In a surprise move, Borissov fired Economy and Energy Minister Traycho Traykov and announced that his country would buy the Russian reactor commissioned for the Belene Nuclear Power Plant that is now under construction.
He added, however, that the unit could instead be installed at the existing Kozloduy nuclear plant which now has two functioning reactors (see background).
The developments appear as a showdown after a protracted period, in which Borissov made noises that he was receptive to the advice of his government's allies in Brussels and Washington, who warned that the project will deepen Bulgaria's energy dependence on Russia.
Bulgaria depends on Russia for 89% of its petrol, 100% of natural gas and 100% of the nuclear fuel needed for its Kozloduy power station. Another reactor is planned at Belene, also with Russian technology, but the project has been frozen over a payment dispute.
Japan's Fukushima nuclear disaster last year prompted Sofia to invoke the need to review safety issues and clarify the conditions of Russian funding for the Belene project.
Environmentalists and lobby groups are staunchly opposed to completing the Belene plant. In contrast, Kozloduy - where four of six reactors have been decommissioned with EU support - has been relatively uncontroversial.
Borissov reacted strongly against a statement by Martin Dimitrov, leader of the small opposition SDS (Union of Democratic Forces), who said that Traykov had been the only safeguard against building the Belene power plant.
The prime minister argued that since Bulgaria had already paid two-thirds of the reactor's price, the best solution was to pay the outstanding amount and to make use of the unit. He also insisted that locating the new reactor at Kozloduy was his idea.
Campaigners against Belene said the developments indicate that Borissov has backed down and that the new power station will be built according to Russia's wishes.