Radio-frequency driven dielectric heaters for non-nuclear testing in nuclear core development

Apparatus and methods are provided through which a radio-frequency dielectric heater has a cylindrical form factor, a variable thermal energy deposition through variations in geometry and composition of a dielectric, and/or has a thermally isolated power input.

FIELD OF THE INVENTION

This invention relates generally to heaters, and more particularly to a radio-frequency driven dielectric heater for use in non-nuclear testing in nuclear core development.

BACKGROUND OF THE INVENTION

Development of space nuclear power and propulsion systems is difficult and costly due to radiation-related health and safety problems during system testing. In order to reduce the health and safety problems, researchers have pursued means for non-nuclear testing of nuclear systems. The non-nuclear testing includes simulation of a nuclear reactor core with electrical heaters. Using a simulated reactor core, various characteristics of the nuclear system that are not directly related to the nuclear operation are tested.

Early stages of conventional nuclear system development utilized resistive electrical heater elements. The resistive electrical heater elements simulate nuclear reactor pins. Conventional resistive electrical heater elements are designed with the same form factor as the simulated nuclear reactor pins, namely a cylindrical structure with an outer metallic clad surrounding a graphite resistive element. The resistive electrical heater elements are bound in a hexagonal packing pattern to simulate the shape of the core structure of the nuclear reactor assembly. Other elements of the reactor design are also incorporated including heat transfer mechanisms, power conversion systems, radiators, and loads. However, the simulated core has thermal loss through the power leads that provide electrical power to the conventional resistance heaters.

Resistance heaters have nuclear pin diameters that are relatively large, greater than 0.5 inches. Power leads of the conventional resistive heaters require relatively large diameter wires to minimize losses in the leads. Since the power leads have electrical resistance, the current passing through the leads produces thermal energy in the leads. In order to minimize the thermal loss, larger diameter wires are used. However, as the lead wire diameter increases, two complications arise. First, the physical placement of the power leads entering the simulated core is problematic. Second, the conduction of thermal energy out of the core via the power leads increases, resulting in a less robust simulation of the nuclear reactor core.

Later reactor designs require smaller pin diameters, such as less than 0.5 inches, and require more pins, such as hundreds, for higher power systems, further exacerbating these problems. These requirements make the use of resistance heaters less attractive.

Inductive heating is commonly used for applications where a metallic structure must be heated. In general, inductive heating uses a coil that carries alternating current (AC) to produce an alternating magnetic field in the test article. The magnetic field induces currents in the test article, which in turn produce thermal energy via resistive losses. However, inductive heating requires high currents in the coil and hence large diameter power leads. Inductive heating also requires that the active heating element be metallic.

For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for a heater with reduced thermal coupling to the outside environment. There is also a need for a heater with a reduced size relative to the power leads. Furthermore, there is a need to generate high thermal energy in a relatively small space to simulate fission reactions for non-nuclear testing in nuclear core development. The above-mentioned shortcomings, disadvantages and problems are addressed by the present radio-frequency driven dielectric heater, which will be understood by reading and studying the following specification.

SUMMARY OF THE INVENTION

A cylindrical radio-frequency (RF) driven dielectric heater includes a conductor, surrounded by a dielectric, which is in turn surrounded by a Faraday shield. An RF electromagnetic field generated by the conductor is dissipated by the dielectric, converting an RF signal into thermal energy. The thermal energy is conducted outward from the RF driven dielectric heater. The Faraday shield confines the RF electromagnetic energy within the perimeter of the RF driven dielectric heater. The RF driven dielectric heater can be made smaller than conventional heaters. The conductor of the RF driven dielectric heater can receive the RF signal either capacitively or inductively, thus requiring no physical coupling for delivery of the RF signal to the RF driven dielectric heater and reducing the thermal coupling to the outside environment.

The present RF driven dielectric heater describes systems, methods, and apparatus of varying scope. In addition to the aspects and advantages of the present RF driven dielectric heater described in this summary, further aspects and advantages of the RF driven dielectric heater will become apparent by reference to the drawings and by reading the detailed description that follows.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description of embodiments of the radio-frequency driven dielectric heater, reference is made to the accompanying drawings that form a part hereof, and in which is shown by way of illustration specific embodiments in which the radio-frequency driven dielectric heater may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the radio-frequency driven dielectric heater, and it is to be understood that other embodiments may be utilized and that logical, mechanical, electrical and other changes may be made without departing from the scope of the present radio-frequency driven dielectric heater. The following detailed description is, therefore, not to be taken in a limiting sense, and the scope of the present radio-frequency driven dielectric heater is defined only by the appended claims.

