
  and maintenance
• Often, the more modern equipment also exhibits
  improved reliability and performance

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 2 Design Changes (contd)
• CATEGORY 1 Revised Licensing Requirements
• Since the original design of Unit 1 was
  completed, some of the codes, standards and
  regulatory licensing requirements have been
  revised to improve consistency and to increase
  the margin of safety. Of the 156 design changes
  being implemented for Unit 2, approximately 50
  are of Category 1.
• New requirement documents were issued by the
  Canadian Regulatory Body to document requirements
  for design, construction, commissioning and
  operation of special safety systems of CANDU
  plants.
• Additional requirements for piping systems
  passing through the containment boundary. 25 out
  of the 156 design changes performed on several
  systems.
• Most of these changes upgrade the ASME class of
  the pipe from the containment wall up to and
  including the first point of isolation outside
  containment boundary.
• In a few cases, additional automatic closing
  isolation valves are installed.

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 2 Design Changes (contd)
• CATEGORY 1 Revised Licensing Requirements
  (contd)
• Revision of the CSA standard which defines the
  requirements for provision of environmentally
  qualified instruments for operator monitoring of
  plant conditions following accidents.
• additional indicator showing the position of a
  critical valve to the Secondary Control Area so
  that the operator can be sure that the valve is
  correctly positioned following a significant
  earthquake. The existing design provides the
  information in the Main Control Room only.

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 2 Design Changes (contd)
• Category 2 Improved CANDU Technology
• Changes to the fuelling machines.
• Fuelling machine ram silicon carbide seals which
  increase the seal lifetime and make maintenance
  easier.
• Modification of the fuelling machine ram drives
  which improves flexibility of operation
• Improved design of an orifice in the Primary Heat
  Transport System. The improved design will
  reduce erosion of the orifice and reduce the
  frequency of maintenance.
• Several of the improvements result in reduced
  maintenance, and also provide the benefit of
  lower radiation dose to station staff.

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 2 Design Changes (contd)
• Category 3 Other Improvements
• Many of the other design changes are based on
  experience in Cernavoda Unit 1 and other stations
  where performance weaknesses have been observed.
  
• Most of the changes are principally to avoid
  frequent maintenance and system unavailability.
• The following are examples
• Some turbine trip instrumentation is improved by
  using 2 out of 3 instruments rather than a single
  instrument. This result in improved reliability
  and avoid spurious trips on instrument failure.
• The capacity of the reheater drain pumps is
  increased to improve reliability. In addition,
  the piping design is improved to avoid flow
  directly to the condenser resulting in wasted
  heat.
• Design changes are made to use Recirculating
  Cooling Water in some applications where Raw
  Service Water was previously used. This prevent
  fouling of heat exchangers due to zebra mussels
  or other deposits from the raw water. This
  improves equipment lifetime and reliability and
  results in less frequent requirements for
  disassembly and cleaning of the heat exchangers.

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Safety Improvements at Cernavoda NPP

• Cernavoda NPP Unit 2 Design Changes (contd)
• Category 4 Replacement of Obsolete Equipment
• Since the purchase of equipment for Cernavoda
  Unit 1, several years have passed and some
  vendors have developed new models based on more
  recent technology.
• In CANDU 6 stations, most significant control
  functions are performed using Digital Control
  Computers (DCC). Important functions controlled
  include
• reactor power,
• water level water in the steam generators,
• steam flow to the turbine generator,
• steam generator pressure
• Cernavoda 2 DCCs are changed to ensure reliable
  operation through readily available spare parts
  and technological support from the vendor.
• The following are other changes made to avoid the
  problems associated with obsolete equipment
• Upgraded control systems for the turbine
  generator and other balance of plant controls
• The measuring instrument being used in the gas
  analysis system
• Air break magnetic type breakers are replaced

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Safety Improvements at Cernavoda NPP

• Safety Improvements at Cernavoda NPP envisaged
  for Units 34
• The reference plant for Cernavoda NPP Units 34
  is Cernavoda NPP Unit 2 as commissioned with
  the following exceptions
• Licensing mandated changes
• modifications and improvements to the Reference
  Plant due to new Safety and Licensing
  Requirements applicable per the Code Effective
  Date (assumed April 01, 2005) and in accordance
  with Licensing Basis Document (LBD)
• Changes due to new codes and standards
• latest versions of the applicable codes and
  standards per the Code Effective Date
• Design changes due to obsolescence
• obsolete equipment which may no longer be
  available
• Plant operating feed back changes
• feed back from various operating CANDU-6 plants

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Safety Improvements at Cernavoda NPP

• Safety Improvements at Cernavoda NPP envisaged
  for Units 34 (contd)
• NSP Design changes compared to reference plant
• 1. CHANGES DUE TO OBSOLESCENCE
• NSP Replacement of DCC
• 2. CHANGES DUE TO SAFETY AND LICENSING
  REQUIREMENTS
• Addition of Gate Valve
• Stainless Steel Liner for Spent Resin Storage
  Tanks and S/B Sumps
• Stronger Fuel Channel Axial Restraint
• Shield Cooling System Improvement
• Relocation of Shield Cooling System Expansion
  Tank to Inside R/B
• Provide Recovery System for Moderator and PHT
• Main Steam Line Routing
• Steam Generator Blow down System
• Addition of Main Steam Isolation Valves
• Design Containment Extensions and Seal Plates for
  MSLB Failure of Dousing
• EPS 48V DC 120V AC Modifications

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Safety Improvements at Cernavoda NPP

• Safety Improvements at Cernavoda NPP envisaged
  for Units 34 (contd)
• NSP Design changes compared to reference plant
  (contd)
• 3. PLANT FEEDBACK CHANGES
• Enlargement of EPS / SCA building
• Valves
• Reactor
• Steam Generator
• D2O Feed
• D2O Transfer
• Communication System
• Pressure and Inventory Control Changes during
  Refueling
• Auxiliary Feedwater Pump
• HVAC Improvements and Dryers Elimination
• Control Room
• Pressure Tube Installation Reversal
• Matte Surface Outside of Calandria Tubes

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Safety Improvements at Cernavoda NPP

• Safety Improvements at Cernavoda NPP envisaged
  for Units 34 (contd)
• BOP Design changes compared to reference plant
• 1. CHANGES DUE TO SAFETY AND LICENSING
  REQUIREMENTS
• Addition of Main Steam Isolation Valves
• Protection of Turbine Building and Associated
  Equipment for Steam Line Breaks
• Steam Generator Blowdown System
• 2. PLANT FEEDBACK CHANGES
• Dual Train Heat Sinks (RSW/RCW) (with electrical
  distribution to support dual train heat sinks)
• Auxiliary Feedwater Pump
• Main Station Connections Simplified One Line
  Diagram
• Steam Turbine DECH Based On DCS Supplier HW
• DCS Supervision and Control Functions Extension
• Condenser Cooling Water Circulation Pumps
• Communications System
• Moisture Separator Reheater