A1.01/03A - Medzamor NPP: I&C Basic engineering and equipment procurement (ref AP 7.3.1.1)

The Armenian Nuclear Power Plant (ANPP) consists of two units of the VVER/440/270 model Soviet type reactor that is a modified version of the VVER/440/230 in view of special seismic design considerations. Unit 1 started its commercial operation in 1976 and Unit 2 in 1980. Both units were shut down shortly after the 1988 Spitak earthquake. Re-commissioning works were performed from 1993 to 1995 and in November 1995 Unit 2 restarted operation.
The original configuration of SG water level system comprised two measurement ranges; one for overall level and another one for an "intermediate level" that was accurate and provided a signal to the feedwater control system. Neither SG water level sensor was environmentally qualified.
In order to control the SG water inventory, each SG is equipped with two independent control valves, one for emergency operation, named Emergency Feedwater (EFW) control valve and one for normal operation (Main Feedwater (MFW) control valve). This section refers essentially to the MFW control system.
During normal operation, the operator manually sets the water reference level to be maintained by the control system for each SG controller. The level inside the SG is maintained around the established set point by the related control valve that is regulated by the control system using measured values of SG level, steam and water flow.

In the original VVER-440/230 design, the steam generator water level instrumentation was used to provide information to the MCR and to the control valves of the feedwater system in order to maintain the water inventory in six Steam generators (SG). There were two issues associated with the SG inventory; i.e. (i) missing reactor protection signals (i.e. reactor trip signal/ESFAS) based on the steam generator level (low/high) only operational interlocks were available, and (ii) no provision for safety graded isolation of SG at the steam lines and feedwater lines was available, only valves classified as normal operational were available.
The IAEA Issue Book for VVER 440/230 reactor types identified an inadequate SG water level measurement in two issues S3 and AA 2.
Consequently the European Commission launched a project to address this issue in the framework of TACIS 96' nuclear safety programme.
The overall objective of this project was to procure and install the new steam generator water level control system and associated I & C interface.

During the 1995 plant annual outage, the ANPP has replaced the old six SG Level Control Valves with improved ones, manufactured by “Fisher-Rosemount”. In addition, the valve actuators were also replaced with electrical linear one, model NMA 3948 DE, manufactured by “Siemens”, which have an extended operating range and a finer tuning capability.
However, these new valves were still operated by the old controllers that used old water level and steam flow measurement system.
This coupling (new valves/actuators and old controllers, which already show many reliability and accuracy problems by themselves) caused additional malfunctions because the controllers were not able to automatically integrate the control valve algorithm, which used proportional/integral type, with the process information.
Besides that the original controllers had already exceeded their operational life time and were lacking environmental qualifications. A replacement concept of the old controllers and associated sensors/transmitters, with new ones was based on reliable transmitters, redundant for the essential functions with live zero and automatic validation capability, and microprocessor based controllers, capable to implement all the needed control algorithms and functions. Both transmitters and controllers should be maintenance free between two refuelling outages.
A conceptual design of the improved steam generator level control system (SGLCS) is shown in the attached Fig. 1.
The replacement of SG water level control system involved the following activities (performed by the EC Consultant):
Analysis of safety functions to be performed by the new valves,
Preparation of procurement technical specifications,
Assistance for SGSV testing, installation and commissioning, and
Support to ANPP in licensing related activities.
The improved SGLCS system maintained the same basic architecture of the existing system, using up to date equipment such as microprocessor based controllers with the addition of few other improvement in the control variables such as Steam and feedwater temperatures and Valve position feedback.
A tender was successfully evaluated and Contract was awarded to the Ukrainian Company Westron in 2005. The factory acceptance tests were conducted in Westron premises August 2006, and the new SG level control system was installed during the annual plant outage in 2006. The final acceptance was issued in December 2008.