The investigation of the utilization of enriched 208 Pb as a coolant to enhance the performance of a long-life fast reactor with a Modified CANDLE (Constant Axial shape of Neutron flux, nuclide densities, and power shape During Life of Energy production) burnup scheme has performed. The analyzes were performed on a reactor with thermal power of 800 MegaWatt Thermal (MWTh) with a refueling process every 15 years. Uranium Nitride (enriched 15 N), 208 Pb, and High-Cr martensitic steel HT-9 were

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... loyed as fuel, coolant, and cladding materials, respectively. One of the Pb-nat isotopes, 208 Pb, has the smallest neutron capture cross-section (0.23 mbarn) among other liquid metal coolants. Furthermore, the neutronproducing cross-section (n, 2n) of 208 Pb is larger than sodium (Na). On the other hand, the inelastic scattering energy threshold of 208 Pb is the highest among Na, nat Pb, and Bi. The small inelastic scattering cross-section of 208 Pb can harden the neutron energy spectrum. Therefore, 208 Pb is a better neutron multiplier than any other liquid metal coolant. The excess neutrons cause more production than consumption of 239 Pu. Hence, it can reduce the initial J o u r n a l P r e -p r o o f 2 fuel loading of the reactor. The selective photoreaction process was developing to obtain enriched 208 Pb. The neutronic was calculated using SRAC and JENDL 4.0 as a nuclear data library. We obtained that the modified CANDLE reactor with enriched 208 Pb as coolant and reflector has the highest k-eff among all reactors. Meanwhile, the nat Pb cooled reactor has the lowest k-eff. Thus, the utilization of the enriched 208 Pb as the coolant can reduce reactor initial fuel loading. Moreover, the enriched 208 Pb-cooled reactor has the smallest power peaking factor among all reactors. Therefore, the enriched 208 Pb can enhance the performance of a long-life Modified CANDLE fast reactor. One of the reactors that can directly consume natural uranium without enrichment is the CANDLE reactor. The CANDLE (Constant Axial shape of Neutron flux, nuclide densities, and power shape During Life of Energy production) reactor is an innovative burnup concept that the shapes of neutron flux, power distribution, and nuclide densities are remaining the same throughout the reactor core at equilibrium state [2] [3] [4] . Then, move in the axial direction with constant shape and speed. Therefore, the excess reactivity is the same throughout the operational time. The scheme divided the core into two regions, namely a fresh fuel region J o u r n a l P r e -p r o o f