Design Study on Power Flattening to Sodium Cooled Large-Scale CANDLE Burning Reactor With Using Thorium Fuel

The CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy production) burnup strategy is a new burnup concept. The CANDLE reactors generate energy by using only natural or depleted uranium as make up fuel and achieve about 40% burnup without fuel recycling of the conventional nuclear energy concept. So far the CANDLE cores feature a relatively large peak-to-average power density and discharge burnup distribution. Peaked power and burnup distribution are undesirable as they deteriorate economical performance. The objective of this paper is to study the feasibility of power flattening of sodium cooled large scale CANDLE reactor toward commercial use by using thorium fuel loading into the inner core zone. When power density profile becomes flat, it is expected that the axial position of burning region is aligned at the same height for each radial position. It makes core height shorter and raises the average power density farther. The shorter core has usually more merits such as smaller loss of coolant pressure obtained during passing fuel channel and more negative coolant void coefficient. For this purpose, thorium is added uniformly to the uranium fuel in the inner core. If we choose the amount of thorium proper, net radial current of neutrons in the inner core becomes zero in the inner core, and at the boundary between inner and outer core enough neutrons leak from the uranium region and the net radial current is still zero at this point. In the outer region the neutrons leak outward. By this way, we can make the power density distribution flat in the inner core. In the present work, the power density profile is intended flatten for the metallic fuel CANDLE reactors by adding thorium uniformly in the inner core region. The maximum axially integrated power density (radial peaking factor) decreases from 1.87 with only uranium fuel to 1.44 with uranium and thorium fuels. We can expect increasing average discharge burnup and decreasing fuel inventory and pressure drop.