RF dielectric heating techniques utilize a rapidly fluctuating electromagnetic field which applies an alternating torque on polar molecules in a material under test. This torque either causes the molecules to flip orientation or to attempt to flip orientation, depending on the material. As a result, thermal energy is produced via the frictional resistance to the molecules motion. This technique is commonly used to heat nonmetallic materials in a very controlled manner including plastics, ceramics, and food products. The effectiveness of the heating process is dependent on the dielectric properties of the test article. Similarly, the ideal choice of frequency of the electromagnetic field is based on the electrical properties of the material. Materials that are very efficient at converting the RF power to thermal power are termed lossy dielectrics.

In some embodiments, RF energy source transmitters are limited to a number of predetermined frequencies. The dielectric constant of materials (e.g., graphite, nuclear pellets, soils, etc.) varies with the frequency that is applied to the subject material. For any given RF frequency, various subject materials are likely to have various different dielectric constants. For example, the impedance of a conventional system has been observed to vary depending on the frequency to which the RF source is tuned and with soil. See, Garbe Et. Al.; “Calculation of Electromagnetically and Thermally Coupled Fields In Real Soil Decontamination”; 15thInternational Wroclaw Symposium on EMC, 2000.

FIG. 1is a block diagram of an apparatus implementation of a radio-frequency (RF) driven dielectric heater100. The RF driven dielectric heater100is also known as a rod or a pin.

The RF driven dielectric heater100includes a cylindrical conductor110. Examples of the conductor110include an antenna, a wire, or any other combination. A signal transmission source (not shown) passes an RF signal into the conductor110, generating an RF electromagnetic field (not shown).

The conductor110is further surrounded by an insulating dielectric130. An insulating dielectric is a dielectric that has electrical insulating properties. The RF electromagnetic field readily passes from the conductor110and into the insulating dielectric130.

The RF electromagnetic field is dissipated by the insulating dielectric130. The RF electromagnetic energy causes rapid fluctuations in the orientation of molecules in the insulating dielectric130, converting the RF electromagnetic energy into thermal energy. The thermal energy is conducted outward from the insulating dielectric130.

In some embodiments, the insulating dielectric130includes zirconia, a carbide, and/or titania. In some other embodiments, the insulating dielectric130includes a non-metallic depleted nuclear fuel, which in various examples can take the form of powder, rods, pellets, liquid and/or gas. The non-metallic depleted nuclear fuel allows precise simulation of thermal energy transfer and thermodynamic properties.

The insulating dielectric130is surrounded by a Faraday shield150. The Faraday shield150provides a system ground for the RF electromagnetic energy and confines the RF electromagnetic energy within the perimeter of the RF driven dielectric heater100. A Faraday shield is also known as a Faraday cage.

In some embodiments, the Faraday shield150is comprised of a refractory metal, such as tungsten, tantalum, molybdenum and/or osmium. In other embodiments, the Faraday shield150is comprised of a non-refractory metal such as aluminum or copper.

The conductor110is an active radiator of electromagnetic energy. The electromagnetic energy has the ability to deliver energy into the surrounding environment, or other dielectric heaters when more than one is being used.

The insulating dielectric130substantially prevents a short circuit from forming between conductor110and the Faraday shield150. For these reasons the insulating dielectric130can not be electrically conductive, but must have some dielectric loss.

In some embodiments, the RF driven dielectric heater100simulates fission thermal energy in a nuclear reactor for non-nuclear testing in nuclear core development. In those embodiments, the insulating dielectric130can be selected to match the thermal characteristics of the nuclear material that is simulated in order to accurately simulate thermodynamics and thermal energy transfer effects and properties of the thermodynamically simulated nuclear material. In some embodiments, the RF energy source ranges from 2,000 to 5,000 volts with RF frequencies up to 50 MHz. Those of ordinary skill in the art would understand that the selection of voltage and frequency is a function of the materials of the conductor110, the dielectric130and the Faraday shield150.

RF dielectric heating has several advantages over other techniques for application to non-nuclear testing of nuclear systems. Unlike resistive and inductive heating techniques, dielectric heating techniques depend on high field potential at low current to produce thermal energy. Therefore, smaller diameter power leads can be used to transmit the RF signal for the frequency range of interest, typically less than 100 MHz. The Faraday shield150provides a return path for the RF signal, therefore only a single power lead is required. Nuclear fuels are typically in the form of ceramic compounds. Therefore, the simulated reactor pin can use depleted forms of the nuclear fuel material as the dielectric material resulting in precise matching of thermodynamic and material characteristics, as shown in figures below.

