Study of Atmospheric Plasma-Based Mass Separation System for High-Level Radioactive Waste Treatment

2. Plasma-Based Mass Separation

2.1. Mass Separation experiments

Plasma mass separation provides a unique opportunity for radioactive waste long-term management, especially for HLRW. Using plasma for element separation based on mass is not new. The first time plasma mass separation used was in Calutrons in the Manhattan Project during WWII [13,14], where the U-235 atoms were separated from U-238 to make a concentrated uranium bomb. The first rotating thermal plasma experiment for ion mass separation was done in Sweden from 1966 to 71 using H, D, He, and Ar gases [15]. Another plasma mass separation experiment was conducted at Yale University in 1980-97 as Plasma Centrifuge [16], which was later refined by vacuum Arc Centrifuge in Australia [17] and Brazil [18]. The original plasma centrifuge had a metal cathode in which materials were subject to be separated. A 4 kV voltage was applied to the cathode to produce a vapor plasma in a magnetic field of 7 kG (0.7 T). The material used for the cathode was various metal isotopes or elements, Cu/Zn and their isotopes, C, Al, Mg, Zn, Cd, Pb, etc. [19]. Although there was no practical device was built to demonstrate the plasma centrifuge in HLRW mass separation, the concept was presented as one of the potential mass separation methods for HLRW management through mass separation by Abraham J. Fettermana and Nathaniel J. Fischa from the Department of Astrophysical Sciences, Princeton University, in 2011 [9]

One of the most prominent plasma mass separation experiments was carried out by a private company named Archimedes Technology in San Diego from 1998 to 2005 on the single-particle ion orbit theory (band gap theory) T. Ohkawa [12]. A demo system was built to generate a helicon plasma in a chamber of 3.9 m length and 0.4 m in diameter magnetic field of 1.5 kG and for separation only a DC voltage component was used. The plasma was heated with ~3 MW, 6 MHz RF power and driven with end electrodes biased at ≤700V [20]. Although no actual HLRW mass separation was performed using the demo device, a few initial noble gas mass separation and Na and Bi metal [21] mass separation experiments showed promising results. The demo device addressed significant mass separation physics and the processes for rotating plasma and high throughput, but no actual separation of HLRW was published [22].

Since 2010, the most active experiments on plasma-based mass separation have been carried out at Irkutsk, Russia [23], plasma mass filter experiment (PMFX) at Princeton Plasm Lab [24] and Spent Nuclear Fuel (SNF) processing at the Joint Institute for High Temperatures in Moscow [25]. Table 1 lists all the previous experiments of plasma-based mass separation till now:

2.2. Plasma-based mass separation for high-level nuclear waste remediation

In all the plasma-based mass separation experiments, Plasma Centrifuge, Archimedes filter, Plasm Mass Filter and SNF Separator System are considered for nuclear waste mass separation, especially HLRW and spent nuclear fuel [9,12,19,25]. Archimedes filter, Plasma Mass Filter, and SNF Separator System are considered to separate elements(atoms) in a mass group rather than individual elements. In traditional chemical separation like UREX (URanium EXtration), PUREX (Plutonium and Uranium Recovery by EXtraction), and TRUEX (TRansUranium Extraction), the separation works on separating certain atoms at a time in a chemical mixture [26]. The process is a time-consuming and multi-stage process. In the case of plasma mass separation for HLRW, the separation will focus on separating the fission products as a group having a mass profile of 80-160 amu rather than, for example, the separation of Cs-137 (standard fission products). The throughput of plasma mass separation will be determined at what rate wastes can be processed in a mass separation system, and the separation factor will depend on a group of species i, in the output and input waste which can be defined as:

$$\mathrm{Separation}\mathrm{factor}\mathsf{\alpha }=\frac{Fraction of group i in output products }{Fraction of group i in the input products}$$

(1)

The target of plasma mass separation is to separate the fission products from spent fuel (group i); Additionally, plasma mass separation provides a greater chance of separating fission products in HLRWs in a single stage than traditional chemical-based separation. The HLRWs mass separation goal is to achieve a cutoff mass between the high and the low mass group. Table 2 lists the separation goal for various HLRW with a mass cutoff in amu.

