**3. Lead-208-based coolant**

Among the realistic liquid metal coolants of fast reactors are sodium, lead, and eutectic alloy of lead and bismuth. **Table 1** shows data on the prevalence of lead, bismuth, and sodium, their world production, and world reserves (according to data [33]).

Natural lead is intended to be used in lead-cooled reactors. The isotopic composition of such lead (*nat*Pb: 1.4% 204Pb − 23.6% 206Pb − 22.6% 207Pb − 52.4% 208Pb) is obtained by averaging all known deposits (about 1500). The isotopic composition of lead in different fields can vary significantly (see **Table 2**).

The large differences in the isotopic composition of lead from different deposits open up great opportunities for optimizing the composition of lead coolant without the use of expensive isotope separation technology. By mixing lead from different deposits, it's possible to obtain a coolant with a given concentration of stable


**Table 1.**

*World production, reserves, and prevalence of certain elements in the Earth's crust.*


**Table 2.**

*Isotopic composition of lead of different deposits, % (wt) [16].*

isotopes, i.e., with given nuclear-physical properties. If it's necessary to minimize VRE, thorium lead with a high concentration of 208Pb (decay product of 232Th) is preferable, and if it is necessary to minimize the production of 210Po, uranium lead with a high concentration of 206Pb (decay product of 238U), and small impurities of 207Pb (decay product of 235U), as well as non-radiogenic lead 204Pb are required. The use of uranium and non-radiogenic lead leads to an increase in VRE.

**Figures 1** and **2** present statistics on the isotopic composition of lead in the monocytes of the Ukrainian shield. The age of the breed is about 2 billion years. An analysis of 49 samples was carried out: along the abscissa axis is the sample number according to [34].

**Figure 3** presents the results of the MCU calculation of the void and density effects of reactivity (Δρ) in a high-power BREST reactor. In **Figure 3(a)**, the dependence of VRE on the content of 208Pb in the coolant is given. **Figure 3(b)** shows the dependence of the effects of reactivity on the density of a coolant based on lead of polymetallic ores. Various drainage scenarios are considered: (1) the entire reactor (when calculating the density effect—the change in the density of the coolant in the entire reactor); (2) the core and the lower reflector (or a change in the density of lead); and (3) drainage of the core. The implementation of VRE involves the complete loss of coolant (drainage of the reactor, core or part thereof), the implementation of the density effect involves a change in the density of the coolant.

**Figure 1.** *The concentration of lead isotopes in different samples of monazite.*

**Figure 2.** *The dependence of the content of the isotope 208Pb in lead (*C*208) on the lead content in monazite (*C*Pb).*

**Table 3** shows the VRE values (calculations according to the MCU code) for various scenarios of drainage of the BREST reactor. For comparison, the last line shows the VER values for the sodium-cooled reactor. The numbers indicate drainage scenarios (see **Figure 3**).


#### **Table 3.**

*VRE values for various drainage scenarios (LOCA WS).*

#### **Figure 3.**

*Studies of the void and density effects of reactivity in a high power BREST reactor, (a) the dependence of VRE on the content of 208Pb in the coolant is given, (b) the dependence of the effects of reactivity on the density of a coolant based on lead of polymetallic ores.*

When using lead with a high concentration of 208Pb as a coolant, the potential risk of reactivity accidents is reduced. The neutron absorption cross section of 208Pb in the spectrum of the BREST reactor is 10<sup>−</sup><sup>3</sup> b (1 b = 10<sup>−</sup>28 m3 ), that is, approximately three times lower than *nat*Pb. The core of such a reactor is characterized by a large volume fraction of lead and a small fraction of fuel, which increases the role of the coolant in changing the neutron macroscopic absorption cross section Σ*a* by core materials. When switching to thorium lead, the Σ*a* decreases, which leads to an increase in the lifetime of prompt neutrons and limits the prompt supercritical when reactivity is introduced that is greater than β. When switching to uranium lead (206Pb), the Σ*a* increases (the neutron absorption cross section of 206Pb is more than three times higher than natPb); as a result, the potential danger of reactivity accidents (the reactor is prompt supercritical) increases.

One of the ways to increase the corrosion resistance of structural materials is the use of technological additives (inhibitors and deoxidants) to the coolant. The possible inhibitors are characterized by a high absorption cross section and quickly burn out in a neutron field, forming slags in the coolant. All metallic impurities contained in liquid lead (except bismuth), and all metallic components that may be present in lead as a result of corrosion and erosion of structural materials, have a lower electrode potential than lead, i.e., these impurities play the role of deoxidizing agents. The deoxidizing agent may also be an alkali metal. Small additives of lithium or potassium form a eutectic with lead, reducing the freezing point of the coolant. Small sodium additives play the role of an alloying additive to lead, increasing the freezing point of the coolant. Lithium is the strongest reducing agent—the best getter of free oxygen. Experiments conducted at the JSC "SSC RF-IPPE" (Obninsk, Russia) show that the cardinal way of changing the initial oxidizing properties of heavy coolants is the use of additives of metal deoxidizing agents that can reduce the level of oxidative potential of the melt. Thus, a technological additive of 1.8% (wt.) potassium to lead allows the formation of a eutectic alloy with oxygen activity five orders of magnitude lower than that of a pure lead melt at temperatures of 500–550°C.

With a decrease in the mass number of nuclei, the role of elastic moderation increases, and the void and density effects of reactivity increase. The relatively high VRE with the use of a coolant based on the eutectic Pb-<sup>7</sup> Li alloy can be reduced by switching from natural lead to 208Pb in the composition of this alloy (see **Figure 4**). However, in this case, VRE is significantly higher than when using lead polymetallic ores.

#### **Figure 4.**

*Change in the reactivity effect Δρ depending on the density of the coolant in the core and the lower reflector during drainage of the BREST high-power reactor from the bottom up (involvement of bubbles in the core), (1) eutectic 7 Li-208Pb; (2)* nat*Pb; (3) eutectic* nat*K-208Pb; (4) 208Pb.*

More detailed results of studies on the use of a coolant based on lead extracted from thorium ores are presented in [16, 35–38]. To ensure acceptable VRE in highpower reactors, it is not necessary to use isotopically pure lead-208. It is enough that the content of 208Pb in the lead coolant is at least 75–80%. This makes it possible to use almost any thorium deposit for lead mining.
