Title:
RADIOISOTOPE FUEL MATERIAL AND METHOD
United States Patent 3790440


Abstract:
A primary alpha-particle-emitting radioactive fuel material and a method for preparing it so that secondary neutron generation by an alpha-neutron reaction is substantially reduced. Those individual isotopes of oxygen, carbon, nitrogen, silicon, and chlorine which have a threshold for the alpha-neutron reaction greater than the maximum energy of the emitted alpha particles are selected for combination with the alpha-particle-emitting radioactive isotope to give the desired oxides, carbides, nitrides, silicides, and oxychlorides.



Inventors:
KESHISHIAN V
Application Number:
05/106071
Publication Date:
02/05/1974
Filing Date:
01/13/1971
Assignee:
ROCKWELL INT CORP,US
Primary Class:
Other Classes:
136/202, 423/249, 423/250, 423/251, 423/252, 423/253
International Classes:
B64G1/42; C01B21/06; C01B31/30; C01G56/00; G21G4/04; (IPC1-7): C01G56/00
Field of Search:
252/31.1R 23
View Patent Images:
US Patent References:



Other References:

Argo, et al.; Neutron Emission by Polonium Oxide Layers, Nuc. Sci. Abs., Vol. 10, No. 11, Abs. No. 3648, 1956, p. 464. .
Terent'Ev, Polonium Oxides, Nuc. Sci. Abs., Vol. 10, No. 17, 1956, p. 791. .
Richter, Ceramic Tubes Developed For External Heat Sources, Nuc. Sci. Abs., Vol. 15, No. 18A, Abs. No. 23676, 1961 p. 3053. .
"Snap Programs", Nuc. Sci. Abs., Vol. 15, No. 10, 1961, Abs. No. 12652, p. 1620. .
Madorsky, et al., Concentration of Isotopes of C1 by the Counter-Current Electromigration Method, J. of Research, N.B.S., Vol. 38, 1947, p. 185. .
Kistemaker et al., Proc. of the Internat. Symposium on Isotope Separation, 1958, pp. 158, 336, 337. .
Calvin, et al., Isotopic Carbon, 1949, Wiley & Sons, p. 4. .
"Stable Isotopes", USAEC Publication, 1948. .
Sheft, et al., Equilibrium in the Vapor-Phase Hydrolysis of Plutonium Trichloride, The Transuranium Elements, McGraw Hill, 1949, pp. 841-847. .
Keshishian et al., Use of O.sup.] with P.sub.[ to Reduce Neutron Yield, Trans. Am. Nuc. Soc., Vol. 9, No. 1, 1966, p. 102. .
Rutherford et al., Preparation of O Reduced in Masses 17 and 18, and Effect on Total Neutrons Emitted from PuO , Trans. Am. Nuc. Soc., Vol. 9, No. 2, 1966, p. 599-600. .
McVey, Possible Requirements for Radioisotopes as Power Sources, Nuc. Sci. Abs., Vol. 15, No. 21, 1961, Abs. No. 27914, p. 3600..
Primary Examiner:
Quarforth, Carl D.
Assistant Examiner:
Tate R. L.
Attorney, Agent or Firm:
Humphries, Lee Kolin Henry L.
Parent Case Data:


CROSS REFERENCES TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 815,131, filed Apr. 10, 1969, which is a continuation-in-part of application Ser. No. 687,945, filed Oct. 25, 1967, which is a division of application Ser. No. 464,702, filed June 17, 1965, now U. S. Pat. No. 3,515,875.
Claims:
1. A primary alpha-particle-emitting radiactive fuel material having a substantially reduced secondary neutron emission by an alpha-neutron reaction comprising a primary alpha-particle-emitting radioactive isotope combined with a component which includes a mixture of individual isotopes of a selected element containing a concentration therein in greater than its naturally occurring abundance of those ones of said individual isotopes which have a threshold for the alpha-neutron reaction greater than the maximum energy of the alpha particles emitted by said radioactive isotope whereby secondary neutron generation by said radioactive fuel by

2. A fuel material according to claim 1 selected from the class consisting of the oxides, carbides, nitrides, silicides, and oxychlorides of plutonium-238, thorium-228, uranium-232, curium-242, curium-244, and americium-241, and the oxide, nitride, and oxychloride of polonium-210, the electronegative component of the radioactive fuel material including a mixture of individual isotopes of an element selected from the class consisting of oxygen, carbon, nitrogen, silicon, and chlorine which contains a concentration therein in greater than its naturally occurring abundance of those ones of said individual isotopes having a threshold for the alpha-neutron reaction greater than the maximum energy of the emitted

