RESULTS OF THE GRADUAL UPGRADING AT BOHUNICE WWER - 440/230 NPP

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1 RESULTS OF THE GRADUAL UPGRADING AT BOHUNICE WWER - 440/230 NPP P. Krupa Ingeneer, Krupa_Peter@ebo.seas.sk Bohunice NPPs Introduction The centre of upgrading activities in VVER NPP is clearly in the Central and East European countries, which are applying for the accession to the European Union. This concerns especially the Slovak Republic, Czech Republic, Hungary and Bulgaria which are running in total 20 VVER NPP. Typical for these countries is a small number of NPP but with a high share (20% to 45%) in the countries total electrical energy production. This means a closure of older plants within a short period, as repeatedly requested by West European politicians, is a difficult and challenging request for this countries. Consequently since the political changes in the early nineties they opted for upgrading their plants up to an internationally acceptable safety level, in order to secure the existing investment. Jaslovské Bohunice V1 NPP consists of two units of VVER-440 type V-230 series. Unit 1 was commissioned at the end of 1978 and the second one at the beginning of The original Russian design of unit s VVER-440/230 series originates from the end of sixties and the beginning of seventies. Rupture of primary coolant circuit with equivalent diameter of32 mm was stated as maximal design basic accident, for managing of which the emergency systems were dimensioned in compliance with those days valid legislation. Positive features of the original design Plant parameters with large margins between operating conditions and safety limits provide flexibility to cope with operational transients and accidents. This is evidenced in design features such as: Low heat production with respect to the fuel weight Large volume of primary water small core/coolant ratio Horizontal steam generator (SG) designs with large water inventories High quality of the manufacturing of main primary components This inherent safety capacity of the WWER gives considerable safety margins ensures residual-heat removal without feeding for 4-5 hours, stable natural circulation for residual-heat removal up to 5 % of nominal power.

2 Deficiencies of the original design However, original plant design had following deficiencies: Insufficient capacity for emergency core cooling Insufficient confinement tightness and strengths Poor physical separation of redundant systems Insufficient instrumentation and control Insufficient qualification of safety systems (including seismic event) Insufficient fire protection measures Safety goals for upgrading of Bohunice V1 plant Deterministic safety goals Main safety goal was to upgrade safety systems, safety support systems and confinement to extent of the list of postulated DBAs in compliance with international practice and IAEA guidelines. Specifically to cope with: LOCA up to break of - 1 x 200 mm D ekv at the most limiting location (conservative approach) - 2 x 200 mm D ekv pressurizer surge line (conservative approach) - 2 x 500 mm D ekv primary loop (best estimate approach) without exceeding of allowed doses in the protected area Main steam header rupture Steam line rupture between the steam header and SG Feedwater line rupture Lifting-up of SG primary collector cover Earthquake 8 MSK 64 (a hor = 0.34 g; a vert = g) Basic design requirements on the new safety systems including their support systems Minimal redundancy 2 x 100 % Physical separation of redundancies (full or to the reasonably achievable level) Physical and electrical separation of safety systems from normal operation systems Seismic resistance up to 8 MSK 64 Demonstration of compliance with the single failure criterion Automatic activation Fire and flooding resistance

3 Qualification for environmental conditions (normal operation and accident conditions) Capacity of consumed media for 72 hours (feedwater, DG fuel, essential service water) Testability of safety systems at full power operation (adopted by plant in the course of design) Approach how to achieve the safety goals Nuclear power plant safety is based on the defence in depth concept. Following this concept, the general design of a nuclear plant includes successive barriers which prevent the radioactive material from leaking to the outside. In a WWER, there are 4 successive barriers, namely: the fuel matrix (1 st barrier), the fuel cladding (2 nd barrier), the primary circuit boundary (3 rd barrier) and the confinement boundary (4 th barrier). Upgrading program was aimed to strengthen defence in depth in a balanced way. Highest priority was given to preventive measures in general, and in particular to those areas where the feasibility of modifications was limited. Prevention of severe accidents with core melt was also systematically addressed by appropriate measures Modifications performed in Bohunice V1 NPP Significant step in safety improvement of Bohunice V1 NPP units was made after 1990, after realisation of more national and international expert missions. Expert missions were focused on evaluation of Bohunice V1 NPP nuclear safety and operational reliability level status. Short - term and long - term measures were the results of evaluation of expert missions, while no one mission approached the conclusion that it is necessary to shut down the Bohunice V1 NPP. Based on recommendations of individual expert missions the Czechoslovak Nuclear Regulatory Authority issued the decision No. 5/91 dated January 11, There were defined 81 measures concerning further Bohunice V1 NPP safety and reliability improvement and decision No. 213/92 dated June 23, 1992, where there were stated another 14 measures. These measures were performed during the period years and they are known as BOHUNICE V1 NPP SMALL RECONSTRUCTION. Preparation of Bohunice V1 NPP gradual upgrading Folloving the decision No. 5/91 of Czechoslovak Nuclear Regulatory Authority on possibility to operate Bohunice V1 NPP after 1995 under the condition, that its nuclear safety will be increased into the European level, the preparation of "PRINCIPAL RECONSTRUCTION" started with working teams of Bohunice NPPs, VÚJE, Škoda and Energoprojekt since Result of activities was issuing the " SAFETY CONCEPT FOR NPP PRINCIPAL RECONSTRUCTION dated