Until 2013, about 120,000 tHM spent fuels from the nuclear reactor were reprocessed worldwide, primarily by the USA, France, India, Japan, Russia, and the United Kingdom. PUREX is the most common chemical separation process for spent fuel reprocessing, where uranium and plutonium are extracted from the spent nuclear fuel in a chemical process. There are a number of issues with the PUREX process. Firstly, the PUREX process is not economical with respect to the direct storage/disposition of waste and using fresh uranium fuel, as the process is a multi-stage continuous solvent extraction process. Currently, most nuclear countries use a once-through fuel cycle where the spent fuels are not reprocessed but stored directly as HLW in dry casks. This is because fresh uranium fuel costs relatively cheap than reprocessing. The second issue with PUREX is it produces a separate stream of uranium and plutonium, which has a greater risk of nuclear proliferation. The third issue with PUREX is, that it created more residual wastes in the process. Although about 96% of the original spent fuel mass can be extracted (95% uranium and 1% plutonium), leaving a large amount of nitric acid solution containing the remaining intensely radioactive 3-4% fission products of the original spent fuel mass. Making the whole solution HLW, with some ILW and LLW containing transuranic elements separated during the process. Plasma mass separation can address the nuclear proliferation issue as the waste will be separated as a mass group of all the transuranic elements instead of individual actinides.

Additionally, replacing multi-stage chemical separation with single-stage plasma separation will reduce residual waste production in chemical separation. It is estimated that after using PUREX to extract uranium and plutonium from the spent nuclear fuel, the remaining 95% of the mass is nitric acid introduced by the reprocessing [27]. This liquid solution is stored as high-level liquid radioactive waste (HLLRW). This liquid waste must be vitrified for long-term storage/disposal for proper immobilization. Since plasma mass separation will operate on high-temperature plasma, no liquid waste will be generated. Additionally, the actual waste output can be collected as vitrified waste that will ensure the immobilization of the waste and dramatically reduce the waste storage/disposal volume in the long term.

In terms of cost, Table 3 lists the life cycle cost of HLRW management with the currently available technologies.

For plasma mass separation to dramatically change HLRW processing in the future, the process should cost $200/KgHM [37] since the plasma mass separation system has a relatively smaller footprint than other systems, in addition to reduced construction, maintenance, and operation costs. Additionally, with zero additional cost of vitrification for waste immobilization, a 2B budget for processing 500 tHM in 20 years would keep the processing cost <$200/KgHM [9].

3. Functional modelling of a generic plasma-based mass HLRW separation system

Most of the current plasma-based mass separation technologies are subject to some practical limitations because they operate in a high vacuum, a minimum energy investment is necessary for ionizing and heating the atoms, and they have a low density relative to atmospheric pressure plasma that restricts their throughput [19]. They are nearly collisionless ion gyro-orbits or ion drifts in electric and magnetic fields. However, highly collisional plasmas are also interesting for mass separation since they occur at relatively high density, which could produce high throughput, and at relatively low temperatures, reducing the need for auxiliary plasma heating. This is where the atmospheric pressure plasma torch can be advantageous in high density, highly collisional, and continuous operation with respect to low density, near collusion less and batch operation. Plasma mass separation is a complex thermochemical process with various stages involved. As previously discussed, Archimedes’ demo filter, based on the band gap ion mass filter principle by T.Ohkawa and colleagues, separates ions in a plasma based on the m/z ratio. It utilizes cross-electric and magnetic fields in cylindrical geometry where only a DC radial parabolic voltage is applied [12]. This paper investigates the principle of mass separation in atmospheric pressure plasma. A convenient way to identify the stages/processes involved will be to design a functional model representing the process with its variables. A functional model represents the functions (activities, actions, processes, operations) within the modelled system or subject area in system engineering. Functions translate needs into a physical structure that will meet those needs. Function modelling is the formal way to define and model functions [38]. In plasma-based mass separation for HLRW, the process has five main tasks or stages: waste preparation, waste injection, waste ionization, waste mass separation, and finally, waste collection. There are multiple activities in each stage. Various physics is involved in each stage of the process. The functional model of a proposed generic plasma mass separation is shown in Figure 1.

