WO2010033927A1

WO2010033927A1 - Radioisotope thermal generator

Info

Publication number: WO2010033927A1
Application number: PCT/US2009/057709
Authority: WO
Inventors: Richard Westfall; Gary Rodriguez
Original assignee: Richard Westfall; Gary Rodriguez
Priority date: 2008-09-22
Filing date: 2009-09-21
Publication date: 2010-03-25

Abstract

A radioisotope thermoelectric generator (RTG) comprises a power source, a heat producing shield that converts gamma radiation into thermal energy, and an electricity generator. The heat producing shield can include an x-ray fluorescence cascade shield (XFCS). The XFCS includes a layered structure that disposes higher atomic number (Z) elements toward the radiation source and lower atomic number elements away from the source. The radiation source can include 207Bi, which emits substantial gamma radiation and decays into the stable and non-radioactive element, 207Pb. The electricity generator can include any device capable of converting thermal energy into electricity, including a free-piston Stirling engine. The RTG provides a reliable, maintenance-free power cell for terrestrial uses or space missions that can last for many years.

Description

RADIOISOTOPE THERMAL GENERATOR

[001] The present invention claims priority to US 61/098,832, which is hereby incorporated by reference.

FIELD OF THE INVENTION

[002] The present invention relates to a radioisotope thermal generator, particularly to such a generator using a low toxicity radiation source.

BACKGROUND OF THE INVENTION

[003] A radioisotope thermoelectric generator (RTG) produces power through radioactive decay of a suitable unstable isotope. Heat released by the decay can be converted into electricity using, for example, an array of thermocouples. Radioactive isotopes having sufficiently long half-lives can power a device for many years or even decades. RTGs can, therefore, be a viable power source for unmanned or unmaintained situations over timeframes that are too long for standard batteries or fuel cells. Such situations can include, for example, satellites and space probes.

[004] Suitable radioactive isotopes will have a half-life that generates a sufficient amount of power over the expected time frame. Assuming several watts of continuous power are required over several decades, only a few dozen atomic isotopes are available from the entire table of nucleotides. The isotope should produce radiation that can be converted to electrical energy. The decay can produce high levels of penetrating alpha radiation. Proper shielding can contain alpha radiation. Gamma emitters have been unsuitable in RTGs of the prior art because gamma radiation is very difficult to shield without large quantities of shielding such as, for example, lead plates. Prior art includes no facile means to shield gamma radiation or to convert it into electricity. ²³⁸Pu has been a preferred isotope because of low beta and gamma emissions, low shielding requirements, a favorable half-life, and massive alpha emissions that can be converted into heat and consequently electrical energy. The alpha emissions can be converted to thermal energy and then to electrical energy.

[005] Still, the decay products of commonly used RTG radioactive isotopes are typically radioactive or otherwise toxic. For these reasons, RTGs have not found utility as terrestrial power sources.

SUMMARY OF THE INVENTION

[006] The present invention describes a RTG that produces reduced levels of neutrons, alpha, and beta radiations. The RTG includes a radiation source, a heat producing shield at least partially surrounding the radiation source, and an electricity generator. The generator is thermally coupled to the heat producing shield.

[007] The radiation source can include any suitable radioactive isotope including those isotopes that emit substantial gamma radiation. Preferably, the isotope will decay into a stable element. The radiation source can comprise, for example, a bismuth isotope such as

²⁰⁷Bi, which has a half-life of 31.55 years. The decay product is the stable, non-radioactive isotope ²⁰⁷Pb.

[008] The heat producing shield comprises an x-ray fluorescence cascade shield (XFCS) comprising a layered structure that collects gamma rays emitting from the power source and converts them to heat. Additionally, the shield substantially contains radiation within the

RTG and reflected radiation can return to the core or the shield. The layered structure of the

XFCS disposes higher atomic number (Z) elements toward the radiation source and lower atomic number elements away from the radiation source. Prior layers closer to the radiation source absorb radiation energy, convert a portion of the radiation energy to lattice phonons

(heat), and then fluoresce at a lower frequency than the original incident radiation to subsequent layers farther from the source. Elements in subsequent layers absorb the fluoresced radiation. The process is repeated in subsequent layers. The layered structure thereby converts the radiation, such as for example gamma rays, from the radiation source into a cascade of thermal phonons.

