**2.5 Modern directions of development of power LMFRs**

As noted, the world is considering two main concepts of medium-power and high-power LMFRs: with sodium and lead coolant. As a possible heat carrier for use in low-power reactors, the eutectic alloy Pb-Bi [15, 21, 22] is considered. In the 1990s, South Korea, China, Japan, and other countries showed interest in fast lead-bismuth reactors. In Russia, projects of such low-capacity reactors are being developed [15, 22]. Bismuth is significantly more expensive than lead. The use of bismuth leads to the working time of the short-lived highly active isotope polonium-210 in the heat carrier. In the 1990s, systems for purifying heat carrier from polonium were actively developed during reactor operation [23].

Sodium coolant is technologically developed. In many countries, it is preferred in the development of LMFRs [15]. In Russia in 2016, the power unit of the BN-800 was put into power operation; the project of the BN-1200 power reactor is being developed [24, 25].

Lead has not yet been used as a coolant. According to the developers of BREST projects, their implementation is possible within the framework of existing Russian technologies [14]. The key works that initiated the development of the BREST direction can be considered articles [13, 26–28].

Works on the development of lead-cooled fast reactors are actively carried out in Russia (technical design and planned construction of BREST-OD-300, work on the concept design of MN fuel BREST-1200 is underway [27, 29]), European Union countries (concept projects ELFR, ELSY, LEADER, ALFRED with MOX fuel have been developed since 2006 [30]), and the USA by 2005–2007, and conceptual projects of STAR series of reactors of different target purpose with UN-fuel have been proposed [22, 30].

### **2.6 Problems of BREST concept during transition to high-power reactors**

In terms of the possibility of avoiding severe accidents among fast reactors, the BREST-OD-300 project is most attractive [14]. It's a low-power reactor. It is well-known that it is much easier to ensure the safety of a low-power reactor. At reactor power increase (BREST-1200), there are problems with inherent safety provision. For example, the VRE is several times greater than β. The achievement of high economic efficiency of the NPP with BREST reactor may be hampered by the relatively high rate of corrosion-erosion of liquid lead on structural materials. If in BREST-OD-300 it is estimated at 39 kg of steel (for core only) per year [14, 31], for BREST-1200 it can be 39 · 4 = 156 kg/year. At the same time, economic estimates should correspond not to the cost of steel but to the cost of fuel assemblies, in which fuel elements will be depressurized.

The use of MN fuel will be accompanied by the release of nitrogen (from the PuN) and its migration to the cladding of fuel pin. In the presence of free nitrogen, the corrosion rate of the inner surface of the claddings increases. (The process is similar to the release of free oxygen from MOX fuel. The only difference is that the corrosion rate in the presence of nitrogen is less than in the presence of oxygen).

The absence of zirconium in reactor core allows deterministic elimination of steam-zirconium chemical reaction. However, hydrogen release is possible in other metal-based chemical reactions implemented at high temperatures. Chromium reacts with steam (water) in the absence of oxide film on the fuel element surface: 2Cr + 3H2O ↔ Cr2O3 + 3H2. In this reaction, heat and free hydrogen are released. The surface of cladding must be carefully protected from possible contact with the steam involved in the core. (Chromium is present as part of the structural steel of fast reactors. The BREST uses a two-circuit cooling systems. Lead is primary circuit, steam is secondary circuit).

According to journalists, the interaction of nitrogen with lead (if there is a lead layer between the fuel and the cladding) can cause the formation of explosive lead azide. Lead does not react with nitrogen and such an event is highly unlikely. Lead azide production requires special conditions and reagents not present in BREST.

#### **2.7 The considered emergency operation**

As part of the deterministic approach to safety analysis [4], ATWS requires priority consideration. In fast reactors with liquid metal cooling, the whole set of emergency situations, including ATWS, is described (simulated) by perturbations of reactivity δp, coolant flow rate δ*G*, and coolant temperature at the core inlet δ*Tin*. When designing LMFRs, the following emergency situations (commonly used abbreviations are specified) and their combinations correspond to various disturbances:


Simulation of emergency modes LOF WS, TOP WS, LOHS WS, and OVC WS was performed using a code version of FRISS-2D and DRAGON-M [1]. DRAGON-M and MCU codes [32] are used to analyze LOCA WS.
