How nuclear fuel is produced and how dangerous it is for the employees of the enterprise
(ORDO NEWS) — Nuclear power for most of us is something mysterious and damn dangerous. Everyone knows that the consequences of an accident at a nuclear power plant are very deplorable, at least because of the spread of radiation.
But where does the fuel for nuclear reactors come from, and why does no one receive lethal doses of radiation in its production?
There are three types of nuclear fuel – plutonium, thorium and uranium. But due to the complexity of the production and processing of the first two and large reserves of the latter, it is uranium, the heaviest metal on our planet, that is used as fuel for nuclear power plants.
In 1938, scientists figured out that this element could be fissioned using a controlled nuclear chain reaction—the fission of a nucleus into two parts, called fission fragments, with the simultaneous release of two or three neutrons, which, in turn, could cause the fission of the following nuclei.
A large amount of kinetic energy is concentrated in fission fragments, and their deceleration in matter is accompanied by the release of a huge amount of heat. In turn, this heat heats the water, turning it into steam, which, in turn, turns the turbines of the power plant.
Nuclear fuel begins its journey to the NPP rector from ore deposits. It is mined either in the mines of the deposit, or in an open way, when the upper part of the soil is removed by bulldozers and the rock is excavated.
In the ore, uranium and its decay products are in radioactive equilibrium. But during mining, there is a risk of internal exposure to radioactive gas radon, its decay products, as well as external gamma and beta radiation.
To minimize the impact of radiation on personnel, effective ventilation of mines and limiting the time of work directly in the mine are used.
In Canada, at one of the world’s largest uranium ore deposits in Saskatchewan, robots and remote-controlled equipment are being used with might and main in order to protect workers from radiation as much as possible.
In open pit mines, of course, dose loads are significantly less, but this method also brings much more significant harm to the environment.
By the way, in 1972, in the Oklo uranium deposit in Gabon (Central Africa), a natural nuclear reactor was discovered in which a spontaneous nuclear chain reaction took place.
True, this reactor worked about 1.8 billion years ago, and today the fuel reserves of the required concentration in this field have already been depleted.
Today, the leaching method is considered the best way to extract uranium. Wells are drilled in the rock, through which a leaching agent based on sulfuric acid, which has a special chemical composition, is pumped into the subsoil.
It dissolves in the depths of ore deposits and is saturated with uranium compounds, which are then pumped out and sent for enrichment.
From uranium purified by chemical reactions in the form of a powder, uranium anhydride or uranium dioxide is obtained, which is just the same nuclear fuel.
By itself, metallic uranium is rarely used as a fuel. It has a low maximum phase transition temperature, in addition, the chain reaction process is accompanied by an increase in the volume of the metal. Toli matters uranium dioxide: it has no phase transitions, it is less prone to swelling.
But before going to the reactor, uranium dioxide is waiting for a few more metamorphoses. To begin with, using a press, small round tablets are formed from it and baked in an oven at a temperature of 1000 degrees Celsius.
Then the tablets with a diameter of a couple of centimeters are packed into zirconium tubes – 30 pieces each – and the ends are sealed. At this stage, prior to being used in a reactor, nuclear fuel cells have a low radiation background.
In a power plant, there are hundreds of fuel cells in the reactor core, which release a huge amount of energy, but their resources are not endless. Periodically, NPP personnel reload nuclear fuel. The process of burning out the elements occurs unevenly, sometimes the fuel cells are interchanged, since the reaction is more intense in the center of the active zone.
The spent elements are sent to a special pool for aging, which is usually located in the immediate vicinity of the reactor. Spent fuel elements contain a large amount of uranium fission fragments; immediately after being removed from the reactor, the element is very strong fonit.
Therefore, it is stored for three to four years in pools with a certain temperature regime under a layer of water that protects personnel from radiation from uranium decay products.
After that, spent elements with weak radiation are either sent for disposal or recycled, since uranium never burns out completely and its remains can be isolated, in addition, plutonium can be obtained from waste (up to 1% of the total mass of spent nuclear fuel). But it is very expensive, dangerous and laborious,
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