Particular implementations of the RF driven dielectric heater100are described below inFIGS. 2–5.FIG. 2is a block diagram of an apparatus implementation of a RF driven dielectric heater200having an insulating dielectric and having layers that comprise certain materials. In the RF driven dielectric heater200, the conductor110ofFIG. 1is a wire210. Furthermore, the insulating dielectric130ofFIG. 1is a depleted uranium oxide (UO2) dielectric230. Lastly, the Faraday shield150ofFIG. 1is a molybdenum tube250.

The depleted uranium oxide dielectric230obviates the need for an insulator around the dielectric. The depleted uranium oxide dielectric230is a substantial insulator. Thus, no short will develop between the wire210and the molybdenum tube250. Therefore, no insulator is required between the wire210and the depleted uranium oxide dielectric230, and no insulator is required between the depleted uranium oxide dielectric230and the molybdenum tube250. Thus, a RF signal is supplied to the conductor110which radiates into the depleted uranium oxide dielectric230at a resonant frequency, yielding thermal energy.

In some embodiments, the RF driven dielectric heater100has a diameter of approximately 0.125″ as required in high power fission core applications. Thus, the RF driven dielectric heater200provides a RF driven dielectric heater with high thermal output, relatively small physical size and a simplified construction that requires no insulators surrounding the depleted uranium oxide dielectric230.

FIGS. 3–5show RF driven dielectric heaters having an insulating dielectric with a variable thermal energy deposition. The variable thermal energy deposition results from variations in impedance of the insulating dielectric. Variation in the impedance of the insulating dielectric is achieved through a variation of at least one characteristic of the insulating dielectric along at least one dimension. The characteristics include the density profile and the geometry of the dielectric. The dimensions include a radial dimension and an axial dimension. One example of varying density is creating or forming air pockets or vacuum pockets in position(s) or area(s) of the dielectric where different thermal energy deposition is desired. The air or vacuum pockets have dielectric properties that are taken into account in a design of a RF driven dielectric heater, but the effect is to alter the dielectric properties on a local level. One example of varying geometry is varying the thickness of the dielectric.

FIG. 3is a block diagram of an apparatus implementation of a RF driven dielectric heater300having an insulating dielectric that varies along an axial dimension. The RF driven dielectric heater300includes a conductor110and a Faraday shield150as inFIG. 1. However, the RF driven dielectric heater300also includes an insulating dielectric330that varies along an axial dimension. The insulating dielectric330varies in density of dielectric material along the axial dimension to create a thermal energy deposition that increases from one end of the axial dimension to the other end of the axial dimension.

FIG. 4is a block diagram of an apparatus implementation of a radio-frequency driven dielectric heater400having an insulating dielectric that varies along a radial dimension. The RF driven dielectric heater400includes a conductor110and a Faraday shield150as inFIG. 1. However, the RF driven dielectric heater400also includes an insulating dielectric430that varies along a radial dimension. The insulating dielectric430varies in density of dielectric material outward from the center to create a thermal energy deposition that decreases outwardly from the center.

FIG. 5is a block diagram of an apparatus implementation of a RF driven dielectric heater500having an insulating dielectric that varies geometrically along an axial dimension. The RF driven dielectric heater500includes a conductor110as inFIG. 1. However, the RF driven dielectric heater500also includes an insulating dielectric530that varies in thickness along an axial dimension. In one embodiment, the insulating dielectric530is thinner in the middle of the axial dimension to create a thermal energy deposition that is greatest in the middle along the axial dimension. This embodiment is particularly well-suited to simulate the thermal energy deposition of fission reactors that have the greatest thermal energy deposition in the middle along an axial dimension. The RF driven dielectric heater500also includes a Faraday shield150that inversely varies in thickness along an axial dimension corresponding to the variance in thickness of the dielectric530.

FIG. 6is a block diagram of an apparatus implementation of a RF driven dielectric heater600including insulators surrounding a conducting dielectric. The RF driven dielectric heater600includes a conductor110that has a form factor (i.e. shape) that is cylindrical. A RF signal passes into the conductor110.

The conductor110is surrounded by an inner insulator620. The inner insulator620presents little impedance to an RF electromagnetic field that radiates from the conductor110. Molecules in the inner insulator620readily flip back and forth with the alternating electromagnetic field produced by the RF signal in the conductor110. Because the molecules in the inner insulator620flip orientation easily, the RF electromagnetic field readily passes from the conductor110, through the inner insulator620and outward.

