Safety Improvements at Cernavoda NPP Dumitru Dina, Nuclearelectrica S.A. Sorin Ghita, Nuclearelectrica S.A. The Eighth Meeting of the EMRAS Working Group.

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


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%.


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.

Nuclear steam supply system


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.

Fuel Handling System

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.


Heat Transport System


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.

Moderator System


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


Cernavoda NPP Unit 1 Major Improvements after 10 years of operation (cont’d)

• 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 R&D Program for CANDU 600 Plants

• Member of WANO Level 3 – Member INPO from 2006

CONCLUSION:CONCLUSION: Annual investments of. 20 ÷ 25 MIL. EURO in development, additional to current costs for O&M


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.


Cernavoda NPP Unit 2 Design Changes (cont’d)

After much review, 156 design changes were selected for implementation on Cernavoda Unit 2

1. 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

2. Changes due to development of CANDU technology. • In general, these changes result in improved performance or reliability of operation

3. Other design improvements that improve system or station performance.

4. 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



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.



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.



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.



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.



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


Safety Improvements at Cernavoda NPP envisaged for Units 3&4

The reference plant for Cernavoda NPP Units 3&4 is Cernavoda NPP – Unit 2 “as commissioned” with the following exceptions:

1. 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)

2. Changes due to new codes and standards• latest versions of the applicable codes and standards per the Code Effective Date 3. Design changes due to obsolescence• obsolete equipment which may no longer be available 4. Plant operating feed back changes• feed back from various operating CANDU-6 plants


Safety Improvements at Cernavoda NPP envisaged for Units 3&4 (cont’d)

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• PAM Electrical Power Supply Modifications• Stainless Steel Liner for Spent Fuel Storage Area



NSP Design changes compared to reference plant (cont’d)

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 Dryer’s Elimination• Control Room• Pressure Tube Installation Reversal• Matte Surface Outside of Calandria Tubes



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• Steam Cycle / Turbine Enhancements

Safety Improvements at Cernavoda NPP Dumitru Dina, Nuclearelectrica S.A. Sorin Ghita, Nuclearelectrica S.A. The Eighth Meeting of the EMRAS Working Group.

Documents

moderator system slide

cernavoda npp slide

heat transport system

fuel handling system

safety improvements

system replacement of

cernavoda unit

reactor fuel channels