A rotating plasma utilizes a cross electric and magnetic field to separate ions inside the plasma chamber based on a cutoff mass. Based on the functional model, a generic mass separation system incorporating two atmospheric pressure inductively coupled plasma (ICP) torches connected in series was proposed. A cross electric and magnetic field is applied in the second ICP chamber. A 2D-asymmetric generic mass separation system was designed in COMSOL for initial noble gas ICP simulation under mass separation conditions. A cross-sectional view of the proposed generic mass separation system highlighting its components is shown in Figure 2, and a 3D model of the system is shown in Figure 3. The parts marked in numbers are listed in Table 4.

3.1. Waste preparation

Waste preparation is the primary stage of the whole process. This stage determines the suitable waste form to inject into the mass separation system for proper ionization in the system plasma. There are mainly two forms of HLRW available, solid waste (spent fuel (SP) assemblies) and liquid waste from spent fuel reprocessing. For mass separation of HLRW, the suitable waste form for RF plasma torch injection is liquid waste through a carrier gas channel. Therefore, the spent fuel assemblies’ solid waste needs to be liquified after shearing and end piece removal. The process is well known and practiced worldwide in PUREX and UREX processes. The spent fuels are liquified by dissolving spent fuel into HNO₃ to make UO₂(NO₃)₂ (aq), Pu(NO₃)₄ (aq) and nitrates of other metals. This aqueous solution is perfect for waste injection into the mass separation chamber. Later this liquid solution can be mixed with the first and second-cycle PUREX raffinate (liquid HLRW) to raise the total solid concentration.

3.2. Waste injection

The liquid waste from the waste preparation phase can then be nebulized through the central carrier channel (parts 124 in Figure 3) of the primary RF ICP torch shown in Figure 2 for ionization in the plasma flame. The plasma torch is an RF ICP torch with argon as the plasma gas supplied through the central and sheath channel (parts 122 and 123 in Figure 3). Nebulization of waste into fine droplets for injection instead of liquid injection ensures rapid, homogenous distribution of waste into plasma with less chance of plasma fluctuation. The vaporized waste by nebulization can then be rapidly heated by the torch's >7000K plasma temperature in the coil region. This temperature was achieved at the center of the plasma flame by an RF power supply generating an 11 kW coil excitation power supplied through the primary RF coils (parts 121x) and with a 13.56 MHz excitation frequency [39] by COMSOL RF ICP simulation and an initial experiment with RF ICP plasma using pure argon gas. Waste injection through the carrier channel will expose the waste to a higher temperature at the center of the plasma for fast ionization. The waste is expected to be partially ionized and produce high-temperature waste vapor before exposure to the separation chamber's central RF plasma. The advantage of using an atmospheric pressure ICP for both the injection and the mass separation of waste is that the waste injection can be continuous, whereas other high vacuum systems are usually capable of batch treatments [40].

