RU2111169C1 - Method of fluorination of metallic uranium to uranium hexafluoride - Google Patents

Abstract

FIELD: conversion of weapon (high-rich) metallic uranium to energic (low-rich) uranium. SUBSTANCE: fluorination is carried out with dilute fluorine in admixture with argon, nitrogen and/or uranium hexafluoride at 623-773 K under reduced pressure in two stages during circulation of gas mixture. In the first stage metallic uranium is fluorinated at partial fluorine pressure of $$$ Pa and at linear rate of fluorinating mixture of 7-12 m/s in reaction zone, in the second stage, products which contain intermediate uranium fluorides are further treated at partial uranium pressure of 2-30 Pa. Process is carried out at total pressure of $$$ Pa. EFFECT: more efficient fluorination method. 2 cl, 1 dwg

Description

[1, 2], избыток которого необходимо выводить из зоны реакции. Во-вторых, при взаимодействии металлического урана с фтором кроме гексафторида урана образуются практически все возможные низшие фториды урана (UF₃, UF_4-x, U_nF_4n+1). Промежуточные фториды урана общей формулы U_nF_4n+1 могут образовываться также (по данным [3]) в результате взаимодействия гексафторида и тетрафторида урана. Низшие промежуточные фториды выносятся из зоны реакции и отлагаются в коммуникациях после фторатора, приводя к их забивкам, как в виде самостоятельной твердой фазы (UF₃, UF_4-x, UF₄, так и вследствие диспропорционирования промежуточных фторидов урана [1]:

Таким образом, при фторировании металлического урана фтором до гексафторида урана должны быть решены две основных проблемы:
- регулирование процесса, т.е. поддержание стабильного теплового режима в зоне фторирования;
- превращение промежуточных фторидов урана, образующихся в результате протекания параллельных реакций в системе уран-фтор-гексафторид урана, в гексафторид урана.The invention relates to the technology of fluorination of metallic uranium and its alloys to uranium hexafluoride. The development of the technology of direct fluorination of metallic uranium to uranium hexafluoride has become especially urgent in connection with the solution of the problem of converting weapons-grade (highly enriched in the ²³⁵ U isotope metal uranium into low-enriched (energy) uranium hexafluoride. Technological features of this process are the specific characteristics of the reaction of uranium with fluorine. first, since uranium is a very active metal reducing agent, and fluorine is the strongest oxidizing agent, the reaction between them is accompanied by the release of b lshogo heat

[1, 2], the excess of which must be removed from the reaction zone. Secondly, in the interaction of metallic uranium with fluorine, in addition to uranium hexafluoride, almost all possible lower uranium fluorides are formed (UF ₃ , UF _4-x , U _n F _{4n + 1} ). Intermediate uranium fluorides of the general formula U _n F _{4n + 1} can also be formed (according to [3]) as a result of the interaction of uranium hexafluoride and tetrafluoride. Lower intermediate fluorides are removed from the reaction zone and are deposited in the communications after fluoride, leading to their blockage, both in the form of an independent solid phase (UF ₃ , UF _4-x , UF ₄ , and due to the disproportionation of intermediate uranium fluorides [1]:

Thus, when fluorinating metal uranium with fluorine to uranium hexafluoride, two main problems must be solved:
- process control, i.e. maintaining a stable thermal regime in the fluorination zone;
- the conversion of intermediate uranium fluorides formed as a result of parallel reactions in the uranium-fluoro-uranium hexafluoride system to uranium hexafluoride.

 Known methods of fluorination of metallic uranium in liquid fluorinating media - chlorine trifluoride, bromine trifluoride, diluted with hydrogen fluoride and uranium hexafluoride, with recirculation of the unused part of the fluorinating reagent and part of the obtained uranium hexafluoride [1]. The use of liquid fluorinating media improves the conditions for heat removal from the reacting surface through the liquid phase to the cooled walls of the reactor. Intermediate uranium fluorides remained in the liquid phase until they were completely converted to uranium hexafluoride, which partially dissolved in the liquid phase and partially sublimated with exhaust gases. At the end of the process, uranium hexafluoride was isolated from the liquid phase and subjected to purification. As experiments [1] showed, in the event of contact of metallic uranium with vapors of halofluorides, the reaction becomes uncontrollable, up to an explosive nature (especially if uranium has a highly developed surface). In addition, it is very difficult to separate the resulting complex mixtures of fluorides.

