Nuclear energy is a source of major electricity production, due to the high-energy capacity of the uranium fuel element during nuclear fission. Uranium is used mainly in the form of UO₂ type compounds. For this purpose, it must be enriched in its U²³⁵ isotope to fulfill this purpose. The needed enrichment level will depend on the type of reactor used, and there are 2 types: Light Water Reactors (LWR), which require an enrichment of U²³⁵ minimum between 3 and 5% to operate, and Pressurized Heavy Water Reactor (PHWR), which uses natural uranium as fuel [1] .

Figure 1 shows the uranium conversion for the production of UF₆, raw material for the enrichment process.

This process reduces the uranium compound, either from UO₃ or from U₃O₈, and obtains UO₂ powders, according to the overall reaction:

Subsequently, UO₂ powder should be treated by hydrofluorination to obtain UF₄ powders:

Finally, the UF₄ compound is taken to the fluorination process to obtain UF₆:

The UF₆ compound, gaseous at 56˚C and atmospheric pressure, is carried to the enrichment process, in order to obtain a concentrated and a diluted fraction in U²³⁵ through centrifugal force reactors [3] , using the different weights of this and the U²³⁸ isotope. This process is shown schematically in Figure 2 [4] .

The gaseous compound UF₆, once enriched, must return to its UO₂ form. To do this, there are 3 methods to carry out this task [7] [8] .

H₂ reduction: The UF_6(g) compound is reduced using hydrogen according to the reactions:

Hydrolyzing: The UF_6(g) compound is hydrolyzed using water:

Subsequently, the ammonium diuranate is reduced using H₂ at temperatures of 600˚C - 800˚C:

Finally, the third method for enriched UO₂ production is through precipitation of ammonium uranyl carbonate instead of ammonium diuranate, according to reaction (10):

All these processes require the use of a fluidized bed reactor for producing UO₂ for both UF₆ production, intended for further uranium enrichment, as for reproduction as UO₂ fuel pellets for nuclear power reactors.

It is for these reasons that this work will evaluate the main operating parameters of a fluidized bed reactor for UO₂ production from UO₃ concentrates using mathematical modeling techniques and the COMSOL Multiphysics software, in its 4.3b version. These parameters are the followings: feed flow, hydrogen concentration and initial temperature.

Conventional UO₃ to UO₂ reduction reactor is shown in Figure 3.

UO₂ production is conventionally performed in a reduction reactor as shown in Figure 3. Here, uranium trioxide and hydrogen are continuously fed, H₂ is normally diluted in an inert gas such as argon or nitrogen. It is also common to use ammonia, which is cracked [11] to obtain a nitrogen and hydrogen gas mixture. This UO₂ production reactor [12] [13] is controlled by the following operational parameters: composition and flow of the reducing gas, UO₃ feed and internal reactor temperature.

The reactor used in the development of experiences is shown in Figure 4.

This reactor is a tubular type, with input and output in conical shape. Its length is 1.66 m . and it has an internal diameter of 4.5 cm . The development of the experiences was performed by setting as parameters the initial temperature of both the gas and the reactor, which was between 400˚C - 500˚C and initial hydrogen concentrations in the reducing gas: 0.25, 0.5 and 0.75 M . The flow feeding the reduction reactor was kept constant at 2.5 L/min. The input speed of this flow was 0.1 m /s, and the cross section of 1.5 cm, so the feed flow regime was laminar.

To develop this model, the 3 transport phenomena mentioned before were occupied: momentum, heat and mass for dilute species. Working conditions are shown in Figure 5:

Figure 5 shows the considered parameters for the modeling: In the case of momentum transport, the reducing gas has inlet and outlet inside the reactor, and there is no slip flow on the walls. For heat transport, it is considered that the initial temperature for uranium conversion processes is 700 K. The walls symbolize heat losses by natural convection, where this system was covered by a thermal jacket at 450 K. The remaining heat will be generated by the exothermic reaction inside the vessel between UO₃ and H₂. Finally, for the transport of diluted species, reducing gas flow has the aforementioned initial hydrogen concentrations. A fraction of this gas is consumed by the UO₃ vessel. The remaining hydrogen at the reactor outlet was burned to prevent gas leakage. In

order to simplify the development of the calculations, the model was proposed using as axial symmetry, in the bottom of Figure 5.

