[en] Some experimental and theoretical results obtained in the investigations undertaken at the Institute of Atomic Physics in Bucharest, Romania concerning the direct nuclear energy conversion into electrical energy are presented. Fission electric cells (FEC) able to deliver significant power at high neutron fluxes and to operate without supply were developed and tested in the VVR-S nuclear reactor at 10⁹ n/cm² s thermal neutron flux. The experimentally obtained values of the FEC parameters were much higher than those reported by other laboratories (Battelle Memorial Institute, Jet Propulsion Laboratory). These investigations led to a better understanding of the main physical processes regulating FEC operation (FEC space current variation law, electron secondary emission, the importance of the geometrical factor). The neutron flux dependence of the FEC output parameters in the 10⁸-10¹² n/cm² s range and the existence of a critical neutron flux value which assures an efficient FEC operation have been evidenced. An estimation of the FEC performance at high values of the neutron flux has been performed on the basis of our experimental data. Our researches would make feasible the design of efficient fission electric cells as potentially usable components in nuclear reactors especially for space reactors. (Author)