Prestress Losses In Containment Of VVER 1000 Units

This report presents results from 20 years of experience in the field of monitoring and analysis of the stress state of containments aimed at controlling the prestress level of the reactor building of units operating in the Russian Federation for 18 years. Three of them concern the initial and modernised types (unit 5 in Novovoronezhskaja NPP, unit 1 and 2 in Kalininskaya NPP), the four remaining units being of the unified series (unit 1, 2, 3, 4 of Balakovskaya NPP). To control the variations of the stress / strain state (SSS) of the containment, in construction phases as well as when they are normally operating, they are fitted with a monitoring system. When they are normally operating checking of the prestress level is obtained by regular measurements of the stress / strain state parameters by means of the monitoring system, in the scope of scheduled control works. Since plants have been operating, such controls on the prestress system have been carried out from 2 to 7 times in the above mentioned containments. The 18-year operational experience in prestressed containments with helical prestress pattern without injection in the cylinder and orthogonal prestress pattern without injection in the dome confirms that the rules which have been developed and the design of the monitoring system allows to reliably control the prestressing level of containments and support their operational suitability regarding their design prestress level. Specificity of containments with helical prestress pattern without injection, influence of wire relaxation and creep of concrete on containment prestress losses for their entire lifespan have been studied and corresponding measures have been taken. For the NPP containments belonging to future new series orthogonal prestress pattern without injection has been considered, together with the use of high strength strands and concrete with improved rheological characteristics