3. A fuel material according to claim 2 selected from the class consisting of the oxides of plutonium-238, thorium-228, uranium-232, curium-242, curium-244, americium-241, and polonium-210 wherein the O16 isotope of the oxygen component is present in a concentration therein in greater

4. A radioactive fuel material according to claim 3 consisting essentially of Pu238 O2 wherein the oxygen component is substantially

5. A radioactive fuel material according to claim 3 consisting essentially of Po210 O2 wherein the oxygen component is substantially

6. A fuel capsule heat source wherein the kinetic energy of nuclear particles emitted by a decaying radioisotope fuel contained therein is converted to thermal energy, the radioactive fuel material being as set

7. A fuel capsule heat source wherein the kinetic energy of nuclear particles emitted by a decaying radioisotope fuel contained therein is converted to thermal energy, the radioactive fuel material being as set

8. A fuel capsule heat source wherein the kinetic energy of nuclear particles emitted by a decaying radioisotope fuel contained therein is converted to thermal energy, the radioactive fuel material being as set

9. A fuel capsule heat source wherein the kinetic energy of nuclear particles emitted by a decaying radioisotope fuel contained therein is converted to thermal energy, the radioactive fuel material being as set

10. A fuel capsule heat source wherein the kinetic energy of nuclear particles emitted by a decaying radioisotope fuel contained therein is converted to thermal energy, the radioactive fuel material being as set

11. The method of preparing a primary alpha-particle-emitting radioactive fuel material selected from the class consisting of the oxides, carbides, nitrides, silicides, and oxychlorides of plutonium, thorium, uranium, curium and americium, and the oxides, nitride, and oxychloride of polonium for an isotope fuel generator wherein secondary neutron generation by an alpha-neutron reaction is substantially reduced, comprising

12. The method according to claim 11 wherein an oxygen component having a natural distribution or the oxygen isotopes oxygen-16, oxygen-17 and oxygen-18 therein is depleted in its oxygen-17 and oxygen-18 content to form an oxygen component substantially enriched in the oxygen-16 isotope, and the enriched component is reacted with a plutonium-238 radioactive isotope to form a radioactive plutonium oxide fuel material wherein said

13. The method according to claim 12 wherein said radioactive plutonium oxide fuel material consists essentially of Pu238 O216.

14. The method according to claim 12 wherein Pu238 Cl3 is hydrolyzed in the vapor phase with O16 enriched water to form a plutonium oxide fuel material consisting essentially of Pu238 O216.

Description:
BACKGROUND OF THE INVENTION

This invention relates to a radioactive fuel material and to a method for preparing such radioactive fuel material for a radioisotope generator. More particularly it relates to an alpha-particle-emitting fuel material for a radioisotope generator wherein neutron shielding requirements are substantially reduced compared with similar generators.

Radioisotope-powered generators are known. Such units are of particular interest for space missions for supplying the power needed by the instruments of the space vehicle. These generators are also of utility in situations where there is need for a remote, unattended, long-lived small power source that is relatively impervious to conditions and hazards of its environment. Such uses include earth-based ones such as navigational aids in remote areas, communication relay stations, forest warning equipment, ocean cable boosters, and the like. This invention is also of interest for use in pacemaker heart devices and artificial hearts which are of medical interest at present for prolonging the life of individuals with certain cardiac deficienies.

For space missions, particularly manned ones, shielding requirements against radiation contribute significantly to the overall weight of the space vehicle. Astronauts present in the vehicle would require shielding not only from external radiation but also from the radiation emitted by the isotopic power unit itself.

In general, isotopes which are alpha-particle emitters are preferred for fuel use in manned mission space flights because they are relatively easy to shield against, alpha radiation being the least penetrating of all. Exemplary of such a suitable isotopic fuel is plutonium-238. However, the alpha-particle-emitting isotopes are not ordinarily usable as fuel in elemental form, but are present in the form of their compounds, alloys and mixtures so as to provide an isotopic fuel with suitable properties with respect to melting point, hardness, ease of fabrication and handling, and other related physical and metallurgical characteristics.

