The nuclear fuel cycle refers to the entire process of obtaining, using, processing, and recycling nuclear fuel. It is an important part of the nuclear industry system. The nuclear fuel cycle is usually divided into two parts: the front end and the back end. The front end includes uranium exploration, uranium mining, ore processing (including beneficiation, leaching, extraction and precipitation processes), refining, conversion, enrichment, component manufacturing, etc.; the back end includes Post-processing of uranium-plutonium separation and radioactive waste processing, storage, and disposal of spent fuel elements after the reactor has been irradiated.
1. Introduction to the front-end part
(1) Geological exploration of uranium deposits
Uranium is the most basic raw material for the nuclear industry. The purpose of uranium geological prospecting is to identify and study the geological conditions under which uranium deposits are formed, to summarize the temporal and spatial distribution of uranium deposits, and to use this rule to guide general surveys and exploration and to detect underground uranium resources. The procedures of general survey and exploration work include regional geological survey, general survey and detailed survey, disclosure evaluation, exploration, etc. At the same time, staff are required to perform a series of basic geological work such as topographic survey, geological mapping, and original data cataloging.
The uranium element dispersed in the earth's crust is continuously concentrated under various geological actions, and finally a deposit of uranium minerals, namely uranium deposits, is formed. Understanding the formation process of uranium deposits has very important guiding significance for uranium census exploration. Not all uranium deposits have the value of mining for industrial use. According to statistics, among the more than 170 uranium deposits and uranium-bearing minerals that have been discovered, only 14-18% have actual mining value. The two main factors affecting the uranium deposit industry are ore grade and deposit reserves. In addition, the evaluation factors include the technical processing performance of the ore, the mining conditions of the deposit, the possibility of comprehensive utilization of useful elements, and the transportation conditions.
(2) Uranium mining
The first step in producing uranium is uranium mining. Its task is to extract industrial-grade uranium ore from underground deposits, or chemically leaching uranium to produce liquid uranium compounds. Because uranium ore is radioactive, there are special methods for uranium mining. There are mainly three types of open-pit mining, underground mining and in-situ leaching. Open-pit mining is generally used for shallow buried ore bodies. The method is to strip the topsoil and covering rocks to expose the ore, and then conduct mining. Underground mining is generally used for buried ore bodies, and the process of this method is more complicated. Compared with the above two methods, in-situ leaching of uranium has the advantages of low production cost and low labor intensity, but its application has certain limitations and is only suitable for deposits with certain geological and hydrogeological conditions. The method is to inject chemical reactants into the ore belt through surface drilling, selectively dissolve the useful component-uranium in the ore through a chemical reaction, and extract the leachate out of the surface without causing the ore to move around the surrounding rock.
(3) Processing of uranium ore
The purpose of uranium ore processing is to process the mined ore with industrial grade or radioactive beneficiation into an intermediate product with higher uranium content, which is commonly referred to as uranium chemical concentrate. This uranium chemical concentrate is refined and further processed into uranium oxide that is easy to hydrofluoride as the raw material for the next step.
The main steps of uranium ore processing include: ore grade, ore grinding, ore leaching, mother liquor separation, solution purification, precipitation and other processes.
In order to facilitate leaching, after the ore is mined, it must be crushed and ground to fully expose the uranium minerals. Then, a certain process is used to selectively dissolve the valuable components in the ore with the help of some chemical reagents (i.e., leaching agent) or other means. There are two leaching methods: acid method and alkali method. Because the uranium content in the leaching solution is low, and there are many types and high content of impurities, the impurities must be removed to ensure the purity of uranium. To achieve this process, you can choose the following two methods: ion exchange method (also known as adsorption method) and solvent extraction method. The last process of hydrometallurgical production is to wash, filter, and dry the precipitate, and then obtain the hydrometallurgical product uranium chemical concentrate, also known as yellow cake.
(4) Enrichment of uranium
The separation of uranium isotopes to increase the concentration of uranium-235 is called enrichment. Enrichment can provide certain reactors with uranium fuel with a uranium-235 concentration that meets the requirements. The enrichment methods used today include gas diffusion, separation, laser, nozzle, electromagnetic separation, chemical separation, etc., among which gas diffusion Method and centrifugal separation method are the concentration methods commonly used in modern industry. The enrichment process is carried out in the form of uranium hexafluoride.
(5) Nuclear fuel elements
Uranium that has been purified or enriched cannot be used directly as nuclear fuel. After chemical, physical, mechanical processing, etc., components of various shapes and qualities must be produced before they can be used as fuel for the reactor. There are many types of nuclear fuel elements, which can be divided into metal type, ceramic type and dispersion type according to group and characteristics; according to geometric shape, there are cylindrical, rod-shaped, ring-shaped, plate-shaped, strip-shaped, spherical, and prismatic elements; According to the reactor, it can be divided into test reactor components, production reactor components, and power reactor components (including nuclear fuel assemblies for nuclear power plants).
Nuclear fuel elements are generally composed of a core and a cladding. Because it works under strong radiation, high temperature, high flow rate and even high pressure environment for a long time, it has high requirements on the overall performance of the chip, the structure of the cladding material and the service life. It can be seen that the manufacturing of nuclear fuel components is a high-tech technology.
2.Introduction to the back-end part
(1) Reprocessing of spent fuel
The irradiated fuel elements always contain a certain amount of unsplit and newly formed fission fuel when discharged from the reactor. The purpose of spent fuel reprocessing is to recover these fission fuels such as uranium-235, uranium-233, and plutonium, and use them to reproduce new fuel elements or use them as nuclear weapons charges. In addition, the conversion of raw materials (uranium-238, cesium-137, strontium-90) is recovered, transuranic elements generated by the extraction process, and certain radioactive fission products that can be used as radiation sources (such as cesium-137, strontium-90, etc.) , Have great scientific and economic value. However, this process is highly radioactive, highly toxic, and prone to criticality accidents. Therefore, safety protection measures must be strengthened when reprocessing spent fuel.
The post-treatment process is generally divided into four steps: cooling and first-end treatment, chemical separation and reduction of uranium and plutonium through chemical conversion, and purification to produce metallic uranium (or uranium dioxide) and plutonium (or plutonium) respectively. Cooling and head-end treatment are cooling to disintegrate the spent fuel assembly, that is, to remove the element cladding and dissolve the fuel pellets. Chemical separation (purification and decontamination process) is to remove fission products from U-Pu, and then separate uranium-plutonium by solvent extraction, and extract them as uranyl nitrate and plutonium nitrate solutions.
(2) Treatment and disposal of the three wastes
In the process of nuclear industry production and scientific research, some solid, liquid and gaseous wastes with varying degrees of radioactivity will be produced, referred to as the "three wastes" for short. In these wastes, although the content of radioactive substances is very low, the harm is great. Ordinary external conditions (such as physical, chemical, and biological methods) basically have no effect on radioactive materials. Therefore, in the process of radioactive waste processing, in addition to relying on the decay of radioactive materials to attenuate the radioactivity, it can only take measures such as multi-stage purification, decontamination, compression and volume reduction, incineration, and solidification to separate radioactive materials from the waste, so that Reduce the volume of waste that contains radioactive materials as much as possible and change its existing state to achieve the purpose of safe disposal. This process is called "three wastes treatment and disposal".