Space--time dynamics of a fast breeder reactor for localized disturbances

Sodium voiding accidents in Liquid Metal Fast Breeder reactors may cause a positive reactivity insertion due to a lack of balance between two competing effects following the voiding: (1) the shift in neutron energies to higher values which leads to a positive reactivity addition, and (2) the increased neutron diffusion which generates a negative reactivity. Since these two effects have strong spatial dependence, the amount of reactivity added in a voiding accident depends strongly on the position in the reactor core where the voiding accident has occurred. A method of analysis is developed for the localized voiding perturbation based on a flux factorization technique, accounting for the flux shape changes taking place near the region of the voided subassembly. Heat transfer equations are solved in conjunction with the neutronic equations and the two are coupled via feedback effects like the Doppler effect. For the purpose of analysis, a typical large (1000 MWe) LMFBR with two radial core zones of different enrichments is considered. Time-dependent void fractions, which are the input quantities in the analysis, are taken to be typical of a channel flow blockage accident. The transients are analyzed for voidings taking place at three different radial locations in the core and for two different rates of voiding. Results of the calculations show that the voiding transients occurring at the center of the reactor core cause most severe effects in terms of the positive reactivity addition and the effects become progressively less severe for voidings occurring outward toward the periphery of the reactor