In this master’s thesis, an advanced simulation method for determining neutron activation in nuclear reactor internals was both tested and evaluated. The activities of the following Krško Nuclear Power Plant core components were calculated: baffle, core barrel, thermal shield, and reactor presssure vessel with inner cladding. Neutron flux and energy spectra were determined using the MCNP program with model NEK_ExCore, developed by Tanja Goričanec and Žiga Štancar. MCNP calculation incorporated a source bias obtained through the ADVANTG hybrid code. Activities and activation products after 60 years of operation time were then computed using the FISPACT-II burnup code.
The results reveal significant, however, symmetrical deviations in neutron flux and activities within each individual component. These deviations stem from the non-cylindrical shape of the core and the cylindrical shapes of the surrounding components. Activation results were compared with the outcomes of the Siempelkamp NIS study. Differences were generally relatively small (up to 15 %). Nonetheless, in components with notably distinct spectra (such as the baffle), larger discrepancies (50 %) emerged in comparison to the NIS results.
In the final section, a sensitivity analysis was conducted on several theoretical operational defects and parametric deviations. The findings indicate the substantial importance of accurate isotopic composition of materials. To ensure the precision of the results, it becomes essential to account for fuel burnup across operational cycles, as activities could deviate by as much as 50 %. Likewise, a cobalt impurity with a concentration of 1000 ppm exhibits a similarly profound impact. Asymmetric defects, such as a 5 mm core eccentricity (displacement) and a 10 K lower temperature of the primary water, were simulated. Both defects have a noticeable impact on the calculated activities (up to 10 % - 15 %). Asymmetric power profile was simulated with a 2 % increase in power in one of the quadrants, which directly resulted in a 2 % difference in total activity between the halves of the components. The results
of the sensitivity analysis indicate the challenge of precisely determining activities through simulation methods. The cumulative numerical uncertainties were, at most, 2 % - 3 %.
With the application of simulation methods, there’s potential to distribute radioactive waste between the two Krško NPP owning countries. In a bilateral contract, it is stipulated that waste must be divided based on a 1 % or 5 % difference in both mass and activity. However, it would be essential to mitigate the impact of the most significant defects through measurements and to adhere to a condition that permits a maximum of a 5 % difference.
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