While alpha-particle emission per se requires but minimal shielding, secondary radiation resulting from interaction of the primary alpha particle with material in the immediate vicinity of the isotope emitter accounts for a significant increase in shielding requirements. The most important secondary source of radiation requiring shielding arises from the alpha-neutron reaction in which an element is transmuted by absorption of an alpha particle, a neutron leaving the excited nucleus.

SUMMARY OF THE INVENTION

It is an object of this invention to provide an alpha-particle emitter and a method for preparing it whereby there results minimal secondary neutron generation.

In accordance with this invention a novel fuel material is provided having utility in a radioisotope generator which includes a fuel capsule and may include a radiation shield in cooperative relation therewith wherein the radioisotope fuel material is an alpha-particle-emitting radioactive isotope combined with electronegative components which essentially have a threshold for the alpha-neutron reaction greater than the maximum energy of the emitted alpha particles. Thereby secondary neutron generation resulting from an alpha-neutron reaction is substantially reduced, with a corresponding reduction in neutron-shielding requirements. While the radioisotope fuel provided by this invention would not necessarily require a formal shield against neutrons for use in a pacemaker heart device, the reduction of secondary neutron emission would still be of considerable importance.

Preferred as radioisotope fuel materials for use in the practice of this invention are alpha-emitting radioisotopes of the actinide series combined in molecular form with particular low atomic number electronegative elements, or with selected isotopes of these elements, which have a threshold energy for the alpha-neutron reaction greater than the maximum energy of the alpha particles emitted by the radioisotope. Particularly preferred as fuel material is radioactive plutonium oxide wherein the plutonium consists essentially of the plutonium-238 isotope and the oxygen consists essentially of the oxygen-16 isotope substantially free of, or with only trace amounts of, the oxygen-17 and oxygen-18 isotopes. Ordinarily, plutonium-238 oxide will emit a primary neutron by spontaneous fission and about 15 times as many secondary neutrons by an alpha-neutron reaction with natural oxygen. Thus complete elimination of secondary neutrons will reduce overall neutron emission by a factor of 16.

BRIEF DESCRIPTION OF THE DRAWING

For a more complete understanding of the invention, reference is made to the sole FIGURE of the drawing showing a perspective view, partly in section, of an embodiment of a radioisotope generator suitable for use in practice of this invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to the drawing, which is intended as illustrative and not restrictive of the present invention, a simplified view of a radioisotope generator 1 is shown from which its principal components may be seen. An outer shell 2, usually in the form of a thin cylindrical can of metal, protects the internal components from contamination and may serve as a heat radiator where required. A radiation shield 3 is required to provide safety requirements in handling the generator during launch and following impact and also to protect astronauts present in the space vehicle during manned space flights. These shields are of high density and prevent the emission of primary and secondary radiation. Exemplary of such shields are lead, depleted uranium, and cast iron. Low density neutron shields such as lithium hydride are also required. These shields all contribute considerably to the weight of the radioisotope generator. This increase in shielding weight is a major disadvantage for space missions. Where the minimizing of shielding weight is a primary consideration, it is preferred to use radioisotopes which are alpha-radiation emitters, since alpha radiation is the least penetrating of all and requires minimal shielding provided no secondary radiation of a penetrating nature occurs.

An energy converter section 4 is used to transform part of the isotope decay heat into electricity. This may consist of an array of thermoelectric elements or thermionic converters. At the heart of the generator is the energy source, shown as a fuel capsule 5 in which a radioisotopic fuel material 6 is enclosed by a capsule wall 7.

The radioisotope generator may be of any desired shape, cylindrical shapes or spherical shapes being more common. In one type of assembly, the energy converters are placed around a space reserved for the fuel capsule. The shield is then wrapped around the converters. The outer shell, except for an end left open for fuel insertion, is soldered or welded around the shield. The fuel capsule is then usually inserted by remote control for safety reasons, and the last piece of the outer shell is then sealed in place. Almost all of the nuclear particles emitted by the decaying radioisotopic fuel are absorbed inside the fuel capable. During the absorption process, the fast nuclear particles collide with the atoms in the fuel capsule, causing them to move more violently and thus raise the capsule temperature. The kinetic energy of the particles is thereby converted to heat. Generally about 5 to 10 percent of the total heat flow, shown by directional arrows, is converted into electricity. The remaining heat energy produced by the fuel flows into the outer shell from where it is radiated or conducted to a surrounding environment.