[009] The electricity generator can include any device capable of converting thermal energy from the heat producing shield into electricity, including a free -piston Stirling engine. The

Stirling engine uses heat to expand a working gas in a chamber thereby driving a piston. A heat sink cools and contracts the gas. As the working gas is alternatively heated and cooled, a magnetized piston head moves through electrical coils in the cylinder wall thereby producing electricity.

BRIEF DESCRIPTION OF THE DRAWINGS

[010] Figure 1 shows an example of an x-ray fluorescence cascade.

[011] Figure 2 shows a cylindrical cross-section of an x-ray flourscence cascade shield of the present invention.

[012] Figure 3 shows a cross-section of a sheet of the x-ray flourscence cascade shield.

DETAILED DESCRIPTION OF THE INVENTION

[013] The RTG of the invention includes a core, a heat-producing shield, and an electricity generator. The core includes a radiation source, that is, a power source. The radiation source can include any suitable radioactive isotope such as, for example, ²³⁸Pu, ²⁴⁴Cm, ⁹⁰Sr, ²⁰⁷Bi, and combinations thereof. The power source preferably includes an isotope that produces substantial quantities of gamma/x-rays. Preferably, the radioactive isotope decays into a stable element.

[014] In embodiments, the power source can include ²⁰⁷Bi. Unlike most metals, bismuth is not toxic to biological organisms. ²⁰⁷Bi undergoes beta decay to form ²⁰⁷Pb, which is stable and not radioactive. Further, ²⁰⁷Bi does not emit alpha particles or neutrons and cannot sustain nuclear fission. Advantageously, ²⁰⁷Pb can be converted back to ²⁰⁷Bi in an industrial nuclear reactor, and has a half life of 31.55 years. The decay reaction is — ²⁰⁷Bi + e >

²⁰⁷Pbm > ²⁰⁷Pb. The electron capture conversion of ²⁰⁷Bi to ²⁰⁷Pbm and subsequent conversion to ²⁰⁷Pb produces 2.203MeV. Electron capture by Bismuth and the internal conversion produces gamma rays, but no alpha, beta or neutron radiation. The decay of ²⁰⁷Bi to ²⁰⁷Pb produces elements which have manageable chemical properties and no dangerous radiological issues.

[015] The 31.55 year half life Of²⁰⁷Bi gives the power source a useful lifespan appropriate for deep space operations and other remote location missions and applications. A power source comprising ²⁰⁷Bi does not require control rods and the levels of electric current and thermal outputs are entirely dependent upon the half-life and mass of the ²⁰⁷Bi.

[016] Without a critical mass required for operation, the process is completely scalable.

The output of a RTG comprising a power source including ²⁰⁷Bi can be scaled by adjusting the mass of the ²⁰⁷Bi to be incorporated into a specific design. Although ²⁰⁷Bi is not chemically toxic and degrades into a non-radioactive element, ²⁰⁷Bi produces gamma radiation that prior art RTGs have found problematic and substantially unusable. The amount of shielding material normally required to capture gamma/x-rays rendered a gamma/x-ray emitter power source impracticable.

[017] The present invention includes a heat-producing shield that converts gamma/x-rays into heat. The shield can include an x-ray fluorescence cascade shield comprising a plurality of layers. The layers include an inner layer around the power source, that is, a radiation source that emits gamma ray, and ending with an outer layer. The number, thickness, and composition of the layers can be altered depending on conditions such as, for example, the power source, electrical generator, the objective efficiency, and desired electrical output. The atomic number of elements in the layers will generally increase from the inner to the outer layers. Conveniently, the outer layer can include carbon in the form of graphite. Graphite is readily formable, inexpensive, and has a very high melting point.

