The paper presents the results of modeling and comparative analysis of long-term scenarios of the expansion of the generating capacities of Ukraine's Power System assuming decommissioning or extension of nuclear power plants' (NPPs) life. Development of scenarios and model calcula-tions was carried out using the optimization modeling tool "WASP-Ukraine", developed on the basis of IAEA software WASP-IV designed to find the economically optimal generation expansion policy within user-specified constraints. The model "WASP-Ukraine" is based on linear programming technique. Modeling baseline and alternative scenarios made for the period up to 2030 and the base year is 2014.
The study examined six scenarios assuming decommissioning and extension of NPP life in ac-cordance with international practice. The share of nuclear power in electricity production in Ukraine in 2030 under different scenarios varies from 11% to 48%. In the scenarios, which minimize the role of nuclear power, compensation of decommissioned NPP is provided by renewables or by conventional thermal power plants.
Comparative analysis of the scenarios is conducted by four criteria: cost efficiency (average production cost of electricity generated in a power system); environmental sustainability (total emissions of sulfur oxides (SO2) and nitrogen oxides (NOx)); reliability of the power system and electricity supply (aggregated loss of load probability (LOLP) and the energy not served (ENS) indicators); provision of power plants with primary energy resources (steam coal). The comparative analysis proved that scenarios assuming extension of NPP life are more acceptable and efficient by all selected criteria comparing to those assuming decommissioning of NPP after their life period. Thus, maintaining the dominant role of nuclear energy in Ukraine's power balance at least until 2030 is justified.

Keywords: power system, generating capacity, nuclear energy, scenarios, long-term development, assessment

Ukraine's electricity market can be considered as rigidly regulated by the state, low competitive and inefficient, and such that does not fulfill its main functions: regulatory, distribution, stimulating, informational ones etc. Its transformation in the context of European integration of the national econ-omy requires the creation of an institutional environment that would enhance its openness, transpar-ency and competitiveness at the expense of domestic and foreign sources and improved government regulation of economic relations on it.
The purpose of the article is to assess the state of price competition in Ukraine's electricity mar-ket, the level of market power of the electricity producing companies and other market participants in terms of pricing and regulation of electricity prices.
The methodological basis of the study is the institutional and behavioral approach to the definition and analysis of institutional conditions, in particular regarding the freedom, restrictions and government regulation of economic activities of the power generating companies on Ukraine's wholesale electricity market, assessment of the preconditions for the emergence and availability of market power in determination of electricity prices and implementation of price policy. The presence and abuse of market power of the electricity producing companies was assessed using a comparative analysis of their profitability.
It has been determined that current conditions and rules of functioning of Ukraine's electricity market essentially determine the freedom of economic activities of power generating companies as regards pricing. In fact, the power generating companies are price-takers and have a limited impact on the terms of goods turnover on the market. Instead, the main players on the market with the highest market power are the government bodies, namely the energy regulator and the corresponding ministry. This situation can be characterized as an institutional monopoly.
Assessment of the profitability of power generating companies in Ukraine according to Lerner coefficients and EBITDA margin indicates that they have no extra high yield (monopoly rent) com-pared to average industry indicators of Ukraine and groups of countries, which indicates their lack of market power or abuse of a dominant position on the market.

Keywords: electricity market, price and price policy, state regulation, competition, market power