United States Patent 3666846

A process of encapsulating an alpha-emitting isotope to form an isotopic heat source. Microspheres are prepared, each containing its own helium gas plenum, an alpha-emitting isotope, and containment cladding. These microspheres are sealed in a capsule with or without a metal matrix.

Sump, Kenneth R. (Richland, WA)
Robinson, Ramon K. (Richland, WA)
Howard, Boyd D. (Richland, WA)
Drumheller, Kirk (Richland, WA)
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G21G4/06; (IPC1-7): G21C21/00
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Primary Examiner:
Padgett, Benjamin R.
Assistant Examiner:
Hellman S. R.
What is claimed is

1. A process of forming an isotopic heat source comprising preparing hollow refractory metal microspheres by forming solid microspheres of a metal, coating the solid microspheres with said refractory metal, and heating the coated microspheres whereby the shell cracks and the material of the solid microspheres can be removed through the cracks, the microspheres being formed of a metal which is compatible with tee coating metal, stable at the coating temperature, capable of cracking the refractory metal shell by thermal expansion and which can be removed from within the refractory metal shell, coating the hollow microspheres thus formed with a material containing an alpha-emitting isotope, coating the coated microspheres with a layer of a refractory metal compatible with the alpha-emitting isotope, and encapsulating the coated microspheres.

2. A process according to claim 1 wherein the refractory metal of the microspheres and the compatible cladding is tungsten and the material containing an alpha-emitting isotope is a rare earth polonide.

3. A process according to claim 2 wherein the coated microspheres are further coated with a different metal to provide strength to the microspheres and with an oxidation-resistant material.

4. A process according to claim 3 wherein the solid microspheres are formed of magnesium, tungsten is deposited on the microspheres by chemical vapor deposition, nd the magnesium is removed by heating the coated microspheres at about 1,500° C.

5. A method according to claim 4 wherein a water-cooled plate is disposed above the tungsten-clad microspheres as they are being heated to about 1,500° C. whereby a large button of magnesium forms on the water-cooled plate, the system being cooled after the button of magnesium forms, the magnesium button then being removed and the system being reheated to a temperature between 1,000° and 1,500° C.

6. A process according to claim 1 wherein the material containing an alpha-emitting isotope is deposited on the hollow microsphere by depositing a rare earth metal on the microspheres and reacting this metal with polonium-210.

7. A process according to claim 5 wherein gadolinium is deposited on the microspheres by vacuum vaporization and the gadolinium layer is reacted with polonium vapor at about 850° C.

8. A process according to claim 7 wherein the gadolinium-coated microspheres are reacted with hydrogen for two hours prior to reacting it with polonium and the polonium is pretreated by heating it at about 800° C. in the presence of tantalum.

9. A method of coating hollow refractory metal microspheres with an alpha-emitting heat source comprising exposing the microspheres to vapor of a rare earth metal until a layer of a rare earth metal is deposited on the microspheres and reacting the said rare earth metal with polonium vapor.


The invention described herein was made in the course of, or under, a contract with the United states atomic energy commission.


This invention relates to a method of making an isotopic heat source. In more detail, the invention relates to a method of making a heat source incorporating an alpha-emitting isotope in which the helium gas formed as a result of the radioactive decay of the isotope is retained in individual particles containing the isotope.

The SNAP Program -- an acronym for Systems for Nuclear Auxiliary Power -- is divided into two parts -- development of systems utilizing the decay heat from radionuclides as the energy source and development of systems utilizing nuclear reactors as the energy source. Among other radioactive isotopes employed in the first of these systems are those emitting alpha particles. Examples of such isotopes are polonium-210, curium-242, and plutonium-238. Polonium-210, for example, decays in accordance with the following equation:

84 Po21082 Pb206 + 2 α4

The alpha particle, of course, being identical to the helium atom. Designers of alpha-emitting heat sources must therefore contend with the high internal gas pressure in the heat source caused by the generation of helium when ensuring the integrity of the capsule to prevent exposure of the alpha-emitting isotope to the environment. This is a difficult and challenging task, since it is estimated that the capsule will endure environmental and thermal exposure, as follows, on reentering the atmosphere following a space mission.

