Design concepts of small CANDLE reactor with melt-refining process

The innovative CANDLE (Constant Axial shape of Neutron flux, nuclide densities and power shape During Life of Energy production) burnup strategy has several advantages over conventional fast reactor designs: namely, the reactor characteristics do not change with burnup, it is possible to withdraw the burnup reactivity control mechanism, there is no need for orifice control along with burnup, and it is easy to extend reactor life by increasing the core height and using natural uranium as fresh fuel. Maintaining the fuel cladding integrity of a CANDLE reactor during operation is one of the key technological challenges that still need to be addressed. The introduction of the melt-refining process has shown great potential for solving this challenge in the high-burnup condition of large CANDLE reactors. The purpose of the present study was to design a small CANDLE reactor with the melt-refining process. With metallic fuel of natural uranium (90 wt% U, 10 wt% Zr), ODS cladding material, and lead-bismuth eutectic (44.5 wt% Pb, 55.5 wt% Bi) coolant, a 300-MWt reference core of 1.3-m radius and 2.2-m length can realize CANDLE burning. The results of our analysis demonstrate that it is possible to design a small CANDLE reactor with the melt-refining process. The analysis focused on the impacts of core homogenization, melt-refining, and the cooling and waiting times of the fuel. From the equilibrium analysis, the burning region velocity, neutron flux distribution, power density distribution, and core nuclide number density distribution were obtained. Applying the melt-refining process reduced the burning region velocity from 0.70 cm/year to 0.59 cm/year. Meanwhile, the minimum effective neutron multiplication factor and core averaged discharged fuel burnup were increased from 1.0047 to 1.0235, and from 435.2 GWd/t to 523.7 GWd/t, respectively.