The inner insulator620is further surrounded by a conductive lossy dielectric630. A conducting dielectric is a dielectric that has electrical conducting properties. The inner insulator620electrically isolates the conductor110from the dielectric630. Notwithstanding the inner insulator620confining the RF signal in the conductor110, an RF electromagnetic field radiates from the conductor110through the inner insulator620and into the conducting dielectric630. One example of a conducting dielectric material is graphite as discussed below in conjunction withFIG. 7.

The conducting dielectric630is surrounded by an outer insulator640. The inner insulator620may be made from the same materials as the outer insulator640, but not necessarily. The outer insulator640is surrounded by a Faraday shield150.

In some embodiments either of the insulators620and640comprise a very efficient dielectric (i.e. not lossy) material in order to minimize loss of the RF electromagnetic energy in the insulator. In some other embodiments, either of the insulators620and640, have insulating properties, as well as sufficient material strength, at an expected maximum operating temperature. In yet other embodiments, either of the insulators620and640are chemically compatible with the adjoining materials over a range of expected operating conditions. Examples of materials that either of the insulators620and640are composed of include oxides and/or oxide compounds (e.g. alumina, silica and magnesia), polymers, carbides, nitrides.

An electrical RF signal (not shown) is received by the conductor110. The insulators620and640on both sides of the insulating dielectric630prevent the signal in the conductor from electrically shorting to the Faraday shield150. Absence of the insulators620and640would allow the RF signal to flow to the Faraday shield150, which would in turn radiate RF electromagnetic energy outward. Thus, the insulators620and640help confine energy within the heater600, which increases the efficiency of the heater600.

FIG. 7is a block diagram of an apparatus implementation of a RF driven dielectric heater700having layers that comprise certain materials and including insulators surrounding a conducting dielectric. In the RF driven dielectric heater700, the conductor110ofFIG. 6is a wire710. Furthermore, the conducting dielectric630ofFIG. 6is a graphite dielectric730. Lastly, the Faraday shield150ofFIG. 6is a molybdenum tube250.

The graphite dielectric730necessitates electrical insulation around the outer and inner sides of the graphite dielectric730. The graphite dielectric730is a poor insulator. Thus, a short will develop between the wire710and the molybdenum tube250. Therefore, an insulator is required between the wire710and the graphite dielectric730, and an insulator is required between the graphite dielectric730and the molybdenum tube250. Thus, the RF driven dielectric heater700provides a heater with high thermal output and relatively small physical size.

FIGS. 8–10show RF driven dielectric heaters having insulators surrounding a conducting dielectric with variable thermal energy deposition.

FIG. 8is a block diagram of an apparatus implementation of a RF driven dielectric heater800having a conducting dielectric that varies along an axial dimension. The RF driven dielectric heater800includes a conductor110and a Faraday shield150as inFIG. 6. However, the RF driven dielectric heater800also includes a conducting dielectric830that varies along an axial dimension. The conducting dielectric830varies in density of dielectric material along the axial dimension to create a thermal energy deposition that increases from one end of the axial dimension to the other end of the axial dimension.

FIG. 9is a block diagram of an apparatus implementation of a radio-frequency driven dielectric heater900having a conducting dielectric that varies along a radial dimension. The RF driven dielectric heater900includes a conductor110and a Faraday shield150as inFIG. 6. However, the RF driven dielectric heater900also includes a conducting dielectric930that varies along a radial dimension. The conducting dielectric930varies in density of dielectric material outward from the center to create a thermal energy deposition that decreases outwardly from the center.

FIG. 10is a block diagram of an apparatus implementation of a RF driven dielectric heater1000having a conducting dielectric that varies geometrically along an axial dimension. The RF driven dielectric heater1000includes a conductor110as inFIG. 6. However, the RF driven dielectric heater1000also includes a conducting dielectric1030that varies in thickness along an axial dimension to create a thermal energy deposition that is greatest in the middle along the axial dimension. In some embodiments, the RF driven dielectric heater1000also includes an insulator1040that varies in thickness in inverse proportion to the conducting dielectric1030along an axial dimension in order to maintain a constant thickness of the RF driven dielectric heater1000in accommodation to the conducting dielectric1030. The RF driven dielectric heater1000is particularly well-suited to simulate the thermal energy deposition of fission reactors that have the greatest thermal energy deposition in the middle along an axial dimension.

FIG. 11is a block diagram of an apparatus implementation of an electrical circuit1100that provides electrical energy to a RF driven dielectric heater. The electrical circuit1100includes a signal generator1110electrically coupled to a 1.5 kW amplifier1120. The electrical circuit1100also includes two electrically identical combiners1130and1180. The first combiner, 4 way splitter1130, is used as a splitter to provide four equal drive signals from the 1.5 kW amplifier1120to four 1.5 kW amplifier modules1140,1150,1160and1170. The second combiner1180takes the output of the four modules1140,1150,1160and1170and combines the output into a single RF signal. Electrical circuit1100yields the RF signal in a desired voltage output such as 2,000 to 5,000 volts with RF frequencies up to 50 MHz.