Because of the physical and metallurgical requirements for the fuel material, an alpha-particle-emitting isotope cannot ordinarily be used in the pure elemental form. For example, Pu238 has a melting point ranging from 120°C. to 640°C., depending upon the particular crystalline structure. Whereas, in the form of PuC its melting point is 1654°C.; PuO2 melts at 2282°C., PuN 2450°C. Similarly, high melting points are shown for ThO2 3050°C., ThC 2655°C., UO2 2500°C., UN 2630°C. Thus the isotopic materials will ordinarily and preferably be used in the form of their molecular compounds, alloys, or non-stoichiometric mixtures.

It is an essential feature of this invention that secondary neutron generation resulting from an alpha-neutron reaction is substantially reduced by selecting the electronegative component that is combined with the alpha-particle-emitting radioactive isotope to have a threshold for the alpha-neutron reaction that is greater than the maximum energy of the emitted alpha particles. Generally a threshold above about 5.6 Mev is required, this threshold value varying somewhat depending upon the particular alpha-particle-emitting source.

The selection of the particular alpha-particle-emitting radioactive isotope and its combining component will be determined by many factors. As mentioned, metallurgical and physical properties play a primary role. Also of importance is compatibility of the fuel material with cladding materials, as well as its chemical stability, availability and cost. Where a space mission of brief duration is contemplated, an isotope such as polonium-210 which has a half-life of 138 days may be suitable; whereas for a Mars mission, which would require approximately two years, a longer-lived isotope, such as plutonium-238, which has a half-life of 90 years, would be required. Also the choice of isotope would be governed in part by the relative freedom of the alpha-particle-emitting isotope from other primary radiation emission such as gamma rays and neutrons.

The alpha-particle-emitting radioactive isotopes of the actinide series are generally preferred in the practice of this invention. Exemplary of suitable alpha-particle-emitting isotopes are: Pu238, Th228, U232, Cm242, Cm244, and Am241, and also Po210. Pu238 is advantageous for use as an alpha-particle-emitting radioactive source because of its long half-life and the relatively small amounts of gamma rays and neutrons which are also primarily emitted. Thus shielding requirements are considerably minimized. Examples of electronegative components with which the electropositive alpha-particle-emitting isotopes may be combined, either as molecular compounds, alloys, or non-stoichiometric mixtures, are oxides, carbides, nitrides, silicides, and oxychlorides. Preferred fuel materials for the practice of this invention are the oxides, carbides, nitrides, silicides and oxychlorides of the alpha-particle-emitting radioisotopes of the actinide series, e.g., Pu238, Th228, U232, Cm242, Cm244, and Am241, as well as the oxide, nitride, and oxychloride of Po210.

In Table I is shown the threshold energy for the alpha-neutron reaction for the relatively light weight natural isotopes. The relative abundance of these natural isotopes is also shown.

TABLE I

Threshold Energies for Alpha-Neutron

Reactions in Natural Isotopes from Li to Ni

Threshold Energy for Isotope % Abundance (α, n) Reaction (Mev) Li6 7.4 4.85 Li7 92.6 5.25 Be9 100 0 (Q pos.) B10 18.8 0 (Q pos.) B11 81.2 0.22 C12 98.9 11.25 C13 1.1 0 (Q pos.) N14 99.62 6.1 N15 0.38 8.15 O16 99.76 15.2 O17 0.04 0 (Q pos.) O18 0.20 0.86 F19 100.0 2.35 Ne20 90.52 6.7 Ne21 0.27 0 (Q pos.) Ne22 9.21 8.52 Na23 100.0 3.47 Mg24 78.6 8.37 Mg25 10.1 0 (Q pos.) Mg26 11.3 0 (Q POS.) Al27 100.0 3.05 Si28 92.3 8.25 Si29 4.7 1.72 Si30 3.0 3.95 P31 100.0 6.58 S32 95.1 9.77 S33 0.74 0.75 S34 4.2 5.06 S36 0.0136 3.40 Cl35 75.4 6.52 Cl37 24.6 4.28 A40 99.632 4.28 K39 93.1 7.77 K41 6.9 3.72 Ca40 96.97 No data Ca42 0.64 6.58 Ca43 0.145 0 (Q pos.) Ca46 0.0033 0.25 Ca48 0.185 0.15 Sc45 100.0 2.45 Ti46 7.94 4.75 Ti47 7.75 0.38 Ti48 73.45 2.92 Ti49 5.52 0 (Q pos.) Ti50 5.34 1.92 V50 0.23 0.20 V51 100.0 2.45 Cr50 4.49 5.30 Cr52 83.78 3.85 Mn55 100.0 3.80 Fe54 5.81 5.95 Fe56 91.64 5.43 Fe57 2.21 1.4 Fe58 0.34 3.84 Co59 100.0 5.42 Ni58 67.7 10.36 Ni60 26.2 8.1 Ni61 1.2 4.17 Ni62 3.7 6.8 Ni64 1.2 4.9

