Patent Application
20180096745
The disclosure pertains to apparatus and methods of separating radioactive and toxic materials from non-toxic base materials and in particular uses a pulsed plasma reactor to remove surface contaminants on the surface of solid waste. The system and device can function as a stand-alone waste treatment system or as a pre-treatment system for solid radioactive wastes generated from decommissioning of nuclear reactors in conjunction with plasma or gas mass separation system.
Significant amounts of nuclear wastes are being generated and in storage in the US and around the world from operating and/or decommissioning nuclear power plants. At present, low and intermediate level nuclear wastes, representing a majority of the total radioactive waste, are disposed of using surface or under-ground repositories that are safe for people and the environment for extended periods of time. While this disposal method is technologically sound, the cost of waste storage and potential leakage to the environment is significant. As such, there is a growing demand for a technology to reduce the volume of such nuclear wastes without creating an additional waste stream.
In recent years, a number of plasma based waste treatment solutions have been proposed. The key benefit of the plasma based waste treatment solution is the use of gaseous plasma medium instead of liquid chemical agents such as a sodium hydroxide (NaOH) containing solution. This allows the plasma treatment to reduce the waste volume without generating secondary waste. For example, the Archimedes Technology Group developed a physical process called “Plasma Mass Filter (PMF)”, where radioactive wastes are separated from non-radioactive materials based on the atomic mass of each material element inside the rotating plasma. In addition, a magnetic centrifugal mass filter has been proposed by the researchers at Princeton University utilizing centrifugal and magnetic confinement of plasma in a cylindrical magnetic trap.
While both technologies present solutions to the need to separate radioactive wastes from non-radioactive materials without utilizing liquid chemical agents, both methods suffer the challenge related to vaporizing and producing plasma from a large amount of nuclear waste. In the case of Archimedes Technology Group, arc plasma injectors, plasma torch and high power lasers are utilized to vaporize the waste and forms the plasma before separating the waste into different atomic masses. All of these methods require a large amount of thermal energy inputs as the entire waste mass is vaporized.
However, there are significant amounts of waste where there is no need to vaporize the entire waste mass. For example, in the case of concrete and contaminated soils from the nuclear reactor sites, these wastes are in the form of solid containing only small fraction of radioactive waste on the surface, as shown in
The disclosure is particularly applicable to a nuclear waste reduction system and method that uses pulsed plasma reactor to perform vaporization of a surface of the waste and it is in this context that the disclosure will be described. It will be appreciated; however, that the system and method has greater utility, such as to being used to reduce other types of radioactive or toxic wastes and may use other types of plasma reactors than those disclosed below. In one embodiment, the system and method may use a pulse plasma generator such as that disclosed in US patent application (US Patent Application US20140263181) as shown in
In the disclosed method and prior to the plasma treatment, the solid wastes may be segregated by the particle size. Pulverization, grinding or milling can be used in connection with particle sieves to control the feed particle size. The desired range of the particle size is between 1 μm and 1 mm.
Typically, the mass density of radioactive nuclear materials and toxic heavy metals are much heavier than the average mass density of rocks and soils. Here are the mass densities of typical radioactive nuclear materials and toxic heavy metals: 19.1 gram/cm3 for Uranium, 19.8 gram/cm3 for Plutonium, 19.4 gram/cm3 for Neptunium, 13.7 gram/cm3 for Americium, 13.6 gram/cm3 for Mercury, and 11.3 gram/cm3 for Lead. In comparison, the typical mass density of soil is only 2.6 gram/cm3 and the mass density is 2.65 gram/cm3 for silica or silicon dioxide, the most common constituent of sand. The big difference in particle mass density can be utilized to remove the particle type 202 from the rest of the waste feed using density based separation methods such as cyclonic particle separation.
