## 2 Approach to Organizing the FNS-ST Fuel Cycle  
According to [6; 7], designing the FNS-ST facility will be a step toward the development of the stationarily operating DEMO-FNS neutron source [8; 9] and the development of some technologies, including fusion FC technology [10]. The FNS-ST will be a stationary operating facility, and its FC structure will be similar to that of the DEMO-FNS [11; 12], with allowance for the specific features of the facility. The fuel cycle of the FNS-ST includes systems for storing the fuel isotopes, pumping the gases from the vacuum chamber, separating the hydrogen isotopes from the mixed pumped-out gases, additionally purifying the chemically bonded hydrogen isotopes, processing the tritium-containing radioactive waste (including the system for removing tritium from the gas-water mixture), producing tritium and extracting it from the carrier gas, separating the hydrogen isotopes to obtain their concentration required to supply the injection systems, as well as the control systems for the sources of hydrogen isotopes in the plasma and the heating and fuel supply systems. In this design stage, we do not consider the possible systems providing circulation of impurity gases (for injecting them into the divertor) or lithium circulation on the first wall, since their effect on the FC operation is moderate. They will be considered in the next design stages. In this work, we calculate the particle fluxes in the FC systems to determine their efficiencies. Optimization of the FC systems will be performed in the future based on the calculations carried out in this work.  
At first, for the FNS-ST project, it was planned to use the off-axis injection of fast atoms with the slanted arrangement of the injectors [1; 2]. Subsequent analysis [13; 14] showed that it is technically difficult to realize slanted injection, and it is not reasonable to tilt the injection axis off the equatorial plane. It was also proposed to inject deuterium beams since the injection of tritium in the form of fast atomic beams creates some technical difficulties. Similar to the DEMO-FNS simulations [11; 15], in this work, we calculate the scenarios for using the D + T or D heating beams. In this case, the calculations are performed relying on injectors based on the positive ion source, which results in the appearance of the beam components with lower energies: E/2 and E/3.  
The calculations based on the consistent model of the bulk and divertor plasmas performed for the DEMO-TIN [15; 16] showed that to maintain the required plasma density, it was insufficient to fuel the bulk plasma with the beams and neutral flows from the divertor. As a result, it became necessary to use pellet injection along with large-scale gas injection. Since for the spherical tokamak, it is difficult to inject the fuel pellets from the high magnetic field side (HFS), the injection at the FNS-ST is foreseen from the low field side (LPS). This results in additional fuel losses due to the outward drift, and to maintain fuel balance, we should increase the fuel inflow with the pellet injection. This results in an increase in the required tritium reserve at the facility.  
The project in [8; 17] provides for the hybrid blanket, but it does not provide for tritium breeding. In the simulations, we use a tritium breeding ratio (TBR) equal to 1.25, which is sufficient to replenish the tritium burnout in the core plasma. The FNS-ST parameters are discussed in detail in the next sections.