3.3. Waste ionization

Partially ionized waste vapour from the primary plasma torch will be directed into the main separation chamber. The waste will be fully ionized and atomized by the central filter plasma generated by a five turns RF coil assemblies (parts 130 in Figure 3) bounded by magnets/electromagnets on the opposite end. The central plasma in the separation chamber utilizes the primary RF ICP plasma outlet as an inlet or plasma gas. The multiphysics simulation using COMSOL shows that using an 11 kW coil excitation power for both primary and central RF coils with a 13.56 MHZ coil frequency, the temperature achieved inside the separation chamber is >9000 K. In mass spectrometry, using an atmospheric pressure ICP ionization method compared to other ionization methods is considered a hard ionization method to ensure complete sample atomization during sample ionization [41] Since the mass separation of waste is solely dependent on the ionization Z=+1 of waste elements for separation in a mass group, two stages of waste ionization will ensure the waste materials are fully ionized with high temperatures generated by the primary RF plasma torch and later by the central plasma before mass separation. This double plasma treatment of waste will ensure maximum ionization of the waste for mass separation.

3.4. Waste mass separation

The band gap ion mass filter principle says that ions can be radially separated based on the mass range with a combination of an axial magnetic field and radial or azimuthal electric field in cylindrical confinement [12]. A cross electric and magnetic field influences the fully ionized waste for ion mass separation inside the main cylindrical separation chamber. The Magnetic field is generated using two circular electromagnets or permanent magnet rings at each end of the central RF coil (parts 140 in Figure 3). An electric field proportional to r can be imposed by a set of concentric ring electrodes at the axial ends of the cylinder. Although the band gap mass filter principle is operated under a high vacuum with low dense collisionless plasma, our study with atmospheric plasma simulation (section 4) shows the operation can be transparent to atmospheric plasma where the plasma is highly dense and heavily collisional. When the electrode’s voltage has AC and DC components, the ion orbit characteristics show a bandgap structure like electrons in a semiconductor with few confined and unconfined spaces [12,42]. The bandgaps can be graphically represented in the α-β space on the x and y axes, respectively, where α represents the ratio of the potential difference across the filter to the total kinetic energy of the ion. While, the parameter β represents the ratio of the band gap's width to the ion's total kinetic energy. The parabolic band curve separates the regions as confined and unconfined regions for the range of α and β values for which ions with a particular mass-to-charge ratio are radially confined in the filter. While the masses above the certain mass-to-charge ratio are not confined radially [12]. For a case of β=0 and the electrode’s voltage with only DC voltage components while V_rf = 0, the ions with mass/charge $>$ are radially unconfined

$$\frac{m_c}{Z}>\frac{e{B}^2{a}^2}{8V_{dc}}\equiv m_{cutoff}$$

(2)

Where, $A_{Cutoff}$ = cutoff atomic weight/mass, $m_c$ =mass of the cutoff atom, z = ion charge, B = megneti field, $R_{Wall}$ = plasma radius, $V_{dc}$= center to wall voltage, e = electron charge. Hydrogen ion mass considering the base mass $m_c= m_H$ and $V_{dc}= V_0$, equation 2 can be rewritten as the

$$A_{cutoff}=\frac{Ze{R}^2{B}^2}{8V_o{m}_H}$$

(3)

Under mass separation conditions, ions above the cutoff mass are not confined and thrown to the radial wall, whereas the ions below the cutoff mass are contained in the center of the chamber's axial direction.

3.5. Mass collection

After the mass separation of waste, the separated high-mass ions above the cutoff mass and the low-mass ions below the cutoff mass are collected in the different waste collectors. Since the high mass ions are thrown into the radial wall, where negative pressure suction channel on the radial wall can take the heavy mass ions out of the chamber. The low-mass ions that accumulate in the center of the plasma and the chamber can be collected through the light mass collection point opposite the waste injection channel shown in Figure 2. One added advantage of the proposed mass separation unit shown in Figure 2 is that the separated low-mass ions will be pushed to the system's edge, where the waste will be collected due to the input pressure. It is important to mention the waste insertion and collection mechanism depending on the temperature. In waste insertion, rapid heating is necessary, whereas rapid cooling is essential for collecting waste output. Both heavy and light collector systems are considered separate units from the mass separation system, so they can be periodically disconnected and cleaned. Additionally, any waste removal through suction would affect the plasma stability and temperature. This paper presents a study of mass separation at atmospheric pressure. Further research must be carried out on the effect of waste collection on the separation and system plasma stability.