4 June 1992, which was reelaborated in February 1993 in to the form of "SAFETY CONCEPT FOR BOHUNICE V1 NPP GRADUAL RECONSTRUCTION" and which was approved by Slovak Nuclear Regulatory Authority decision No. 1/94 dated February 24, Elaborating of BASIC ENGINEERING for Bohunice V1 NPP Regarding importance and scope of Bohunice V1 NPP reconstruction works there were selected more foreign companies for submitting their tenders. Westinghouse and Siemens submitted satisfactory tenders at the beginning of Selection of tenders finished in the first quarter of 1994 for the benefit of Siemens KWU and on May 5, 1994 there was signed the contract for Basic design for Bohunice V1 NPP gradual reconstruction.basic Engineering was finished in November The results of engineering work were assessed and commented by Bohunice V1 NPP and independent organisations and finally they were submitted to Slovak Nuclear Regulatory Authority for approval. During Basic Engineering elaborating the activities of suppliers and customer were co-ordinated in 15 expert working groups, which discussed and solved their problems. Elaborating the DETAIL DESIGN and REALIZATION Elaborating of realisation documentation, equipment production, implementation, testing and putting into operation for whole gradual reconstruction were provided by MAIN CONTRACTOR - consortium REKON consisting of Siemens and the Slovak Nuclear Power Plants Research Institute (VÚJE), according the contract that was signed in April The works associated with the reconstruction of each out of 15 functional process entities were implemented mainly during extended unit refueling outages. The normal duration of unit outage (approximately 40 days) was extended up to days according to the scope of the work planned. It was in line with the global philosophy of the investor to implement the reconstruction work gradually at the unit 1 and at the unit 2 during particular outages without the need to shut the unit down for a long time. This philosophy required large efforts to be spent to the consistent planning and co-ordination of activities during outages. NPP gradual reconstruction implementation was made from the end of 1996 and last modifications were made during refuelling in The progress of reconstruction was regularly reported to IAEA during Governing Committees of IAEA extrabudgetary programme, as well as to European Commission (EC) during meetings of working groups CONCERT (regulatory bodies

5 of Eastern, Middle and Western Europe) and NUSAC (co-ordination of help in the area of nuclear safety). The Government of the SR was regularly yearly informed on the reconstruction progress. FULL SCOPE OF BOHUNICE V1 NPP GRADUAL UPGRADING was divided into fifteen systems as follows: 1. SAFETY AND RELIEF VALVES reconstruction, from pressuriser to relief tank, to fulfil the BLEED function for Bleed and Feed on primary circuit. 2. EMERGENCY FEEDWATER SYSTEM supplementing, to steam generators to fulfil the FEED function for Bleed and Feed on secondary circuit. System is now designed for 72 hours of operation without the need of external make- up water. 3. STEAM RELIEF VALVES from SG to the atmosphere supplementing, to fulfil the BLEED function for Bleed and Feed on secondary circuit. 4. THIRD ELECTRIC SOURCE supplementing, to supply home electric consumption from the nearby hydroplant Madunice, in case of Black - Out. 5. EMERGENCY CORE COOLING SYSTEM reconstruction into the two separate and independent redundancies, to fulfil the core cooling function for extended scope of LOCAs and FEED function for Bleed and Feed on primary circuit. Each redundancy consists of two HPSI pumps an one LPSI pump. System is designed to cope with LOCA spectrum and to ensure subcriticality after secondary side breaks and after seismic event (new 80 m 3 boron tank). The system is able to provide for the primary side Residual Heat Removal. 6. FIRE PROTECTION (14 measures) improvements. 7. ELECTRIC SYSTEMS reconstruction, to have two independent and separated redundancies. The Emergency Electrical Power Supply System, s capacity (DGs power) has been doubled for each safety train in order to cover the energy demand of new installed ECCS system during LOCA accident. The capacity of Accumulator Batteries has been increased from 30 minutes to 2 hours. exchange of the obsolete motorgenerators reconstruction of DG control and excitation systems reconstruction of 6 kv, 0,4kV, 220 DC + AC switchgears 8. I&C SYSTEMS reconstruction: (RPS) RTS + ESFAS exchange by new Siemens digital system TELEPERM XS Reactor Power Control System exchange