For a SNAP 29 type mission

A. 135 days at 800° t0 1,200° C. in inert gas

B. one hour at 1,650° C. in an oxidizing and ablative atmosphere (arbitrarily selected temperature excursion due to mission abort or reentry)

C. X years at 1,350° C. decreasing to ambient temperature in an oxidizing atmosphere (soil burial).

For a Thermionic Application

A. X days at 1,200° to 1,800° C. in inert gas

B. one hour at 2,000° C. in an oxidizing and ablative atmosphere (reentry)

C. X years at 1,500° C. decreasing to ambient temperature in an oxidizing atmosphere (soil burial).

Isotopic heat sources have heretofore been constructed employing a very strong, heat- and corrosion-resistant capsule to withstand the pressure of the helium formed in the isotope. This pressure and the resultant cladding stress are quite high even if a void volume is supplied within the capsule to accommodate expansion of the helium.


According to the present invention, hollow microspheres containing an alpha-emitting isotope are encapsulated to form an isotopic heat source. The procedure found most suitable includes the following steps:

1. An expendable mandrel is coated with a cladding material.

2. The mandrel is removed.

3. The hollow microsphere thus formed is coated successively with:

a. a material containing an alpha-emitting isotope,

b. a layer of a metal compatible with the isotope,

c. if necessary or desirable, a layer of a metal which will provide strength to the microsphere, and, if necessary,

d. a layer of an oxidation-resistant material.

4. The coated microspheres are loaded into a thin-wall capsule with or without metal matrix powder, and the capsule is compressed.


Although the invention applies broadly to the use of other alpha-emitting isotopes, the discussion hereinafter will be restricted to polonium-210, since this isotope is peculiarly suitable to serve in a radioisotope heat source. Factors which make it particularly suitable are its high specific activity, high specific power, low shielding requirements, and relative ease of production.

In general, the process comprises preparing small particles, each including polonium-210, a void volume and an impervious cladding. It is desirable, due to strength considerations, that the particles be spherical in shape. These particles are preferably prepared by coating 125-149 μ magnesium spheres with a 7.5 μ coating of a refractory metal such as tungsten. This is accomplished by chemical vapor deposition wherein a tungsten halide is reduced with hydrogen in a fluidized bed of the magnesium spheres. After deposition of the metal on the magnesium spheres, the spheres are heat-treated in vacuum at temperatures up to 1,500° C. for several hours. As a result of this treatment, a small crack forms in the coating and the magnesium is vaporized through the crack, leaving a thin-wall, hollow, microsphere of tungsten. Other core and cladding materials could also be used. For example, microspheres of copper could be clad with tantalum. Where copper is used as the mandrel material, leaching is used to remove the mandrel.

Polonium-210 is then deposited on the hollow microspheres by first depositing a rare earth metal thereon by vacuum vaporization and then exposing the coated microspheres to polonium vapor. If desired, the rare earth metal layer may be activated by exposing it to hydrogen gas. In the vapor state the polonium reacts readily with the activated rare earth metal. Rare earth metals such as gadolinium, dysprosium, lutecium, and erbium can be used.

While the above is the preferred procedure, the rare earth polonide can also be deposited directly on the hollow microspheres by depositing the polonide downward, using an electron-beam gun or by sputtering while agitating the microspheres. As an alternate to depositing the fuel layer on the outside of the hollow microsphere, it would also be possible to prepare a hollow microsphere with an internal rare earth metal cladding. This is accomplished by cladding an expendable core with a rare earth metal followed by deposition of the refractory metal, subsequent removal of the core and reaction of the rare earth metal with polonium vapor.

