Research on Confinement Strength During Large Break of Primary Coolant System for Armenian Nuclear Power Plant (ANPP)
Nuclear and Radiation Safety Center of Armenian Nuclear Regulatory Authority
Armenian Nuclear Power Plant (Armenian NPP)
Armenian NPP (ANPP) unit 2 was re-started after implementation of the first phase of safety upgrading programme in November 1995 after 6 years of shutdown. Further continuous upgrading is planned to be implemented up to the year 2004 .
Confinement of ANPP units with reactors of V270 type is specific. Net volume is approximately 12,500 m3  and it was designed for overpressure of 100 kPa . It is protected against over pressurization by safety valves (1?520, 8?1130) with opening pressure of 75 kPa (small valve), respectively 80 kPa (big valves) with direct blow off to the atmosphere . After loss of coolant accidents (LOCA) in the confinement it is necessary to ensure its integrity and safe separation.
In the original design of the ANPP safety systems as emergency core cooling system (ECCS), confinement and its equipment such as spray system (SS) were sized only to the level of the Design Basis Accident (DBA), which was defined as a primary break with the diameter of 32 mm .
Civil engineering confinement structures were sized for the design overpressure of 100 kPa. Confinement relief flaps were installed as a protection against exceeding this limiting pressure and assure confinement integrity even in the case with larger breaks 32 mm, up to the break of 200 mm . However, it is true that all this equipment was significantly over-sized, and these margins could later be utilized during safety upgrading of Armenian NPP units.
During the years 1995-2001 a comprehensive safety upgrading programme has been developed and implemented at ANPP Unit 2.
Plan for safety upgrading process of ANPP Unit 2 was aimed to improve defence in depth concept while keeping positive features of this type of the reactor.
At present a new safety upgrading program to be implemented before 2004 is being developed. At the same time the Regulatory Authority develops requirements to further safety improvement of the ANPP Unit 2. At that the IAEA recommendations for safety upgrading of WWER-440/230 NPP’s are taken into account [5,6]. The regulatory requirements firstly will deal with the maximal design base accident. The requirement of the Regulatory Authority is to increase the maximal design accident from LOCA 32 mm to LOCA 200 mm . To fulfill this requirement there are required relatively small expenditures for modification of the exiting ECCS. However, there raises the problem of hermetic compartments (confinement) toughness in case of LOCA in different compartment of confinement and sufficiency of safety valves with direct blow off to the atmosphere and allowing to remove the peak pressures during first seconds of accidents.
The objective of the proposed project is to investigate the behavior of accident localization system that includes confinement, safety valves and spray system for assessment of overpressure boundary values and negative pressure in confinement with respect to the new value of the maximal design accident and simultaneous seismic impact. As well as the assessment of the sufficiency of the existing safety valves and toughness of the existing confinement structures, and development of recommendations for the confinement modification.
Three institutions from Armenia will participate in the Project:
1. Nuclear & Radiation Safety Center of Armenian Nuclear Regulatory Authority (NRSC);
The main scientific and engineering potential of Armenia in the field of nuclear power plant safety, modeling of physical-mechanical processes, stress analysis, risk assessment is concentrated in these institutions.
Implementation of the Project will support the collection of “as-built” structural and configuration data. Based upon the created database the specific analytical models will be developed and using developed models large break accidents and confinement behavior during these accidents will be analyzed. New and modern approaches to assessment of safety will be applied.
Project will be transferred, in a usable format as a recommendations to appropriate state, organizations and companies interested or responsible for managing and mitigating the risk of accident.
The research should be conducted in accordance with recommendations of the IAEA and appropriate International Safety Standards.
The Project will provide weapon scientists and engineers opportunities to redirect their knowledge and skills to peaceful activities.
Foreign collaborators have been actively involved with the development of this project and will continue to collaborate with participants of the project in the following areas:
– Serve on the technical advisory committees for each area of the project providing input and guidance for the project implementation.