RU2112744C1 - Method of processing high-concentration uranium - Google Patents

Abstract

FIELD: uranium processing. SUBSTANCE: invention relates to processing high- concentration uranium into power-destination low-concentration uranium by way of diluting the former. Method includes oxidation of uranium metal, fluoridation of oxides obtained into uranium hexafluoride (residual uranium content in the cinders is 10-25% based on the weight of uranium fed into fluoridation stage), afterpurification of uranium hexafluoride to remove plutonium hexafluoride by selectively sorbing the latter on sodium fluoride, preparation of uranyl nitrate solution from fluoridation cinders, extraction-sorption purification of uranyl nitrate, and its denitration to produce uranium oxides which are returned into fluoridation stage. Time and temperature of uranium hexafluoride afterpurification process is chosen in dependence of conditions determined by equation:

where C_o and C are concentrations of plutonium in uranium hexafluoride before and after afterpurification process, respectively; τ uranium hexafluoride-sorbent contact time, s; and T sorbent temperature, K. EFFECT: optimized process parameters. 2 cl, 1 tbl

Description

 The invention relates to a technology for processing highly enriched uranium (HEU), especially weapons grade, into low enriched uranium (LEU) for energy purposes by diluting HEU.

 Known methods of diluting highly enriched uranium with low enriched uranium in the form of oxide powders obtained in one way or another [1]. A common drawback of diluting HEU and oxide matrices in powder form is that these methods cannot produce a sufficiently homogeneous product; therefore, none of the options based on the use of this method is suitable for the production of LEU-based fuel [1, p. 85].

A known method of diluting HEU in the gas phase, which consists in the fact that both HEU and LEU are first converted into uranium hexafluoride, which are then mixed [1, p. 85]. The operations in this method are performed in the following sequence:
metal HEU is first converted to uranium dioxide:
uranium dioxide is converted into uranium tetrafluoride by reaction with hydrofluoric acid:
UO ₂ + 4 HF → UF ₄ + 2H ₂ O
uranium tetrafluoride is burned in a fluorine torch with the formation of HEU hexafluoride:
UF ₄ + F ₂ → UF ₆
highly enriched uranium hexafluoride is diluted with a matrix of low enriched uranium hexafluoride.

При фторировании продукты распада накапливаются, в основном, в нелетучих остатках (огарках), а поскольку некоторые из них (особенно ²⁰⁸Tl и ²¹²Bi) обладают жестким γ - излучением, это создает неблагоприятные условия для работы обслуживающего персонала и выдвигает требования принятия достаточно сложных технических мер по обеспечению радиационной безопасности. Другим недостатком этого способа является отсутствие очистки ВОУ от плутония, часть которого при фторировании также остается в нелетучих остатках, а другая часть сопутствует гексафториду урана.However, this method does not take into account that the real HEU being processed contains plutonium and decay products of uranium - 232. The latter are formed according to the scheme [2]:

During fluorination, the decomposition products accumulate mainly in non-volatile residues (cinders), and since some of them (especially ²⁰⁸ Tl and ²¹² Bi) have hard γ-radiation, this creates unfavorable conditions for the work of staff and puts forward requirements for the adoption of rather complex technical measures to ensure radiation safety. Another disadvantage of this method is the lack of purification of HEU from plutonium, part of which during fluorination also remains in non-volatile residues, and the other part is accompanied by uranium hexafluoride.

Closest to the claimed method is the processing of HEU, which includes the following operations [3]: oxidation of HEU with atmospheric oxygen at a temperature of 700 - 1100 ^o C, fluorination of oxides with elemental fluorine with excess of stoichiometry by 5 - 50%, re-condensation of the obtained uranium hexafluoride, and then uranium hexafluoride is sent to a cascade of gas centrifuges, where they are purified from fluorides of elements with a molecular weight lower than the molecular weight of uranium hexafluoride, a mixture of gas high and low enriched uranium hexafluoride and arivanie commercial product (prototype).

 However, the prototype does not provide for the cleaning of highly enriched uranium from plutonium and decay products of uranium - 232, which are contained in real HEU.

 The objective of the invention is the processing of HEU into uranium hexafluoride, suitable in purity for mixing with low enriched uranium hexafluoride to produce energy-grade uranium.

где
C_o, C - концентрация плутония в гексафториде урана до и после очистки, соответственно;
τ - время контакта гексафторида урана с сорбентом, c;
T - температура сорбента, K,
а остатки от фторирования, содержащие уран в количестве 10 - 25% от поданного на фторирование, растворяют в азотной кислоте, полученный раствор уранилнитрата направляют на экстракционно-сорбционную очистку, очищенный раствор подвергают денитрации, полученные при этом оксиды урана возвращают на стадию фторирования.The problem is solved in that in the method of processing highly enriched uranium into uranium hexafluoride, suitable for mixing with low enriched uranium hexafluoride to produce energy-grade uranium, including oxidation of highly enriched metallic uranium to oxides, fluorination of oxides with elemental fluorine to obtain uranium hexafluoride and residues from fluorination uranium hexafluoride from impurities, the fluorination process is carried out to a residual uranium content in the residues equal to 10 - 25% of the amount of uranium, under nnogo on fluorination obtained uranium hexafluoride is sent for cleaning hexafluoride plutonium selective sorption of the latter with sodium fluoride, the contact time and temperature are selected from the conditions defined by the equation

Where
C _o , C is the concentration of plutonium in uranium hexafluoride before and after purification, respectively;
τ is the contact time of uranium hexafluoride with the sorbent, s;
T is the temperature of the sorbent, K,
and the fluorination residues containing uranium in an amount of 10 - 25% of the fluorination feed are dissolved in nitric acid, the resulting uranyl nitrate solution is sent for extraction and sorption purification, the purified solution is denitrated, and the uranium oxides obtained are returned to the fluorination step.

