Over the next few years, Kozloduy 5 and 6, two VVER-1000/320 units put into operation at the end of the 1980s, will be subjected to a rigorous modernisation programme to bring them into line with western safety standards.
The programme was devised by Bulgarian institutions, aided by EdF and Russian institutes (MOHT), and has been assessed by the Bulgarian Nuclear Safety
Authority, the IAEA and Riskaudit. It will be performed by a consortium called European Consortium Kozloduy (ECK), comprising Siemens, Atomenergoexpert and Framatome.
In 1998, ECK was contracted to perform basic engineering for 53 measures outlined in the modernisation programme. The aim of this first stage was to fix the final scope of supply and services for those measures.
The basic engineering phase is now finalised. Selected measures that had to be implemented urgently (modernising the UPS, and replacing boronmeters and separators) were contracted separately.
In the summer of 1999, the first redundancy of the UPS at unit 5 was replaced and the new equipment is now in use.
Scope of ECK activities
The main contract for the elaboration of detailed design and the supply and installation of measures with hardware and safety analysis was signed between NEK and ECK in July 1999. The scope of the activities contracted with ECK includes:
•Safety analysis: internal and external hazard analysis, mechanical analysis, preparation of operation documents.
•Fire protection improvements, seismic upgrading and hydrogen detection and recombination.
•Electrical and I&C
systems modernisation: UPS, diesel generators, 6kV distribution, power breakers.
•Diagnostic systems: leaks, loose parts, fatigue, electrical equipment.
•System improvement: thermal insulation replacement, design modification of emergency systems (long-term core cooling), radiation monitoring, secondary system improvements (eg steam generation safety valve replacement), exchange of tube bundles in condenser and heat exchangers.
•Maintenance, inspection and repair procedures and equipment.
Steam generator safety valve (SGSV) replacement
The main task of the SGSV is the protection of the secondary circuit from over-pressurisation.
The modernisation programme will upgrade SGSV to meet improved safety requirements for the VVER-1000/320. The SGSV must be able to cope with two scenarios not considered in their original design. The first assumes a primary to secondary leak where the primary circuit water would quickly fill the steam generator and the part of the steam line that cannot be isolated. The SGSV must therefore be qualified to operate with water and water and steam mixtures to ensure re-closing after opening and to prevent a containment bypass leakage discharging the primary water.
The second scenario is based on probabilistic studies: in some of the analysed transients, it is necessary to remove the decay heat from the primary circuit by feeding the steam generators and relieving them through the steam generator relief valves as well as the SGSV. In this case the SGSV must be useable at low pressure.
The enhanced SGSV will ensure stability in: earthquake conditions (considering the loss of the operational power supply); main heat sink loss; station black-out (automatic pressure limitation and manual opening for bleed function); main steam line leaks behind the main steam isolation valve; and emergency or accident conditions resulting in blow down of water or two phase flow instead of steam. Bleeding will be possible at low system pressures down to 0.5MPa, but the main valve will be tight enough to prevent leaks even at very high pressure. The valves will be opened remotely from the main and emergency control rooms. There will be a positive indication of the SGSV and pilot valves.
The existing lever-type pilot valves have to be removed because they do not met the requirements of the Russian regulation PNAEG-7-008-89, which is in use in Bulgaria.
SGSV modification
The proposed improvement of the SGSV system will take place in three stages.
The first will replace the complete main safety valves (MSV) with new valves from the manufacturer Chekhov-Power. These valves function according to the same principles and have the same dimensions as the existing SGSV, so no changes to the main steam line or valve controls will be necessary.
The second part of the work will replace the existing lever-type pilot valves with spring-loaded pilot valves that have additional solenoid force. The pilot valve is qualified with the main valve.
Finally, each of the eight main safety valves will be equipped with an extra motor-operated pilot valve (MPV) to allow the implementation of the bleed function.
Siemens-Chekhov co-operation
Siemens has co-operated with valve manufacturer Chekhov for several years in the technological development of common products like pilot-operated safety valves, steam generator control valves, high-pressure globe and gate valves and quick-acting globe valves. Chekhov-Power is the main manufacturer for live steam pilot operated safety valves in Russia and eastern Europe. Siemens’ experience is in design, function, material combinations and examining valve technology.
Its integral valve concept uses analysis and valve monitoring to offer a high level of reliability and availability. This safety technology transfer has provided Chekhov with the know-how to manufacture the new SGSV for Kozloduy.
The new valves
The new Chekhov SGSV, designed in co-operation with Siemens, has become the standard for VVER 1000 plants. The valves are installed mainly in steam lines because they have excessive actuation forces. Using this type of valve a safe discharge of large volume flows is possible, even under varying physical conditions, like steam, a mixture of steam and water and water.
The MSV safety function is performed with the loaded concept, the inlet pressure acts by opening the pilot valve. The MSV is actuated by live steam and not by springs or a compressed air drive, to allow adequate safety margins for opening and operation with steam or water. These safety margins have been verified by calculations and testing at the Siemens facility and during in-service testing where the valves have been installed.
The MSV comprises two pilot valves arranged in a functional unit in each SGSV:
•One spring-loaded pilot valve with additional solenoid force in the opening and closing direction. This is controlled by the pressure signal. The advantage of this design is the passive functionality of the pilot valve in case of loss of power supply. This is qualified together with the MSV.
•A second pilot valve is motor-operated to allow bleeding. It allows forced remote opening and closing, also at low system pressure.
Both pilot valves are controlled by switches at the main and reserve control board.
Siemens is responsible for the engineering, hardware supply and erection of the SGSV at Kozloduy 5 and 6. To realise the modification, the company will perform:
Engineering (basic and detailed design)
•Part of the process engineering
•Valves
•Part of the electrical and I&C
•Part of the civil works
•Arrangement of the new control lines
•Erection instructions
•Test procedures
•Modification of the plant documentation
Supply
•16 SGSV, including spare parts
•16 functional units of the pilot valves containing the SPV and the MPV, including spare parts
•Austenitic pipes for the control lines
•New electrical and I&C equipment
Erection and tests
•Preparation of the modification (dismantling the thermal insulation, dismantling the valves, removing the existing valves)
•Cutting out the existing SGSV
•Preparation and welding of the new SGSV
•Examination of the weld seams
•Assembly of the new SGSV
•Installing the new functional units of the pilot valves
•Installing new control lines
•Examining weld seams of the control lines
•Installation and modification of cable trays and cable support structures
•Part of the installation of the necessary cables of the electrical and I&C
•Hydraulic tests
•Installation of the thermal insulation
In 1998 and 1999 the basic engineering of the main steam generator safety valve was performed and the results were approved by Kozloduy. The replacement of the MSV is scheduled for the beginning of the on-site activities of the modernisation programme and will take place in 2002.