In accordance with the teaching of this invention, the electro-negative component forming the oxides, carbides, nitrides, silicides, and oxychlorides will include a mixture of the individual stable isotopes of oxygen, carbon, nitrogen, silicon, and chlorine, these individual isotopes having different thresholds for the alpha-neutron reaction both less than and greater than the maximum energy of the alpha particles emitted by the radioactive isotope. However, this mixture is treated to contain a concentration in greater than its naturally occurring abundance of those ones of the individual isotopes which have a threshold for the alpha-neutron reaction greater than the maximum energy of the emitted alpha particles, i.e., O16, C12, N15, Si28, and Cl35. Hence, by reacting the electronegative component containing the concentrated isotope with the electropositive component containing the alpha-particle-emitting radioactive isotope, the formed primary alpha-particle-emitting radioactive fuel material will have substantially reduced secondary neutron generation as a result of an alpha-neutron reaction. If the alpha-emitting isotope is used in the form of its carbide, e.g., uranium carbide, the C13 isotope would be eliminated by chemical or physical treatment either prior to or subsequent to the formation of the compound so that the carbide would be substantially free of the C13 isotope and would consist almost exclusively of the C12 isotope, e.g., U232 C12. Thereby, since the threshold energy of the C12 isotope is 11.25 Mev (Table I), no alpha-neutron reaction would occur and secondary neutron emission would be substantially reduced or eliminated. Similarly, if plutonium oxide (PuO2) were used, the radioactive fuel would consist essentially of Pu238 O216 since the threshold energy of O16 required for the alpha-neutron reaction is 15.2 Mev; whereas the threshold energy of the O17 and O18 isotopes is considerably less. This radioisotope fuel, Pu238 O216 is particularly preferred therefore in the practice of this invention because of its elimination of secondary neutron generation as well as its desirable chemical and physical properties.

Generally, the desired electronegative isotope will be enriched in greater than its naturally occurring abundance prior to formation of the desired fuel. Thereby, standard chemical reactions which are well known in the art can then be used to prepare the oxides, carbides, nitrides, silicides, and oxychlorides of the desired alpha-particle-emitting radioisotopes of the actinide metals, and the oxide, nitride and oxychloride of polonium. The enriched electronegative isotopes of oxygen, carbon, nitrogen, silicon and chlorine are either available in the form of directly usable compounds, or can readily be converted to usable compounds by standard chemical reactions.

The stable isotopes of the elements of the electronegative component that are utilized for compound formation are not only selected from the class of oxygen, carbon, nitrogen, silicon, and chlorine isotopes, but also will have present in greater than its naturally occurring abundance those individual isotopes of these electronegative elements which have a threshold for the alpha-neutron reaction greater than the maximum energy of the emitted alpha particles. Referring to Table I, it is seen that the electronegative component would therefore be concentrated in the following isotopes: O16, C12, N15, Si28 and Cl35.

The following enriched isotopes are available from the Isotopes Development Center of Oak Ridge National Laboratory, operated by Union Carbide Corporation for the U.S. Atomic Energy Commission: carbon-12 in inventory form as elemental carbon in an isotopic abundance of greater than 99.9 percent (naturally occurring abundance 98.892 percent); chlorine-35 in the form of NaCl in an isotopic abundance of greater than 98 percent (naturally occurring abundance 75.529 percent); silicon-28 in the form of SiO2 in an isotopic abundance of greater than 98 percent (naturally occurring abundance 92.21 percent).

