The paper gives a description of the contemporary methods used for the fast neutron fluence determination applied to the reactors of the Dukovany VVER-440 nuclear power plant (NPP). Measurement methods include measurements in containers with supplementary surveillance program and in the reactor cavity. Retrospective dosimetry using scraps of reactor pressure vessel cladding material is being prepared. Information is also given about the analogous methods prepared for the reactors of the Temelin VVER-1000 NPP. The results at measured points (reactor cavity, surveillance) are recalculated to the reactor pressure vessel inner wall by using the conversion factors obtained in mock-up experiments carried out for both VVER- 440 and VVER-1000 at the LR-0 research reactor. The experimental works are completed with fluence calculations for which two methods have been developed. The DAVKA code is the LEPRICON system adaptation for the VVER reactors and the INCAL 3D code makes use of the influence functions of the given volume of the fuel element to the total fluence. Measurements can be completed by gamma scanning of surveillance specimens rest activities.
Standard surveillance specimen programs for materials in the reactor pressure vessels of VVER-440/V-213 type reactors were designed with a high lead factor (approx. equal to 12 for base metal and 18 for weld metal). Thus, end-of-life neutron fluence levels are obtained within the first three years of operation. The maximum irradiation time is five years after which there is no active monitoring of neutron fluence inside reactor pressure vessels. Also, the distribution of neutron flux along surveillance specimen chains creates a large difficulty in grouping of specimens for static fracture toughness testing.
A reevaluation of the standard surveillance program for NPP Dukovany has been performed. This reevaluation includes additional irradiations and a very detailed analysis of all results, and provides a basis for a design of a Supplementary Surveillance Program. Its realization started in 1997 by loading the first chains with specimens.
The principles of this Program are explained in this paper. This program will provide: 1) new results under a realistically low lead factor, 2) new data on cladding properties under irradiation as well as after annealing and re-irradiation, and 3) data on the re-irradiation rate of vessel materials. Also the fourth part of the Program will provide a permanent neutron monitoring through the real vessel end-of-life. Within the whole program, the IAEA reference material JRQ will be also used, and this material will serve as a correlation monitor with other irradiation experiments.
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