Once the particle type 202 is removed from the rest of the waste feed, the remaining waste particles may be fed into a plasma reactor 400 for surface decontamination operating as shown in
As shown in
In the case of PN plasma generator shown in
At these gas pressures, even a modest reactor volume contains a large number of gas atoms and molecules that will get ionized and turned into a plasma. For example, in the case of argon gas pressures at 5 torr and at 50 torr, the corresponding gas number densities are 1.4×1017 cm-3 and 1.4×1018 cm-3. For a plasma reactor volume of 100 cm3, the amounts of gas atoms in the reactor are 1.4×1019 and 1.4×1020 respectively. With the presence of these high number of gas atoms or molecules, it is then possible to couple a significant amount of radio frequency electrical energy to the waste particles via plasma generation. In comparison, many of the radio frequency inductive plasmas used at semiconductor materials processing operates from 1 mtorr to 50 mtorr pressure range with the corresponding gas density on the order of 3×1013 cm-3 to 1.5×1015 cm-3. Since there are not enough gas atoms and molecules in these reactors, it is typically not possible to generate significant heat flux to the waste particles at these low operating gas pressures.
The plasma generation at these gas pressures between 1 torr and 1000 torr involves significant heat generation in the plasma medium and subsequent heat transfer to surrounding structures, including but not limited to the antenna, an enclosure of the pulsed radio frequency generator, one or more walls of the reactor chamber, a substrate for surface treatment, nozzles in the inlet, a material collection system such as filter and collectors. By limiting the pulse duration of the plasma source operation between 10 μs and 10 ms, it is then possible to alleviate or eliminate cooling requirements to the affected surfaces, thus simplifying the engineering requirement and improving the reliability and overall power efficiency.
The use of short pulse duration requires high pulse power in order to generate high density plasma via which a significant amount of heat flux can be delivered to the particle surface. As discussed by Park [5], the typical energy budget of plasma generation is 100 eV-500 eV of energy cost for ionization of one pair of an electron and an ion from a gas atom where 1 eV equals to 1.6×10−19 J. Assuming an energy budget of 150 eV, in order to ionize a 5% of gas in the reactor volume, an energy input from pulsed RF power system to the plasmas needs to be 17 J for the reactor volume of 100 cm3 and a gas pressure of 5 torr, which results in the plasma density of 7×1015 cm-3. In the case of 50 torr gas operation, the amount of energy input needs to be 170 J for the reactor volume of 100 cm3, which results in the plasma density of 7×1016 cm-3. For the pulse duration of 10 μs, the required pulse powers are 1.7 MW for 5 torr and 17 MW for 50 torr. For longer pulse duration of 1 ms, the required pulse powers are 17 kW for 5 torr and 170 kW for 50 torr. As such, high power coupling to the plasma during the pulse is needed for the radio frequency pulsed plasmas to be useful in the preferred embodiment.
If the plasma power during the pulse is too weak, the rate of vaporization will be too slow for practical uses. On the other hand, if the pulsed duration is too long, the surface heating from the plasma will propagate to the central region of the particles, leading to particle melting and mixing of radioactive nuclear waste with non-radioactive recyclable base materials. A preferred embodiment will utilize the localized surface heating and subsequent vaporization of radioactive from the particle surface using high pulse power and short pulse plasma generator such as the PN plasma generator.
