This paper shows a methodology to obtain metallic uranium through a magnesiothermy process. Chile has two experimental reactors operated by the “Chilean Nuclear Energy Commission” (CCHEN). One is 5 MW and the other is 10 MW. In order to fulfill international agreements about nuclear energy for testing purposes of these reactors, CChEN purchased 19.9% enriched uranium hexafluoride, also known as the limit of Low Enriched Uranium (LEU). Due to the capacity of these reactors, they need high-density uranium compounds for their fuel, in order to work with LEU. For this reason, the uranium needs a previous conversion into metallic uranium. The conversion laboratory carried out experiences for reduction of UF 4 with Mg. The main purpose of this study was to analyze the operating conditions under which the reduction reaction takes place, the designed method and the equipment and materials used. The raw material used was dehydrated UF 4, prepared by electrolytic reduction and commercial purity Magnesium. The reaction took place in a cylindrical reactor made of low alloy steel, with a conic section in the lower part. The internal zone was coated with a 2.5 cm thick layer of CaF 2. The process started by applying external heating, according to a heating program, developed specially for this purpose. The reduction reaction took place starting at 650°C. The result was a cylinder of uranium metal and MgF 2 slag. The crossed cut uranium cylinder showed a smooth and homogeneous surface without inclusions of slag, pores or blisters. The yield of the reaction was of the order of 75% with respect to the expected theoretical value. The uranium cone obtained fulfilled the required conditions for source material for nuclear fuel fabrication, with a uranium content of 97.5%.
In the decade of the 80s, the Chilean Nuclear Energy Commission (CChEN) bought LEU in form of UF6. The objective of this purchase was the development of U3O8 fuel type compounds for these reactors. However, enrichment reduction international program (RERTR) launched in 1978, affected the continuity of this project, because U3O8 uranium compound allows limited uranium load. For this reason, CChEN needed to convert UF6 (LEU) into metallic uranium, suitable for the manufacture of higher neutron density fuels. This fact allowed using higher density compound, as U3Si2. Uranium and silicon form several different stoichiometric compounds including USi2, USi (or U34Si34.5), U3Si2, U3Si [
The literature mentions that the methodology to obtain metallic uranium consists in reducing, by exothermic reaction, UF4 with a metal, specifically Mg or Ca granules, according to the pre-established operating conditions [
• Direct reduction of UO3 with H2 or other organic reducer to obtain UO2.
• Hydrofluoration of UO2 in fluidized bed with HF at a temperature between 450˚C and 480˚C.
• Electrolytic reduction of the U6+ ions to U4+ and its precipitation with F1− ion, or Reduction of the U6+ ions with SnCl2.
The remaining methods are feasible to be used to reach UF4 directly. The hydrolysis reaction of UF6 that takes place is: [
U F 6 + 2 H 2 O = U O 2 F 2 + 4 H F (1)
In the UO2F2 compound, the uranium ion has its highest oxidation state valence (U6+). Therefore, a reduction process transforms it into tetravalent uranium (U4+). An electrolysis process or reduction with stannous chloride achieves this process [
U O 3 + 2 H F = U O 2 F 2 + H 2 O (2)
The electrolysis process is based on the following ionization reaction:
U O 2 F 2 = U O 2 2 + + 2 F − (3)
The overall reaction that takes place in the electrolysis cell is as follows:
U O 2 2 + + 2 H + + 4 F − = U F 4 ⋅ 3 4 H 2 O + 1 4 H 2 O + 1 2 O 2 (4)
Or what is the same
U O 2 F 2 + 2 H F = U F 4 ⋅ 3 4 H 2 O + 1 4 H 2 O + 1 2 O 2 (5)
Magnesiothermic reduction employs metallic magnesium or calcium as a chemical reducer of uranium. Magnesium is mixed with stoichiometric excess to uranium tetrafluoride (UF4), according to the following reaction:
U F 4 + 2 M g = U + 2 M g F 2 → Δ H = − 82 k c a l / m o l (6)
UF4 is a greenish substance which, mixed with magnesium, can be reduced to uranium metal under adequate thermal conditions. This reaction is intensely exothermic. The reaction products utilize the resulting exothermic heat and melt to form the uranium ingot at the bottom of the crucible and the slag. The supernatant slag, which contains essentially MgF2, solidifies at the top of the ingot.
