**5.4 ATF based on MN and uranium nanopowder**

Further the improvement of fuel characteristics while maintaining high melting temperature is possible at transition to fuel based on MN nanopowder (100–500 nm). To achieve a density of 85–95% theoretical, the required sintering temperature 1800–1900°C and a 10 time of 10–30 hours. At the same time, up to 15% plutonium is evaporated from the fuel [48, 49]. When using such fuel, the nature of time dependence of the maximum temperatures in emergency modes does not change qualitatively.

The best and cheaper alternative is fuel based on micrograins MN and nanopowder U. The beginning of MN decomposition by 450–500°C exceeds the sintering temperature of fuel tablets during their manufacture. Significant improvement in reactor performance and safety can be expected with MN-U fuel due to high density and thermal conductivity, equal role of Doppler reactivity coefficient in TOP WS and LOF WS, and neutron balance improvement (BRC = 1). By neutron balance and nature of emergency modes, MN-U fuel is close to U-Zr and U-Pu-Zr [11] but much higher temperature.

The most relevant is the possibility of using fuel based on fine-grained MN and nanopowder U in lead-cooled reactors with a lead layer between the fuel and the cladding. The chemical bond energy in UN and PuN is markedly lower than in oxides (for UN, 5.464 ± 0.217 eV; PuN − 4.857 ± 0.651 eV; UO − 7.805 ± 0.174 eV; UO2 = UO O − 7372 ± 0.304 eV; PuO − 7.459 ± 0.217 eV [46]). Although the operating temperature of MN fuel is much lower than MOX, nitrogen can exit MN. The absorption of nitrogen from MN fuel by metal uranium can slow corrosion of internal surface of cladding.
