[en] The actual Nuclear Power Plant units design, looking to their performance and efficiency, have to be improved and upgraded. Looking to the secondary circuit global efficiency the upgrading possibility exists. Necessity to increase by design the nuclear power and corresponding the electrical power is possible but the steam turbine becomes high volume, especially on the low-pressure stage. Discussing about the global NPP unit efficiency, the transmission of the heat from the reactor by means of the steam generator is performed with a high efficiency; also the steam turbines have good conversion efficiency. In the thermal power plant units, based on different thermal cycle, the secondary thermal circuit evacuates the heat through the condenser. The main desire or goal of the power plant unit is to produce electrical power, so to convert the thermal power in electrical power by any means. One of the solutions is to use appropriate cooling agents that have effective thermal and chemical properties. To use an appropriate thermal cycle with the optimum heat transport agents, in the secondary circuit of a NPP unit, is very important. Using different design on the BOP of NPP unit performs this; some of the effects are on the accident risk and also to the NPP unit reliability. The paper presents different nuclear power plant units that use advanced fuel cycles and efficient thermal cycles on the secondary thermal circuits. Some examples of how is evaluated the accident risk and the plant unit reliability are presented in the paper in order to optimise the design and the investment costs for an accepted accident risk level and a given NPP unit reliability. (authors)