Objectives

In the framework of the Tacis Nuclear Safety Programme, it was decided to increase the nuclear safety of the units 1 through 6 at the Zaporozhye NPP by improving the protection of the primary circuit from overpressure under normal and emergency modes.

The Zaporozhye NPP consists of six units of the VVER 1000/320 type. In order to protect the primary circuit from overpressure, the pressurizer of each unit is equipped with 3 independent safety relief valves which are installed in parallel on the steam dump pipeline. During construction, units 1 – 4 of the power plant were fitted with pressurizer safety valves manufactured by the German valve manufacturer, Sempell. Units 5 and 6 were fitted with pressurizer safety valves manufactured in the Soviet Union.

Due to functional reliability problems with the pressurizer safety valves installed on units 5 and 6, it was first decided to replace these valves through competitive tendering. This work was covered by the present contract and is described below. For units 1 – 4 on the other hand, it was established that the desired safety improvements could be achieved with the greatest cost/benefit ratio by upgrading the already installed pressurizer safety valves (see project U1.03/02B2).

At the units 5 and 6, each of the 3 independent pressurizer safety valves installed in parallel on the steam dump pipeline is controlled by 2 pilot valves actuated by solenoids or directly by the operating medium.

The existing pressurizer safety valves system had the following shortcomings:

• The main valves were not equipped with position indicators;
• The safety valves were not qualified for operation with water and water-steam mixture;
• The solenoid drives and the position indicators of the safety valves did not function reliably;
• The design of the unit for checking and adjusting the closure pressure of the pilot valve required a decrease of the reactor coolant system parameters in order to carry out the adjustment works;
• The design of the safety valves system featured a large number of flange connections which degraded the system reliability.

The generic purpose of the intended valve replacements was to:

• Eliminate the deviations from the Ukrainian safety regulations and rules;
• Enable the core heat removal under emergency conditions (Bleed-and-Feed function);
• Enable the protection of the primary vessel from cold overpressure.
• Enable the application of state-of-the-art testing methods for the valves and thus to exclude potentially dangerous and imperfect testing methods.

In order to achieve the above goals, the design of the new pressurizer safety valves included the following:

• Provision of the possibility to close the safety valve in case of failure of the pilot valve;
• Provision of the possibility to remotely control (open - close) any and each of the 3 safety valves from the main control room, as well as the emergency control room;
• Provision of the possibility to achieve the "Bleed and Feed" function and to protect the primary circuit against cold overpressure;
• Qualification of the safety valves for operation with steam, water, water-steam mixture and nitrogen;
• Installation of position indicators (opened/closed) on all safety valves and pilot valves, with the possibility to transmit the related signals to the main and emergency control rooms;
• Provision of the possibility to adjust the safety valves, to check the functionality and to test at low parameters of the primary circuit;
• Provision of the possibility to perform a diagnosis of the valve condition.
• Provision of the possibility to connect a mobile system for adjusting and testing the pilot valves;

The equipment to be supplied included 2 complete sets of 3 pressurizer pilot-operated safety relief valves complete with the required control systems (control cabinets, pressure sensors, position indicators, terminal boxes and cabling, etc). The supply also included a system for adjusting and testing the valves.

The Supplier’s scope of work comprised the design, manufacture, testing, certification, delivery and supervision of the installation of the equipment. The Supplier’s scope also included the development of all necessary documentation including that required for licensing purposes, the provision of training to the End User’s personnel and warranty on the supplied equipment.

Project Results

The supply contract became effective on 21 November 2003 by signature of the Supplier. The kick-off meeting with all involved organisations was performed at ZNPP in December 2003.

The design documentation preparation process started and continued until September 2004. The documentation was approved by the Beneficiary and sent to the SNRCU for approval. The Supplier started the manufacturing process.

The valve qualification process started in parallel and was concluded by acceptance by the Ukrainian nuclear regulator SNRCU in May 2004. Later, however, Riskaudit, who was providing EC (Tacis) financed support to the SNRCU for the licensing of the project (see project U3.01/97), raised some concerns regarding the qualification of the valves for operation with different fluids and under conditions representative of the VVER-1000 reactor. The Supplier agreed to perform additional analyses and to provide additional information to support the completion of the qualification process, which was concluded to the satisfaction of SNRCU.

Manufacturing was completed for Lot 1 (Unit 5) in February 2005, followed by Factory Acceptance Testing from 28 February – 4 March 2005.

The equipment of Lot 1 was delivered on 13 June 2005 and customs clearance was obtained by 1 July 2005.

The valves were installed by ZNPP, with the assistance of the Supplier, during outage in August/September 2005. SNRCU gave conditional approval for installation and requested additional documentation concerning the control system. A large additional effort was required by the Supplier and the End User to satisfy the request. Following installation, SNRCU gave approval for commissioning and testing. The valves were tested according to the approved test programme in the period from 5 to 17 September 2005.

The additional documentation provided by the Supplier was accepted by SNRCU some weeks after the testing and the Provisional Acceptance Certificate (PAC) was signed on 2 November 2005. Pilot unit operation started and continued until March 2006. In parallel the manufacturing of Lot 2 was completed and the Factory Acceptance Testing of Lot 2 was performed in September 2005, followed by an agreed additional quality inspection programme at the Supplier’s site in October 2005.

The equipment was delivered on 19 December 2005 followed by customs clearance. Installation, commissioning and testing of the valves of Lot 2 was performed, with the assistance of the Supplier, during Unit 6 outage in April/May 2006.

The PAC was signed on 10 May 2006. The warranty periods were monitored and no failure reports were issued.