Summary The dual fluid reactor (DFR) is a novel concept of a very high‐temperature (fast) reactor that falls off the classification of generation 4 international forum (GIF). DFR makes best of the two previous designs: molten salt reactor (MSR) and lead‐cooled fast reactor (LFR). In this paper, we present a new reactor design Dual Fluid Reactor metallic (DFRm) with the liquid eutectic U‐Cr metal fuel composition and the lead coolant of which general idea was patented recently. By performing the first steady state neutronic calculations for such a reactor (the neutron flux density as a function of energy, the burnup, the effective multiplication factor/reactivity), we show that this 250‐MWth reactor is critical, and that it can operate almost 20 years without refuelling. We also optimise the geometry (reflector thickness, fuel tubes pin pitch) with respect to the multiplication factor. The optimisation together with some other opportunities for the liquid metal fuel design (eg, the use of electromagnetic pumps to circulate the medium) allows DFRm to be of a small size. This rises economy of the construction as expressed nicely in terms of the energy return on invested (EROI) factor, which is even higher than for the molten salt fuel design (DFRs). Last but not least, we show that DFRm has all the (fuel, coolant, reflector) temperature coefficients negative, which is an important factor of the passive safety.

Summary Motivated by some previous steady‐state burnup calculations (J. Sierchuła et al. IJER 43, 3692 (2019)) we study the Dual Fluid Reactor metallic (DFRm) eutectic design with different fuel compositions. It is just the spent nuclear fuel (SNF) from Light Water Reactors with reactor‐grade Plutonium as the fissile material. The isotope vector used here contains all important isotopes up to 243Am. Two SNF fuel compositions are studied. One of them is appended with some fraction (0.578%) of 235U (natural Uranium), while another is just the typical SNF without this extra isotope. In both cases one achieves fuel burnup above 225 MWd/kg, higher yield of average number of neutrons trueν¯ (up to 2.94), and higher conversion ratios (up to 1.42) as compared to previous DFRm Uranium fuel composition (enriched 13.44% Uranium). However, the application of a standard SNF with no natural Uranium requires the modification of core geometry as compared to a previous design. Our results also show a significant reduction of 237Np and Am isotopes while keeping high breeding capabilities. With SNF Plutonium fuel, as compared to just enriched Uranium fuel, one is able to extend more the DFRm operation time without fuel replacement so that this reactor may act as a 20‐year lasting nuclear battery.

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