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The highest efficiency in the usage of nuclear energy resources can be implemented in fast breeder reactors of generation IV. It is achieved thanks to the ability of consuming minor actinides (MAs) in energy production. One of the options to use this benefit is full recycling of MAs to close the nuclear fuel cycle. Monte Carlo burn up (MCB), an integrated burn-up calculation code, deals with the complexity of the burn-up process which is applied to the European Lead-cooled Fast Reactor (ELFR). MCB uses continuous energy representation of cross section and spatial effects of full core reactor model; however, it automatically calculates nuclide production in all possible reactions or decay channels. Multi-recycling of MAs can cause an intensified build-up of curium, berkelium and californium. Some of their isotopes are strong neutron emitters from spontaneous fission, which hinders fuel recycling. The implementation of a novel methodology for trajectory period folding allows us to trace the life cycle of crucial MAs from the beginning of the reactor life towards the state of adiabatic equilibrium. The result of the analysis performed is presented, showing the sources of strong contribution to the neutron production rate. The parametric sensitivity analysis method for selected nuclide reactions is applied, revealing sensitivity of transmutation chains for the production of neutron emitter isotopes.

Abstract: The Sodium-cooled Fast Reactor (SFR) is one of the most promising Generation IV systems with many advantages, but has one dominating neutronic drawback – a positive sodium void reactivity. The aim of this study is to develop and apply a methodology, which should help better understand the causes and consequences of the sodium void effect. It focuses not only on the beginning-of-life (BOL) state of the core, but also on the beginning of open and closed equilibrium (BOC and BEC, respectively) fuel cycle conditions. The deeper understanding of the principal phenomena involved may subsequently lead to appropriate optimization studies. Various voiding scenarios, corresponding to different spatial zones, e.g. node or assembly, have been analyzed, and the most conservative case – the voiding of both inner and outer fuel zones – has been selected as the reference scenario. On the basis of the neutron balance method, the corresponding SFR void reactivity has been decomposed reaction-, isotope-, and energy-group-wise. Complementary results, based on generalized perturbation theory and sensitivity analysis, are also presented. The numerical analysis for both neutron balance and perturbation theory methods has been carried out using appropriate modules of the ERANOS code system. A strong correlation between the flux worth, i.e. the product of flux and adjoint flux, and the void reactivity importance distributions has been found for the node- and assembly-wise voiding scenarios. The neutron balance based decomposition has shown that the void effect is caused mainly by the influence of the sodium removal on the neutron spectrum, and this has been confirmed by the perturbation method. The neutron balance based decomposition has also shown that the prime consequence of the sodium voiding is the reduction of the absorption rate (especially of 238U capture), which provides the strongest positive component of the void reactivity. The changes in neutron production and leakage provide smaller and negative contributions. Sensitivity analysis results for individual isotopic reactions have been found to be, in a relative sense, consistent with the neutron balance based isotope-wise decomposition. Finally, the neutron balance method has been applied to analyze the void reactivity differences between the three different SFR fuel cycle states considered, viz. BOL, BOC and BEC. [Copyright &y& Elsevier]

• Basic parameters on optimization of heavy water cooled thorium breeding reactor. • Effect to feasible design region for breeding and negative void reactivity coefficient. • Significant parameters are moderator to fuel ratio, fuel diameter, fuel power density, void faction. • It shows 14.5 mm is the optimum pin diameter for optimum enrichment and conversion ratio. Basic study on optimization of heavy water cooled thorium breeding reactor has been investigated to estimate the feasible design region of breeding and negative void reactivity coefficient. Those parametric surveys are moderator to fuel ratio, fuel pin diameter, fuel pellet power density, and void faction. A fuel pin diameter of 14.5 mm shows the optimum feasible fuel breeding and less required U-233 fissile content. The systems require the enrichment less than 8% and it gives some possible area of breeding. The effect of moderator is essential to make the negative void reactivity. Void fraction effect is still good enough to make more negative void reactivity, however, for no more heavy water as moderator, it obtains less negative void reactivity or becomes slightly positive. It has been shown that several significant parameters affect to the feasibility breeding windows and potential negative void reactivity based on heavy water-cooled thorium reactor. [ABSTRACT FROM AUTHOR]