There are many types of reactors, but it mainly consists of an active zone, a reflective layer, an outer pressure shell and a shield. The active zone consists of nuclear fuel, moderator, coolant and control rods. Of the reactors currently used in atomic power stations, pressurized water reactors are the most competitive type (about 61%), boiling water reactors account for a certain percentage (about 24%), and heavy water reactors are used to a lesser extent (about 5%). The main features of pressurized water reactors are:
1) common water, which is inexpensive and available everywhere, is used as the moderator and coolant,
2) in order to keep the cooling water, which has a very high temperature inside the reactor, in a liquid state, the reactor is operated at a high pressure (the water pressure is about 15.5 MPa ), which is why it is called a pressurized water reactor;
3) as the water inside the reactor is in a liquid state, the steam that drives the turbo generator set of steam must be generated outside the reactor; this is achieved with the help of a steam generator, cooling water from the reactor, i.e., the first circuit water flows into the side of the steam generator heat transfer tube, the heat will be transferred to the other side of the heat transfer tube of the two-loop water, so that the latter is converted into steam (the second circuit steam pressure of 6-7 MPa, the average temperature of the steam is 310 ℃, to the Daya Bay (the average temperature of steam in the second loop is 310℃ at Daya Bay Nuclear Power Plant);
4) Since ordinary water is used as the moderator and coolant, the thermal neutron absorption cross-section is large, so it is not possible to use natural uranium as the nuclear fuel, and it is necessary to use enriched uranium (the content of 2-4% of Uranium-235) as the nuclear fuel. Boiling water reactors and pressurized water reactors belong to the same category of light water reactors, which, like pressurized water reactors, also use ordinary water as moderator and coolant. The difference is that steam is generated in boiling water reactors (at a pressure of about 7 MPa) and goes directly into the turbine to generate electricity without the need for a steam generator, and without the distinction between the first and the second circuits, so that the system is particularly simple, and the working pressure is lower than that of pressurized water reactors. However, the steam from a boiling water reactor is radioactive and requires shielding measures to prevent radioactive leakage. Heavy water reactors use heavy water as a moderator and coolant. Since its thermal neutron absorption cross section is much smaller than that of ordinary water, natural uranium can be used as the nuclear fuel for heavy water reactors. The so-called thermal neutrons are neutrons whose speed is reduced to 2200 m/s with an energy of about 1/40 eV after the slowing down of the fast neutrons emitted during the fission of the U-235 atomic nucleus. Thermal neutrons are 190 times more likely to cause fission of the U-235 nucleus than to be captured by the U-238 nucleus. In this way, the nuclear fission chain reaction can continue in a heavy water reactor fueled by natural uranium. Because heavy water is not as effective at slowing down neutrons as regular water, the core of a heavy water reactor is much larger than that of a light water reactor, making pressure vessel fabrication difficult. Heavy water reactors still need to be equipped with a steam generator, where the heavy water in the first loop brings heat to the steam generator, which is transferred to the ordinary water in the second loop to produce steam. The biggest advantage of heavy water reactors is that they do not use enriched uranium but use natural uranium as nuclear fuel, but one of the major reasons hindering their development is that heavy water is hard to come by, as it accounts for only 1/6500 of the natural water. In order to protect against neutrons, γ-rays, and thermal radiation, it is necessary to set up a shielding layer around the reactor and most of the auxiliary equipment. It is designed to be inexpensive and space-saving. For γ-ray shielding, the usual choices are steel, lead, plain concrete and heavy concrete. Steel has the best strength, but is more expensive; lead has the advantage of high density, so lead shielding is less thick; concrete is cheaper than metal, but less dense, so the shielding is thicker than all the others.
Gamma rays from reactors are so strong that they heat up when absorbed by the shield, so there are cooling water pipes permanently in the gamma-ray shield immediately adjacent to the reactor. Some reactors have heat shielding between the core and the pressure shell to minimize neutron-induced irradiation damage to the pressure shell and radiation-induced heating of the pressure shell.
Neutron shielding needs to have a large neutron capture cross-section of the elemental material, usually containing boron, sometimes concentrated boron -10. some shielding materials capture neutrons after radiation γ-rays, so there should be a layer of γ-rays shielding outside the neutron shielding. The outermost layer of shielding should normally be designed to reduce the radiation to below the allowable human dose level, often referred to as biological shielding. The outermost layer of shielding for nuclear power reactors is generally made of ordinary concrete or heavy concrete. Nuclear power traveling wave reactors borrow their name from the traveling wave tube of radio technology, but the physical nature is very different. A traveling wave tube uses electrons fired from an electron gun to transfer energy to microwaves transmitted in the same direction in a focusing system, thereby amplifying the microwave signal. A nuclear power traveling wave reactor, on the other hand, utilizes a small amount of highly enriched uranium-235 fission-produced fast neutrons at the starting end to bombard depleted uranium (almost exclusively uranium-238) to produce plutonium-239. plutonium-239 captures the neutrons and then fissions to produce as many as 300 types of various intermediate-mass atoms and generates an average of 2.5 neutrons and 200 million electron volts of energy. The fission energy is absorbed by liquid metallic sodium or other heat-carrying media to generate electricity, and the newly generated neutrons keep the nuclear reaction in the core moving forward until the entire core is "burned up". This is why traveling wave reactors are called traveling wave reactors.