During the operation of a nuclear reactor, neutrons are absorbed in stable nuclei of structural materials through the process of activation, inducing their radioactivity. In the decommissioning phase of a nuclear power plant, these activated materials represent a significant portion of the radioactive waste, whose quantity and composition must be determined. In recent years, developments in nuclear technology have shifted towards smaller and more modular designs with a power output of up to 300 $\mathrm{MW_e}$, which differ from large, $GW$-scale reactors in several aspects. One of these differences is that components would be manufactured in factories with shorter construction times on site. However, their reduced size results in poorer neutron economy, meaning that more neutrons per unit of power escape from the reactor and interact with the nuclei of structural materials, such as the reactor pressure vessel.
In this master's thesis, neutron leakage in the small modular reactor NuScale was estimated using the transport equation in the diffusion approximation. NuScale is currently the only modular reactor approved by the U.S. Nuclear Regulatory Commission (U.S. NRC). The results were compared to the neutron leakage from the larger reactor at the Krško Nuclear Power Plant. The comparison was repeated using results from the Monte Carlo neutron transport code MCNP and the activation analysis code JSIR2S. The results are in agreement with theoretical predictions and indicate higher component activation and neutron leakage in the smaller reactor. The diffusion coefficients of the materials used in NuScale reactor components were also calculated, along with contact dose rate estimations. The results show that nuclides which contribute the most to the total activity are $\mathrm{^{55}Fe}$, $\mathrm{^{51}Cr}$, $\mathrm{^{63}Ni}$ and $\mathrm{^{56}Mn}$. According to calculations, the most activated components of the NuScale reactor are the reflector and the core barrel.
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