Design and Analysis of a Metallic Uranium Reactor Type-Pump Using the Magnesiothermy Process

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 UF4 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 UF4, 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 CaF2. 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 MgF2 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%.


Introduction
In the decade of the 80s, the Chilean Nuclear Energy Commission (CChEN) bought LEU in form of UF 6 . The objective of this purchase was the development of U 3 O 8 fuel type compounds for these reactors. However, enrichment reduction international program (RERTR) launched in 1978, affected the continuity of this project, because U 3 O 8 uranium compound allows limited uranium load. For this reason, CChEN needed to convert UF 6 (LEU) into metallic uranium, suitable for the manufacture of higher neutron density fuels. This fact allowed using higher density compound, as U 3 Si 2 . Uranium and silicon form several different stoichiometric compounds including USi 2 , USi (or U 34 Si 34.5 ), U 3 Si 2 , U 3 Si [1] [2]. The uranium density and thermophysical properties of high uranium content uranium silicides (U 3 Si 2 and U 3 Si) make them an attractive material from both an economic and safety point of view as a replacement for UO 2 [3]. Experience from research uranium fuel reactors indicate that U 3 Si exhibits too much swelling under irradiation for use as a nuclear fuel. Additionally it decomposes into U 3 Si 2 and solid solution U above 900˚C, which is below some expected temperatures in uranium silicide fueled pins. Fortunately, U 3 Si 2 has a very promising behavior under irradiation in research reactor fuels and maintains several advantageous properties over UO 2 . U 3 Si 2 have 17% more uranium atoms in a set volume than in the same volume of UO 2 , given a constant percentage of theoretical density for both samples. This superior uranium loading has the potential to enable power uprates, extend cycle length in LWRs, or reduce enrichment, all of which are economically beneficial [4]. This fact raised the need to conduct studies to determine the most favorable conditions, by which uranium metal is prepared.
The literature mentions that the methodology to obtain metallic uranium consists in reducing, by exothermic reaction, UF 4 with a metal, specifically Mg or Ca granules, according to the pre-established operating conditions [5]. This reaction takes place in a closed reactor in an inert atmosphere. The most common method to obtain UF 4 is through the following chemical reactions [6]: • Direct reduction of UO 3 with H 2 or other organic reducer to obtain UO 2 .
• Hydrofluoration of UO 2 in fluidized bed with HF at a temperature between 450˚C and 480˚C. • Electrolytic reduction of the U 6+ ions to U 4+ and its precipitation with F 1− ion, or Reduction of the U 6+ ions with SnCl 2 . The remaining methods are feasible to be used to reach UF 4 directly. The hydrolysis reaction of UF 6 that takes place is: [7] In the UO 2 F 2 compound, the uranium ion has its highest oxidation state valence (U 6+ ). Therefore, a reduction process transforms it into tetravalent uranium (U 4+ ). An electrolysis process or reduction with stannous chloride achieves this process [8]. For this stage of the study, the raw material was uranium tetrafluoride prepared by electrolysis. This method of preparation consists in obtain-World Journal of Nuclear Science and Technology ing a synthetic solution of UO 2 F 2 from the solubility of UO 3 depleted with HF.
According to the following reaction: [9] The electrolysis process is based on the following ionization reaction: The overall reaction that takes place in the electrolysis cell is as follows: Or what is the same Magnesiothermic reduction employs metallic magnesium or calcium as a chemical reducer of uranium. Magnesium is mixed with stoichiometric excess to uranium tetrafluoride (UF 4 ), according to the following reaction: According to other authors, reaction (6) starts at 600˚C -650˚C [10]. However, this heat generated must be enough to reach the melting reaction products; starting from room temperature, since U melts at 1132˚C and MgF 2 at 1255˚C.
To have a proper separation between uranium and slag, the molten products need a viscosity low enough to allow the breakup between them. The Effect of impurities, such as magnesium or uranium oxides, has been found to yield poor slag-metal separation probably due to incomplete reduction and high viscosity of oxide slag [11].
Another important aspect of this process is the presence of humidity and acid coming from the aqueous obtaining process MgO 2HF MgF H + = + (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 [12]. World Journal of Nuclear Science and Technology

