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Spent nuclear fuel from a VVER-440-type power plant is stored in Slovakia in the Interim Spent Fuel Storage (ISFS) pools in special KZ-48 compact racks. They are made of chromium-nickel stainless steel with guaranteed boron content min. 1.10 wt. %, which serves as a thermal neutron absorber. However, the required boron content in steel may potentially decrease due to its fluctuation in the base material during production itself, in the process of welding of absorbent cases or by absorption of thermal neutrons. The aim of the study is to verify the distribution of boron content in selected steel samples, including the application of experimental welding joints by using the same welding methods used for the manufacture of the absorbent cases for the KZ-48 compact racks. Methods of optical (OM), scanning (SEM) and transmission microscopy (TEM) as well as spectroscopic methods (ICP-MS, GDOES) were used.

The thermal ageing monitoring of nuclear power plant primary circuit materials is the issue solved only since 2010. Monitoring of this degradation process is necessary in aspect of NPPs long-term operation as one from the main material degradation process of the safety important component of the primary circuit. The conception of the thermal ageing monitoring program of nuclear power plant primary circuit materials as well as the realized activities related with this program is described in this paper.