According to other authors, reaction (6) starts at 600˚C - 650˚C [
Another important aspect of this process is the presence of humidity and acid coming from the aqueous obtaining process The presence of H2O or HF leads to evolution of hydrogen by pre-reaction during heating at lower temperatures itself (380˚C - 600˚C), according to the following reactions: [
M g + H 2 O = M g O + H 2 (7)
M g + 2 H F = M g F 2 + H 2 (8)
U O 2 F 2 + H 2 = U O 2 + 2 H F (9)
U F 4 + 2 H 2 O = U O 2 + 4 H F (10)
M g O + 2 H F = M g F 2 + H 2 (11)
For this reason, it is very important to eliminate both water and acid traces before the beginning of the reaction, along with a preheating of the reactants before the reaction for this process [
The experience of reducing UF4 with Mg was carried out in a pump-type reactor, which was constructed of low alloy steel, with an effective volume of 30.000 cc. Initially the mass of UF4 was 4500 g . and the amount of Mg corresponds to the stoichiometric plus 20% excess. The electric oven heated the reactor using an electric oven of 5.5 kW of power. Three resistors distributed the heat, two located on the sides and the third at the bottom. A programmer regulated the power supply and temperature.
To prevent undesirable oxidation reactions in the dehydrated UF4-Mg mixture, the powders were contained in an inert atmosphere to remove water from both humidification and crystallization before their use in the process of reduction to metallic uranium. The X-ray diffraction analysis (DRX) of
The UF4 pellet diameter, about 1.5 to 2 mm, was prepared via an electrolysis of uranyl fluoride (UO2F2) solution. The pellets was filtered to obtain the related crystals. The natural moisture from the aqueous obtainment process belongs to the uranium tetrafluoride lattice. To eliminate it, a drying furnace (
The final temperature reached by the system once the reaction finished, was calculated based on the specific heat and reaction heats, including heat losses. According to these calculations, the melt temperature should reach 1450˚C. This temperature allowed obtaining a fluid slag, which in turn allows the reduced U
to decant. The reaction is exothermic and produces 82 kcal per mol formed, once it reaches the indicated temperature.
To determine the degree of preliminary preheating, the reactor needs to achieve the reaction conditions for the spontaneous reaction. The system needs a preliminary thermal balance.
The reactor at the beginning of the test contained 4500 g of UF4, equivalent to 12.74 moles. In these conditions, reaction (6) formed 25.48 moles of MgF2. The temperature expected inside the pump is around 1450˚C. At this temperature, the U formed melts and decant at the bottom of the reactor. The final state of the slag, MgF2 in this case, and U at this temperature are both liquid.
Elements and compounds | Specific heat Cp Cal/g˚C | Sensitive heat. cal/˚C | Melting Heat kcal/g |
---|---|---|---|
U | 0.028 | 6.7 | 4.35 |
MgF2 | 0.238 | 29.5 | 13.9 |
Fe | 0.11 | 3462 | - |
CaF2 | 0.26 | 2600 | - |
Mg | 0.234 | 13.6 | 0.85 |
Uranium = 6.7 Cal/˚C mol |
---|
MgF2 = 29.5 Cal/˚C mol |
Total heat = 36.2 Cal/˚C mol |
Pump and insulator = 6.06 Kcal/kg˚C |
Heat generated by the reaction | Heat to melt U + 2MgF2 + 20% Excess Mg | Sensitive Heat Products U + 2MgF2 + 20% excess | Excess heat |
---|---|---|---|
82 Kcal/mol | 33 Kcal/mol | 6.7 + 29.5 + 2.3 = 38.5 Cal/˚C | 49 Kcal |
Heat generated by the reaction (Kcal/mol) | Heat to melt U + 2MgF2 + 20% Excess Mg | Heat available Kcal/mol |
---|---|---|
90% × 82.0 = 73.8 | 33 Kcal/mol | 40.8 Kcal |
However, the balance also considered a yield of 90% for reaction (6). For a 90% UF4 conversion, the heat available to reach 1450˚C is the following.
