The Neutronic Transmutation Characteristics of Tc-99 in PWR

We conducted a systematic study on neutronic characteristics of Tc-99 transmutation in pressurized water reactor (PWR) in this paper. We divided the core into three zones. The U-235 enrichment is 1.8%, 2.4% and 3.1% respectively. The loading patterns of Tc-99 include homogeneous loading and heterogeneous loading. In the homogeneous pattern, the mass fraction of Tc-99 raises from 0 to 2% stepped by 0.25%. The calculation result shows that the k_eff of the core decreases significantly when the Tc-99 mass fraction reaches 2%. In the practical situation, the Tc-99 mass should not exceed 2% in a homogeneous pattern. We choose the Tc-99 coating of fuel pellet as the transmutation rod form in the heterogeneous pattern, which can reduce the self-shielding effects. There are 44 transmutation rods per assembly, replacing part of fuel rods directly. We evaluate the effect of the arrangement of transmutation rods on the characteristic values such as k_eff and neutron spectrum of the core. In both homogeneous and heterogeneous patterns, the assembly flux near the outer edge of the core has an increasing trend. Our further study shows that much more epithermal neutrons and fast neutrons enter the reflecting layer and bounce back to the core again after moderation and reflection.