A1.01/00 C2 - Accident localisation Safety System Equipment

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 accident localization system which consist of confinement structure, and equipped with passive and active components to handle the harsh environmental conditions following the LOCA, is typically improved by improving the confinement thermal hydraulic response, limiting the pressure peak, and making leak-tight the leaking areas inside the confinement building such as defects on wall liner, floor liner, manholes, cable and pipe penetrations, etc. While a robust air compressor that was delivered to the plant under separate TACIS project enabled pressurizing the entire confinement and thus helped identifying leakages for subsequent repair, the Jet vortex condenser (JVC) was considered to improve a thermal hydraulical behaviour of the entire confinement especially in short term period following the LOCA, before the active spray system would become fully operational.

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 and install a Jet Vortex Condenser that would improve the thermal hydraulic behaviour of ANPP confinement structure especially in early stages of loss of coolant accidents (LOCA).

The improvement of Accident Localization System involved several measures such as improvement of the confinement tightness by implementing series of measures to make containment penetrations (cable, instrument, pipe, etc.) tight by application of appropriate sealant material, a delivery of a suitable compressor to enable performing the confinement leak tests, and improve capacity and performance of the confinement spray system.

Besides above measures, the improvement of the confinement response following the wide spectrum of LOCA required another solution that would ensure a fast pressure reduction in the confinement before the spray pumps could become fully operational. The JVC helps the steam condensation and thus reducing the confinement pressure loads. JVC is passive equipment that starts working automatically on a pressure difference in the confinement.

The project was divided into two parts: part I. related to the feasibility study of the applicability of the Jet Vortex Condenser to the confinement of Armenian NPP, and part II. to the JVC procurement. The JVC design solution has been invented by VNIIAES (All Russian Research Institute) and the JVC itself was already installed at WWER 440 Units at Novovoronezh NPP, Russia.

Sogin, the EC Consultant performing On-Site Assistance at ANPP, placed a subcontract (Project C1) for elaboration of a feasibility study with the original designer VNIIAES; this subcontract was endorsed in October 2003. Project part C1 resulted in the demonstration of the applicability of JVC at the confinement of ANPP.

The present contract, a direct agreement contract for the procurement of JVC and its related components (Project C2) was endorsed in December 2003. Consequently, the JVC steel structures and components were delivered to ANPP in June 2005. The provisional acceptance was issued in July 2005 on the basis of the factory acceptance tests because this type of equipment could not be tested directly on-site (LOCA conditions are needed to test the functional performance). The warranty period expired and the contract was concluded in July 2006 without installation of the equipment.

ANPP has applied for licensing of the design modification and JVC installation permit at the Armenian Nuclear Safety Authority (ANRA). JVC belongs to the safety modification, and therefore the design and associated safety documentation was subjected to an in-depth assessment that was jointly performed by ANRA and Riskaudit (EU TSO assisting the ANRA in licensing related reviews).

The in-depth assessment concluded that the JVC functional performance was not sufficiently demonstrated for large spectrum of LOCA. Consequently, ANRA requested the plant to further elaborate the JVC licensing related documentation, in particular to provide for additional calculations and demonstration test that would prove the JVC functionality under different spectrum of LOCA.