Considering this criterion,
The value of 56.2 Cal/g˚C is the closest estimation for the expected results of this experience. At the proposed temperature, the U + MgF2 mixture has a viscosity low enough to allow the separation during the process.
The trouser mix system of
For insulation inside the reactor, the walls were covered with CaF2 technical grade, under 180 μm before filling the reactor with the mixture. The density of the compacted mixture is 3 g /cc approximately. With the reactor loaded, the argon cylinder connecting to the pump generates an inert atmosphere to prevent
Heat remaining Kcal | Estimated Cp.m for the mass of U and MgF2. | Preheating to reach U + MgF2 melting. ˚C | Temperature difference to reach 1450˚C |
---|---|---|---|
40.8 | 56.2 | 694 | 756 |
the oxidation of the reactants, as undesirable reactions [
To know the temperature at which the reaction begins, the differential thermal analysis (DTA) from
The noise produced and the temperature increase confirmed the start of the reaction. At this point, the temperature control system shutdown the power
supply to the oven, allowing it to cool freely by natural convection. After 24 hours, the reactor was disassembled and the load removed.
According to the heat and mass balance, the reaction starts when the outside temperature of the oven reaches 650˚C. The proposed system obtains it after 21 hours.
The start of the reaction showed the following phenomenon:
• Heavy noise and vibrations.
• Increased pressure in the reactor pressure gauge and the argon flow regulator. This parameter reached a maximum value of 15 psig.
• Sudden temperature increase, up to the range of 1350˚C - 1400˚C.
With this phenomenon detected, the control system turned the oven off and allowed to cool freely until the next day.
The reaction step lasted 12 hours. At this time, the argon gas flow was stopped because the slag formed in the process prevented the oxidation of the obtained uranium. The cooling was by natural convection, during a period of 16 hours before opening the reactor. The following figures show the aspects observed in the obtained product. Three aspects are highlighted: a porous area within the insulating material, is attributed to gases present during the process that come from moisture not previously removed at the beginning, or decomposition of some loaded product.
Another area corresponds to a dense and compact material immediately after the reactor wall, is CaF2, used as insulating material.
Metallic uranium | Slag before grinding | Ground slag | U weight contained in the UF4 loaded | Operation performance |
---|---|---|---|---|
2582.0 g | 2470.0 g | 2444.6 g | 3410 g. | 76% |
It is worth noting that the appearance of the uranium button does not show pores, blisters or inlays inside. This indicates that there was no trapped slag inside the product of interest or the temperature was low at some interior point. It
Denomination Sample | Origin Material | Properties | DRX Analysis | Differential Thermal Analysis |
---|---|---|---|---|
Sample U CONV-13 | Fine insulating CaF2 material | Easy to grind | CaF2-SiO2-UO2-UF5 and MgF2 | Weight variation 1.19% |
Sample U CONV-14 | Insulation material contaminated with black particles | Black particles do not grind easily | CaF2.-SiO2.-UO2 and MgF2. | No weight loss |
Sample U CONV 15 | CaF2 contaminated with black particles | Difficult to disintegrate. it was milled in impact mill | CaF2.-U3O8.-UO2.-MgF2.-MgO. Mg. U and Si | It was not subjected to this analysis |
Sample U CONV 16 | Slag material. MgF2 | Medium hard. grounded with difficulty under 80 mesh | CaF2.-MgF2.-UO2.-MgO.-U.-UF5. MgSiO3 | There is no variation in weight. |
Sample U CONV.17 | Material of the reactor center. CaF2. | Easy to grind by contact with the lid | CaF2. SiO2. UO2. MgSiO3 and UF5 | It was not subjected to this analysis |
Chemical analysis in% | Sample U CONV-13 | Sample U CONV 14 | Sample U CONV 15 | Sample U CONV 16 | Sample U CONV 17 |
---|---|---|---|---|---|
U | 2.9 | 0.15 | 34.0 | 10.8 | 0.44 |
Ca | 29.9 | 33.4 | 2.56 | 17.3 | 38.6 |
Mg | 0.41 | 0.45 | 7.1 | 3.79 | 0.232 |
Fe | 0 | 0 | 0 | 0 | 0.15 |
Si | 1.2 | 3.7 | 1.1 | 0 | 0 |
presents a smooth, even and uniform fracture that indicates a total fusion and above the melting temperature of the U. This confirms that the system reached the indicated temperature of 1300˚C, according to what the system needs to reach the fusion of the U and its total runoff to the bottom of the reactor.
