Background
The containment of a VVER 1000 nuclear reactor is the final barrier to radioactive release into the environment. The piping systems penetrating the containment are designed with containment isolation valves to allow isolation of the containment after specific incidents, such as "Loss of coolant accident" (LOCA). For to ensure reliable isolation of the containment by radioactive release inside the containment, isolation valves are installed inside and outside the containment. Reliable operation of the isolation valves especially under harsh accident condition inside the containment is required. The fast acting isolation valves of the main systems at Zaporozhye NPP are actuated by air controlled servo valves. The installed servo valves caused a large number of failures and malfunctions during the first years of operation due to operation and environment conditions. The failure reasons were eliminated by the plant but nevertheless the old design servo valves installed inside the containment were not in compliance with the requirements for large LOCA conditions.
In total 735 servo-valves of two different modifications (old and new design) are installed at three units: 260 at Unit 1, 255 at Unit 2, 220 at Unit 3. To provide the required reliability, 3 valves (in some systems – 2 valves) are installed in sequence in one line; one valve is located inside the containment, the other outside the containment. The new modification meets all actual requirements; the old modification (about 60% of the valves) is not in accordance with the requirements.
It was decided to ensure that all valves installed inside the containment meet all the requirements. This requires replacement of the servo-valves of the old design inside the containment with valves of the new design as follows:

• Relocation of new design valves from outside containment to inside containment by the End user
• Purchasing and mounting of missing valves of new design (subject of this contract – 50 new valves)
• Purchasing of spare parts for new and old design servo-valves.

Objectives
The subject of the project was

• the replacement of 50 old servo valves inside the containment with new design valves (Part B1)
• purchasing of spare parts for the new design valves (part B2)
• purchasing of spare parts for old design valves (part B3).

The project includes:

• supply of servo valves, which would be compliant with the technical requirements in view of their technical characteristics and would actuate the main valves with the required level of reliability.
• the supply of spare parts for the old type of valves for an new supplier (the original supplier/manufacturer refused to supply spare parts)
• design, manufacture, acceptance tests, certification, delivery, supervision of installation and commissioning and training of End user personnel.

Project Results
The tender procedure for part B1 was launched in July 2002 and the supply contract became effective on 8 July 2003. The supply of spare parts, project part B2 and B3 were included into the B1 contract by a contract addendum became effective on 16 April 2004. The installation of the servo valves in Unit 3 took place in September/October 2004. The entry control for the spare parts according part B2 and B3 was performed in December 2004. The equipment and services were provided in accordance with the contract requirements. The Final Acceptance Certificate has been signed on June 30, 2006.
The achievement of this project increased significantly the reliability of the servo-valves to actuate the main valves with the required level of reliability under harsh operation conditions (LOCA) which increased the safety and reliability of the plant.