Each fueled microsphere requires cladding which is compatible with the rare earth polonide and which provides the strength to resist the internal pressure generated by the helium pressure buildup from polonium-210 decay. While the same metal may be used to serve both of these functions, another procedure is to use different metals such as tungsten and rhenium. The thickness of the cladding should be about 38 μ. The metal is deposited by chemical vapor deposition, wherein a metal halide is reduced by hydrogen in a fluidized bed of the microspheres. The temperature used will depend on the particular halide employed.

If necessary or desirable, an oxidation-resistant coating can also be deposited on the microspheres. The material used must be stable during the entire life of the mission at 1,000°-1,500° C.; it must withstand high temperatures during brief excursions at atmospheric conditions; and it must be as thin as possible to avoid reduction of the power density of the heat source. Silicides, platinum group metals and the refractory oxides are possible materials. The choice is iridium. If a diffusion barrier between the iridium and tungsten is necessary or desirable, a very thin layer of zirconium oxide may be used.

The hollow microspheres formed as described are then loaded into a small cylindrical thin-wall capsule to 65 percent theoretical density. A refractory metal powder of < 44 μ particle size may be loaded into the voids among the microspheres.

Placing the fuel in the shell of a hollow thin-wall microsphere as above described results in a higher power density and operating temperature than can be attained with possible alternatives, such as one in which the isotope is present in the particle in a low density pure form or is dispersed in a porous matrix. Placing the fuel in the shell of the microspheres permits maximum void volume in the individual particle, the maximum number of spherical particles in a given area and good transfer of heat, permitting a high surface temperature with only slightly higher core temperature than in possible alternatives.

The invention will next be described in somewhat more detail, including details of certain experimental runs designed to establish optimum operating parameters for critical steps in the process.

Magnesium microspheres were obtained commercially and screened to remove all spheres smaller than 125 μ in diameter and larger than 149 μ in diameter. The spheres are then preferably heated in a fluidized bed for 30 minutes at 600° C. to improve their sphericity. A total of 140 grams of tungsten was then deposited on these microspheres. The desired coating thickness was 7.5 μ. The operating parameters for the tungsten runs were as follows:

Wf6 temperature of 50° C.

Reactor temperature of 500° C.

1 to 3 grams of magnesium spheres per run.

Wf6 flow rate of ≉80 cm3 /min.

Operating time of ≉12 minutes.

H2 flow rate of about 4 liters/min.

These coated pellets were then heated by resistance heating in a vacuum of ≉10-5 Torr at a temperature of up to 1,500° C. for several hours. Preferably a water-cooled plate is employed 1/4 inch above the container holding the tungsten-clad hollow microspheres. Heating should be slow to prevent the spheres from "jumping" and adhering to the top of the vessel. Magnesium removal begins at about 700° C. in a vacuum of ≉10-5 Torr. After a large button of magnesium is formed on the cooled plate, the system is cooled, the magnesium button removed and the system reheated to a temperature between 1,000° and 1,500° C. This procedure eliminates problems which make it necessary to restrict the amount of spheres processed in a single run.

The hollow microspheres are then fueled by coating them with 3 to 6 μ of gadolinium and reacting the gadolinium coating with polonium-210. Gadolinium is deposited on the hollow microspheres by vacuum vaporization. Vacuum vaporization is accomplished by an electron beam gun which melts the end of a 1/4-inch diameter rod as it is fed through a hole in a copper cooled block. The molten end of the rod is held just at the opening of the hole in the copper cooled block, allowing a small molten drop to form. Evaporation is from the molten drop. The microspheres are located below the evaporation device and are vibrated during the deposition. The following table gives the parameters of a number of runs on various rare earths.