Reactor Design Based on Heat and Mass Balance
The experience of reducing UF 4 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 UF 4 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. Figure 1 shows the system used for the experience.
To prevent undesirable oxidation reactions in the dehydrated UF 4 -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 Figure 2 shows that the material used is dehydrated UF 4 , with a water level that does not affect the reduction reaction. In this case, the inert gas removed the humidity before the reduction process started.
The UF 4 pellet diameter, about 1.5 to 2 mm, was prepared via an electrolysis of uranyl fluoride (UO 2 F 2 ) 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 (Figure 3), heated at 150˚C with a ceramic crucible, eliminated the water of crystallization.
The Mg was also dried as a precaution for possible adhered moisture.
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 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. Table 1 shows the values used for the estimation of heat needed for the initial heating. Table 2 shows the heat requirement for the products obtained by reaction (6). Table 3 shows the heat generated by reaction (6) Table 4 shows estimations for the specific heat of the uranium and the MgF 2 slag, in their molten state.   However, the balance also considered a yield of 90% for reaction (6). For a 90% UF 4 conversion, the heat available to reach 1450˚C is the following.
Considering this criterion, Table 5 shows the estimation for the specific heat of the molten mixture of U + MgF 2 to reach this temperature: 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 + MgF 2 mixture has a viscosity low enough to allow the separation during the process.

Experimental Development
The trouser mix system of Figure 4 mixed the dried and dehydrated UF 4 product with 20% Mg excess, for 40 min. at 16 R.P.M.    the oxidation of the reactants, as undesirable reactions [13]. The argon inside the pump evacuates the air inside. Finally, the argon outlet sent the air to a water column to maintain a positive pressure, equivalent to 1500 mm water column.
This positive pressure prevents the entry of gases from the outside. With this pressure achieved, the heating program started through the oven. The reaction started after 16 hrs and at a temperature of 620˚C -650˚C.
To know the temperature at which the reaction begins, the differential thermal analysis (DTA) from Figure 6, determined that the required temperature for the uranium reduction is between 600˚C -650˚C. Over this temperature, reaction (6) provides the required heat for the reaction.
The noise produced and the temperature increase confirmed the start of the reaction. At this point, the temperature control system shutdown the power World Journal of Nuclear Science and Technology supply to the oven, allowing it to cool freely by natural convection. After 24 hours, the reactor was disassembled and the load removed.

Results Obtained
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. Figure 7 shows the start of the pump-type reactor, once the reduction reaction started, at 650˚C: With this phenomenon detected, the control system turned the oven off and allowed to cool freely until the next day. Table 6 shows the obtained products.
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 CaF 2 , used as insulating material.   Figure 7. Magnesiothermy reactor, once the reduction reaction started.  Figure 9 shows the porous zone inside the insulating material. These zones result in heat losses during the pre-heating and the metallothermic reaction process. This aspect consider the 10% of heat loss during the reduction reaction. Figure 10 shows the uranium obtained at the bottom of the reactor and the MgF 2 slag at the end of the process. The conic form of the billet allows obtaining a homogeneous final product and the separation from the undesired material. Another important aspect is the empty space during the cooling time. The geometry assured that the contraction suffered by the slag do not reach the uranium billet. In this case, leaving a space of 100 cm. between the slag and the billet. Figure 11 shows the presence of a portion of metallic uranium during the slag removal. Despite having reached the temperature to reach the molten state, its viscosity was not enough for a correct separation, causing the entrapment inside the slag.     Table 7 shows the compositions of the products formed inside the reactor. Table 8 shows the chemical analysis of the compounds of the reactor and the contamination of every part with residual uranium Table 8 shows a high uranium content of impurities in the sample coming from the external insulating material. This was because of the high pressure exerted by the energy produced by the reduction reaction. Figure 12 shows disassembly of the reactor and the obtained uranium billet:

Chemical Analysis
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     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. Table   9 shows the chemical composition of the uranium product obtained.
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 UF 4 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 UF 5 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)   However, Table 11 shows the results using the consideration of a 90% yield for the degree of progress of the reaction: The difference between the 76% recovery achieved in the billet shown in Fig-ure 5 and the theoretical 90% used for the calculations is because of the uranium trapped in the molten MgF 2 slag and the CaF 2 insulator. In the case of the uranium in MgF 2 , this happened because this section did not reached the fusion temperature needed to produce the separation. In the case of the insulator, the uranium was trapped because of the high pressure of reaction (6). According to

Conclusions
The main conclusions of this study are the following: 1) The proposed system allowed obtaining metallic uranium as a final product,  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 MgF 2 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 MgF 2 slag, for the total mass of the compounds is consistent with the results obtained in the experience.