Renewable Distillation of Spent Nuclear Fuel

Abstract

Nuclear waste is one of the most important environmental problems of nuclear power plants. A novel renewable distillation method has been proposed for the direct on-site recycling of spent nuclear fuel and the separation of its valuable components from fissile isotopes, which is especially applicable for reactors using liquid fuels. This dry separation technique can be applied in two single, parallel total-reflux columns with integrated separation stages for chlorinated nuclear waste. According to theoretical calculations, high separation accuracy of the UCl4-NpCl4, PuCl3-UCl3, CmCl3-SmCl3, and EuCl3-CsCl fractions could be achieved using twenty-six separation stages and five total-reflux repetitions, demonstrating the high efficiency of the method proposed. A scheme of the future pyroprocessing separation plant is also presented.

1. Introduction

Since the beginning of the commercial use of nuclear energy, the issue of the disposal of nuclear waste has remained largely unresolved. Currently proposed recycling methods [1], such as hydrometallurgical–water extraction processes or pyrometallurgical salt-mediated separation and subsequent electrorefining, use numerous solvents or are still ineffective and environmentally unfriendly. This is especially true for the widely applied extraction method PUREX (Plutonium–Uranium Recovery by Extraction), which allows for the separation of plutonium and uranium in a liquid–liquid system (see [1,2,3] for an overview). A similar method can also be applied for the separation of other elements such as americium and curium [3,4].

Volatilization of metallic constituents as oxides or contained in oxide material by chlorination followed by distillation is also a common separation technique for the extraction of selected materials. Since the 1940s, it has been applied in the large-scale production of titanium from titanium-bearing ores and oxide materials—known as the Kroll process (see [5]). There are a lot of other processes of metal chloride separation such as niobium and tantalum extraction as pentachlorides from iron–niobium or iron–tantalum alloys [6] or NdFeB magnet recycling through the chlorination or volatilization of non-rare-earth components and subsequent evaporation/distillation, as described in [7]. Distillation of uranium or actinide separation has been demonstrated on a laboratory or small scale for the recycling of molten salts (MS) using Molten Salt Reactor technology [8]. Similarly, the distillation-based recovery of FLiNaK salts [9] and the continuous distillation of mainly LiF-BeF₂ and ZrF₄-MF₃ (M = REEs: rare earth elements) [10] have also been shown. The purification of LiCl and other chloride salts (e.g., CsCl, SrCl₂, BaCl₂, or CdCl2), mainly via vacuum distillation or in a closed chamber distillation unit system, is presented [11,12,13,14,15,16,17,18,19,20,21,22], as well. On the other hand, for the general separation of chlorides from metallic uranium or uranium oxide materials, electrochemical processes for the production of metallic uranium are also applied [23].

Herein, we present a novel recycling method based on the distillation of spent nuclear fuel (SNF), which is particularly (but not only) useful for the new generation of nuclear reactors working at high temperatures with liquid fuels. The most developed concepts are the Molten Salt Reactor (MSR) or Dual Fluid Reactor (DFR) [24,25]. Whereas the former applies fluorides or chlorides of fissile isotopes simultaneously as fuel and coolant, the latter has separate loops for liquid chloride or metallic fuel and metallic coolant, with this being liquid lead. The fast-neutron version of MSR and DFR can even burn non-fissile isotopes of uranium and plutonium and, in combination with the distillation-based reprocessing of the SNF of older reactor generations, can lead to an environmentally friendly solution for modern nuclear power plants.

First of all, the method proposed is a dry method without any solvents and can be used for the separation of a large number of chemical elements, which significantly reduces the amount of nuclear waste and recovers the most valuable parts of it. This process has already been presented in our previous study [26] devoted to the recovery of rare earth elements (EERs) from NdFeB magnets. The separation technique is based on two single, parallelly working total-reflux columns with integrated separation stages for chlorinated nuclear waste. As will be shown herein, the method is very effective and of large separation accuracy, limiting the burden on the environment.

2. Pyroprocessing Separation Plant

In Figure 1, a model of a pyroprocessing separation plant that takes into account the different physical and chemical properties of recycled materials is presented. Starting with SNF in the form of spent uranium rods, a mechanical separation unit will be necessary to prepare the material for the following chlorination process. The zirconium cladding material of the fuel rods can also be shredded and recycled. Chlorination can be performed by means of carbon dioxide-forming tetrachloromethane (see our own study in [27]). The application of chlorides with their strongly different boiling points will allow for the volatilization of the recycled material and the use of distillation as the main separation method.

After chlorination, the actinides can be separated from fission products in the pyroprocessing separation unit (PPU) and further used as fuel material for liquid fuel reactors. The distillation process should be designed in such a way that individual chemical elements have to be separated with high accuracy. Electrorefining [28,29,30] can be additionally used here for the eventual reduction of chlorides or metallic components, depending on the type of fuel needed for the liquid fuel reactor.

