Presentation on theme: "Safety Improvements at Cernavoda NPP Dumitru Dina, Nuclearelectrica S"— Presentation transcript:
1 Safety Improvements at Cernavoda NPP Dumitru Dina, Nuclearelectrica S 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 – June 1st, 2007
2 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 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 MWh electric power and the gross capacity achieved since coming into commercial operation was 87.55%.
3 Safety Improvements at Cernavoda NPP Nuclear steam supply systemReactorThe 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.
5 Safety Improvements at Cernavoda NPP Nuclear steam supply systemFuel Handling SystemRefuels the reactor with new fuel bundles without interruption of normal reactor operation;It is designed to operate at all reactor power levelsProvides facilities for the storage and handling of new fuel;Transfers the irradiated fuel remotely from the reactor to the storage bay.
7 Nuclear steam supply system Heat Transport System Safety Improvements at Cernavoda NPPNuclear steam supply systemHeat Transport SystemThe 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.
9 Safety Improvements at Cernavoda NPP Nuclear steam supply systemModerator SystemThe 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.
11 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 1 Major Improvements after 10 years of operationReplacement of refrigerating units “chillers”Improvement of Pumphouse strainer systemReplacement of ECCS inlet filtersFinalization of Spent Fuel Intermediate Storage Facility (Modules 1, 2 & 3)Replacement of defected Vertical Flux DetectorsReplacement of Gaseous Fission Products Monitoring systemReplacement of D2O in H2O Leak Detection systemReplacement of Area Gamma Monitors systemReplacement of Thermal Cycle Bellows AssembliesRefurbishment of Failed Fuel Location SystemFinalization of New Alternative Emergency Control Centre
12 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 1 Major Improvements after 10 years of operation (cont’d)Replacement of Fire Protection system pipesNew system: SERGI type fire extinction system for Plant Power TransformersFinalization of integrated Probabilistic Safety Assessments (PSA Level 1) for Internal and External EventsImplementation of Risc Based Decision Making Process (EOOS)Developed capacity of performing Thermal-Hydraulic Analyses for all CANDU 600 Design Basis AccidentsMember of COG R&D Program for CANDU 600 PlantsMember of WANO Level 3 – Member INPO from 2006CONCLUSION: Annual investments of. 20 ÷ 25 MIL. EURO in development, additional to current costs for O&M
13 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 2 Design ChangesIn 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 worldDuring the design and construction of new plants and operation of existing plants, improved ways of doing things are continually developedAs 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.
14 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 2 Design Changes (cont’d)After much review, 156 design changes were selected for implementation onCernavoda Unit 2Design changes to meet revised licensing requirements.in response to revision of codes, standards or regulatory requirement documentsprovide increases in the margin of safetyChanges due to development of CANDU technology.In general, these changes result in improved performance or reliability of operationOther design improvements that improve system or station performance.Replacement of Obsolete Equipment, and modernizationResult in improved availability of spare parts and maintenanceOften, the more modern equipment also exhibits improved reliability and performance
15 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 2 Design Changes (cont’d)CATEGORY 1: Revised Licensing RequirementsSince 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.
16 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 2 Design Changes (cont’d)CATEGORY 1: Revised Licensing Requirements (cont’d)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.
17 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 2 Design Changes (cont’d)Category 2 – Improved CANDU TechnologyChanges 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 operationImproved 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.
18 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 2 Design Changes (cont’d)Category 3 – Other ImprovementsMany 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.
19 Safety Improvements at Cernavoda NPP Cernavoda NPP Unit 2 Design Changes (cont’d)Category 4 – Replacement of Obsolete EquipmentSince 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 pressureCernavoda 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 controlsThe measuring instrument being used in the gas analysis systemAir break magnetic type breakers are replaced
20 Safety Improvements at Cernavoda NPP Safety Improvements at Cernavoda NPP envisaged for Units 3&4The reference plant for Cernavoda NPP Units 3&4 is Cernavoda NPP – Unit 2 “as commissioned” with the following exceptions:Licensing mandated changesmodifications 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 standardslatest versions of the applicable codes and standards per the Code Effective DateDesign changes due to obsolescenceobsolete equipment which may no longer be availablePlant operating feed back changesfeed back from various operating CANDU-6 plants
21 Safety Improvements at Cernavoda NPP Safety Improvements at Cernavoda NPP envisaged for Units 3&4 (cont’d)NSP Design changes compared to reference plant1. CHANGES DUE TO OBSOLESCENCENSP Replacement of DCC2. CHANGES DUE TO SAFETY AND LICENSING REQUIREMENTSAddition of Gate ValveStainless Steel Liner for Spent Resin Storage Tanks and S/B SumpsStronger Fuel Channel Axial RestraintShield Cooling System ImprovementRelocation of Shield Cooling System Expansion Tank to Inside R/BProvide Recovery System for Moderator and PHTMain Steam Line RoutingSteam Generator Blow down SystemAddition of Main Steam Isolation ValvesDesign Containment Extensions and Seal Plates for MSLB + Failure of DousingEPS 48V DC & 120V AC ModificationsPAM Electrical Power Supply ModificationsStainless Steel Liner for Spent Fuel Storage Area
22 Safety Improvements at Cernavoda NPP Safety Improvements at Cernavoda NPP envisaged for Units 3&4 (cont’d)NSP Design changes compared to reference plant (cont’d)3. PLANT FEEDBACK CHANGESEnlargement of EPS / SCA buildingValvesReactorSteam GeneratorD2O FeedD2O TransferCommunication SystemPressure and Inventory Control Changes during RefuelingAuxiliary Feedwater PumpHVAC Improvements and Dryer’s EliminationControl RoomPressure Tube Installation ReversalMatte Surface Outside of Calandria Tubes
23 Safety Improvements at Cernavoda NPP Safety Improvements at Cernavoda NPP envisaged for Units 3&4 (cont’d)BOP Design changes compared to reference plant1. CHANGES DUE TO SAFETY AND LICENSING REQUIREMENTSAddition of Main Steam Isolation ValvesProtection of Turbine Building and Associated Equipment for Steam Line BreaksSteam Generator Blowdown System2. PLANT FEEDBACK CHANGESDual Train Heat Sinks (RSW/RCW) (with electrical distribution to support dual train heat sinks)Auxiliary Feedwater PumpMain Station Connections Simplified One Line DiagramSteam Turbine DECH Based On DCS Supplier HWDCS Supervision and Control Functions ExtensionCondenser Cooling Water Circulation PumpsCommunications SystemMoisture Separator ReheaterSteam Cycle / Turbine Enhancements