A Study of Irradiation-Induced Growth of Modified and Advanced Zr-Nb System Alloys after Irradiation in the VVER-1000 Reactor Core at Temelin NPP

Since 2014, the irradiation of material samples manufactured from advanced Russian zirconium alloys has been performed in the VVER-1000 reactor core of Temelin NPP Unit 1, focused on the study of irradiation-induced growth (IIG) and related microstructural changes. The material samples differ from each other by alloying elements and final heat treatment, which provides a variety of initial microstructural characteristics. The test matrix allows the evolution of these different types of microstructures to be studied with increasing neutron irradiation and how it corresponds with macroscopic IIG. The expected outcome of this large research program is to obtain new experimental data on the IIG of Zr-alloys and the underlying microstructural changes that occur under standard irradiation conditions in a commercial VVER-1000 reactor. Six material cluster assemblies (MCAs) containing ampoules with longitudinal segments of cladding tubes with a size of 50 × 6 × 0.6 mm were irradiated in five irradiation cycles to achieve six different values of neutron fluence. One/two MCAs were removed from the reactor core after each irradiation cycle and then cooled in the spent fuel pool. A unique cutting device called POMA installed in the transport container pit is used to separate the ampoules from the remaining part of the MCA. Irradiated materials are being evaluated in the hot-cell facilities at UJV Řež and Research Centre Řež. The material tests and analyses include the determination of neutron fluence based on the activities of neutron activation monitors, geometry measurements of samples and their evaluation, TEM, SEM, and LOM analyses and microhardness measurement. The irradiation of all material samples has been completed. The IIG has been measured for the first four batches and related to the accumulated neutron fluence and to the microstructural changes. The alloys of multicomponent Zr-Nb-Sn-Fe systems exhibit higher values of IIG compared with the Zr-Nb-(Fe,O) alloys at the same neutron fluences.