**6. Conclusions**

There is potential to further improve LMFR reliability and safety within the existing technologies.

In order to ensure acceptable VRE in lead-cooled LMFR of high power, the minimum content of isotope 208Pb in the lead heat carrier should be about 75–80%. This is typical of thorium ores.

The use of tungsten coatings of fuel element cladding helps to reduce VRE, eliminate reaction of chromium with steam in emergency situations, and exclude hypothetical possibility of lead azide formation. With the simultaneous use of tungsten coatings and heat carrier based on lead extracted from thorium ores, it is possible to ensure a good balance of neutrons in the core.

The potential and reserves of MOX fuel in ensuring the safety of large LMFR with sodium or lead coolant have not yet been exhausted. Pellet fuel based on finegrained MOX and nanoscale U has no advantage over pellet MN fuel.

The prospects for LMFR development may be related to the use of fuel based on fine-grained MN and nano-sized metal uranium powder.

Uranium nanopowder additives will not only increase the density and thermal conductivity of nuclear fuel, which is important in the problem of reactor safety,

### *Nuclear Materials*

but also significantly improve the neutron balance (by directing "excess" neutrons to ensure safety: achievement of condition BRC = 1 even on oxide fuel), almost completely eliminate the conflict character in the requirement for Doppler reactivity coefficient in terms of self-protection against ATWS in large LMFR, which opens up wide possibilities for the optimization of core layout without deterioration of safety.

The complex use of the proposed innovations allows to ensure the safety of large LMFR. So, pellet MN fuel with nanopowder of metallic uranium, a coolant based on lead extracted from thorium ores, and steel claddings of fuel elements with a tungsten coating together can significantly increase the safety of the reactor. This is accident-tolerant materials for LMFR.

The results presented can be easily summarized into other types of reactors. The research can be useful in the development of new-generation reactors and the improvement of technical solutions to existing nuclear reactors.
