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The operational lifespan of all four units at the Hungarian Paks Nuclear Power Plant (NPP) has been extended by 20 years beyond the original 30-year term. This extension has been implemented gradually, in one- to two-year increments, since 2013. The Hungarian Atomic Energy Authority, responsible for granting approval for these life extensions, has prescribed the continuous and rigorous monitoring of the reactor pressure vessel (RPV) throughout the plant’s extended service life. Among the key requirements is the validation of the neutron transport calculations, with specific attention to the large Lead Factor (LF), which reaches around 12 at the reactor core’s center. To meet these regulatory requirements, neutron transport calculations and reactor dosimetry measurements using neutron activation detectors (neutron monitors) have been conducted both inside and outside the RPV. Inside measurements were taken in the RPV surveillance channel, while outside measurements were conducted in the cavity adjacent to the outer wall of the RPV. These efforts aim to determine the key parameters of the neutron irradiation and validation is achieved if the experimental results agree with the neutron transport calculations at both the surveillance position and the RPV's outer wall (in the cavity), ensuring that the calculations are also valid for the inner wall of the RPV. This paper presents the validation of experimental data used to confirm the accuracy of the neutron transport calculations. The results of irradiations conducted over one- and four-year periods are compared and analyzed to demonstrate the authenticity of the experimental data for validation of the calculation results.

Following an initial Round Robin inter-comparison of gamma spectrometry measurements reported in 2014, this paper presents results from the first part of a second Round Robin inter-comparison commissioned by the European Working Group on Reactor Dosimetry in 2018. Measurements were performed by thirteen European organisations on a set of irradiated neutron activation detectors representative of those commonly used by the Reactor Dosimetry community to measure neutron fluence using gamma spectrometry methods. The radionuclides measured were 110mAg, 58Co, 60Co, 54Mn, and 46Sc. The purpose of the exercise was to demonstrate the level of consistency between participating organisations in blind tests of measurements. The samples used were disks of iron, nickel, titanium, and two standard alloys of aluminium, one of 1% cobalt and the other 1% silver. They were irradiated in the MARIA reactor operated by National Centre for Nuclear Research, Poland. Participants provided their results to an independent referee who collated and compared the data. The results are presented in an anonymised form together with discussion and conclusions which may be drawn from the exercise. Good agreement was obtained with standard deviations for individual measurements between 2.0% (54Mn) and 5.6% (60Co). Overall, the results obtained from the latter Round Robin show less consistency than those from the first Round Robin, which is attributed to the participation of a wider pool of organisations.