**4. Construction materials**

The greatest successes have been achieved in the field of creating new structural materials. For fast sodium reactors, specialists from Russia, the USA, Japan, China, France, and Ukraine consider radiation-resistant heat-resistant materials based on dispersion—hardened by thermally stable nanooxides Y2O3, Y2O3-TiO2 or Al2O3 (3–5 nm) ferritic-martensitic steels with good strength and mechanical properties capable of working at neutron fluences (with kinetic energies of more than 0.1 MeV) up to 2 × 1016 cm−<sup>2</sup> s<sup>−</sup><sup>1</sup> to damaging doses of 160–180 sleep at temperatures of 370–710°С (see [39–41] and links to these works).

While ensuring a good neutron balance in the core due to the use of innovative fuels, coolant, and structural materials, two advantages of additional absorbers in structural materials can be distinguished.

Firstly, the fast neutron absorption cross section of structural materials that cannot leave the core when it is drained favorably affects the development of the LOCA emergency: they contribute to the reduction of VRE. In the manufacture of structural materials, the content of alloying additives is regulated by regulatory documents. The content of alloying additives can vary in a given range. A negative factor may consist in a change in the sign of VRE depending on the content of alloying additives in the composition of structural steel (see **Figure 5**).

Secondly, the removal from the core of erosion and corrosion products (of structural materials) that strongly absorb neutrons helps to prolong the campaign of the reactor, increasing reactivity, similar to the action of burnable absorbers in thermal reactors (see **Figure 6**). The negative factor is a decrease in the thickness of the cladding of the fuel rods and slagging of the contour (increase in erosion rate) by corrosion and erosion products that have not settled on the "cold" section of the coolant circulation path. As a result, the processes of corrosion and erosion of structural steels contribute to solving the problem of minimizing the reactivity

#### **Figure 5.**

*VRE in a BREST reactor of high power under conditions of uncertainty in the concentration of alloying additives in structural steel within the permissible (MCU).* H*—The boundary of the medium "lead-vacuum." conservative assessment: Horizontal neutron leakage is equal to zero; 100% replacement of lead by vacuum are not taken into account, (a) the drainage boundary moves from top to bottom (lead discharge, in BREST the event is ruled out deterministically), (b) the drainage boundary moves from bottom to top (involvement of bubbles in the core during depressurization of the lead-water heat exchanger tubes).*

#### **Figure 6.**

*Change in the reactivity Δρc for microcampaign T as a result of corrosion of the structural steels and the fuel burnup.*

margin for fuel burnup. In BREST projects it is proposed to use steel without nickel but with increased silicon content [14]. Silicon promotes the formation of protective oxide films on the outer surfaces of cladding of fuel element.

The calculated analysis made it possible to conclude on the possibility of using tungsten coatings of fuel element cladding from the outside and inside. This will lead to the improvement of reliability and safety of BREST reactors without deterioration (and possibly with improvement) of economic characteristics of NPPs with such reactors [16, 42, 43]. The application of tungsten coverings will allow to reduce the corrosion rate and erosion in liquid lead and will lead to the deterioration in neutron balance (and perhaps, reactor costs); minimization will be promoted by the VRE (large absorption cross section of fast neutrons) and will open possibilities of use of the lead more polluted by impurity.

Tungsten does not react with nitride fuel at temperatures below 1485°C. This temperature is not achieved even in ATWS [14, 16]. Tungsten interacts weakly with lead. Conditions for the formation of lead (PbWO4) tungsten in the reactor core are poor: insufficient oxygen, high-energy neutrons break chemical bonds.

Tungsten in combination with chromium improves mechanical properties of steel [44]. Due to the small (compared to iron) values of the cross section of neutron interaction with tungsten nuclei, accompanied by the yield of gaseous products (less than 10<sup>−</sup><sup>2</sup> b), and the high threshold of such reactions (10 MeV), tungsten does not swell and does not become brittle at high fluxes of fast neutrons.

Tungsten coatings produced by plasma spraying have a layered structure, do not crack, or peel even when bent [45]. Tungsten plasma spraying technology is well developed. The dense (nonporous) tungsten film completely covers the steel surface. Corrosion and erosion resistant coatings in a wide range of temperatures (higher than ATWS temperatures) in liquid metal melts, including under conditions of additional abrasive wear (practically excluded by tungsten coatings).

Like silicon, tungsten contributes to the formation of a protective oxide film on the surface of the shell (to a lesser extent because it holds oxygen slightly worse: bond energy SiO = Si + O is 8.24 ± 0.10 eV, WO = W + O is 6.94 ± 0.43 eV [46]). Due to the leakage of the circuit, oxygen will always be present in the lead. The oxide film (WO2, W4O11, WO3) holds well (up to 923°C) and serves as an additional naturally formed protective coating.

The tungsten content of the sprayed layer is about 1%. As a result, the increase in the cost of the structural material may not be essential. Among the neutronphysical aspects of the use of tungsten-sprayed cladding, the following are noted. The deterioration of the neutron balance may not be essential when thin coatings are used. (At that, it is possible to reduce thickness of cladding).

The MCU code calculations showed that an increase in the proportion of tungsten in the structural material from 0 to 100% leads to a marked decrease in VRE: by 1.76 \$ with hypothetical drainage of the core and the lower end reflector and

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

**Figure 7.** *The dependence of kin on* C*W.*

when 98% 208Pb in the lead coolant. The most realistic and most dangerous scenario of such an emergency situation is not related to the complete drainage of the reactor but to the involvement of bubbles in the core during depressurization of the pipes of the "lead-steam (water)" heat exchanger. At that, reactivity effect is maximum at the reduction of lead density in the core and lower end reflector from 10.5 to 7.0 g/cm3 [35]. In such a scenario, the density effect of reactivity is \$0.06 when used as a structural material of corrosion-resistant steel and minus \$ 0.86 when switching to tungsten cladding. To obtain a conservative estimate, an infinite array of fuel elements is chosen, formally corresponding to the reactor of infinite power.

**Figure 7** shows the dependence of the infinity multiplication factor kin on the mass content of tungsten *C*W in the structural steel of the BREST-OD-300 core. It is possible to fully compensate for neutron losses due to their absorption by tungsten nuclei by increasing the proportion of 208Pb in the heat carrier even when using shells made entirely of tungsten.

The other way is connected with self-organizing coatings of internal surface of fuel element cladding. It is proposed to place liquid lead saturated with zirconium or liquid eutectic alloy 97.53% (wt.) Pb, 2.25% Mg, and 0.2% Zr [47]. When the reactor is operating, a protective coating based on zirconium nitride is spontaneously formed on the inner surface of the cladding. Self-healing of accidental damages of coating is provided.
