Objectives
The original confinement structure of VVER 440/V-230 units consists of a reinforced concrete structure, internally lined in stainless steel. It comprises several compartments in which the steam generators, and reactor coolant system pipe work is located. The Confinement (as built) was characterized with high leakage rate that was about 5000 volumetric % of the leaked confinement volume per 24 hours, as well as low reliability of the confinement isolation safety function on demand. Besides that, there was no demand in the plant operating limits and conditions to perform regular leak tightness test after each outage. In fact that the plant was not equipped with sufficient air compressor that could be able to pressurize the confinement structure owing to its large leak rate.
The IAEA Issue Book for VVER 440/230 reactor types (TECDOC 640) identified this issue (S06) in Category III i.e. issue of high safety concern, where defence in depth is insufficient, and immediate corrective action is necessary.
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 necessary equipment that would enable pressurizing the ANPP confinement structure so that to allow for identifying the volume and location of confinement leaks.
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 its operation.
The confinement leak tightness is typically improved by identifying the leaking areas inside the confinement building such as defects on wall liner, floor liner, manholes, cable and pipe penetrations, etc. In order to identify the leaking places, it was necessary to install a robust air compressor which could enable pressurizing the entire confinement and thus helped identifying leakages for subsequent repair. ANPP however did not have a suitable air compressor that could be used for confinement leak tests.
Project Results
The confinement tightness improvement project involved a delivery of a suitable compressor to enable performing the confinement leak tests.
The EC Consultant prepared procurement technical specifications, and the European Commission organized respective tender. The ALSTOM Power Service SA was selected as the compressor supplier.
The air compressor was delivered on site in April 2001, and the plant started with pressurizing the confinement and identifying leakages during next outages.
There were number of places identified in the confinement that were leaking; e.g. missing welds on confinement liners, insufficient sealing of containment cable penetrations, leaking hermetic doors and covers, leaking valves on containment isolation system, etc.
The EC Consultant subcontracted a local engineering company Atomservice that developed a confinement test procedures, which was needed for proper air compressor line up, pressurizing the confinement, leakage identification and depressurizing the confinement. Since 2001, the confinement leak test is performed in every outage. This project helped the ANPP to substantially reduce the confinement leaks.