6 Excore Neutron Flux Measures exchange Incore Temperature Control Measures exchange Reactor Power Limitation System supplementing PAMS - more parameters supplementing Radiological Monitoring System supplementing 9. CONFINEMENT SPRAY SYSTEM reconstruction into the two separated and independent redundancies. 10. PRESSURE SUPPRESSION SYSTEM supplementing, after LOCA, inside confinement. 11. CONFINEMENT ISOLATION improvement by isolation valves supplementing, into ventilation pipes crossing through confinement boundary. 12. CONFINEMENT STRENGTH increasing, to assure confinement resistance against maximum over and under pressure. 60/120 kpa over pressure for DBA Ø 200/DBA Ø500, and 15 kpa under pressure. 13. SERVICE WATER SYSTEM reconstruction and supplementing for cooling safety systems, to separate and create essential service water system. New Essential Service Water System was constructed and designed for 72 hours of operation without the need of external make-up water. 14. VENTILATION SYSTEMS reconstruction and supplementing, for cooling rooms, where new I&C and Electrical systems are placed. 15. SEISMIC improvement on a lot of equipment that can have negative influence to safety systems. Conclusion Modernising a NPP is at least as challenging as building a new plant. The renewed and the remaining plant systems have to match properly in order to achieve the goal of increased nuclear safety and plant availability and finally to guarantee the integral function of the modernised plant. Carrying out the gradual reconstruction we achieved the internationally acceptable nuclear safety and operational reliability level of the V1 plant. This has established technical assumption for reliable, safe, economical and ecological operation Bohunice V1 NPP units. In the final report from conference organised by EU, OECD and IAEA held in Vienna on June 14 18, 1999 dealing with safety upgrading of reactors in Eastern Europe, the implementation measures at the Bohunice V1 NPP was rated as an example of the safety level which should VVER 440/V230 reactors achieve.

7 Bohunice V1 NPP as the most modern WWER 440/V230 plant is a good example how eastern type of units can be successfully upgraded and improved, in cooperation with western companies, into the internationally acceptable safety level and further more in reasonable costs. In , 18 international expert missions visited Bohunice and none of them required to shut the nuclear power plant down. The results of the Gradual Reconstruction at Bohunice V1 were appreciated several times at the international level, and in this way also the excellent technical work of Slovak technicians, designers and engineers in co-operation with the well-known foreign company Siemens. In addition to IAEA mission, V1 NPP was evaluated by SIEMENS mission, mission appointed by the Austrian Government, WANO (World Association of Nuclear Operators) mission, WENRA (West European Nuclear Regulatory Association), then it was evaluated within the first evaluation meeting in compliance with Nuclear Safety Agreement, Conference on Nuclear Power Plants Safety in the countries of Middle and Eastern Europe, working group within the high level committee between EC and SR, consortium appointed by EC and currently by a Group for nuclear issues appointed by EC. The successful completion of the extensive reconstruction of safety related systems and the achieved high level of safety is in a sharp contrast with the decision of the government of the Slovak Republic to close prematurely Bohunice V1 unit 1 in 2006 and unit 2 in One can hope that under the pressure of technical and economical arguments, the government s decision about the premature closure of Bohunice V1 will be revoked and the units will generate and supply reliably and cheaply electricity for consumers.