4. Mass separation simulation of ICP for noble gas inside mass separation unit

A simulation analysis was conducted using the COMSOL multiphysics simulation tool using the proposed generic mass separation unit in section 3 to investigate the mass separation effect under atmospheric pressure. Pure, noble gases, argon and helium, are used to analyze mass separation under atmospheric pressure in separate simulations under the same parameters. The model in Figure 2 was developed using COMSOL Multiphysics software, version 6.1, released in December 2022. In COMSOL, RF, ICP is modelled with an equilibrium discharge (ED) interface. The interfaces for ED are appropriate for modelling any thermal plasmas (i.e. arcs or inductively coupled discharges). Where partial to complete local thermodynamic equilibrium (LTE) conditions of the thermal plasma are assumed [43]. These plasmas can be modelled using the magnetohydrodynamics (MHD) equations because these kinds of plasma can be considered conductive fluid mixtures at the macroscopic level. The motion of the conducting fluid in an electromagnetic field is described by MHD, which later combines the Navier-Stokes, heat, and Maxwell's equations. The plasma's chemical makeup is disregarded in this case. Modelling plasma in COMSOL multiphysics using the ED interface is based on a set of assumptions that lead to simplifications of the MHD equations:

• The plasma is fully ionized (a mixture of electrons and ions)
• The plasma optically thin is under local thermodynamic equilibrium (LTE) conditions
• The plasma is considered a locally neutral Newtonian fluid mixture
• The plasma flow is laminar and quasi-incompressible under atmospheric pressure

4.1. Mathematical and physical model

Four physics interfaces are used in ED for the mass separation model simulation in COMSOL; Magnetic Fields (MF), Heat Transfer in Fluids (HTF), Electric Current (EC) and Laminar Flow (LF) interfaces. There are additional multiphysics couplings; Megnetohydrodynimic (M) (Magnetic Fields and Laminar Flow), Equilibrium Discharge Heat Source (EDHS) (1. Magnetic Fields and Heat Transfer in Fluids, 2. Electric Current and Heat Transfer in Fluids), Equilibrium Discharge Boundary Heat Source (EDBHS) (1. Electric Current and Heat Transfer in Fluids, for the anode, 2. Electric Current and Heat Transfer in Fluids, for cathode), Static Current Density Component (SCDC) (from Electric Current and Magnetic Field), Induction Current Density Component (ICDC) (from Magnetic Field to Electric Current), and finally, Nonisothermal Flow (NF) (Laminal Flow and Heat Transfer in Fluids). The ED can be described by a set of equations defined in the physics interfaces constituting the different multiphysics interfaces. Note that all the physics features in the individual physics interfaces are also available in the multiphysics interface. The MF interface solves Ampère–Maxwell’s equations formulated using the magnetic vector potential and, optionally for coils, the scalar electric potential as the dependent variables. The M couples Lorentz force and Electromagnetic force. In addition to Ampere’s Law, the following equations are solved under the MF interface and M multiphysics node in the frequency domain, assuming time-harmonic fields:

$$\nabla . J=0$$

(4)

$$\nabla . H=J$$

(5)

$$B=\nabla . A$$

(6)

$$J=\sigma E+j\omega D+\sigma v\times B+J_e$$

(7)

$$E=-j\omega D$$

(8)

$$F=\frac{1}{2}Re(J \times B^{*})$$

(9)

$$J=\sigma \left(E+ v\times B\right)$$

(10)