Calibra- Rare Earth Average tion, Coating Rare Power, A/1000 Rate, Thickness Date Earth W Hz A/min on sphere, μ __________________________________________________________________________ 4-9 Dy -- 2780 Dy -- 2800 Dy 250 2800 840 3.5 4-17 Dy 250 2800 840 4 4-18 Dy 250 2800 840 5 4-23 Dy 250 2800 840 4 5-3 Er 200 785 Er 250 785 5-6 Er 250 780 150 4 5-14 Dy 250 2800 840 5.6 5-30 Gd 350 3200 -- 4.9 6-4 Lu 350 958 6-4* Lu 350 958 140 7.2 6-5 Er 250 770 160 4 6-10 Gd 350 1500 Gd 350 1500 150 3.32 6-11 Er 200 780 78 3 6-12 Dy 250 1590 -- 7.75 __________________________________________________________________________

The coated microspheres are then reacted with polonium vapor. To contain the polonium and prevent contamination of the polonide, the reaction is carried out in an evacuated tube reaction vessel in a tube furnace. The first GdPo reaction was carried out at 850° C. for 1 hour and with a purification time of 45 minutes at 900° C. Because the vessel was loaded in a cold furnace and brought to temperature over a period of 11/2 hours, the reaction time may actually have been longer. Ten runs were subsequently made using four different rare earths (Dy, Er Gd and Lu) and up to 0.3 gram of polonium-210 was used in a single reaction system. The fuel appeared to be of good integrity and remained on the microspheres during subsequent coating runs. Up to 1.9 × 10-6 grams of polonium were incorporated in a single microsphere in subsequent runs. Best results were attained by reacting the rare-earth-coated microspheres with hydrogen for two hours at 650° C. and pretreating the polonium by heating it at 800° C. in the presence of tantalum powder to eliminate oxygen contamination of the rare earth and of the polonium.

The fuel is contained within the microsphere by cladding consisting of a relatively thick layer of tungsten or a layer of tungsten plus a layer of rhenium. This is necessary to prevent polonium from escaping from the microspheres at high temperatures.

The first hot run was a deposition of tungsten onto GdPo using hollow tungsten microspheres as a substrate. The total substrate loading was 0.5 gram of GdPo-fueled microspheres and 5.0 grams of nickel spheres (used as filler material). The operating parameters were as follows:

Reactor temperature 600° C. WF6 temperature 45° C. WF6 flow rate 65 cc/min H2 flow rate 3.9 liters/min Operating time ≉10 min

The run was prematurely terminated when plugging of the nozzle with tungsten occurred. However, a tungsten cladding of 3 μ was successfully attained in this period of time.

Tungsten was deposited on DyPo-fueled microspheres in the second hot run. The operating parameters were:

Reactor Temperature 550°C. WF6 temperature 45°C. WF6 flow rate 60 cc/min H2 flow rate 3.9 l/min Operating time 40 min

Metallography revealed a 20 to 25 μ thick coating of tungsten. A rhenium coating was then deposited over the above tungsten coating. Operating parameters were:

Reactor temperature 250°C. ReF6 temperature 50°C. ReF6 flow rate 60 cc/min H2 flow rate 3.9 l/min Operating time 45 min

Temporary loss of the fluidized bed at the start of this run resulted in some spheres adhering to the reactor nozzle. Approximately 5 μ of rhenium was deposited.

Tungsten was deposited onto LuPo in the sixth hot run. Metallographic examination revealed a good tungsten cladding approximately 12 to 15 μ thick. Rhenium was deposited on GdPo, LuPo, and ErPo in the seventh, eight, and ninth hot runs. Metallographic examination of the rhenium-clad GdPo revealed a coating approximately 10 μ thick.

A further oxidation-resistant coating -- for example, iridium -- may also be employed. If a diffusion barrier is desired inside of the iridium this may be a very thin layer of zirconium oxide.

The coated hollow microspheres are then loaded in capsules which may be formed of tantalum.

The capsules are sealed shut and may be used as such or may be compressed. Refractory metal powder of < 44 μ particle size may, if desired, be loaded into the voids among the microspheres. An over-all density of 82 to 83 percent theoretical is obtained by using a vibratory loader to load the refractory metal powder. The matrix can then be further densified by pneumatic impaction to reduce the void volume to 21/2 percent or less. Only a little deformation of the microspheres is noted.

It will be understood that the invention is not to be limited to the details given herein but that it may be modified within the scope of the appended claims.