TVEL is a fuel element. Nuclear reactor
Although nuclear power is not completely safe today, reactors and power plants around the world are building more than closing down. So in the United States of America, the number of operating reactors has just exceeded one hundred, in France (second place by the number of peaceful atoms on the planet) - about 60, and they provide about 80% of the electricity generated in the country.
Fuel for a nuclear reactor is TVEL. This is an element in which the controlled reaction chain directly flows. How are the "firewood" of the nuclear boiler, how are they made and what happens to the fuel in the heart of the power plant?
What is a nuclear chain reaction
It is known that atomic nuclei consist of protons and neutrons. For example, the nucleus of the uranium atom contains 92 protons and 143 or 146 neutrons. The repulsive force between positively charged protons in the uranium nucleus is simply enormous, about 100 kgf in a single (!) Atom. However, nuclear forces do not give way to the core.When a free neutron enters the uranium nucleus (only a neutral particle is capable of approaching the nucleus), the latter is deformed and spreads into two halves plus two or three free neutrons.
These free neutrons attack the nuclei of other atoms, and so on. Thus, the number of collisions increases exponentially and in a fraction of a second, the entire mass of the radioactive metal decays. This decay is accompanied by scattering of fragments at all speeds in all directions, and their collisions with environmental molecules cause heating to several million degrees. This is a picture of a conventional nuclear explosion. TVEL directs this phenomenon to a peaceful course. How does this happen?
Controlled nuclear reaction
For a nuclear reaction to sustain itself, become a chain, a sufficient amount of radioactive fuel is needed (the so-called “critical mass”). In nuclear weapons, this issue is solved simply: two ingots of the weapons-grade metal (uranium 235, plutonium 239, etc.) with a mass of each slightly smaller than the critical one are combined by means of an explosion of an ordinary TNT.
For peaceful use of the atom, this method is not suitable.The figure shows schematically the device of the simplest atomic reactor. Each fuel element (fuel element - uranium fuel) is less critical than its mass, but their total weight exceeds this mark. Being in close proximity to each other, the fuel rods "exchange" free neutrons. Due to this mutual neutron bombardment in the reactor, the nuclear chain reaction is maintained. Graphite rods play the role of a kind of "brake" of the nuclear process. Graphite is a good neutron absorber, the reaction dies out when rods of this material are placed between the fuel elements. This completely stops the exchange of free neutrons.
Thus, the reaction is under the constant control of automation. The decay is accompanied by movement in the medium of the coolant fragments of uranium nuclei, which heat it to the required temperature.
How electricity is generated
Further device of a nuclear power plant is not much different from the usual heat, running on gas, fuel oil or coal. The difference is that heat is generated in a CHP plant by burning fossil hydrocarbons, while in a nuclear power plant, the coolant is heated by a fuel rod of nuclear reactors.
The heat carrier brought to a temperature of 500–800 ° C (superheated water, molten salts, and even liquid metals can play its role) in a special heat exchanger heats the water, turning it into dry steam. The steam rotates a turbine, mounted on one shaft with a generator, in which electric current is generated.
What they are
The first nuclear reactors were homogeneous devices. They consisted of boilers in which there was nuclear fuel (often liquid, less often gaseous). This is a melt of uranium salts or poorly enriched uranium, sometimes a suspension of uranium dust, etc. The process was regulated by introducing a moderator in the active zone in the form of plates or rods of material that slows free neutrons well. Heat was transferred to water by means of heat exchangers located directly in the active zone, like grates in a coal furnace.
Our figure shows a heterogeneous nuclear reactor, which is now the absolute majority in the world. Such "nuclear boilers" are easier to maintain, change the fuel in them, repair, they are safer and more reliable than the old homogeneous ones.
Another bonus of using uranium fuel elements is the generation of uranium nuclei of such an element as plutonium 239 as a result of neutron irradiation.which is then used as fuel for small-sized nuclear reactors, as well as a weapon metal.
Where is the fuel for nuclear power plants
Uranium is mined in many countries of the world by open-cast (quarrying) or mining method. Initially, the ore does not even contain uranium itself, but its oxide. Isolation of metal from oxide is the most complicated chain of chemical transformations. Not every country in the world can afford to acquire enterprises for the production of nuclear fuel.
A further task is enrichment of mined uranium. Less than 1% of uranium 235 is contained in natural material, the rest is isotope 238. It is extremely difficult to separate these two elements. Uranium enrichment centrifuges are the most complex devices.
In order for uranium to become highly enriched (the content of the isotope 235 increased to 20%), it will have to turn into gas and go up to a thousand stages of processing.
How does TVEL
Engineers get into the hands of engineers enriched uranium, but it is still on nuclear fuel. The production of this fuel is akin to powder metallurgy. The powdered metal (or its chemical compounds) is pressed into small tablets with a diameter of about a centimeter.
Products made from uranium metal are better adapted to withstand the hellish conditions inside the reactor, but the clean element is very expensive to manufacture. Uranium dioxide is much cheaper, but so that it does not crumble from the enormous pressure and heat it is necessary to bake under enormous pressure at a temperature of more than 1000 ° C.
TVEL is a set of such washers with a length of about 2-4 meters, placed in a tube made of steel or alloys of iron with molybdenum. TVELs themselves are recruited into a bundle of several tens or even hundreds. Such a set is called a fuel assembly (fuel assembly).
FAs are installed directly in the heart of an atomic reactor. In one reactor, their number can reach several hundred. As uranium decays, fuel elements lose their ability to produce heat, then they are replaced. But one kilogram of technical uranium, enriched to a 235 isotope content of 4%, has time to produce as much energy in a nuclear reactor as it would have if burning 300 standard two hundred-liter fuel oil barrels.