Where, $\nabla$, $H$, $J$, $B$, $A$, $E$, $D$, σ, v, $J_e$, $j$, are the divergence, magnetic field intensity, current density, magnetic flux density, magnetic vector potential, electric field intensity, electric displacement or electric flux density, electrical conductivity, the velocity of the conductor, externally generated current density, imaginary unit, and angular frequency, respectively. In the HTF interface and EDHSs, multiphysics coupling features solve the energy conservation equation under frequency stationary study: In fluid,

$$\rho C_{p}u . \nabla T+ \nabla .q=Q+Q_p+Q_{vd}$$

(11)

$$q= -k\nabla T$$

(12)

$$\rho =\frac{p_A}{R_{s}T}\mathrm{in\; an\; ideal\; gas\; domain}$$

(13)

In solid,

$$\rho C_{p}u . \nabla T+ \nabla .q=Q+Q_{ted}$$

(14)

Where the source of heat Q (W/m³) includes three source/sink components:

1. Resistive heating (ohmic heating)

$$Q=J .E$$

(15)

2. Volumetric net radiation loss Q_rad is defined by the total volumetric emission coefficient, which is a material property

3. Enthalpy transport (energy carried by the electric current)

$$\frac{\partial }{\partial t}\left(\frac{5k_{B}T}{2q}\right)\left(\nabla T. J\right)$$

(16)

Where, 𝜌, $C_p$, $u$, $k_B$, T, q, $Q_p$, $Q_{vd}$, $Q_{ted}$, $p_A$, $R_s$ are density, specific heat at constant pressure, mass averaged fluid velocity vector, Boltzmann constant, temperature, thermal conductivity, Pressure work, viscous dissipation, thermoelastic damping, absolute pressure, and specific gas constant, respectively. The EC interface and EDBHSs, SCDC, and ICDC multiphysics coupling features solve a current conservation equation based on Ohm’s law using the scalar electric potential as the dependent variable. Ohmic heating and induction heating when heat transfer is coupled together with electrical or electromagnetic interfaces

$$\Delta .J=Q_{j,v}$$

(17)

$$J=\sigma E+J_e$$

(18)

$$E=-\nabla V$$

(19)

$$J_{mf}=J_{e,mf}$$

(20)

$$J_{e,mf}=-\sigma \nabla V$$

(21)

$$J_{ec}=-\sigma \nabla V+ J_{e,ec}$$

(22)

$$J_{e,ec}=0$$

(23)

For anode

$$-n .\left(-k\nabla T\right)= Q_b$$

(24)

$$Q_b= \mid J . n\mid {\Phi }_s$$

(25)

For cathode

$$Q_b= -J_{ele} {\Phi }_s +J_{ion}{V}_{ion}$$

(26)

$$J_{elec}=if(\mid J . n\mid >J_R, J_R,\mid J . n\mid )$$

(27)

$$J_R=A_R{T}^{2}exp\left(-\frac{q{\Phi }_{eff}}{k_{B}T}\right)$$

(28)

$$J_{ion}=\mid J . n\mid - J_{elec}$$

(29)

Where, $Q_{j,v}$, $J_{mf}$, $J_{e,mf}$, $J_{ec}$, $J_{e,ec}$, $k$, ${\Phi }_s$, $Q_b$, $J_{ele}$, $J_{ion}$, $V_{ion}$, $J_R$, $A_R$, $q$, ${\Phi }_{eff}$, are the volumetric source of current, current density due to magnetic field, the external current density in the magnetic fields, current density due to electric current, the external current density in the electric currents, thermal conductivity, surface work function for anode and cathode, boundary heat source, electron current, ion current, the ionization potential of the plasma, Richardson-Dushman current density, Richardson’s constant, electronic charge, the effective work function of the surface, respectively. Finally, the LF interface and NF multiphysics coupling solve continuity, momentum and energy equations as follows under stationary and weakly compressible flow conditions:

Continuity

$$\nabla .\left(\rho u\right)=0$$

(30)

Momentum

$$\rho \left(u . \nabla \right)u =\nabla . \left[-pI+K\right]+F$$

(31)

$$K=\mu \left(\nabla u+{\left(\nabla u\right)}^T\right)-\frac{2}{3}\mu \left(\nabla .u\right)I$$

(32)

$${\mathrm{Q}}_{\mathrm{vd}}=\mathsf{\tau } : \nabla u$$

(33)

The Lorentz force acting on the fluid defined in the magnetic field interface

$$F= J \times B$$

(34)

Where, p, τ, $Q_{vd}$, F, μ, I, andKare the pressure, viscous stress tensor, viscous dissipation, volume force vector, dynamic viscosity, identity matrix, and dimensionless resistance coefficient, respectively.

