Evaluation of the Effect of Thermal Stratification on the Residual Life of the Surge Line of the Pressure Compensator in the No. 5 Unit of the Novovoronezh NPP

Abstract

This article is concerned with investigations of temperature stratifi cation of the fi rst-loop coolant in the surge line of the pressure compensator in the No. 5 unit of the Novovoronezh NPP. The actual stress-strain state and accumulated structural damage in the surge line in different operating regimes was analyzed by means of multiparametric computational and experimental monitoring. The experimental data demonstrate thermal stratifi cation, giving rise to a temperature differential over the cross section ranging from 30°C under normal operating conditions to 150°C, as a result of disruption of the normal operating conditions, and high thermal stresses. The analysis shows that the cyclic strength of the weld connection of the surge line can be exhausted by the end of the 60-yr life of the No. 5 unit. It is concluded that the actual residual stresses must be measured and calculations of the cyclic strength and the residual life of the surge line must be performed. It is suggested that a technology be developed to strengthen the weld-joint metal in order to reduce the formation of operational surface defects. Since 1980, tens of operating defects and even through cracks formed in pipelines because of stratifi cation were discovered in NPP with PWR and VVER [1, 2]. Defects in the form of shallow surface cracks in transitional surfacing were found in the piping of the pressure compensation system of the No. 5 unit of the Novovoronezh NPP during operational monitoring of the weld connections. According to the recommendations of the International Insurance Inspectorate the infl uence of thermal stratifi cation on the damage rate for the surge line of the pressure compensator should be determined. The function of the pressure compensation system is to create and maintain the coolant pressure in the fi rst loop in order to prevent the coolant from boiling in stationary regimes and to limit the deviation in transitional and emergency regimes. The pressure compensator communicates with the coolant in the hot string of the fourth loop of the main circulation pipeline (MSP) via the 426 × 40 mm connecting surge line (Fig. 1). While the reactor is operating, the surge line is heated owing to the closed circulation of the coolant. When the average temperature of the fi rst-loop coolant changes in transitional regimes associated with the disruption of equipment operation and accompanying a change in the load, part of the coolant fl ows from the pressurizer into the loop or from the loop into the pressurizer along the surge line. In addition, a limitation on the deviation of the pressure from the nominal value is attained by compression or expansion of the steam blanket in the pressurizer. Thus, the integrity of the surge line is directly related with the reliability of fi rst-loop operation. In the course of operation the following loading factors act on the pipeline: mass of the structure, internal pressure, temperature change, and temperature stratifi cation in transient regimes. Thermal stratifi cation is associated with the separation of the coolant fl ow over the section of the pipeline into a cold layer at the bottom and hot layer at the top. The temperature gradients in the coolant result in beyond design-basis stresses in