5.6.Hệ thống, thiết bị và linh kiện được thiết kế đặc biệt hoặc chế tạo đặc biệt để sử dụng trong nhà máy làm giàu bằng trao đổi hóa học hoặc trao đổi ion

INTRODUCTORY NOTE
The slight difference in mass between the isotopes of uranium causes small changes in chemical reaction equilibria that can be used as a basis for separation of the isotopes. Two processes have been successfully developed: liquid-liquid chemical exchange and solid-liquid ion exchange.
In the liquid-liquid chemical exchange process, immiscible liquid phases (aqueous and organic) are countercurrently contacted to give the cascading effect of thousands of separation stages. The aqueous phase consists of uranium chloride in hydrochloric acid solution; the organic phase consists of an extractant containing uranium chloride in an organic solvent. The contactors employed in the separation cascade can be liquid-liquid exchange columns (such as pulsed columns with sieve plates) or liquid centrifugal contactors. Chemical conversions (oxidation and reduction) are required at both ends of the separation cascade in order to provide for the reflux requirements at each end. A major design concern is to avoid contamination of the process streams with certain metal ions. Plastic, plastic-lined (including use of fluorocarbon polymers) and/or glasslined columns and piping are therefore used.
In the solid-liquid ion-exchange process, enrichment is accomplished by uranium adsorption/desorption on a special, very fast-acting, ion-exchange resin or adsorbent. A solution of uranium in hydrochloric acid and other chemical agents is passed through cylindrical enrichment columns containing packed beds of the adsorbent. For a continuous process, a reflux system is necessary to release the uranium from the adsorbent back into the liquid flow so that ‘product’ and ‘tails’ can be collected. This is accomplished with the use of suitable reduction/oxidation chemical agents that are fully regenerated in separate external circuits and that may be partially regenerated within the isotopic separation columns themselves. The presence of hot concentrated hydrochloric acid solutions in the process requires that the equipment be made of or protected by special corrosion-resistant materials.
5.6.1. Liquid-liquid exchange columns (Chemical exchange)
Countercurrent liquid-liquid exchange columns having mechanical power input, especially designed or prepared for uranium enrichment using the chemical exchange process. For corrosion resistance to concentrated hydrochloric acid solutions, these columns and their internals are normally made of or protected by suitable plastic materials (such as fluorinated hydrocarbon polymers) or glass. The stage residence time of the columns is normally designed to be 30 s or less.
5.6.2. Liquid-liquid centrifugal contactors (Chemical exchange)
Liquid-liquid centrifugal contactors especially designed or prepared for uranium enrichment using the chemical exchange process. Such contactors use rotation to achieve dispersion of the organic and aqueous streams and then centrifugal force to separate the phases. For corrosion resistance to concentrated hydrochloric acid solutions, the contactors are normally made of or protected by suitable plastic materials (such as fluorinated hydrocarbon polymers) or glass. The stage residence time of the centrifugal contactors is normally designed to be 30 s or less.
5.6.3. Uranium reduction systems and equipment (Chemical exchange)
(a) Especially designed or prepared electrochemical reduction cells to reduce uranium from one valence state to another for uranium enrichment using the chemical exchange process. The cell materials in contact with process solutions must be corrosion resistant to concentrated hydrochloric acid solutions.
EXPLANATORY NOTE
The cell cathodic compartment must be designed to prevent re-oxidation of uranium to its higher valence state. To keep the uranium in the cathodic compartment, the cell may have an impervious diaphragm membrane constructed of special cation exchange material. The cathode consists of a suitable solid conductor such as graphite.
(b) Especially designed or prepared systems at the product end of the cascade for taking the U⁺⁴ out of the organic stream, adjusting the acid concentration and feeding to the electrochemical reduction cells.