 A known method of fluorination of highly enriched uranium alloys, which consists in dissolving the alloy in a molten mixture of sodium fluoride and zirconium tetrafluoride, first blowing hydrogen fluoride and then one of the fluorinating reagents - fluorine, chlorine trifluoride or bromine trifluoride [1]. Intermediate uranium fluorides remain in the melt until they are completely converted to uranium hexafluoride when treated with fluorinating reagents. An insurmountable disadvantage of this method was severe corrosion of equipment at a temperature of 873–973 K.

A known method of fluorination of metallic uranium, in which the task of removing the heat of reaction of uranium with fluorine is solved by diluting fluorine with a gas having a high specific heat capacity, helium, as well as carrying out the process in a fluidized bed of calcium fluoride particles at a linear velocity of 0.122 - 0.183 m / s and partial fluorine pressure 5.0 • 10 ³ - 5.0 • 10 ⁴ Pa [2]. The disadvantages of the method are associated with the fluidized bed technique: it is the need to filter a highly dusty and highly aggressive gas stream leaving the fluoride, and the loss of uranium with the spent fluidized bed material.

 A known method of fluorination of uranium-containing materials with diluted fluorine (a mixture of fluorine with argon, nitrogen and / or uranium hexafluoride) at a temperature of 723 - 823 K with forced circulation of a fluorinating gas mixture [4]. The process is first diluted, and then pure fluorine. This method is essentially the closest to the claimed and adopted as a prototype. The disadvantage of this method is that it also does not solve the problems caused by the presence of intermediate uranium fluorides in the gas stream after the fluorination operation, if applied to the fluorination of uranium metal.

 The objective of the invention is to develop a method of fluorination of uranium metal, providing a given process speed, thermal stability and the absence of intermediate uranium fluorides in the gas stream after the fluorination operation.

The problem is solved in that in the method of fluorination of metallic uranium with diluted fluorine (for example, a mixture of fluorine with argon, nitrogen, helium and / or uranium hexafluoride) at a temperature of 623 -773 K and reduced pressure, the process is carried out in two stages, while the first stage is supported the partial pressure of fluorine (2.5 - 15.0) • 10 ² Pa, and the linear velocity of the fluorinating mixture is 7-12 m / s, and the resulting gaseous reaction products containing intermediate uranium fluorides are subjected to additional fluorine treatment at a partial pressure of fluorine (2 - 30) • 10 ² Pa.

The process is preferably carried out at a total pressure in the reactor of (6-8) • 10 ³ Pa.

A feature of maintaining a given thermal regime in the claimed method is the removal of heat from the reaction of uranium with fluorine not through the wall of the reactor, but by a fluorinating gas stream. Our studies have established that it is possible to sustainably maintain a stationary thermal regime in a reactor and to control the process speed by changing the partial pressure of fluorine in a fluorinating gas stream only if the fluorination of uranium occurs in the kinetic region of a heterogeneous reaction. This condition is realized under such a hydrodynamic situation in the reaction zone, when the linear velocity of the gas fluorinating stream relative to the free cross section of the reactor is at least 7 m / s. In this case, the process of interaction of the surface of metallic uranium with fluorine is determined by the rate of a chemical reaction
U + 3F ₂ → UF ₆
and does not depend on the speed of supply of fluorine molecules to the metal surface. In this case, the concentration of fluorine on the reacting surface is approximately equal to its concentration in the core of the fluorinating gas stream.

 The value of the gas flow velocity in the reaction zone of metallic uranium with fluorine, equal to 7 m / s, is the lower limit of this parameter in the claimed method.

 With a decrease in the flow velocity of less than 7 m / s, the thermal regime in the fluorination zone becomes unstable and the reaction “breaks down” into the diffusion region of the heterogeneous reaction, which is characterized by high temperatures (up to melting of uranium) for the reaction of metallic uranium with fluorine.

 The upper limit of the speed of passage of the fluorinating gas mixture through the reaction zone (12 m / s) is due to the fact that above this value the thermal regime also becomes unstable, but "in the opposite direction": the gas stream cools the reaction zone of uranium with fluorine and quenches the reaction.

 A typical dependence of the temperature in the reaction zone on the speed of the fluorinating gas stream is shown in the drawing.