4.2. Results and discussion

A 2D-axisymmetric model of the mass separation unit was designed in COMSOL, shown in Figure 4, for separate argon and helium plasma-based generic mass separation simulation. Although a mass separation simulation using mixed argon and helium gas would be a more accurate and obvious choice, the current COMSOL multiphysics version 6.1 does not support a mixed gas ICP plasma modeling under the ED interface rather with a single plasma gas (i.e, air, argon, helium, nitrogen, etc.) Table 5 shows the parameters used to develop the mass separation unit and the ICP simulation for different noble gases. Quartz was used in the simulation as the material for the three-channel primary RF IPCT. Both the primary and central RF coils material was defined as copper. The electromagnetic field coils material was also defined as copper. In the case of the main separation chamber, the bottom wall and the radial wall section underneath the central RF coils are defined as quartz. At the same time, the top wall as concentric rings and the radial wall section underneath the electromagnetic field coils are defined as graphite for conductivity. The chamber was 0.35 m long and had a radius of 0.14 m in the coil region.

Using the parameters from Table 5, simulation was carried out in multiple stages. First, only the primary RF ICP torch (ICPT) connected to the separation chamber was analyzed, using an RF power of 11 kW and an average gas velocity of 0.12 m/s through all three channels of the primary RF ICPT (laminar gas flow) The carrier, central and sheath channels had a thickness of 2 mm, 2.2 mm, and 3.5 mm, and an inner radius of 3.7 mm, 18.8 mm, and 25 mm, respectively. An argon gas flow of 0.31 L/min, 7.2 L/min, and 4.2 L/min and with helium gas a flow rate of 31 L/min, 72 L/min, and 42 L/min was supplied through the three channels respectively. Three turns were used for the primary RF coil with an 11 kW excitation power and 13.56 MHz excitation frequency, and a diameter of 8 mm. A temperature of 10000 K was achieved for the pure argon simulation at the beginning of the main separation chamber shown in Figure 5.a

Now, changing the plasma gas to pure helium, a temperature of 6500 K was achieved at the beginning of the main separation chamber with the same coil excitation power, shown in Figure 5.b. We know at LTE that T = T_e = T_i = T_g, Where T = plasma temperature, T_e = electron temperature, T_i = ion temperature, T_g = gas temperature are all considered in thermohydraulic equilibrium. Although the T_e for helium is almost twice the argon plasma, helium has ten times higher first excitation potential under the same operating conditions leading to a much lower electron density for helium plasma. This leads to more deviation of plasma temperature in equilibrium for helium than argon plasma [44]. The temperature for both argon and helium ICP does not change much with increasing the power. At the same time, increasing the inlet velocity raises the plasma axially. A temperature close to 10000 K can partially ionize the radioactive waste and prepare for mass separation.

In the second simulation stage, both primary and central RF ICP were provided with the same RF power, and excitation frequency and kept the same gas velocity at the primary inlets. The central RF coil has a diameter of 15 mm and five turns. With both primary and main RF ICPT being active, an average temperature inside the separation chamber reaches over 7000 K for pure argon gas ICP (Figure 6.a) and over 4000 K for pure helium gas ICP (Figure 6.b). It should also be mentioned that the coil's power and plasma are strongly coupled in here. This means electron heating only occurs in the region of the skin depth of the plasma. According to Bahour et al., in the intense region of the plasma, the electron and ion density shows similar evolution [45], which is also the case here in ED conditions assuming fully ionized plasma. With a frequency of 13.56 MHz, the plasma is more uniform in density in argon ICP than in helium ICP.