The equations used to the model development were obtained from previous studies [15] . In this work, the reduction kinetics of uranium trioxide was developed based on the formation of an intermediate compound, U₃O₈, according to the following reactions:

The reduction rates for both equations were determined by using Arrhenius’s Law, according to Equation (18):

where: UO₃ reaction rate, cH₂: hydrogen concentration in the reducing gas, n: reaction order for hydrogen concentration, R: universal gas constant, T: absolute temperature, E_a: activation energy.

The corresponding reaction rates for Equations (17) and (18) are, respectively:

According to other authors [16] , the conversion process is usually made from U₃O₈ compounds, as the uranium oxide with greater chemical stability. This implies that reaction (19) occurs uneventfully. Moreover, reaction rate (20) involves the formation of other intermediate oxides of uranium, such as U₃O₇ or U₄O₉, which, as already discussed in other studies [15] [16] , decrease the overall speed of the process at temperatures above 700˚C. For this reason, reaction kinetics (21), which is the controlling step, will be used for modeling the reduction reactor.

The obtained results were as follows:

According to Equations (12) and (13), it can be said that the progress of these reactions is quantified through water production. Figure 6 shows the UO₃ reduction kinetics is so fast at these temperatures, almost no hydrogen exists in the vicinity of the vessel, where the reaction occurs. The following figure shows the linear profiles generating water into the reactor, along the axial symmetry.

Figure 7 shows that, for all hydrogen concentrations in the reducing gas, they are consumed almost entirely in the UO₃ vessel.

However, the most important parameter for the development of experiences is the temperature profile along the reactor, and the exothermic peak reached by the system due to the release of reaction heat, as this defines the capacity of the system to work autonomously, without the need of additional energy. This profile is determined by the release of energy from the exothermic reaction between uranium oxides and hydrogen. The profiles obtained for the hydrogen concentrations in the reducing gas, are shown in Figure 8.

According to bibliographic data [15] , the ideal temperature for the development of these experiences is 500˚C - 600˚C. The objective of this result is to check whether the process can operate autonomously, keeping the temperature within this range. For this reason, the process must be performed at 0.5 M H₂ concentrations to ensure this stationarity condition.

The results of the experiments carried out in this reactor are shown in the following figure:

Figure 9 shows that UO₃ reaction kinetics is virtually instantaneous in contact with the reducing H₂ gas, since for temperatures of 500˚C and 600˚C, the transformed fraction of UO₃ to UO₂ it is above 90%.

In the case of temperature, the experimental exothermic peaks obtained are compared with the modeling system.

Table 1 shows that the temperature system, operating continuously, can be predicted in an acceptable way using mathematical modeling techniques, since the data obtained for the proposed working conditions show relative errors of less than 10%.

・ From the previous data of UO₂ production, it is possible to predict the reduction of UO₃ compounds with hydrogen using mathematical modeling techniques.

・ The fluidized bed reactors can be monitored effectively with the gaseous products, which means that it is not necessary to manipulate the UO₃ vessel. This fact allows the safety during the data collection.

・ For the studied kinetic parameters, the UO₃ reduction reactions occur almost instantly, as the results show a hydrogen conversion to water almost completely.

・ The condition that allows continuous operation for the reduction reactor is feeding with reducing gas at 0.5M H₂.

・ The temperature profiles and relative errors allowed to conclude that the reduction of U₃O₈ compounds to UO₂ provided the required energy to maintain the working temperature in the required ranges.

The Chilean Nuclear Energy Commission acknowledges the assistance provided by University of Santiago, Chile, in the use and license of computer software COMSOL Multiphysics v4.3b, “CFD Module User’s Guide”, Modeling Single Phase Flow, Page 27-144. License Number: 2079130, Host ID: 5c260a04780b, 1.998-2.103.

Pedro Orrego,José Hernández,Jorge Manríquez, (2016) Modeling Operational Parameters for Uranium Dioxide Production Reactor through Uranium Trioxide Reaction Using Hydrogen. World Journal of Nuclear Science and Technology,06,131-139. doi: 10.4236/wjnst.2016.63014