Another parameter affecting the thermal regime and the fluorination rate of uranium is the partial pressure of fluorine in the fluorinating gas mixture. We found that the optimal values of this parameter lie in the range 2.5 • 10 ² - 15.0 • 10 ² Pa, when the main products are uranium hexafluoride and pentafluoride. A decrease in this value to less than 2.5 • 10 ² Pa leads to the formation of predominantly lower, non-volatile fluorides in the fluorination zone, which sharply reduces the yield of uranium hexafluoride in the target product. An increase in the partial pressure of fluorine in the mixture supplied to the fluorination of uranium in excess of 15.0 • 10 ² Pa leads to the transition of the process to the region where the thermal regime and the reaction rate of uranium with fluorine are not amenable to stable regulation.

The problem of the absence of intermediate uranium fluorides in the gas stream after the operation of fluorination of uranium in the claimed method is solved by the fact that the specified gas stream is subjected to additional fluorination at partial pressures of fluorine (2 - 30) • 10 ² Pa. In this case, intermediate uranium fluorides contained in the gas stream are completely converted to uranium hexafluoride. The lower limit of the partial pressure of fluorine (2 • 10 ² Pa) is due to the need to ensure full conversion of intermediate uranium fluorides to uranium hexafluoride. With a decrease in this value, “jumps” of intermediate uranium fluorides through the additional fluorination zone are possible. Exceeding the partial pressure of fluorine over Z0 • 10 ² Pa violates the balance of the operations of fluorination of metallic uranium and additional fluorination of the resulting gas stream.

 The method is carried out in a dual-zone fluorine reactor. Chip from uranium metal is loaded into the first zone of the reactor, the second zone is filled with a nozzle of nickel rings.

After evacuation, a pre-prepared gas mixture of fluorine with uranium hexafluoride is fed into the reactor, in which the partial pressure of fluorine is (2.5 - 15.0) • 10 ² Pa. The second zone is filled with a nozzle of nickel rings, heated to 673 - 773 K, then the first zone is heated to 623 - 673 K. The process is carried out when the gas mixture is circulated with a linear velocity of 7 - 12 m / s, calculated on the free cross section of the first zone. After the first zone, fluorine is introduced into the gas stream in an amount that ensures the partial pressure of fluorine (2 - 30) • 10 Pa at the entrance to the second zone. The gas mixture leaving the second zone of the reactor is directed to the condensation of a part of uranium hexafluoride, after which the gas stream containing unreacted fluorine and non-condensed uranium hexafluoride is returned to the first reaction zone after preliminary adjustment for fluorine, which ensures its partial pressure in the first zone (2, 5 - 15.0) • 10 ² Pa.

 The process is monitored by temperatures in the first and second zones of the reactor, pressure in the reactor, gas flow rate entering the reactor, and fluorine concentrations in the streams entering the first and second zones of the reactor.

 Experimental verification of the claimed method of fluorination of metallic uranium showed that its implementation provides a given process speed, thermal stability and the absence of intermediate uranium fluorides in the gas stream after the operation of additional fluorination.

Sources of information
1. Stoler S., Richards R. Reprocessing of nuclear fuel. - M .: Atomizdat, 1964.

 2. Zuev V. A., Yakhonin I.F. Kinetics and mechanism of fluorination of uranium, plutonium and neptunium compounds with fluorine and halofluorides. VNIIHT, Information issue N 12, 1974.

 3. Yahata T., Iwasaki M.J. Nucl. Sci. And Technology, 9 (6), 1972.

 4. M. Bourgeois, P. Faugeras. Traitement des combustibles par voie seche Etudes realisees en France. Center d'etudes nucleaires de Fontenay - aux - Roses, France. 1968.

Claims (2)

1. The method of fluorination of metallic uranium to uranium hexafluoride with diluted fluorine, taken in a mixture in argon, nitrogen and / or uranium hexafluoride, at a temperature of 623 - 773 K and reduced pressure, characterized in that the process is carried out in two stages, while in the first stage the partial pressure of fluorine is maintained (2.0 - 15.0) • 10 ² Pa and the linear velocity of the fluorinating mixture in the reaction zone is 7 - 12 m / s, and in the second stage, reaction products containing intermediate uranium fluorides are subjected to additional fluorine treatment partial pressure (2 - 30) February ¹⁰ Pa. 2. The method according to p. 1, characterized in that the process is carried out at a total pressure of (6 - 8) • 10 ³ Pa.