Oxygen-16 is obtainable in the form of O16 -enriched water or O16 -enriched oxygen gas. Water enriched in the oxygen-16 isotope is obtainable from Volk-Isotopes, Westwood, New Jersey, with a depleted content of 0.007 percent O17 and 0.007 percent O18, with the balance O16. Oxygen-16 is conveniently obtained in gaseous form by electrolysis of the O16 -enriched water. The resulting enriched oxygen-16 content is 99.986 percent compared with the naturally occurring abundance in oxygen of 99.759 percent O16.

Nitrogen-15 is available from the Isomet Corporation, Palisades Park, New Jersey, in 99 percent enrichment in the form of NH3, compared with its naturally occurring abundance of but 0.38 percent. Nitrogen-15 can be prepared by a variety of methods. The Nitrox process involves exchange of NO with HNO3. See L. Gowland and T. F. Johns, "A Laboratory-Scale Plant for the Enrichment of 15 N Using the `Nitrox` Process," AERE-Z/R-2629 (1961), Atomic Energy Research Establishment, Harwell, England. The principal reaction is

N15 O(g) + HN14 O3(soln) ➝ N14 O(g) + HN15 O3(soln)

Other methods are ammonia-ammonium ion exchange reactions and distillation of NO.

Various standard chemical reactions may be utilized for preparing the desired oxides, carbides, nitrides, silicides, and oxychlorides. Americium metal forms the same compounds with electronegative elements of Group III to VII as do the other actinides. The pink sesquioxide Am2 O3 and the black dioxide AmO2 are known, as well as a number of non-stoichiometric oxides. The dioxide may be obtained by ignition of americium compounds in oxygen-16 gas.

Polonium oxide can be prepared by reacting the metal with oxygen-16 gas at 250°C. according to the reaction

Po + O2 ➝ PoO2

This method for the preparation of the oxide is reported by M. Haissinsky in "Polonium" MLM-1165 (tr)(1964), Mound Laboratories, Miamisburg, Ohio. Thorium and plutonium oxides can be prepared by the same reaction except at a higher temperature, 700°C.

Thorium and plutonium carbides can be prepared by arc-melting the metals with carbon-12, which is the form supplied by Oak Ridge. The reactions are M + C ➝MC where M is plutonium or thorium.

The carbides of americium and of curium may be similarly prepared since both metals have been prepared in macco (multigram) amounts. See L. B. Asprey et al "The Chemistry of the Actinides" Chem. and Eng. News, pages 75-91 (July 31, 1967). Since americium and curium from some of their other reactions are seen to react like true actinides, a straightforward preparation of the carbides would be to arc-melt the metal with the carbon-12 available from Oak Ridge. The reactions are

Am + C ➝AmC

and

Cm + C ➝CmC

which take place rapidly above the melting point of the carbide. Another standard procedure is to hydride the metals mix with carbon, press and sinter. The reactions are

Am + H2 ➝ AmH2

AmH2 + C ➝AmC + H2

and

Cm + H2 ➝ CmH2

CmH2 + C ➝ CmC + H2

The preparation of americium nitride has been reported by K. Akimoto in J. Inorg. and Nuclear Chem., Vol. 29, pages 2650-2652 (1967) by the reaction AmH2 + NH3 800°C AmN + 2.5H2. Utilizing this reaction, the hydrides of americium and curium may be reacted with ammonia which is supplied 99% enriched in the nitrogen-15 isotope to form the respective nitrides as follows:

AmH2 + NH3 ➝ AmN + 2.5H2

CmH2 + NH3 ➝ CmN + 2.5H2

Thorium and plutonium nitride can be prepared by reacting the metals with ammonia which is supplied containing nitrogen-15. The reactions are

Pu + NH3 ➝ PuN + 1.5H2

and

3Th + 4NH3 ➝ Th3 N4 + 6H2

M. haissinsky, supra, has also reported the preparation of the polonium nitride Po3 N7 by decomposition of ammonia hexabromo polonite at 200°C.

