Nuclear power generation

In 1954, the Obninsk nuclear power plant in the former Soviet Union was connected to the grid for power generation, opening the prelude to the use of nuclear energy for power generation, and mankind entered the era of peaceful use of nuclear energy. The process of nuclear power generation is the process of converting nuclear energy released by the fission reaction of nuclear fuel in a nuclear reactor into electrical energy. The basic principle is as follows: Atoms are composed of nuclei and extranuclear electrons, and nuclei are composed of protons and neutrons. As shown in Figure 1, when the nucleus of uranium-235 in the reactor is bombarded by foreign neutrons, one nucleus will absorb a neutron and split into two nuclei with smaller masses, and emit 2 to 3 neutrons at the same time. The neutrons produced by the fission bombard other uranium-235 nuclei, causing new fission and a chain reaction.

Nuclear power generation
Figure 1 – Schematic diagram of chain fission reaction

The chain reaction generates a large amount of heat energy. It uses circulating water or other materials to remove the heat while avoiding the reactor from being burnt due to overheating. The heat is then exchanged to convert the water into steam, thereby driving the gas turbine to generate electricity (Note: The heavy elements that can be used for nuclear power generation include plutonium-239, uranium-233, etc.).

At present, the commonly used reactors in nuclear power plants in the world include pressurized water reactors, boiling water reactors, heavy water reactors, improved gas-cooled reactors, and fast reactors. For different types of nuclear reactors, the corresponding nuclear power plant systems and equipment are quite different. At present, the most widely used is the pressurized water reactor, which uses ordinary water as the coolant and moderator. It is the most mature and successful reactor type developed on the basis of military reactors.

The pressurized water reactor nuclear power plant is mainly composed of pressurized water reactor, reactor coolant system (referred to as primary circuit system), steam and power conversion system (also known as secondary circuit system), circulating water system, generator, power transmission and distribution system and its auxiliary systems .

The primary circuit system consists of a nuclear reactor, a main coolant pump (also known as a main circulation pump), a voltage stabilizer, a steam generator, and corresponding pipelines, valves, and other auxiliary equipment. The high-temperature and high-pressure cooling water circulates in the primary circuit system driven by the main circulation pump. When the cooling water flows through the reactor, it absorbs the heat generated by the nuclear fuel fission, and then flows into the steam generator and transfers the heat to the secondary feed water outside the steam generator tube. After cooling, the cooling water flows into the inlet of the main coolant pump, and after being boosted by the main cooling pump, it is re-introduced into the reactor. This is repeated to form a closed loop.

The secondary circuit system consists of steam-water separator, steam turbine, generator, condenser, condensate pump, feed water pump, feed water heater, deaerator and other equipment. The secondary circuit feed water absorbs heat in the steam generator and becomes water vapor, and then enters the steam turbine to perform work, and the steam turbine drives the generator to generate electricity. The exhaust steam after the work is discharged into the condenser to be condensed into water, and sent to the heater by the condensate pump, and then sent to the steam generator after heating, thus forming a closed circulation loop of the secondary circuit.

The secondary circuit system of a nuclear power plant is similar to the power circuit of a conventional thermal power generating unit. The steam generator and primary circuit system are equivalent to the boiler system of a thermal power plant.

Nuclear power generation has the following characteristics:

1. The economy of raw material transportation and storage

Nuclear energy is a highly enriched energy source. The energy produced by fission of 1 ton of uranium is equivalent to 2.4 million tons of standard coal. The energy density of nuclear fuel is several million times higher than that of fossil fuels, so the fuel used in nuclear power plants is small in size and convenient for transportation and storage.

2. Environmental protection

Nuclear power generation is different from thermal power generation in that it does not require fuel storage, waste residue sites, etc., and does not directly produce SO2, NOx, mercury or other pollutants related to the combustion of fossil fuels, nor does it directly produce CO2. It is a clean energy source. As long as safe operation can be ensured, the impact of nuclear power plants on the environment is minimal. The radioactive gas discharged into the environment is under strict supervision and control, and the surrounding residents receive less than 1% of the natural background radiation dose.

3. Security

The nuclear reactor of a nuclear power plant will not explode like an atomic bomb. The reactor has characteristics similar to a “tumbler”, and its design is inherently safe. When the outside world destroys the balance of the reactor and the release of nuclear energy is too fast, the reactor will not rely on outside intervention within a certain range, and the fission reaction will naturally terminate, returning the reactor to its original state and automatically shutting down.