[018] In embodiments, the shield includes a plurality of layers, and will often include at least about four layers. The layers include an inner layer around the power source and consisting essentially of a first element having a first atomic number, and an outer layer around the inner layer and consisting essentially of a second element having a second atomic number. The first atomic number will be greater than the second atomic number. It will be appreciated that the elements can include mixtures or alloys of elements having similar atomic numbers. Effectively, a prior layer absorbs radiation, converts some of the radiation's energy to heat, and then fluoresces radiation at a lower frequency to a successive layer. This process proceeds from the first layer to the outer layer from prior layers to successive layers. [019] The first element of the inner layer can include a suitably high atomic number element such as, for example, ²³⁸U, ²⁰⁹Bi, or tantalum. The inner layer can absorb gamma/x-rays from the power source, convert at least a portion of the radiation to heat, and fluoresce at least a portion of the radiation at a lower frequency to a subsequent layer. The subsequent layer can absorb the fluoresced radiation, convert at least a portion of the fluoresced radiation to heat, and fluoresce at least a portion of the radiation at a still lower frequency to a second subsequent layer. This cascade can continue through the plurality of layers until it reaches the outer layer. Figure 1 depicts a cascade of this type through four layers of a shield 1. The first layer 10, that is, the inner layer or layer closer to the radiation source, absorbs radiation and fluoresces at least a portion of the radiation at a lower frequency to a second successive layer 12, which fluoresces to a third successive layer 13, which then fluoresces to a fourth successive layer 14.

[020] For example, a shield can include an inner layer of depleted uranium. The inner layer can absorb radiation in the 1 to 2 MeV region and fluoresce in 100 to 500 KeV range to a subsequent layer. Successive layers each translate a single gamma photon into a shower of lower-energy phonons. It will be appreciated that a single gamma ray photon has sufficient energy to produce a cascade of thermal phonons.

[021] By way of further explanation, an X-ray Fluorescence Cascade Shield (XFCS) for gamma radiation shielding includes a layered structure comprising a plurality of materials. Materials possessing higher atomic number elements are oriented toward the radiation source and lower atomic number elements are oriented away from the radiation source. The inner layers of the shield selectively absorb the hard electromagnetic radiation from the radiation source and recursively translate high-energy gamma/x-ray radiation into a flux of lower energy fluorescences until such repetition results in lattice phonon radiation and heat energy. This selective absorption of electromagnetic radiation by large atomic number layers and the sequential absorption of the resultant reduced energy phonons by ever-decreasing atomic number convert high energy gamma/x-ray photons to thermal phonons. In this manner, a single gamma photon can produce large numbers of thermal phonons. [022] The highest energy phonon spectra is absorbed by the atoms with the deepest electron energy levels and re -radiated at ever lower phonon energies. The sequence of smaller atomic number layers is ordered so as to match the energy level, that is, the frequency, to be absorbed with the element needed to absorb that range of energy and re-radiate less energetic phonons for phonon absorption by the next layer.

[023] In embodiments, ²⁰⁷Bi emits a gamma photon as it decays to produce ²⁰⁷Pb. The XFCS includes an inner layer of a high atomic number element such as, for example, ²³⁸U, and the layered structure incorporates lower atomic number layers outward from the power source and the radioisotope. Each layer is of thickness sufficient to absorb an appreciable number of high energy gamma/x-ray photons. Penetration depth means the thickness of a layer capable of absorbing high energy phonon radiation and reradiating it as Bremmstrahlung (braking) radiation. Without intending to be bound by this explanation, large atomic number elements include d-orbitals and f-orbitals, and can absorb high energy gamma/x-ray photons. When the excited d-orbitals and f-orbitals drop back to unexcited or metastable states, lower energy photons are emitted as fluorescence. The remaining energy is converted to lattice phonon energy, that is, heat.

