One of the current engineering challenges is to design next generation (Generation IV) Nuclear Power Plants (NPPs) with significantly higher thermal efficiencies compared to those of current NPPs to match or at least to be close to thermal efficiencies reached at thermal power plants (43–55%). A Sodium-cooled Fast Reactor (SFR) is one of six concepts considered under the Generation IV International Forum (GIF). This concept is the only one from the Generation IV reactors, which is actually in operation in Russia.

In general, there are 3 possibilities for an SFR in terms of the secondary cycle:

1. Subcritical-pressure Rankine-“steam”-cycle through a heat exchanger (current approach used in Russian and Japanese power reactors).

2. Supercritical-pressure Rankine-“steam”-cycle through a heat exchanger (new approach).

3. Supercritical-pressure CO2 Brayton-gas-turbine-cycle through a heat exchanger (US approach).

The BN-600 reactor is a sodium-cooled fast-breeder reactor built at the Beloyarsk NPP in Russia. It has been in operation since 1980 and adopts the secondary subcritical-pressure Rankine-“steam”-cycle with heat regeneration. Steam extractions are taken from High-Pressure (HP), Intermediate-Pressure (IP) and Low-Pressure (LP) turbines.

The basic method of increasing the thermal efficiency of power plants is to improve it by increasing the operating pressure and temperature. With the advent of modern super alloys, the Rankine-“steam”-cycle has progressed into the supercritical region of the coolant and is generating net efficiencies into the mid 40% range. Calculations of thermal efficiency of a secondary sub- and supercritical-pressure Rankine-“steam”-cycle with heat regeneration are presented in the paper.

The Brayton-gas-turbine cycle is under consideration for future nuclear power reactors. The higher operating temperatures will be achieved, the higher thermal efficiency will be. Supercritical CO2 cycle is a new approach in Brayton-gas-turbine cycle. Carbon dioxide has a critical pressure of 7.38 MPa and a critical temperature of 31.0°C, which is significantly less than that of water (22.064 MPa and 373.95°C). However, liquid sodium is more compatible with SC CO2 than with water. Therefore, thermal efficiency of this SC CO2 cycle is also calculated.

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