**5.3 ATF based on MOX and uranium nanopowder**

For MOX-fueled reactors, it is possible to offer pellet fuel based on fine-grained MOX and nanopowder U. Fuel density (at 18% U) under normal conditions of about 12 g/cm3 , thermal conductivity—12 W m<sup>−</sup><sup>1</sup> К−<sup>1</sup> . Increasing the density and thermal conductivity of MOX-U fuel with increasing U fraction leads to changing the role of Doppler reactivity coefficient in providing self-protection against accidents of LOF WS type. As a result, the role of Doppler reactivity coefficient in providing self-protection against accidents of type TOP WS and LOF WS becomes the same (or almost the same), which helps to resolve the conflict situation when optimizing the core layout. This makes it possible to significantly increase reactor safety.

**Figure 8** shows the results of modeling LOF WS mode (FRISS-2D code) in LMFR using different fuels. The relationship between maximum temperature of fuel and heat carrier is presented when using pellet MOX fuel in BN-800 (a), MN fuel in reactor of type BREST-OD-300 [14] (b), and BN-800 (c). The thin line corresponds to the conservative approximation: constant coolant temperature at the core inlet in emergency mode. **Figure 8(d)** shows the time dependence of the maximum temperature of the MOX-based pellet fuel and uranium nanopowder in the TOP WS and LOF WS modes and when they are superimposed (dotted line in **Figure 8(d)**). For correct comparison of variants at use of different fuel, the same fuel assemblies and grids of the core BN-800 [53] are accepted.

LOF WS is initiated by de-energizing of all main circulation pumps of the primary circuit at coastdown time 30 s (for BN-800) or by reduction of flow rate in 20 s (for BREST-300). TOP WS is initiated by reactivity input \$0.3 for 10 s.

In order to increase the self-protection of reactors against the TOP WS accident, it is necessary to increase the negative Doppler reactivity coefficient by absolute value. In LOF WS mode, when MOX fuel is used, the maximum fuel

**Figure 8.**

*Time dependence of pellet fuel temperature in LOF WS mode for BN-800 reactors with MOX fuel (a), БРЕСТ-300 (b), and БН-800 (c) with MN fuel, BN-800 with MOX-U fuel (d).*

temperature decreases with time. At the same time, it is possible to start boiling sodium. In order to increase self-protection, it is necessary to modulo reduce the negative Doppler reactivity coefficient. The use of vibration-compressed MOX fuel with uranium getter [19, 20] does not significantly change these dependencies, and the role of Doppler reactivity coefficient in TOP WS and LOF WS modes remains the opposite.

When using MN fuel in LOF WS mode, an intermediate scenario between metal and oxide fuel is implemented. To minimize maximum temperatures, the negative Doppler reactivity coefficient must be increased modularly, as well as to ensure self-protection against TOP WS accidents. If the maximum fuel temperature is reduced to less than the nominal value, it is necessary to modulo reduce the negative Doppler coefficient, but due to significant temperature reserves in this case, the role of Doppler coefficient has no determining value. This is the pattern (**Figure 1(d)**) observed in the emergency mode LOF WS when using a tablet fuel based on fine-grained MOX and nanopowder U (18% wt.). At its low U content, the scenario of emergency mode development is the same as when using tablet or vibrocompressed MOX fuel with finely dispersed (100 μm) U-getter. At high U content (up to 50% by weight), the nature of the change in maximum fuel temperature over time in the LOF WS mode is similar to that shown in **Figure 1(b)**. At mass content of uranium nanopowder, 18% fuel is slightly lower than the tablet MN in density and 1.5 times in thermal conductivity. As a result, when switching to MOX-U, the maximum fuel temperature is significantly lower than when using a tablet MOX fuel and approximately the same as the MN fuel (with larger fuel pellet diameter). The nature of the change of maximum temperatures in emergency modes TOP WS and LOF WS in fast reactors of medium and high power with MN fuel and fuel based on fine-grained MOX and nanopowder U are the same. In both cases BRC = 1.

The LOHS WS and OVC WS are not dangerous.

### *Accident Tolerant Materials for LMFR DOI: http://dx.doi.org/10.5772/intechopen.90703*

The temperature of MOX-U fuel even in ATWS is lower than the oxide when the reactor operates at rated power. This helps to reduce oxygen release from the Pu4O7 in nominal and emergency modes of reactor operation.

When the temperature increases (in emergency modes), the MOX-U fuel can contain nanodroplets of liquid metal between MOX grains. By neutron balance this fuel is closer to the pellet monoxide or MN, by the melting point—closer to UO2- Pu4O7. So, when using nanotechnologies in the manufacture, MOX-U-fuel opens up large "reserves," approaching it in properties to MN.
