Metals and alloys for nuclear energy
There are two important ways in the making of nuclear energy: nuclear fission and nuclear fusion. Fusion and fission have a different working mechanisms, thus other materials are used. In a nuclear reactor fuel rods are bombarded with neutrons causing fission of the fuel. This way energy is released. With this energy water is heated to steam and with the use of heat exchangers and steam turbines electricity is generated. As fuel is usually uranium used, but also plutonium and thorium are possible energy sources. To protect the environment from radioactivity, the fuel is cladded. For the cladding is usually zirconium alloys or stainless steels used because it should be able to let the neutrons pass and be corrosion resistant. This cladding has also the advantage to give strength to the rod. The reaction in the nuclear reactor is controlled with control rods. These control rods should be able to absorb the neutrons, so cadmium and boron are most of the time used. This reaction takes place in a stainless steel pressure vessel. Nuclear fusion takes typically place in a tokamak. This is a toroidal chamber with magnetic coils. In this chamber fusion takes place under the form of plasma. A lot of energy is released. The vacuum vessel is made of stainless steel that is constantly cooled because the plasma is very hot. The magnetic coils are made of niobium-tin superconductors causing a magnetic field to keep the plasma away from the walls of the vacuum vessel.
One of the most common reactions in nuclear fission is the attack of an 235U atom by a neutron. The result of this collision is that the uranium splits in 91Kr, 142Ba and 3 neutrons and energy is released, see Figure 1.
Figure 1: Typical nuclear fission reaction
Other reaction products are also possible. These energy is then used to heat up water. This hot water or steam (depending on the type of nuclear reactor) is used to generate electricity with the help of heat exchangers and steam turbines, see Figure 2. In Figure 2 a Boiling Water Reactor (BWR) is shown. The generated steam of the reactor pressure vessel is directly used to run the steam turbine. Nowadays, most nuclear reactors have an extra circuit so the water that is heated in the reactor vessel can’t contaminate other parts of the system.
Figure 2: Boiling Water Reactor system
The 3 neutrons that are released during the fusion reaction can be used to attack other uranium atoms with a nuclear chain reaction as a result. The nuclear chain reaction can be controlled by pushing control rods between the fuel rods. The neutrons can also penetrate into the material of the nuclear reactor causing lattice defects. These defects inhibit dislocation movement and so the material is hardened. On the other hand, this irradiation causes atoms to transform to another metal causing the formation of alloys and radioactivity of the material.
In this paper, the materials that are used for the reactor vessel, the control rods and for the fuel rods (cladding and fuel) are investigated.
The control rods in a nuclear reactor are used to control the nuclear chain reaction, so the used materials should be capable to absorb neutrons: the nucleus of the absorbing material catches an neutron, forming an isotope of the original material. The chance that this happens is expressed by the effective neutron-capture cross-section. It is said that the effective neutroncapture cross-section of the used materials has to be big. Looking at Table 1, it can be seen that the materials that are mostly used, are cadmium and boron. Properties Be Mg Zr Al Effective neutron- 0.01 0.063 0.18 0.23 capture crosssection, barn Melting point, °C 1280 651 1845 660 Table 1: Effective Neutron-capture Cross-section (Gulyaev) Fe 2.4 Ni 4.5 Hf 115 B 720 Cd 2400
1539 1455 2130 2000 321 and...
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