A design and safety features of small Candle fast reactor

Shapes of neutron flux, nuclide densities and power density distributions of CANDLE burn-up remain constant but move upward (or downward) along its core axis. Small long life fast reactor with CANDLE burn-up concept has investigated with depleted uranium as replaced fresh fuel. Both core diameter and height are chosen to be 2.0 m, and the thermal power is 200 MW. Lead-bismuth is used as a coolant, and nitride (enriched N-15) fuel are employed. This burn-up strategy can derive many merits. The change of excess reactivity along burn-up is theoretically zero for ideal equilibrium condition, and shim rods will not be required for this reactor. The reactor becomes free from accidents induced by unexpected control rods withdrawal. The core characteristics, such as power feedback coefficients and power peaking factor, are not changed during life of operation. Therefore, the operation of the reactor becomes much easier than the conventional reactors. The transportation and storage of replacing fuels becomes easy and safe, since they are free from criticality accidents. The burn-up 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%. It means following extreme merits. If a light water reactor with a certain power output has been operated for 40 years, the CANDLE reactor can be operated for 2000 years with the same power output and with only depleted uranium left after fuel production for the light water reactor. The system does not need any reprocessing or enrichment. Safety analysis for this reactor has been performed for USDRW (unprotected shut down rods withdrawal), ULOF (unprotected loss of flow) and ULOHS (unprotected loss of heat sink) accidents. The burn-up of this reactor does not change the safety features and the safety analysis were performed at the steady state. The former 3 accidents were simulated by neutronic-thermal hydraulic calculation coupled with stationary diffusion calculation. The analysis results show that the proposed small CANDLE fast reactor can survive all the accidents without any active protection. (authors)