Objectives
The original pressurizer safety valves were not qualified for the two-phase flow and for the seismic loads. The pressurizer safety valves and piping did not allow the primary „Bleed and Feed“ mode of the residual heat removal. Besides that, there was no cold overpressure protection of the reactor pressure vessel in the temperature range close to the brittle fracture temperature (p -T curves have not been available).
The IAEA Issue Book for VVER 440/230 reactor types identified this issues (S09,10,11) in Category II.
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 pressurizer safety valves and associated I&C interface.
Background
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 pressurizer safety valves (PSV) comprised the four valves, each 25% capacity. PSV were not qualified for the two-phase flow; they were partially qualified for seismic loads. PSV and associated piping did not allow the primary „Bleed and Feed“ mode which may be required in certain accident mitigation sequences. Besides that, there was no cold overpressure protection of the reactor pressure vessel in the temperature range close to the brittle fracture temperature (p -T curves have not been available).
Project Results
The replacement of pressurizer safety valves 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 PSV testing, installation and commissioning, and
• Support to ANPP in licensing related activities.

The new PSV design involved two safety valves, each 100% capacity, are capable to perform the following functions:

1. Primary circuit over pressure protection during design basis accidents, and anticipated operational occurrences,
2. Automatic pressure reduction in the primary circuit during cold shutdown to prevent cold over pressurization of the reactor pressure vessel,
3. Residual heat removal by discharging the water pumped into primary circuit by emergency core cooling pumps (Feed&Bleed).

The Supplier SEBIM was selected in the EC tender procedure, and respective supply contract was placed in April 1999. SEBIM delivered PSV on site in August 1999. The associated I&C was delivered at the end of 1999.
Gidropress, the Russian architect designer of ANPP reactor coolant system performed associated T/H analysis and elaborated corresponding design files, i.e. installation design of SEBIM PSV and associated piping at ANPP. The PSV I&C design was performed by Russian designer NIAEP. The installation activities were performed by the ANPP staff.
The installation of I&C system of PSV feed & bleed and cold over pressurization logic circuitry required special, environmentally qualified cables which were not available in Armenia; therefore the EC Consultant prepared another technical specification for procurement of these cables. Instrumentation cables were delivered on site in 2000 just before PSV installation. All other mechanical components (piping, fittings, pipe supports, etc.) were procured by ANPP.
The licensing related activities were performed on the 2+2 basis; the European Consortium Riskaudit worked together with Armenian nuclear safety authority (ANRA) to review PSV design files and amendments to the plant technical safety justification report.
The new PSV were installed at ANPP Unit 2 during the plant outage in 2000. The PSV commissioning tests were successfully performed. The plant design and operating documentation (operating limits and conditions, operating manual, safety justification) was updated to reflect the PSV settings, operating and testing requirements. The plant restarted again in December 2000.