The next stage of the simulations was carried out by applying mass separation conditions using a cross electric and magnetic field into the previous stage's plasma while keeping the primary and main RF ICPT to the same excitation power, coil frequency and plasma gas velocity at the inlets. Two electromagnets located at the opposite end of the central RF coil are provided with a 2.5 kA current to generate a 253 G (0.0253 T) magnetic field in the axial direction of the chamber. Additionally, an electric field in the radial direction was induced into the plasma by 25 V DC voltage to the central most concentric ring electrode at the end of the chamber. A parabolic electric field profile was introduced by gradually lowering the applied voltage to the later concentric rings. A mass cutoff point, 6 amu calculated by Equation 3 using Table 5 parameters, was applied in argon and helium plasma.

With the applied cross electric and magnetic field fully coupled with the plasma physics, a lower temperature is observed, shown in Figure 7.a, at the center of the chamber and higher close to the radial wall for argon ICP. On the other hand, figure 7.b, shows a lower temperature close to the radial wall, and a much higher temperature is observed at the center of the chamber for helium ICP, an opposite behavior compared to the argon ICP. Under ED plasma simulation in COMSOL, the plasma is simulated in partial or complete LTE conditions where the electron temperature T_e is approximately equal to the gas T_g or ion T_i temperature. According to Bahouh et al. [45] and Lei et al. [46], in ICP plasma, in atmospheric pressure and laminar flow, the electron and heavy ion temperature follow the same distribution of the particle density. The hottest region has high electron and heavy ion density and vice versa in the coldest region of the plasma, confirming the LTE condition. So, figure 7.a and 7.b shows an apparent mass separation of noble gases under the 6 amu cutoff mass condition, where the low-mass helium ions are concentrated in the central region of the separation chamber, and the high mass argon ions are observed to be concentrated in the radial edge of the separation chamber.

In the next stage of the simulations, focus on the mass separation effect changing the cutoff point. By keeping the electric field constant and varying the magnetic field by varying the current in the electromagnetic coils listed in Table 6 below,

In the case of argon plasma, the separation effect of raising the cutoff point is shown in Figure 8.a,b, where changing the mass cutoff point from 6 amu (Figure 7.a) to 9 amu (Figure 8.a) and 12 amu (Figure 8.b) creates a colder region in the chamber's center as more argon ions are separated radially.

A further simulation in the case of helium plasma observes the separation effect of raising the cutoff point from 6 amu (Figure 7.b) to 9 amu (Figure 9.a), creating more hottest region in the center as more helium ions are concentrated in the center. Additionally, changing the mass cutoff from 9 amu to 12 amu (Figure 9.b) slightly changes the temperature in the center.

5. Conclusion

Currently, there is no commercial permanent HLRW disposal site in the world. HLW mass separation provides a unique solution for the HLRW to the future nuclear-based power generation worldwide. Many possible mass separation processes can be helpful for HLRW mass separation, as shown in Table 1. Rotating plasma-based mass separation could be the future of how the HLRWs are processed. Using the principle of band gap ion mass filter operation, this paper proposes a mass separation system using atmospheric pressure plasma torches. COLSOL Multiphysics simulation of the proposed mass separation system shows a mass separation effect for noble gas plasma under a cross electric and magnetic field in atmospheric pressure. However, generic physics and technical issues like the charge state, neutral atom separation, molecules ionization and disassociation, ion exchange and recombination, ion collisions, rotation speed, plasma fluctuations, radiation and off-gas processing and many other issues must be resolved before a practical separation device for HLRW mass separation can be achieved. Besides, the system needs to be high throughput and economical before an industrial-scale system with actual HLRW processing. There is a great opportunity to develop new ideas, codes and experiments on this front. Small-scale experiments and simulations like this research work can be designed to test specific unique ideas and applications or improve existing techniques to address some issues.