The RF signal is transmitted to a RF driven dielectric heater1190through transmission medium1195. Examples of the RF driven dielectric heater1190include RF driven dielectric heaters100,200,300,400,500,600,700,800,900and1000. In some embodiments, the transmission medium is an electrically conductive physical wire, such as a copper wire. In other embodiments, the transmission medium1195includes a capacitive coupling means. In yet other embodiments, the transmission medium1195includes an inductive coupling means. The capacitive and inductive coupling means provide a means of transmitting the RF signal without a physical connection to the RF driven dielectric heater1190. The lack of a physical connection increases the thermal isolation of the RF driven dielectric heater1190.

FIG. 12is a block diagram of an apparatus1200for non-nuclear testing in nuclear core development that includes a RF driven dielectric heater. Apparatus1200includes a steam generator1210. The steam generator1210is substantially full of water. The steam generator1210includes a plurality of RF driven dielectric heaters1220immersed in the water. The plurality of RF driven dielectric heaters1220are bundled in close proximity and typically require the same volume of physical space as a fission core. In some embodiments, the plurality of RF driven dielectric heaters1220are electromagnetically coupled1230to a thermally isolated power source (not shown). Thermal energy from the plurality of RF driven dielectric heaters1220vaporizes some of the water in the steam generator1210, yielding pressurized steam1240. The pressurized steam drives a steam turbine electrical generator1250, yielding electricity1260. The waste steam is cooled to a liquid state1280by a condenser1270and returned to the steam generator1210. In the aggregate, the steam generator1210, the steam turbine electrical generator1250and condenser1270are also known as a power converter. Thus, apparatus1200satisfies the need to generate high thermal energy in a relatively small space to simulate fission reaction for non-nuclear testing in nuclear core development.

FIG. 13is a block diagram of an apparatus for non-nuclear testing in nuclear core development that includes a RF driven dielectric heater. Apparatus1300receives alternating current (AC)1305into an RF signal generator1310. The RF signal generator1310transmits an RF signal to an RF amplifier1315. An amplified RF signal is transmitted from the RF amplifier1315to an RF splitter1320. Split RF signals are transmitted from the RF splitter1320through capacitive RF power coupling1325. The capacitive RF power coupling1325is mounted in a thermal barrier1330that encapsulates a plurality of RF driven dielectric heaters1220. The thermal barrier1330also encapsulates a refractory core1335. A heat pipe1340transfers material that is heated in the refractory core1335to a heat exchanger1345. Thermal energy from the heat exchanger1345is transferred to a Stirling Engine1350. The Stirling Engine1350powers an electrical generator1355. The electrical generator1355produces an alternating current (AC)1360. The AC is transmitted to a Stirling Control1365.

The Stirling Control1365includes a rectifier1366that converts the AC to direct current (DC)1383. The Stirling Control1365also includes a resistive load1367and a control circuit1368. The DC1383exits the Stirling Control1365and enters a thruster control1370. The thruster control1370includes a control circuit1375and a DC/DC converter1380to convert the DC1383to a required voltage and current. The DC1383is received by an ion thruster1385, providing thrust1390. Thus, apparatus1300satisfies the need to generate large amounts of thermal energy in a relatively small space to simulate a fission reactor in non-nuclear testing.

CONCLUSION

A radio-frequency (RF) driven dielectric heater has been described. Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. The RF driven dielectric heater is not limited to any particular conductor, dielectric and/or insulator. For sake of clarity, simplified conductors, dielectrics, insulators and/or Faraday shields have been described. This application is intended to cover any adaptations or variations of the present RF driven dielectric heater. In particular, one of skill in the art will readily appreciate that additional layers of materials may be disposed between any two layers of materials to isolate the layers of two materials that would become reactive in direct contact at high temperature without departing from the scope of embodiments of the radio-frequency driven dielectric heater. The additional materials may be selected to ensure the dielectric heating occurs primarily in the dielectric material. The additional layers of material may also include non-solid forms of matter such as a gas interface, or a liquid interface, depending on the particular requirements. One of skill in the art will readily recognize that embodiments of the radio-frequency driven dielectric heater are applicable to future non-nuclear core development.

The terminology used in this application with respect to is meant to include all dielectric and heater environments and alternate technologies which provide the same functionality as described herein. Therefore, it is manifestly intended that this radio-frequency driven dielectric heater be limited only by the following claims and equivalents thereof.