During the pulse, the plasma electrons can transfer its energy to gas molecules via collisions, resulting in gas heating. While the collision frequency is high, the rate of energy transfer efficiency is low for each collision due to very large discrepancy in mass. The general expression for gas heating by plasma electrons is given as dTgas/dt˜ne<συ>Te(me/mgas), where Tgas the gas temperature, ne is the electron density in the plasma, σ is the electron gas collision cross section, υ is the electron velocity, Te is the electron temperature and me/mgas is the mass ratio between electron and gas atoms. In the case of argon operating at 10 torr, the plasma electron density is 1.4×1016 cm-3, assuming a 5% ionization fraction. The plasma density can be controlled by the radio frequency (rf) pulse input power, while the average electron temperature is between 5 and 10 eV in a wide range of input powers, based on the experimental and theoretical database of rf thermal plasmas in steady state. It is noted that the plasmas establish its density and temperature equilibrium for a given rf input power in a very short time, typically within a few μs due to the rapid response of electrons to the applied rf electric fields. We will use 5 eV electron in this example, resulting in electron thermal velocity of 9.4×107 cm/s and electron temperature of 57,000 Kelvin. At 5 eV, electron-gas collision cross section is approximately 1×10-15 cm2 for argon gas. By putting the above numbers in the gas heating expression, dTgas/dt is 1×108 Kelvin/s or 100 Kelvin/μs. This result shows that the gas temperature in the pulsed plasma reactor will gradually increase over the pulse duration by electron gas heating. For a fixed plasma density of 1.4×1016cm-3, the gas temperature will rise to 500 C, (from 20 C inlet temperature) for 5 μs pulse duration and to 1,000 C for 10 μs pulse duration, in this simplified estimate. Thus, the pulse duration control provides a powerful yet convenient method to adjust the reactor gas temperature, especially for pulse duration between 10 μs and 10 ms. Since the gas temperature is one of the critical parameters to determine the heat flux to the waste particles, this results provide the basis for the surface waste contaminants removal rate of the pulse plasmas. It is noted that we have ignored the energy loss mechanism by gas via conduction, convection and radiation, so the temperature rise will be less than this value and the gas temperature will likely saturate between 5,000C and 10,000C. It is also noted that the gas heating rate can be controlled by plasma density or rf pulse power.
It is noted that the pulse plasma reactor only generates a small amount heat during the pulse operation. This is because the heat capacity of the gas is very small, 12.6 J/(mole×Kelvin). Since the pulse plasmas heat only a small amount of gas molecules, 2.3×10-5 mole of argon gas for 100 cm3 reactor volume at 5 torr, the total amount of heat from the high temperature gas is relatively small. Even at 5,000C, the argon gas in the pulsed reactor will only contain 1.5 J of energy at the above condition. Even at very high repetition rate of 1 kHz, the total gas heating results in 1.5 kW, equivalent to a household hair dryer. This is because of the short pulse nature of pulsed radio frequency plasma operation. By limiting heat generation during the pulse, it is then possible to utility modest amount of electrical power to remove the surface contaminants of radioactive and toxic materials. In addition, the pulsed localized heating on the particle surface can accelerate the surface chemical reactions to speed up the vaporization of waste material when used with the chemically reactive precursors containing hydrogen, oxygen, chlorine, and fluorine.
Here we discuss a set of preferred operating conditions for the plasma decontamination reactor using a PN plasma generator operating with 200 kW rf power and 200 μs pulse duration is set forth below. For a feed particle size of 100 μm diameter, the surface area of each particle is 3.14×10-4 cm2 and the volume of the particle is 5.24×10-7 cm3. By controlling the particle feed rate and the gas flow in the fluidized bed configuration, the reactor may operate with 5 grams of waste feed particles inside the reactor, corresponds to treating 3.7 million particles. Due to a small particle size, the total surface area of the 3.7 million particles is 1.1×103 cm2, comparable to or exceeding the reactor chamber surface area. As such, a significant fraction of the rf power to the plasma will be deposited to the particle surface during the pulse. Assuming 30% of rf power to the plasma is deposited to the particle surface, the plasma delivers 3.3×10-6 Joule (J) of heat flux to each particle surface during 200 μs pulse. This corresponds to 520 kW per square meter of pulsed heat flux to the particle surface. In comparison, the average solar radiation to the Earth's surface is 340 W per square meter.