Sorption of PuF _{6 is} carried out at a temperature of 653 - 773K.

 We conducted experiments on the basis of which the form and parameters of the specified kinetic equation were established and its correctness was checked to solve the problem of achieving the purification of uranium hexafluoride to a predetermined value.

 The obtained experimental results are shown in the table.

 As can be seen from the table, the experimental results are adequately described by the proposed equation.

 The method is as follows.

Example 1. A portion of uranium oxide, obtained by oxidation of a metallic HEU (100 kg U), was fluorinated at a temperature of 400-450 ^° C (673-723K). The degree of fluorination was monitored by the magnitude of the γ - background in the reactor from the decay products of uranium - 232. As the fluorination was carried out, the γ - background increased due to a decrease in the effect of screening (absorption) of γ - radiation by the remaining uranium in the reactor. The process was stopped when the γ - background reached 6 - 8 μR / s, i.e. the amount permissible for the staff to work for a time sufficient to carry out manual operations without taking additional radiation protection measures. The amount of uranium remaining in the reactor was 10 kg, i.e. 10% of the amount submitted for fluorination, therefore, the value of 10% is the lower limit of the amount of non-fluorinated uranium left in the reactor.

The uranium hexafluoride obtained in the fluorination step was contaminated with plutonium hexafluoride in amounts of (1 - 8) • 10 ² / μg Pu / kg U. To purify UF ₆ from PuF _{6, the} gas mixture after fluoride was passed through an adsorption column filled with sodium fluoride granules and heated to 450 - 500 ^o C. The rate of transmission of gases through the sorbent was maintained so that the contact time was 18 - 22 s.

 As shown by measurements of the plutonium content in the gas stream leaving the sorption column, compliance with these conditions ensured the production of uranium hexafluoride with a residual plutonium content of 1.0 - 7.0 μg Pu / kg U, which meets the technical specifications for purified HEU.

 Example 2. The experiments were carried out according to the method similar to that described in example 1.

The initial HEU nitrous oxide containing 10 ² - 10 ⁴ μg Pu / kg U was fluorinated until the γ-background from the cinders reached the level of 3 - 4 μR / s. The amount of uranium left in the reactor was 25 kg, i.e. 25% of the feed for fluoridation.

 The value of 25% is the upper limit of the amount of non-fluorinated uranium left in the reactor, and its increase is impractical due to an excessive increase in the load on the operations of hydrometallurgical redistribution of cinder.

The process of sorption purification of uranium hexafluoride from plutonium hexafluoride was carried out at a temperature of (400 ± 10) ^o C and a contact time of 40-50 s. After purification of UF ₆ from PuF ₆ , a HEU hexafluoride product containing an admixture of plutonium of not more than 0.1 μg Pu / kg U was obtained.

 In examples 1 and 2, the mixture of non-volatile fluorides (cinder) remaining in the reactor, containing uranium oxyfluorides, plutonium fluorides and decay products of uranium-232, was directed to dissolution, extraction purification of uranium, sorption trapping of plutonium and thorium and, after denitration of the purified uranyl nitrate solution, was obtained uranium oxide was returned to fluorination.

 Thus, the implementation of the proposed method allows to obtain HEU hexafluoride pure according to ASTM specifications, suitable for producing energy uranium hexafluoride from it, while reducing the necessary power of hydrometallurgical redistribution by 5-10 times, reducing the volume of radioactive waste solutions, and simplifying the process design for the reduction in equipment units compared with technology involving hydrometallurgical redistribution of the total amount of HEU entering and recycling, as well as to improve the nuclear safety conditions.

Sources of information
1. A.Makhidjani, E.Makhidjani. Nuclear materials through dull glass? - IEEP PRESS, 1995.

 2. Tables of physical quantities. Handbook Ed. I.K. Kikoina. - M.: Atomizdat, 1976.

 3. RF patent N 2057377, G 21 C 19/42, 19/48, publ. 03/27/96 (prototype).

Claims (2)

1. A method of processing highly enriched uranium into uranium hexafluoride, suitable for mixing with low enriched uranium hexafluoride to produce energy uranium, including the oxidation of highly enriched metallic uranium to oxides, fluorination of oxides to produce uranium hexafluoride and cinder from fluorination, and purification of uranium hexafluoride, which differs in the fluorination process is carried out to a residual content of uranium in the cinder, equal to 10 - 25% of the amount of uranium supplied for fluorination, the obtained hexafluoride ysokoobogaschennogo directed to uranium hexafluoride purification of plutonium selective sorption of the latter with sodium fluoride, the contact time and the process temperature is selected from the conditions defined by the equation:

where C _o , C is the concentration of plutonium in uranium hexafluoride before and after purification, respectively;
τ is the contact time of uranium hexafluoride with the sorbent, s;
T is the temperature of the sorbent, K,
and fluorination cinders containing 10–25% of uranium supplied to fluorination, and 10–25% of uranium supplied to fluorination are dissolved, the resulting uranyl nitrate solution is sent for extraction and sorption purification, the purified solution is subjected to denitration, obtained by this uranium oxides return to the stage of fluorination.  2. The method according to claim 1, characterized in that the sorption of plutonium hexafluoride is carried out at 653 - 773 K.

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