For the reactor tests, the most important impurities in the uranium are boron and cadmium. In this case, the normal content of these impurities should be less than 1 ppm (or µg/g), because of their capacity of neutron absorption during the uranium fuel burning tests. The cadmium and boron levels are appropriate. Other important impurities are oxygen and nitrogen. Their presence was because the insulation conditions were not enough to prevent its oxidation.
The absence of UF4 in the slag indicates that the reaction was total or in a percentage of the order of 90% to 95%. The X-ray diffraction diagram indicated the presence of UF5 that may correspond to material trapped in the insulating material. The control of the atmosphere and the overpressure allowed maintaining a constant pressure throughout the experience and prevented most of the oxidation of the initial components. Maintaining the same heating power through time guaranteed a constant temperature profile, with a minimum difference between center and edges. This allowed developing the reaction (6) through the entire reactor. The initial yield of the process achieved a value of 76%. This proves that the system reached the temperatures according to what is needed to reach the fusion of the U to the bottom and the MgF2 slag to the top of the reactor.
However,
The difference between the 76% recovery achieved in the billet shown in
The main conclusions of this study are the following:
1) The proposed system allowed obtaining metallic uranium as a final product,
U % | Zn µ/g | Cd µ/g | Co µ/g | Ni µ/g | Fe µ/g | B µ/g | Mg µ/g | Cu µ/g | Ca µ/g | Al µ/g | O2 µ/g | N2 µ/g |
---|---|---|---|---|---|---|---|---|---|---|---|---|
97.5 | 7 | <0.1 | 0.42 | 26.2 | 152 | 0.51 | 596 | 52 | 157 | 212 | 214.2 | 2.523 |
Remnant heat. cal | Specific heat. Cp | Temperature of the reactants at the beginning of reaction (6) | Temperature immediately after the reaction. |
---|---|---|---|
45,700 | 56.2 | 620˚C | 1433˚C |
Remnant heat. cal | Specific heat. Cp | Temperature of the reactants at the beginning of reaction (6) | Temperature immediately after the reaction. |
---|---|---|---|
41,130 | 61.2 | 620˚C | 1403˚C |
with a yield of the order of 76%. The chemical analysis showed a uranium content of 90% - 95% and a level of impurities that allowed producing high-density uranium compounds for Chilean research reactors.
2) The temperature conditions reached allowed the fusion of U and MgF2 slag. These conditions separated the desired uranium product from the slag. The system achieved temperatures of 1433˚C.
3) The design of the reactor based of thermodynamic considerations of specific heat and latent heat of fusion for the metallic uranium and the MgF2 slag, for the total mass of the compounds is consistent with the results obtained in the experience.
The authors declare no conflicts of interest regarding the publication of this paper.
Dides, M., Her- nández, J. and Olivares, L. (2020) Design and Analysis of a Metallic Uranium Reactor Type-Pump Using the Magnesiothermy Pro- cess. World Journal of Nuclear Science and Technology, 10, 9-22. https://doi.org/10.4236/wjnst.2020.101002