Enrichment of uranium-235

uranium

Atomic number: 92, atomic weight: 238.029, element name: uranium, element symbol: U

The refined uranium products obtained from uranium mines are natural uranium products. Uranium-235, which can be fissioned by thermal neutron bombardment in pure natural uranium, accounts for only 0.71%, 99.27% ​​of uranium-238, and thermal neutron bombardment does not cause fission reactions. That is to say, only 7 out of every 1000 uranium atoms can be fissioned by thermal neutron bombardment. To be used as fuel for nuclear power plant light water reactors (pWRs and boiling water reactors), the concentration of uranium-235 needs to be increased to 2.5% to 5%. If a research reactor is to be used for fuel, the uranium-235 concentration needs to be increased to 10% to 90%. To be used to make atomic bombs, the uranium-235 concentration needs to be increased to more than 90%.

Uranium-235 and uranium-238 are isotopes of the same element, and they are chemically indistinguishable, with only minor differences in quality, differing by about 1%. Therefore, to achieve the separation of uranium-235 and uranium-238, the enrichment of uranium-235 is technically difficult. Methods for increasing the concentration of uranium-235 (called enrichment) are now mainly gas diffusion, high-speed centrifugation and laser.

1. Gas diffusion method using molecular diffusion

The gas diffusion method converts uranium into uranium hexafluoride gas to form a high pressure feed stream. When they pass through a special porous membrane, the lighter weight uranium-235 molecules move at a higher rate and are easily permeable to the membrane. The heavier uranium-238 molecules have a lower velocity of movement and are less permeable to the membrane. Diffusion membranes are very demanding, with hundreds of millions of micropores on 1 square centimeter, requiring corrosion resistance and high pressure differentials. The gas diffusion method separates uranium-235 from uranium-238. In fact, the effect of one diffusion separation is very low. To increase the concentration of uranium-235 from 0.71% to 3%, more than one thousand separation units need to be connected in series, so the uranium diffusion plant is a huge factory and a large power consumer.

Second, the use of mass difference gas centrifugation

Centrifugation is the process of converting uranium into uranium hexafluoride gas and feeding it into a high-speed centrifuge. The speed of the centrifuge is very high, and the speed per minute exceeds a thousand revolutions. Under the action of high-speed rotating centrifugal force, the heavier uranium-238甩 is near the wall of the centrifuge drum, and the lighter weight uranium-235 gathers near the central axis of the drum. Thus, uranium-235 and uranium-238 are separated. Because the production of a single centrifuge is very small, many centrifuges are required to be used in series to obtain a separate product that meets the needs.

The centrifugal method consumes less electricity than the diffusion method, has high economy, and has a long service life. It has gradually replaced the diffusion method to enrich uranium-235. It is expected that by 2010-2015, gas centrifugal technology will replace gas diffusion technology. The fifth-generation centrifuge rotor developed by the United States is more than 15 meters long, with a diameter of 76.2 cm, a rotational speed of 10,000 rpm, and a stand-alone production capacity of 400-600 kg.

Third, the use of spectral differences in the laser method

The laser method is based on small differences in the absorption spectra of uranium-235 and uranium-238 atoms (or molecules). The laser method is divided into an atomic laser method and a molecular laser method. The former direct concentrated uranium metal vapor, while the latter is separated from uranium hexafluoride gas. At present, the development of atomic laser method is relatively mature.

The atomic laser method melts metal uranium and evaporates to form an atomic vapor beam. A specific laser is used to interact with the uranium atomic vapor beam to selectively excite the uranium atom. Uranium-235 atoms are excited to ionize, forming a plasma, and uranium-238 atoms are not excited. The excited uranium-235 deflects under the action of an electric field, draws it out, and collects it on the concentrate plate. Uranium-238 atoms are not excited, are still neutral, and are collected on lean plates, thus achieving separation of uranium-235 and uranium-238.

Laser separation of uranium isotopes is a newly developed technology. It has the advantages of high separation factor, low power consumption, flexible production, few separation stages, and small plant scale. It is developing into industrial applications.

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