Design study on small CANDLE reactor

Abstract

A new reactor burn-up strategy CANDLE was proposed, where shapes of neutron flux, nuclide densities and power density distributions remain constant but move to an axial direction. Here important points are that the solid fuel is fixed at each position and that any movable burn-up reactivity control mechanisms such as control rods are not required. This burn-up strategy can derive many merits. The change of excess reactivity along burn-up is theoretically zero, and shim rods will not be required for this reactor. The reactor becomes free from accidents induced by unexpected control rods withdrawal during power operation. The core characteristics, such as power feedback coefficients and power peaking factor, are not changed along burn-up. Therefore, the operation of the reactor becomes much easier than the conventional reactors especially high burn-up reactors. The transportation and storage of replacing fuels become easy and safe, since they are free from criticality accidents. In our previous works it appeared that application of this burn-up strategy to neutron rich fast reactors makes excellent performances. Only natural or depleted uranium is required for the replacing fuels. The average burn-up of the spent fuel is about 40% of total charged fuel. It is equivalent to 40% utilization of the natural uranium without the reprocessing and enrichment. This reactor can be realized for large reactor, since the neutron leakage becomes small and its neutron economy becomes improved. In the present paper we try to design small CANDLE reactor, whose performance is similar to the large reactor, by increasing its fuel volume ratio of the core, since its performance is strongly required for local area usage. Small long-life reactor is required for some local areas. Such a characteristic that only natural uranium is required after second core is also strong merit for this case. The core with 1.0 m radius, 2.0 m height can realize CANDLE burn-up with nitride (enriched N-15) natural uranium as fresh fuel. Lead–Bismuth is used as a coolant. From equilibrium analysis, we obtained the burning region velocity, power density distribution, core temperature distribution, etc. The burning region velocity is less than 1.0 cm/year that enables a long-life design easily. The core averaged discharged fuel burn-up is about 40%.