National Report of The Slovak Republic , september 2001


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2.1.3.2 Implementation of the Gradual Upgrading Project


The implementation of the gradual upgrading of NPP V-1 was organized by the REKON Consortium under a general delivery contract. The scope of the general delivery included:

development of projects for construction permits for the individual systems,
development of executive projects for the individual systems,
drafting of the safety analysis report and conduction of analyses,
development of programs of pre-operational testing,
drafting of operating procedures,
delivery of equipment, systems and components, including delivery of technical documentation, equipment maintenance schedules, and staff training plans,
assembly of the systems, including pre-operational testing, commissioning and handing over to the client.

The gradual upgrading programme was partly implemented during normal operation of V-1 units, but mainly during planed unit refueling outages and overhauls (OH). The outages were extended depending on the extent of the upgrading works to be performed. The gradual upgrading commenced during unit 2 OH in 1996 and was completed during unit 1 OH in 2000.

The implementation works were divided among the following 15 functional technological systems:

  1. Upgrading of pressurizer safety valves and the relief line from pressurizer to the relieve tank, that cover the “BLEED” function for “Primary Side Bleed and Feed”.
  2. Upgrading of the emergency feed water system, to cover the „FEED” function for “Secondary Side Bleed and Feed”. The system is currently designed for 72 hours of operation without the need of being supplied by feed water.
  3. Upgrading of the steam generators relieve valves, to cover the “ BLEED” function for “Secondary Side Bleed and Feed”.
  4. Grid III Madunice - off-site emergency power supply from the near-by hydroelectric power station, to improve black-out management.
  5. Upgrading of the emergency core cooling system to provide for two separate and independent redundancies, to cover the function of core cooling for extended LOCA accidents and the “FEED” function for „Primary Side Bleed and Feed“. Every redundancy comprises two high-pressure pumps and one low-pressure pump. The system has been designed to be able to cope with the LOCA range and to ensure core subcriticality upon secondary system breaks and upon seismic events (a new 80 m3 boric acid concentrate tank). The system is designed ensure core residual heat removal.
  6. Improved fire resistance - 14 measures.
  7. Upgrading of the electro system to provide two independent and separate redundancies. The capacity of the emergency supplies (from DG) has been doubled to provide power supply for newly installed emergency systems. The capacity of the batteries was raised from 30 minutes to 2 hours.
    Replacement of outdated motor generators,
    Upgrading of the DG control and excitation systems,
    Upgrading of 6 kV, 0.4 kV, 220V DC/AC distribution switchboards
  8. Upgrading of the instrumentation and control system (I&C) :
    Replacement of reactor protection system (RTS+ESFAS) for a new digital system TELEPERM XS from Siemens,
    Replacement of the reactor power control system,
    Replacement of the external neutron flux measurement system,
    Replacement of the in-core measurement system,
    Installation of a reactor power limitation system,
    Improvement of PAMS (higher number detectors),
    Improvement of radiation monitoring system.
  9. Upgrading of the containment spray system to provide two separate and independent redundancies.
  10. Installation of an accidents localization system within the confinement after LOCA.
  11. Improvement of the confinement integrity, through adding of fast-acting flaps to the air conditioning piping at the confinement border. The tightness improvement of units 1 and 2 confinement is illustrated in Fig. 2.1.2.

Fig.2.1.2 Results of the NPP V-1 units confinement leak tightness improvement


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  1. Confinement reinforcement, to provide for confinement resistance against overpressure and negative pressure. For overpressure the resistance is 60kPa for DBA, i.e. break of O200mm, and 120 kPa for BDBA, i.e. break of O500mm. Resistance for negative pressure is 15 kPa.
  2. Installation of the essential service water system for cooling of safety systems and its separation from old service water system (non safety related). The new system of essential service water was designed to ensure 72 hours of operation without the need of other external water source.
  3. Upgrading of air conditioning systems for cooling the new electric and I&C systems compartments.
  4. Improvement of the seismic resistance of equipment that may negatively effect safety systems.

A simplified draft of safety systems of the NPP V-1 primary circuit following the gradual upgrading is on Fig. 2.1.3.(a new 80 m3 boric acid concentrate tank not included).

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Fig. 2.1.3    Safety systems of NPP V-1 units following the gradual upgrading

1. Reactor
2. Steam generator
3. Main circulation pump
4. Main isolation valve – hot leg
5. Main isolation valve – cold leg
6. Pressurizer
7. Pressurizer relieve tank
8.HP emergency core cooling pump

9.   Low-pressure emergency core cooling pump
10. Spray system pump

11. Emergency boric acid storage tank
12. Heat exchangers

13. Spray system nozzles
14. Containment


Fig. 2.1.4 illustrates time schedule of the whole gradual upgrading of NPP V-1.


Fig. 2.1.4 Implementation time schedule of NPP V-1 gradual upgrading

Having implemented the NPP V-1 gradual upgrading programme, the objectives set by ÚJD Decision No.1/94 were met, representing a precondition for granting approval for further operation of the NPP. The completion of the reconstruction works resulted in a marked improvement of NPP V-1´s nuclear safety standard, thus achieving the internationally acceptable standards of safety and operating reliability. At the same time, preconditions were created for a safe, reliable, economical and environmentally-friendly operation of NPP Bohunice V-1 to be continued at least through the end of its designed service life.

Previously, ÚJD approved further operation of NPP V-1 units conditional upon meeting the above criteria connected with the progress of works on safety improvement. The approval remained valid for always one fuel cycle. Based on a review of the safety report following the gradual upgrading of the units, ÚJD is about to issue a decision on the approval of continued operation of NPP V-1 unit I, stating conditions listed in the annex of the decision.

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