At typical plasma temperatures of 10,000 Kelvin to 100,000 Kelvin, the electrons and ions of the plasma do not penetrate deep into the particle surface. Instead, the plasma deposits most of its energy to a very thin surface layer, typically on the order of 10 to 100 atomic layers. In the case of Americium, an example of a radioactive nuclear material of interest, the surface thickness of 50 atomic layers is 8.7 nm based on its average atomic layer of 0.17 nm. Using the mass density of Americium at 13.7 gram/cm3 and the specific heat capacity of 62.7 J per mole per Kelvin, the heat pulse of 3.3×10-6 J will increase the temperature of the 50 Americium atomic layers by 3400 degrees in Kelvin during the pulse. In comparison, the boiling point of Americium is only 2,880 Kelvin. While this simple estimate ignores the thermal diffusion, it illustrates the capability of pulsed plasma to deposit a significant amount of heat to the particle surface layer and to vaporize radioactive nuclear waste such as Americium from the surface of feed particles.
While the amount of removed surface layers may be small for each pulse, a sufficient rate of surface removal can be obtained by utilizing many pulses with a high repetition rate of the pulse. Operating at a pulse repetition rate of 50 Hz, the average power consumption for the PN plasma generator is 2 kW at 200 kW rf pulse power with 200 μs pulse duration. In the case of surface contaminations covering 1 μm of surface layers or 5,800 atomic layers of radioactive Americium on the particle surface, the pulsed plasma can provide a waste treatment throughput of 0.9 kG per hour, if each pulse can remove 1 nm of surface layers operating with 5 grams of waste particle in the reactor. In addition, the pulsed plasma reactor can control the thickness of removed surface layer by varying the treatment time of the waste particles inside the plasma reactor along with the pulse power, duration and repetition rate.
In addition, unlike the continuously operating plasma reactors, the pulsed plasma reactor provides time windows when the electrical noise from the plasma is greatly suppressed as shown in
The data from the in-situ measurements can be used to optimize the treatment efficiency as well to provide the end-point of plasma treatment, as shown in
One of the challenges of the plasma reactor is the damage and contamination of the electrodes in contact with the plasma. In addition, with the use of particles in the reactor, frequent arcing can occurs on the surface electrodes contributing to the electrode damage. Also, the use of reactive gas species in the plasma can create complications regarding the compatibility of electrode materials. However, by using rf power between 50 kHz and 10 MHz with externally placed antenna, the inductive plasma generation by the PN plasma generator addresses these challenges.
The plasma treatment in the preferred embodiment will generate two products from the feed waste particles: the removed surface wastes in the vapor phase and the residual particles free from surface contamination. After the plasma treatment, the removed wastes in the vapor phase contain most, if not all, of the surface radioactive or toxic materials, while their total mass is much smaller than the initial waste particle feed. This can be achieved by the use of in-situ monitoring and end-point detection or pre-programmed plasma treatment process. Since they are in the vapor form, the removed waste can be easily transferred out of the reactor volume into the collector via a waste exhaust 403 as shown in
After the plasma treatment, the residual particles will contain little or no radioactive materials, while the total mass of the residual particles is comparable to the feed particles. The residual particles can be collected from the plasma reactor using various methods. By temporarily stopping the gas flow, the residual particles can fall down to the bottom of the reactor by gravity with the appropriate collectors such as particle filter 412 and can be removed from the reactor, if their mass is sufficiently heavy. For small particles where the gravitational force is weak, the filter can also be used to capture them as they are removed from the plasma reactor by generating the air flow toward the particle. In the case of the plasma reactor in
The foregoing description, for purpose of explanation, has been described with reference to specific embodiments. However, the illustrative discussions above are not intended to be exhaustive or to limit the disclosure to the precise forms disclosed. Many modifications and variations are possible in view of the above teachings. The embodiments were chosen and described in order to best explain the principles of the disclosure and its practical applications, to thereby enable others skilled in the art to best utilize the disclosure and various embodiments with various modifications as are suited to the particular use contemplated.
| Number | Date | Country | |
|---|---|---|---|
| 62402668 | Sep 2016 | US |
