# Heat Storage as a Way to Increase Energy Efficiency and Flexibility of NPP in Isolated Power System
## 1 Introduction
If renewable energy sources are used as a power supply source, load regulation is not possible in principle due to dependence on weather conditions [32]. One of the most promising and actively developing areas in the energy supply of mineral and raw materials objects as well as fuel and energy complexes in isolated energy systems is stationary low-power nuclear power plants (NPP) [33; 34]. This direction of energy supply for remote consumers has several advantages--long fuel overload cycles, a large amount of energy per fuel unit, efficient cogeneration of heat and electricity, and zero CO\({}_{2}\) and CO emissions [35; 36]. In the future, small-power NPPs may become a serious alternative to diesel piston and gas turbine power plants in isolated power systems [35]. Leading countries of the world are developing low-power reactors: water-water reactors (PWR), fast-neutron reactors, and gas-cooled reactors [33]. Water-to-water reactors are currently the most common and technically mature technology in this industry. In Russia, this direction of nuclear energy development is based on the experience of creating and operating shipboard nuclear power installations, such as RITM-200 and KLT-40 of the PWR type [14; 37; 38]. The floating NPP on the icebreaker \"Akademik Lomonosov\" is operated in Russia, in the city of Pevek, with two nuclear reactors (KLT-40S) and an installed capacity of 70 MW [39]. A ground-based low-power nuclear power plant with two RITM200 reactors is now being studied as part of a rough makeup design [40]. One of the trends in the development of stationary low-power NPPs is an increase in the initial parameters of working fluid [33]. So, a low-power NPP based on the RITM 200 reactor will operate on an overheated steam. Each RITM-200 reactor has a built-in steam generator, which can be thermally connected to one or more turbo-generating units [41]. Slightly superheated steam is supplied to the turbine [41]. The turbine is a single-cylinder assembly with intermediate steam separation and consists of oppositely directed high- and low-pressure parts [37]. The regeneration system of a steam turbine plant has low-pressure heaters, a deaerator, and high-pressure heaters [37]. The planned fresh steam pressure is 3.5 MPa, and the temperature is 295 \({}^{\circ}\)C [37]. Thus, conceptually, a stationary low-power NPP is a two-circuit NPP with a water-water reactor on light water, a steam generator, and a steam turbine with a regenerative system. In general, this is very similar to a stationary nuclear power plant of high power (for example, with VVER-1200 reactors). The differences are in scale, another reactor design (ship reactor), the presence of steam overheating (turbines of Russian stationary nuclear power plants operate on wet steam), and reduced fresh steam pressure (3.5 MPa at low-power NPPs vs. 7 MPa at high-power nuclear power plants). The nuclear power plants used as a source of electricity exacerbate the problem of balancing the generated and consumed power in the energy system. The reason for this is the low maneuverability of nuclear power plants, which is related to the safety issues of nuclear power plant operation due to the principles of operating nuclear reactors and restrictions [34; 42; 43]. Traditionally, nuclear power plants are not considered as a maneuverable source of energy supply [23; 44]. So, a nuclear power plant has several restrictions on the number and power changes in cycle speed. NPP operation in a maneuver mode with fast increases and decreases in power does not provide 100% operational safety [34; 44]. The maneuverable modes of operation of NPPs hurt the operation of the equipment in the reactor core [29]; this risks damage to fuel rods, damage to control rods, and uneven fuel burnout. Working in such modes, the effective duration of operation for a nuclear reactor without fuel overload decreases, and the amount of liquid radioactive waste increases [42]. However, higher requirements are imposed on small-power nuclear power plants compared to large nuclear power plants.