[024] Table 1 shows an embodiment of the XFCS that begins with an inner layer of a high atomic number metal and continues to an outer layer comprising carbon. This embodiment shows twelve layers selected for their instrinsic electron shell structures and their utility as a workable material. The layers have been "tuned" so that the emission frequency of a prior level matches the absorption frequency of a subsequent layer.

[025] The layers can be directly contacting but conveniently the layers can include an interposing layer to ensure chemical compatibility. The interposing layer can include, for example, a metal such as nickel or steel, or a plastic such as kapton or mylar. In one process, a polymer film is coated on successively lower atomic number layers to produce a laminate. As shown in Figure 2, in order to capture radiation from the power source of an RTG, the laminate can be rolled up to form a cylinder 20. The laminate 20 includes a fuel core 21 encased in a receptacle 22 and surrounded by a first layer of depleted uranium 23, and successive layers of bismuth-207 24, interposing layer 25, copper 26, interposing layer 27, and graphite 28. A casing 29 surrounds the laminate 20. The casing is typically metal to protect the RTG and to capture stray radiation.

[026] The XFCS cylinder includes individual layers that overlap on themselves to produce complete layers of each atomic number. Plugs or discs of the laminate can be used to close the ends of the cylinder. These solid layers and films could also be assembled in a bulkier but flexible format by assembling loose fabric weaves of metal wires, which could then be wrapped about a form in a manner identical to composite structures such as aircraft fuselages. [027] Table 1 Stackup with Z Factor Sequencing

[028] The emerging matching model is based upon the frequency of photons and phonons. Since the frequency of radiation is directly proportional to frequency, and inversely to period, a simple conversion allows for the radiation phenomenon to be treated as a frequency filter problem and so to draw upon a large corpus of filter experience. The conversion is a simple relationship, E_photon=hv, where E is the energy of a photon, h is Planck' constant (4.14xlO^~15 eV), v is frequency. Each layer can absorb incident particles and convert them to a larger number of output particles. For example, a 2 MeV gamma photon can generate 10⁷ infrared phonons.

[029] The electrical generator can include any device capable of using heat to produce electricity. The electrical generator can include, for example, a thermoelectric converter or a free-piston Stirling engine. The former, although having no moving parts, has a conversion efficiency no greater than 5%. A Stirling engine can have efficiencies of over 30%, and has demonstrated efficiencies of up to 39%.

[030] The Stirling Engine is a Carnot Cycle engine such as diesel and gasoline-powered reciprocating engines. Unlike those engines, the Stirling engine permanently contains a working gas within the engine. The working gas can include any convenient gas and preferably includes a relatively inert gas such as, for example, any one of the noble gases, fluorocarbons, or combinations thereof. A heat source expands the working gas in a cylinder and drives the piston on a downstroke. A heat sink cools and contracts the working gas permitting the piston to return to the starting position. A free-piston Stirling engine allows the piston to oscillate between the cylinder ends. The moving piston generates electrical energy typically by imbedding magnets, such as high-Gauss, rare-earth permanent magnets, in the piston body and electrical coils within the cylinder walls. The magnet and the coil are preferably electrically coupled as tightly as possible if the system is to deliver efficient performance. Accordingly, the cylinder walls should be abrasion resistant and preferably self-lubricating.

[031] In embodiments, a RTG includes about 1 kg (about 100 ml) of ²⁰⁷Bi, an x-ray fluorescence cascade shield, and a free-piston Stirling engine. The RTG can be installed in a number of applications that require long-term uninterrupted service. The RTG can be positioned and operated passively for years on submersibles, bouys, towers, blimps and other sensor platforms in littoral, riparian, mountainous, polar and aviation zones, supporting unattended missions for decades. Advantageously, the RTG can be used to charge a capacitor for bursts of power. For example, the RTG can charge a capacitor for power surges of greater than 100 Farads for radar and sonar applications. The RTG of the present invention emits no harmful radiation, is not a fissionable material, does not emit harmful chemicals, and decays to non-radioactive elements.

