Secondary side feed and bleed upgrade at Bohunice V-1

The utility Slovak Electric operates four VVER-440 reactors at the Bohunice site. Two of them, called collectively V-1, are of the older V-230 type. They were connected to the grid for the first time in November 1978 and April 1980, respectively.

The operational record of these two units shows good performance in terms of safety even in comparison with Western pressurised water reactors. No major incident, rated at INES (International Nuclear Event Scale) level 2 or higher, has occurred and anomalies rated at level 1 had a frequency of only 0.25 per reactor-year. However, some weaknesses have been identified in various areas of the design which have required corrective actions.

A number of modifications have been implemented since the commissioning of the units. In addition, a significant programme of safety upgrading was started at the plant in 1991. The programme was divided into two major steps. The first, called “Small Reconstruction”, was carried out in 1991-1993. The second, “Gradual Safety Upgrading”, began being implemented in 1994 and is scheduled to be completed in 1999.

The basic upgrading programme has been designed by the Bohunice plant together with Siemens. The detailed design was done by the REKON consortium, made up of Siemens KWU and VUJE Trnava Inc, which is also managing its implementation. Most of the work is carried out by local subcontractors. All activities are funded from national sources.

FEED AND BLEED

Sixteen areas have been selected for plant modifications in the Gradual Safety Upgrading programme. Two of these are related to the secondary side feed and bleed function. These are:

• Installation of power-operated steam dump relief valves (PORVs), which release steam to atmosphere, on the main steam lines from the steam generators.

• Upgrading of the “super-emergency” feedwater system to the level of a safety system.

With these modifications the secondary side feed and bleed function is assured even in the case of loss of main and auxiliary feedwater systems.

Steam generator PORVs

The new steam generator PORVs are installed on the six main steam lines upstream of their fast acting isolation valves. They are qualified to release steam, water, or a steam–water mixture. This arrangement allows controlled steam bleed, which acts as a heat sink, in the case of a loss of normal operating systems, eg turbines or bypass turbine valves (in the event of, for example, an earthquake, fire, loss of off-site power and diesel generators).

The capacity of the PORVs is sufficient to remove residual heat from the steam generators to atmosphere. Valve actuation is initiated by a high steam generator pressure signal.

Power for the PORVs is supplied from “essential” busbars which are seismically qualified. No additional auxiliary system is needed for their operation.

Manual control by operators in both main and emergency control rooms is possible to provide a smooth cooldown of the plant from 260°C down to 130°C. Later, the residual heat removal (RHR) system can be put into operation.

Super-emergency feedwater system

The original Soviet design included two auxiliary feedwater pumps. Comprising the auxiliary feedwater system, these are located in the turbine hall and are connected to other process systems such as feedwater tanks and main feedwater pumps. It is not realistic to qualify this auxiliary feedwater system as a safety system since it can be affected by common cause failures such as flooding, fires and high energy line breaks – although, it has been used for start-up and shutdown purposes.

However, the Bohunice V-1 units also have a “super-emergency” feedwater system – the only VVER-440 V–230s to be so equipped. Installed as part of the original equipment this super-emergency feedwater system, consists of two pumps installed in a separate building outside the turbine hall (ie a total of two pumps per the two units of the V-1 station). The suction lines for these pumps are connected to demineralised water storage tanks and their discharge lines to steam generator feedwater lines. During the “Small Reconstruction”, a new parallel discharge line was installed, connected to the steam generator blowdown lines.

It is this super emergency system that has been selected for upgrading. This is because it has minimal connections to normal operating systems and of the common cause failures only earthquakes need to be considered. The upgrade measures are:

• Modification of the demineralised water storage tank system to provide water for 72 hours of residual heat removal per reactor. This required a realignment of the suction lines of the pumps supplying water to the main condensers during normal operation. The tanks were also upgraded for seismic events.

• Installation of two additional super-emergency feedwater pumps to comply with the general plant upgrading concept to meet the single failure criterion for both units. As a consequence, each unit is now equipped with two trains of safety systems, including the super-emergency feedwater system.

• Seismic upgrading of the two old super-emergency feedwater pumps and modifications of the pumps and motor cooling systems in such a way that they will be independent of other auxiliary systems (originally providing non-essential service water).

• Realignment of the super-emergency feedwater pump discharge lines. In the upgrade, there are two lines for each steam generator, a primary and a back-up. The primary line is connected to the normal feedwater line near the steam generator feedwater nozzle, downstream of a newly installed check valve. The back-up line is connected to the steam generator blowdown system. Both of them are installed in a seismically qualified area.

• Installation of safety grade I & C to control the system. Thanks to the high steam generator secondary water inventory, there is no need to actuate the system within the first 30 minutes into an event. Consequently, only manual actuation of the system by operators from either main or emergency control room is envisaged.

• As the super-emergency feedwater pumps are driven by electric motors, they are supplied from essential busbars which are connected to diesel generators as a back-up in case of accident. Appropriate measures were taken in the cabling arrangement.

UPGRADE BENEFITS

What is the result of the modifications described above from the point of view of ensuring the heat sink safety function by feed and bleed on the secondary side? They ensure that a multi-level defence in depth concept is in place.

The large cooling water inventory in the reactor coolant system and in the steam generators relative to the core power provides enough time for corrective actions to be taken when the normal heat removal function is lost. This is a very significant inherent safety feature.

On the steam side, several systems are available when the turbine stop valves are closed. For normal and abnormal operations, steam bypass to the main condenser (60% of rated output capacity) is actuated. Additional systems, included in the original design, are the atmospheric steam dump valves installed on the main steam header (downstream of the main steam isolation valves) and the residual heat removal system installed on the secondary side (consisting of a steam pressure reduction station and a condenser cooled by non-essential service water). These systems are able to reduce the frequency of actuation of the new steam generator PORVs. An ultimate means of steam release are the steam generator safety valves – there are three on each main steam line with total capacity of 100% of rated output.

On the feedwater side, in the case of a loss of main feedwater system, the auxiliary (start-up and shut-down) feedwater system is available.

The auxiliary feedwater system is qualified as a safety-related system. A large inventory of preheated water in two feedwater deaerator tanks is available. This creates favourable conditions for the steam generators (small temperature differences and thermal stresses), reduces the frequency of super-emergency feedwater actuation and contributes to the overall reliability of feedwater supply. The original design of the auxiliary feedwater system is such that the feed function of the feed and bleed feature will be provided even in the case of a fire affecting one train and a single failure in another train of the super-emergency feedwater system. In addition, the auxiliary feedwater system of the other reactor can be used as a back-up system. Interconnection between the two units is possible at the level of the auxiliary feedwater systems and of the super-emergency feedwater systems.

The design incorporates some accident management features. There is a connection from a nearby hydro-plant to the essential power busbars, as well as supplementary cabling separate from the busbars in the case of plant blackout, which can be connected directly to selected consumers. There is even the possibility of using a fire truck to feed the steam generators.

All the modifications implemented aim to ensure that the emergency heat sink function provided will achieve a reliability target in compliance with the International Atomic Energy Agency INSAG-3 guidelines.

The work has been performed during extended annual refuelling outages. It was completed on unit 2 in late 1997. For unit 1, the system is now operable but some details are to be completed in 1998.