The PPU utilizes an entirely new reprocessing approach. For safety reasons, a closed distillation system is supposed in order to exclude leakages and emissions of radioactivity or chemically harmful components down to well below the ppm range. Contrary to the chemical industry, the operation time of the facility needed for the separation of different chemical elements is much less important than their final purity of the ppm range. Thus, the distillation column concept proposed herein follows the principle of a fractionated discontinuous batch separation column under high reflux ratios, where fractions are usually obtained as a condensed distillation product due to the decreasing volatility of the subsequently separated fractions. We are going considerably further, however, and would like to apply the total-reflux principle according to which the distillation column would operate as a closed system in two cyclically repeating operating steps for each separable fraction.

The two operation steps of the distillation column are illustrated in Figure 2. In the first operating step, the distillation feed mixture is initially fed into the evaporator while the distillation column is preheated. Then, a steady-state total-reflux condition is reached in the column, and the entire reflux is collected in the reflux tank from where the distillation product is completely recycled back to the distillation column. In the second operating step, the distillation product is removed by closing the reflux tank and removing the product by closing valves V2 and V3 and opening product removal valve V5. During the removal of the distillation product, insertion through the open V4 maintains a constant pressure in the system. Meanwhile, the remaining MS mixture circulates in the distillation column, with it being directly condensed out from the condenser and directly fed back to the column. The two main operating steps should be repeated in progressing process cycles (PPC) until all distillation product fractions have been obtained.

3. Total-Reflux Distillation Process

In the following, we would like to demonstrate the effectiveness of the proposed total-reflux distillation process for the reprocessing of nuclear waste. Figure 3 displays the melting and boiling points of some chlorides representing both fissile actinides and fission products.

Highly volatile chlorides require special distillation treatment, as already discussed in our previous paper [27]. The basic, middle, and heavy boilers can be then divided into less and more volatile chloride groups defined by the 1600 K boiling point, defined by the critical temperature of uranium tetrachloride of 1598 K [31]. The two groups will undergo the distillation procedure in two different distillation columns. Additionally, there is a small group of non-volatile unchlorinated components that can be treated separately as more precious metallic components [27].

Since no experimental data on the vapor–liquid equilibria of any of the binary mixtures of the herein-discussed chlorides are available, the only method to simulate the total-reflux column is to utilize the idealized phase equilibria condition. Two models were used for the simulation of the total-reflux column. The first one is an independently developed code written in Octave (GNU Octave Version 7) (freely available software for solving mathematical and numerical problems [32], which essentially requires no critical data of the pure components. The calculations are based on Raoult’s law using partial vapor pressures. To validate the simulation results of the Octave model, a second model, the ChemSep Lite total-reflux column model according to [33], was also applied which requires considerably more substance property data, however, including critical data on the pure substances. The ideal phase equilibrium conditions and ideal enthalpy calculation of heat capacities for the ChemSep total-reflux column model could be easily adjusted analogously to the Octave model.

The composition of the main “thermal fission” products of the spent nuclear uranium rods is taken from [34,35,36] and used as the chlorinated feed material. For the simulation of the total-reflux distillation column, the feed composition of fuel material with a maximum fission rate of 5% U-235 was used. To simplify the simulation results, only the fission products listed in

Table 1. Feed composition of the most important spent nuclear fuel (SNF) fission product components in mole% from fission product yield data for U-235 [34,35,36]. The data are recalculated for the case of the complete chlorination and separation of non-volatile non-chlorinated material such as residual oxides, precious metals, or other metallic refractory metal components.

CsCl	SrCl2	SmCl3	EuCl3	NdCl3	NpCl4	PuCl3	AmCl3	CmCl3
12.563	11.571	1.147	0.911	15.564	2.378	50.034	5.579	0.253

were simulated and others were neglected. Fission products for which no material data exist, such as berkelium or californium trichloride, were also neglected in the feed composition. The data for chlorides assuming a 100% conversion rate were used for chlorination. The unchlorinated metallic components should first be separated, for example, via electrorefining, and the feed mixture should then be split into two different distillation columns for more and less volatile parts. The feed mixture consists of 94 mole% uranium tetrachloride, the most chemically preferred uranium chloride compound, followed by 1 mole% uranium trichloride. Other actinide chlorides make up less than 1 mole% and the rest are the fission products. The chlorinated feed composition, except the uranium chloride, is given in Table 1. The composition values of Lanthanum trichloride, barium chloride cerium chloride, and presodymium trichloride are also neglected due to non-validated substance property data. Non-chlorinated materials like technetium, ruthenium, rhodium, or molybdenum and other refractory or precious materials, as well as unchlorinated oxide materials, will be retained from the distillation process.

In the simulation of the stationary total-reflux distillation condition, the uranium chlorides can be predominantly separated in the first column for more volatile components together with cadmium and cesium (see Figure 5). However, some small contributions are still visible in the second column for the less volatile part (see Figure 4). Contrary to the first column, the second one should work in more than only one PPC.

Figure 4. Simulation results of total-reflux column operations for the use of distillation. The separation results are presented for four of the five progressing process cycles (PPCs), each consisting of 26 separation stages, which is necessary to achieve a few ppm of substance contamination. Each PPC represents a part of the steady-state total-reflux column operation setting and product removal according to Figure 2.