The standard preparative techniques listed in "Refractory Hard Metals" by P. Schwarzkopf and R. Kieffer, Macmillan, New York, 1953, may be used for the preparation of the actinide metal silicides, i.e., those of Pu, Th, U, Cm and Am. For example, using SiO2, the form in which the enriched silicon-28 isotope is available from Oak Ridge, the reaction MO2 + SiO2 + 4C ➝MSi + 4CO↑ which is carried out at elevated temperature in vacuum can be utilized. Also, the SiO2 can be converted to the metal by the reactions

SiO2 + 4HF➝SiF4 + 2H2 O

and

SiF4 + 4Na ➝Si + 4NaF

The product silicon-28 can then be arc-melted with the metal actinide

M + Si ➝ MSi

as described in the General Procedure and Monthly Progress Report, ANL 6658 (1962), Argonne National Laboratories, Argonne, Illinois.

The preparation of americium oxychloride is described by G. T. Seaborg and J. J. Katz in The Actinide Elements, New York, McGraw-Hill, 1954, page 513, utilizing the vapor phase hydrolysis of AmCl3.

AmCl3(s) + H2 O(g) = AmOCl(s) + 2HCl(g)

Alternatively, to prepare AmOCl and CmOCl, HCl gas is prepared from NaCl, which is supplied by Oak Ridge with enriched Cl35, by the reaction

NaCl + H2 SO4 ➝NaHSO4 + HCl(g)

Then the reaction AM+++ (in solution) + 3(OH)-➝Am(OH)3 would be carried out followed by Am(OH)3 + 3HCl➝AmCl3 + 3H2 O using the enriched HCl gas. The hydrolysis reaction would then be carried out using water prepared from oxygen-16,

AmCl3 + H2 O ➝AmOCl + 2HCl

according to the procedure outlined above. The same reaction would be used to prepare CmOCl.

Plutonium oxychloride can be prepared by the following sequence of reactions:

Pu+++(in soln) + 30H-➝Pu(OH)3

Pu(OH)3 + 3HCl ➝PuCl3 + 3H2 O

using HCl enriched in chlorine-35.

PuCl3 + H2 O(g) ➝PuOCl + 2HCl(g)

using H2 O enriched in oxygen-16. In the case of thorium, the reactions are

Th+4 (in soln) + 4OH-➝ Th(OH)4

Th(OH)4 + 4HCl ➝ThCl4 + 4H2 O

The ThCl4 is then heated with water vapor

ThCl4 + H2 O(g) ➝ThOCl2 + 2HCL(g)

to give the product.

All of the thorium and plutonium compounds are discussed in The Actinide Elements, supra.

Polonium oxychloride PoOCl2 has been referred to in Nuclear Science Abstracts, Vol. 19, pages 24524 (1965).

In general, the oxides, carbides and silicides of the actinides are high melting, or stable to high temperature. ThO2 melts at .about.3000°C., PuO2 is stable to 1000°C. but apparently converts to a lower oxide above 1000°. Am2 O3 and Cm2 O3 are probably stable to at least 2000°C. ThC melts at 2650°C. PuC decomposes above 1650°C. and AmC and CmC should be stable to at least this temperature. PuSi is stable at 1500°C. so the other silicides should also be stable to this temperature. The actinide nitrides probably will decompose above 1000°C. The oxychlorides are in general prepared by hydrolysis with water vapor above 600°C. so they are stable to at least 500°C.

Maximum benefits in reducing shielding requirements are obtained where there is present as combining component only the table isotope of the electronegative element which haa threshold for the alpha-neutron reaction greater than the maximum energy of the alpha particles emitted by the radioactive isotope. However, substantial improvement in shielding requirements is obtained even where lesser or trace amounts are present of the undersired isotopes which have low threshold values for the alpha-neutron reaction.

For purposes of illustration, without in any sense being limited thereby, a specific method of practicing this invention, and the advantages obtaining thereby, will be illustrated with reference to the radioactive isotopic compounds Pu238 O216 and Po210 O216.

EXAMPLE 1

Preparation of Enriched Oxygen-16

Enriched oxygen-16 is conveniently prepared by electrolysis of heavy water, D2 O, which is enriched in the heavier oxygen isotopes. This heavy water process is followed by hydrogen sulfide exchange and then followed by distillation to yield a product containing 90% D2 O. The 90% D2 O is then electrolyzed to produce 99.75% D2 O. During electrolysis, the lighter O16 isotope comes off initially as a gas, the heavier oxygen isotopes concentrating during this production of D2 O by electrolysis. Using such a process H2 O is obtainable with a depleted content of 0.007% O17 and 0.007% O18, with the balance O16. Oxygen-16 is then conveniently obtained in gaseous form by electrolysis of this O16 -enriched water.

