Comparison of characteristics of subcritical and critical reactor systems with lead-based coolant

Report considers reactor systems that are supposed to play key roles in the future three-component structure of nuclear power sector, namely, the reactors-breeders of nuclear fuel and reactors-burners correcting nuclide composition of radioactive wastes. Goals in the development of reactors with Pb-Bi coolant and subcritical systems with an external neutron source are discussed. A brief review of types of reactors suggested now as reactors-burners is presented. Some economic evaluations for reactors with Pb-Bi coolant, specific features of critical and subcritical reactors with Pb-Bi coolant and target are considered. Degree of readiness of various critical and subcritical Pb-Bi reactor technologies is discussed. For comparison, the following systems were considered: the 900 MW(t) power critical reactor RB-EC-2 of existing design (cooled by Pb-Bi coolant, with three-zone flattening of energy release in the core); two variants of subcritical systems (SS-1 and SS-2) with the same subcriticality level, both designed on the basis of RB-EC-2 reactor installation, but with the larger core size. The choice of two variants of subcritical systems was determined by desire to consider two limiting cases for the target size: minimum possible and maximum possible ones at selected size of the core. Number of fuel assemblies in each physical zone of the core for both considered subcritical systems was retained the same as in RB-EC-2 reactor. To reach the necessary subcriticality level, the amount of loaded fuel was varied. For simplification, power of both subcritical systems was selected to be equal to RB-EC-2 power (with allowance for power of external source of neutrons). Results of neutron-physical calculations of characteristics for RB-EC-2, SS-1 and SS-2 reactor systems during their fuel cycles are presented. For one of subcritical systems (SS-2) the change of source intensity during the fuel cycle and respective change of efficiency were studied. Magnitude of damaging dose for structural materials was evaluated, and also the analysis and comparison of neutron spectra for the considered types of systems were performed. It was shown, that the choice of a ratio between size of the target and size of Pb-Bi buffer surrounding this target depends on the type of optimized functional, e.g., the magnitude of neutron source intensity, nonuniformity of energy release, etc. Safety analysis including the study of behavior of considered systems in TOPWS and LOFWS accidents, as well as the evaluation of asymptotic safety potential for such systems were carried out. The principal difference in the behavior of considered systems in accidents was shown, and the role of feedbacks in each case was evaluated. (author)