Figure 4. Simulation results of total-reflux column operations for the use of distillation. The separation results are presented for four of the five progressing process cycles (PPCs), each consisting of 26 separation stages, which is necessary to achieve a few ppm of substance contamination. Each PPC represents a part of the steady-state total-reflux column operation setting and product removal according to Figure 2.

Figure 5. Comparison of the total-reflux simulation results between the Octave and ChemSep models shown as one single progressing process cycle (PPC 1), like PPC 1 in Figure 4.

Figure 5. Comparison of the total-reflux simulation results between the Octave and ChemSep models shown as one single progressing process cycle (PPC 1), like PPC 1 in Figure 4.

Figure 4 shows exemplary total-reflux simulations over five PPCs within the Octave model of a twenty-six-stage total-reflux column. The distillation products already obtained in a previous cycle will be removed from the feed mixture of the next cycle. In the first PPC, the high-purity mixture of uranium and neptunium tetrachloride free of other chlorides is clearly evident in the first separation stage. In the last separation stage, plutonium trichloride is purified as a heavy-boiling product with possible impurities of uranium trichloride of about 3·10⁻³ to 5·10⁻³ mole%. In the second PPC, essentially the same chloride components as in the first PPC are enriched but with higher proportions of uranium tetrachloride as a light boiler and higher uranium trichloride impurities. Since light and heavy boiling components are withdrawn after each PPC, the boiling range of the chlorides remaining in the column decreases after each PPC. Thus, the increase in component contributions with the stage number is not so strong for higher PPCs. In the third PPC, curium trichloride is obtained as a high-boiling distillation product with minor impurities of samarium trichloride within the range of 3·10⁻³ to 6·10⁻³ mole%, and neodymium trichloride is obtained as a high-pure heavy boiling product free from other chloride components. In the subsequent fourth and fifth PPC, the number of impurities then increases significantly due to the broadening of the range of medium-boiling components occurring, which then leads to a higher number of product impurities. Thus, in the fourth PPC, samarium trichloride is obtained as a high-boiling component, with the impurities of americium trichloride, cesium chloride, and strontium dichloride shown in Figure 4, as well as a high-boiling product composition of cesium chloride and europium trichloride with some impurities of americium trichloride in the ppm range. In the fifth PPC, only cesium chloride with the low americium trichloride impurities shown can possibly be obtained as the heavy-boiling distillation product. For higher separation accuracies for these chlorides, a separation column with more separation stages would be required.

In Figure 5, simulations performed within the Octave and ChemSep models are compared. They are in excellent agreement. A small deviation in the separation ability of uranium tetrachloride in the sixth separation stage is visible on the logarithmic scale. This probably arises from different data used for the vapor pressure of uranium tetrachloride.

All simulations shown for the total-reflux column were carried out under ideal phase equilibrium conditions. To check the theoretical calculations with the experimentally obtained results, which are usually influenced by zeotropic mixtures, the titanium tetrachloride distillation in the Kroll process according to [5] can be considered. One such zeotropic mixture is the titanium tetrachloride distillation in the Kroll process according to [5]. For simplicity, a mixture of titanium tetrachloride, silicon tetrachloride, and vanadium tetrachloride is taken into account, which is purified in a five-stage separation column with almost complete reflux of the distillate stream, similar to that of an infinitely high reflux ratio. The calculations were carried out within Ocave and ChemSep models [37] and are presented in Figure 6. The dotted lines show the experimental top (first separation stage) and bottom product composition (fifth separation stage). The experimental and theoretical separation results are in very good agreement with the main components of light-boiler silicon tetrachloride and heavy-boiler titanium tetrachloride. For vanadium tetrachloride and iron trichloride, however, the experimental data lie between the results of the computer simulations. These substances are of very low molecular fractions, so it is not clear whether the differences arise from the model calculations or are due to experimental uncertainties.

4. Conclusions

We can conclude that the Octave model shows reliable and realistic simulation results under idealized phase equilibrium conditions. On the other hand, the depletion of uranium tetrachloride as a high boiler component can also be observed in the enrichment of cesium chloride in the seventh separation stage. Both distillation products are impurified with a small amount of cadmium dichloride but free of the other components. The calculations unambiguously confirm that the separation accuracy better than 10⁻⁴ mole% can be achieved for both uranium tetrachloride and cesium chloride. This is especially important for the uranium component, which makes up 95 mole% of all recycled material. From the remaining 5 mole%, most of the chloride components can also be recovered as high-purity distillation products, including other actinide chlorides e.g., curium trichloride or plutonium trichloride.

Summarizing our results, we have shown that the herein-proposed pyroprocessing plant utilizing a distillation process with total-reflux columns in a closed system can separate SNF very effectively with very high accuracy. The recycling method enables the segregation of nuclear waste into individual chemical elements, which allows us to take into account the lifetime of radioactive isotopes. In this way, the amount of radioactive material intended for storage for more than 300 years can be significantly reduced and, at the same time, valuable elements can be recovered for further use. While the study was focused on recycling standard uranium fuel rods, the proposed chlorination and distillation method can be particularly well applied to the next generation of fast nuclear reactors operating at high temperatures using actinide chlorides as liquid fuel.