# Heat Storage as a Way to Increase Energy Efficiency and Flexibility of NPP in Isolated Power System
## 1 Introduction
So, to achieve development in isolated power systems, NPPs of low power should be characterized by the following: the maximum effective duration of operation for the nuclear reactor without fuel overload, increased maneuverability and safety, and the absence or a low amount of radioactive waste [33]. Thus, the low maneuverability of nuclear power plants is an obstacle to the widespread integration of nuclear power plants in isolated power systems. One of the ways to increase the maneuverability and safety of NPPs, and reduce the amount of radioactive waste and the negative impact of maneuvering modes on the core elements, is by using heat accumulators in NPP circuits [21; 34; 45]. The application of a heat accumulator allows for one to store excess reactor heat and then use this heat to generate electricity during hours of increased demand [36; 46; 47]. Considering the regions of the Far North, it is impossible not to discuss regional features. So, the extreme conditions of the Arctic do not create direct problems for the operation of the main equipment of the NPP. So, the NPP equipment is reliably protected from environmental influences by the shells of buildings [37]. Therefore, the impact of extreme conditions on the operation of storage equipment is not considered in this article. The main regional feature is the work in an isolated power system. One of the actively developing and promising areas in the field of heat storage is heat accumulators with phase change material (PCM) [48]. Such accumulators are characterized by a high density of stored energy and a stable temperature operating mode during charge and discharge [48]. The peculiarity of such accumulators is the use of latent melting-solidification heat, which ensures the storage of a large amount of heat without significant volume changes [49]. Such accumulators are applied in solar energy [50], electronics [51], building [52], and heat supply [53]. PCMs are classified into three categories based on their chemical: organic (low temperature), salts (medium temperature), and inorganic (high temperature) [54]. The main factor in choosing a PCM is the temperature regime of the system in which the accumulator is planned to be used. Among those suitable for the melting point of the PCM, the most optimal is chosen based on the following: chemical compatibility of the container materials, stable physical properties, higher density, higher heat of melting, thermal conductivity, and heat capacity. The design of the accumulator is determined by the required heat exchange power and the parameters of the hot and cold heat transfer fluids. In general, there are two types of approaches to improving thermal performance, namely, extending the heating surfaces or modifying phase change materials [55]. Heat accumulators with a PCM are generally divided into two categories: capsule and shell-and-tube [56; 57]. At the same time, when it comes to high-power accumulators (industrial applications), shell-and-tube accumulators are more often mentioned [58; 59; 60]. It is known that several pilot installations use heat accumulators with a PCM for industrial use with a capacity of up to 1.6 MW [61; 62; 63]. These installations are used for heating heat transfer fluids, including steam generation, and, in general, much is said about the operability of this technology. Thus, this direction of heat storage is actively developing. So, it is fair to reason and evaluate the options for using thermal accumulators with a PCM with a capacity of about several MW in various industries, including nuclear power plants. In this paper, we exactly consider accumulators with a PCM for application in NPPs. As for the method of determining the design of t heat accumulator, there is no simple universal technique. Due to the unsteadiness of the heat exchange processes occurring in such accumulators, the variety of designs, sizes and heat storage materials, each application is unique and should be considered separately.