EXAMPLE 2

Preparation of Pu238 O216

a. By reaction with Plutonium Metal

Plutonium metal is produced on a continuous basis by electrolysis. The plutonium metal is then heated in O16 gas at a temperature of 400°C. to form PuO216.

b. From Plutonium Oxide by Hydrofluorination

PuO2 containing nautral oxygen and prepared by low firing at a temperature below 480°C. is converted to PuF4 by hydrofluorination at 450°C. The PuF4 is reduced to Pu metal by reaction with calcium and iodine. The Pu metal is transferred to a closed system and converted to PuO216 as above described.

c. From Plutonium Oxide by Treatment with Phosgene

Plutonium oxide containing natural oxygen is heated at 400°C. with phosgene to convert it to plutonium trichloride. The chloride is then reduced with calcium and iodine to form plutonium metal which is then converted to PuO216 by reaction with O16 gas as above described.

d. From Plutonium Trichloride.

The plutonium trichloride is hydrolyzed in the vapor phase with O16 -enriched water to form PuO216. The reaction that occurs is as follows:

PuCl3 + 2H2 O➝PuO2 + 3HCl + 1/2H2.

EXAMPLE 3

Decreased Neutron Yield from Alpha-Neutron Reaction

Two samples of radioactive polonium-210 are compared. The samples are obtained as PoCl4 deposited on glass. One sample is dissolved in distilled water containing oxygen of natural abundance (99.759% O16, 0.037%O17, 0.204%O18) and the other in water enriched in O16 (.007% O17, 0.007% O18, balance O16). A small amount of HCl is added to both solutions to prevent deposition of Po on the walls of the container. The two solutions are then made to the same volume in the same size containers so that the ratio of the count rates observed with a neutron in a fixed geometry is the desired ratio of the neutron production rates. The neutrons from the source solution are thermalized with paraffin so as to permit use of a boron trifluoride neutron detector. This detector is efficient to about 10 percent for neutrons entering the sensitive volume. The cross-sectional area for the detector is selected to be about 20 cm2. For a desired count rate of 1,000 counts/min. from the solution of normal water, and a 3-cm thickness of paraffin, about 105 neutrons/min. is required of the radioisotope source. One curie of Po210 will produce 1.33 × 105 n/min. Using a curie of Po210 for each of the samples, approximately 1,000 counts/min. is obtained for the solution of normal water and approximately 1,000/30 or about 30 counts/min. for the O16 -enriched water (i.e., O18 depleted water by a factor of 30). Polonium-210 emits alpha particles with an energy of 5.3 Mev, compared with 5.5 Mev for Pu238.

According to the data of SERDIUKOVA et al., "Investigation of the (α,n) Reaction on Oxygen," Bull. Acad. Sci. USSR (Phys. ser.) Vol. 21, p. 1018 (1957), the neutron source from an alpha-neutron reaction on oxygen is proportional principally to the O18 concentration. In the present process, by depleting both the O17 and O18 isotopes, a significant improvement in reducing the emission of secondary neutrons is obtained by using an O16 -enriched radioactive alpha-emitting oxide.

Where pure Pu238 O216 is used, with the O17 and O18 isotopes of oxygen completely eliminated, the overall neutron source can be reduced by a factor of 16. But even with available O16 -enriched water, where the water is depleted to a content of 0.007% O17 and 0.007% O18, there is a reduction of total neutron emission by a factor of about 10, or a reduction in the secondary alpha-neutron source by a factor of about 27. Thus in one design where an 11-inch thick lithium hydride neutron shield for a Pu238 oxide isotope source is required using natural oxygen, a reduction in shield thickness to 5 inches is obtained by using the above available O16 -enriched source. Thus for a space vehicle using as isotope source Pu238 O2 containing normal oxygen, the 11-inch thick lithium hydride shield would weigh about 1000 pounds. With Pu238 O2 containing the enriched O16, 5-inch thick shield weighs about 400 pounds, resulting in a saving in weight of 600 pounds. This reduction in weight is of course highly significant in a space mission.

While the principles of the invention have been described above in connection with specific materials and processes, it is to be clearly understood that this description is made only by way of example and not as a limitation to the scope of the invention as set forth in the objects thereof and in the accompanying claims.