Studies on physics parameters of metal (U-Pu-Zr) fuelled FBR cores

Full text: For faster growth of nuclear power in India, it is essential to shift to use of metal fuels in fast breeder reactors (FBR), which gives a higher breeding ratio (ER) and lower doubling time (DT). Also, future commercialization of the FBR fuel cycle necessitates the use of metallic fuel along with the pyro-process recycling, which can be less costly than oxide fuel reprocessing. A study of physics parameters of various metal-fuelled cores using XSET -98 cross-section set and 2-D diffusion computations is done as a function of the various reactor parameters like reactor power, fuel smear density, zirconium content in the fuel and number of rows radial blankets. Metal fuels considered here are an alloy of uranium, plutonium and zirconium. More emphasis is given to designs which gives higher breeding ratio and low doubling time. To investigate the effect of zirconium on the fuel performance, two metal fuels are considered. First fuel type is with 10% zirconium, while the other is without zirconium. The main physics parameters investigated here are breeding ratio, doubling time, Doppler constant, plutonium enrichments required to attain the required core excess reactivity, linear powers, delayed neutron fraction and sodium void worth. It is found that a 1000 MWe fast reactor core with U-Pu fuel (ie metal fuel with no zirconium) can attain a breeding ratio of 1.61 and a reactor fuel doubling time of 6.6 years. In general, due to harder neutron spectrum, metal-fuelled cores having higher sodium void worth, which is an important safety parameter for sodium cooled fast reactors. It is found that a 1000 MWe core fuelled with U-Pu-Zr (10%) is having a sodium void worth of 8.1$. Therefore, without much compromise on breeding ratio, methods to reduce its value are also to be investigated. Two ways of reducing the sodium void worth is presented. In the first method, the upper axial blanket is replaced with sodium plenum, the value of sodium void worth drops to 4.6 $ with a slight reduction of breeding ratio from 1.38 to 1.31 for U-Pu-Zr core. In the second method, using a fuel with lower smear density reduces sodium void worth. It is found that a 1000 MWe core fuelled with U-Pu also have a sodium void worth of 8.0$. With a decrease of smear density from 0.75 to 0.62 reduces the sodium void worth from 8.0$ to 7.4$ with a slight reduction in breeding ratio. Use of sodium plenum is thus the most effective method for reducing sodium void worth. As metal fuel pins have gas plenum in the top region, it is easy to have the sodium plenum region above the fuel by removing the axial blanket region