[032] In other embodiments, the x-ray fluorescence cascade shield can be used in place of standard shielding that relies exclusively on high atomic number. For example, an XFCS could replace lead shielding in medical or laboratory applications. As shown in Figure 3, the XFCS can be produced as a sheet. Its weight would be significantly less than that of lead. Figure 3 includes an XFCS 31 comprising a depleted uranium layer 32 facing a radiation source (not shown). Successive layers can include bismuth-207 33, copper 34 and graphite 35. Interposing layers 36 can separate the layers. The XFCS 31 can be encased in a casing 37, 38. The casing can include a nickel-steel alloy.

[033] Numerous modifications and variations of the present invention are possible. It is, therefore, to be understood that within the scope of the following claims, the invention may be practiced otherwise than as specifically described. While this invention has been described with respect to certain preferred embodiments, different variations, modifications, and additions to the invention will become evident to persons of ordinary skill in the art. All such modifications, variations, and additions are intended to be encompassed within the scope of this patent, which is limited only by the claims appended hereto.

Claims

CLAIMS:

1. An x-ray fluorescence cascade shield for producing heat from radiation characterized by a plurality of layers, each layer consisting essentially of an element having an atomic number, wherein the atomic number of a prior layer is greater than the atomic number of a successive layer.

2. The x-ray fluorescence cascade shield of claim 2 characterized by the layers of the x- ray fluorescence cascade shield being tuned so that an emission frequency of the prior level matches an absorption frequency of the successive layer.

3. The x-ray fluorescence cascade shield of any one of claims 1 or 2 characterized by the successive layer having a thickness at least equal to a penetration depth of an emission from the prior layer.

4. The x-ray fluorescence cascade shield of any one of claims 1 to 3 characterized by at least one layer of the x-ray fluorescence shield comprising depleted uranium.

5. The heat producing shield of any one of the preceding claims characterized by the prior layer comprising a first layer consisting essentially of depleted uranium and second, third and fourth layers consisting essentially of bismuth-209, copper and graphite, respectively.

6. The x-ray fluorescence cascade shield of any one of the preceding claims characterized by the prior layer comprising a first layer consisting essentially of depleted uranium and subsequent layers selected from a group consisting of bismuth-209, tantalum, samarium, neodymium, antimony, silver, niobium, copper, vanadium, aluminum, and carbon.

7. The x-ray fluorescence cascade shield of any one of the preceding claims characterized by the prior layer comprising a first layer consisting essentially of depleted uranium and subsequent layers consisting essentially of bismuth-209, tantalum, samarium, neodymium, antimony, silver, niobium, copper, vanadium, aluminum, and carbon, respectively.

8. The x-ray fluorescence cascade shield of any one of the preceding claims characterized by at least two layers of the x-ray fluorescence shield being separated by an interposing layer.

9. The x-ray fluorescence cascade shield of claim 8 characterized by the interposing layer comprising a metal or plastic.

10. The x-ray fluorescence cascade shield of claim 9 characterized by the metal comprising nickel or steel.

11. The x-ray fluorescence cascade shield of claim 9 characterized by the plastic comprising kapton or mylar.

12. A radioisotope thermoelectric generator comprising a radiation source, a heat producing shield at least partially surrounding the radiation source that converts radiation into thermal energy, and an electricity generator in thermal contact with the heat producing shield, characterized by the heat producing shield comprising the x-ray fluorescence cascade shield of any one of the preceding claims.

13. The radioisotope thermoelectric generator of claim 12 characterized by the radiation source emitting radiation consisting essentially of gamma radiation.

14. The radioisotope thermoelectric generator of any one of claims 12 or 13 characterized by the radiation source comprising ²⁰⁷Bi.

15. The radioisotope thermoelectric generator of any one of claims 12 to 14 characterized by the heat producing shield forming a cylinder around the power source.

16. The radioisotope thermoelectric generator of claim any one of claims 12 to 15 characterized by the electricity generator comprising a thermoelectric converter or a Stirling engine.