EXPLANATORY NOTE
These systems consist of solvent extraction equipment for stripping the U⁺⁴ from the organic stream into an aqueous solution, evaporation and/or other equipment to accomplish solution pH adjustment and control, and pumps or other transfer devices for feeding to the electrochemical reduction cells. A major design concern is to avoid contamination of the aqueous stream with certain metal ions. Consequently, for those parts in contact with the process stream, the system is constructed of equipment made of or protected by suitable materials (such as glass, fluorocarbon polymers, polyphenyl sulphate, polyether sulphone, and resin-impregnated graphite).
5.6.4. Feed preparation systems (Chemical exchange)
Especially designed or prepared systems for producing high-purity uranium chloride feed solutions for chemical exchange uranium isotope separation plants.
EXPLANATORY NOTE
These systems consist of dissolution, solvent extraction and/or ion exchange equipment for purification and electrolytic cells for reducing the uranium U⁺⁶ or U⁺⁴ to U⁺³. These systems produce uranium chloride solutions having only a few parts per million of metallic impurities such as chromium, iron, vanadium, molybdenum and other bivalent or higher multi-valent cations. Materials of construction for portions of the system processing high-purity U⁺³ include glass, fluorinated hydrocarbon polymers, polyphenyl sulphate or polyether sulphone plastic-lined and resin-impregnated graphite.
5.6.5. Uranium oxidation systems (Chemical exchange)
Especially designed or prepared systems for oxidation of U⁺³ to U⁺⁴ for return to the uranium isotope separation cascade in the chemical exchange enrichment process.
EXPLANATORY NOTE
These systems may incorporate equipment such as:
(a) Equipment for contacting chlorine and oxygen with the aqueous effluent from the isotope separation equipment and extracting the resultant U⁺⁴ into the stripped organic stream returning from the product end of the cascade;
(b) Equipment that separates water from hydrochloric acid so that the water and the concentrated hydrochloric acid may be reintroduced to the process at the proper locations.
5.6.6. Fast-reacting ion exchange resins/adsorbents (Ion exchange)
Fast-reacting ion exchange resins or adsorbents especially designed or prepared for uranium enrichment using the ion exchange process, including porous macroreticular resins, and/or pellicular structures in which the active chemical exchange groups are limited to a coating on the surface of an inactive porous support structure, and other composite structures in any suitable form including particles or fibres. These ion exchange resins/adsorbents have diameters of 0.2 mm or less and must be chemically resistant to concentrated hydrochloric acid solutions as well as physically strong enough so as not to degrade in the exchange columns. The resins/adsorbents are especially designed to achieve very fast uranium isotope exchange kinetics (exchange rate halftime of less than 10 s) and are capable of operating at a temperature in the range of 373 K (100°C) to 473 K (200°C).
5.6.7. Ion exchange columns (Ion exchange)
Cylindrical columns greater than 1000 mm in diameter for containing and supporting packed beds of ion exchange resin/adsorbent, especially designed or prepared for uranium enrichment using the ion exchange process. These columns are made of or protected by materials (such as titanium or fluorocarbon plastics) resistant to corrosion by concentrated hydrochloric acid solutions and are capable of operating at a temperature in the range of 373 K (100°C) to 473 K (200°C) and pressures above 0.7 MPa.
5.6.8. Ion exchange reflux systems (Ion exchange)
(a) Especially designed or prepared chemical or electrochemical reduction systems for regeneration of the chemical reducing agent used in ion exchange uranium enrichment cascades.
(b) Especially designed or prepared chemical or electrochemical oxidation systems for regeneration of the chemical oxidising agent (agents) used in ion exchange uranium enrichment cascades.
EXPLANATORY NOTE
The ion exchange enrichment process may use, for example, trivalent titanium (Ti⁺³) as a reducing cation in which case the reduction system would regenerate Ti⁺³ by reducing Ti⁺⁴.
The process may use, for example, trivalent iron (Fe⁺³) as an oxidant in which case the oxidation system would regenerate Fe⁺³ by oxidising Fe⁺².