Assessment of irradiation conditions in WWER-440(213) RPV surveillance location

The key component of WWER is the Reactor Pressure Vessel (RPV). The evaluation and prognosis of RPV material embrittlement and the allowable period of its safe operation are performed on the basis of impact test results on irradiated surveillance specimens (SS). The main problem is that the SS irradiation conditions (temperature of SS, neutron flux and neutron spectrum) have not been determined yet with the necessary accuracy. These conditions could differ from the actual RPV component conditions. In particular, the key issue is the possible difference between the irradiation temperature of the SS and the actual RPV temperature. It is recognized that the direct measurement of temperature by thermocouples during reactor operation is the only way for receiving reliable information. However, this is a complex engineering task. In addition, the neutron field's parameters for surveillance specimens have not been determined yet with the necessary accuracy, and these (neutron flux and spectrum) are different from the RPV irradiation parameters. The use of state of the art dosimeters (such as the AMES common reference dosimeters) can provide high accuracy in the determination of the neutron exposure level. In order to solve the above-mentioned problems, a joint project (COBRA project) is being carried out in the frame of the European Programme Copernicus 2, with the participation of organizations and researchers of the European Union, the CEECs and the NIS. In this project surveillance capsules were manufactured which contain state of art dosimeters and temperature monitors (melting alloys). In addition, thermocouples were installed throughout the instrumentation channels of the vessel head to measure directly the irradiation temperature in the surveillance position during reactor operation. The selected reactor was the Unit 3 of Kola NPP situated in the arctic area of Russia. Irradiation of the capsules and online temperature measurements started in September 2001. In this paper preliminary project results of the irradiation temperature measurements with thermocouples will be presented, together with a description of the experimental set up and the equipment developed for the project, including state of the art dosimeters.