The intensifying decarbonisation policy and the international situation related to the war in Ukraine require faster implementation of technological solutions allowing to maintain the strategy of limiting the emission of harmful substances while ensuring energy security. One of the technologies that provide both conditions is nuclear power. In recent years, modular nuclear reactors have gained special importance, and have also become the subject of investment considerations on the Polish market. The natural place to use nuclear modular reactors is heating, which in Poland is based on coal. A heating installation that meets the conditions of high-efficiency cogeneration may be qualified for the support scheme. The entry condition is the fulfillment of the 10% PES (Primary Energy Savings) condition. At the stage of works on coupling study, an issue was identified that should be further considered. Namely, in the Polish legal system there is a cogeneration regulation which precisely defines the algorithm for calculating PES for fossil fuels, biomass, biogas, etc. However, this regulation does not define the method of calculating PES for nuclear fuel. This article addresses this issue and outlines a possible procedure for introducing such a regulation into Polish energy legislation. Regulating this issue will contribute to the possibility of obtaining financial support for nuclear technologies qualified as high-efficiency cogeneration.