DE2523863A1 - RADIO ISOTOPE HEAT SOURCE AND METHOD FOR PRODUCING IT - Google Patents

Description

PATENT ADVOCATE

cable: DIPL-ING. KARL H. WAGNER telephone

PATLAW 8000 MUNICH 22. (089) 29 85

ST. ANNAPLATZ 6 (WEST GERMANY)

R-474,548

United States Energy Research And Development Administration, Washington, D.C. 20545, U.S.A.

Radioisotope heat source and process for its production.

The invention relates to a radioisotope heat source and to a method for a long-lived electric generator.

Radioisotope heat sources are used in many applications where a relatively long life is required. These uses include the generation of thermal energy for thermoelectric Energy conversion generators, heating devices and the like in the field of space travel, military, medical and similar use cases.

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Because of the radiological and toxic nature of many of the radioisotopes used in such heat sources, strict safety conditions are required to ensure that the radioisotopes remain in the heat source under all normal operating conditions, including any abnormal conditions that may arise. As the thermal energy and / or useful life requirements increase, the inclusion of radioisotopes becomes a greater problem. This inclusion is also difficult when radioisotopes, such as are used plutonium that react undesirably with numerous container materials, and ί, ^ άτ especially when operating or construction temperature and under ambient conditions, which may be exposed to the heat source.

Are plutonium radioisotopes, and particularly plutonium-238 attractive as long-life radioisotope heat sources because of the long half-life of the isotope and because of the fact that these are predominantly an alpha emitter and only a limited emitter of more penetrating radiations. However, plutonium is extremely poisonous to humans when inhaled even in small amounts and anyone who uses plutonium The heat source must be designed to withstand the internal temperatures or pressures generated by the radioisotope itself be withstood without a harmful reaction, and moreover, such a heat source must withstand every possible shock or withstand other loads and temperatures that come from outside the heat source.

Many of the common high temperature and / or high strength metals or other materials normally used for containers or shock resistant members are adversely affected in their containment properties when in a radioisotope heat source application can be used, which makes them unsuitable for such applications. In addition, many of these materials are used

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not readily welded or otherwise brought into the desired heat source shape or consist of or contain materials that react with the radioisotope or other materials commonly used in conjunction with it.

Radioisotope powered thermoelectric generators are currently used in various aerospace and similar applications used. A typical generator, such as that described in U.S. U.S. Patent 3,388,008 can be approximately generate an electrical output of 1 milliwatt using wire thermocouples. These generators can be used in their electrical energy output, in the thermal energy generated by their heat source and / or in their Lifetimes be limited. If attempts are made to increase the electrical or thermal energies, so take it generator lifetimes generally decrease due to the more severe environmental conditions encountered by the radioisotope containing materials. The increase in service life, thermal energy and electrical output variables generally suggests the use of extremely rare and expensive materials and machining processes in manufacture the device, which increases the cost of the heat source excessively, and alternative power supplies makes it more attractive. Chemical power sources can be considered, but generally have an essential one shorter life than is desirable and they must be replaced or they can in such applications cannot be used where the power source is not readily accessible. In addition, the chemical power sources are generally relatively large and heavy compared to comparable radioisotope generators.

The present invention aims to provide a plutonium radioisotope heat source with a long life and to provide high reliability. Another object of the invention is to provide a plutonium radioisotope heat source which is extremely safe radiologically and biologically.

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The invention also provides such a heat source that can be used in a thermoelectric generator. The invention also relates to a method for producing such a heat source.

Further advantages, objects and details of the invention result from the description, the most important features being specified in particular in the claims.

The invention thus relates to a radioisotope heat source and to a method for producing one Source in which bits of plutonium dioxide are mixed with particles of yttrium within a sealed layer of tantalum alloy or layers, with a heat treatment at a temperature of about 1570 K to about 1720 K. for at least about 1 hour, followed by encapsulation in an outer nickel alloy composite he follows.

In the following the invention is described with reference to the drawing; in the drawing shows:

1 shows a cross section of a thermoelectric generator which contains the heat source according to the invention;

FIG. 2 shows a cross section of the radioisotope heat source used in the generation of FIG. 1;

Figure 3 is a graph of the hardness of the tantalum alloy liner walls without the treatment prescribed by the invention;

Figure 4 is a graph of the hardness of the tantalum alloy liner across its cross-section, namely after the heat source certain heat treatments and Operations has been suspended.

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The invention aims to provide a thermoelectric radioisotope generator with a life of up to about 15 to 16 years with an electrical output of about 25 milliwatts or more in a radiologically safe form. Such a generator can, for example the radioisotope operated thermoelectric generator 10 shown in FIG. The generator 10 For example, a radioisotope heat source 12 - which will be described in detail with reference to FIG. 2 - may be suitable supported within a cavity in a cylindrical insulator block 14 and an outer jacket or an outer layer 16 have. The heat source 12 can be pressurized against a suitable planar thermocouple assembly or thermopile (Thermopile) 18 are held by a spring biasing member 20 via an insulating plug 22, which is in a bore of the Isolator block 14 is supported at one end of the generator 10. A transition element 24 is between the heat source 12 and the thermopile 18 to provide good thermal contact therebetween, whereas the thermopile is electrically isolated from the heat source. The thermopile is supported by an end wall 26, a mounting block 28 and a transition member 30, with the end wall 26 suitably attached to the shell 16 to provide a rigid Form support for the mounting block 28 and the transition member 30. The transition link 3O generates just like that Transition member 24 has good thermal conductivity to the mounting block 28, whereas there is electrical insulation causes the thermopile opposite the mounting block. The electrical connection to the thermocouples in the Thermopile 18 can be made by a suitable electrical connection and conductor 32 or a circuit board 33. The inner of the generator 10 can be evacuated or pumped down in a suitable manner and then again with an inert gas, such as xenon, for example, to be filled in order to stabilize the thermal conductivities through the pinch tube 35.

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In order to meet the requirements with regard to the 25 milliwatts of electrical power, the thermocouples used require an extreme aspect ratio (i.e. cross-sectional area to length ratio) do this, as is the case with the resistivity, the Seebeck coefficient, the temperature differential, the load voltage and electrical power requirements are determined. A typical thermopile assembly 18 is shown in U.S. Pat. Patent application Serial No. 290,685 dated September 20, 1972. In this thermopile, the thermocouples are a tall one silicon-germanium alloys exhibiting resistivity, which are formed into wafers approximately 0.18 mm thick and 2.5 3.8 mm in size. The tiles are with each other connected in a stack by a suitable glass material using suitable interconnections at opposite ends of the platelets to form the desired number of thermocouples. In the described Generator 10 can use one or two stacks of approximately 26 wafers at a time and bond them together to provide 25 milliwatts of electrical power from each stack at a thermal differential of about 700 ° K.

As mentioned above, the heat source 12 is in firm contact with the thermopile 18 through transition member 24 and the Spring biasing member 20 pressed. To ensure the correct thermal contact over the life of the generator 10 between To maintain the heat source and the thermopile, the spring biasing member 20 should be designed so that a relative constant spring pressure is maintained even when the isolator plug 22 in its dimensions in a should be unstable to some degree. Other biasing arrangements can also be used, such as one Tension rod or a combination of tension rods arranged around the heat source 12 and with it and the base plate 26 are mechanically coupled to generate the desired compressive forces over the life of the generator are constant across The transition members 24 and

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30 disks made of boron nitride, glass or ceramic materials which produce the desired thermal conductivity and electrical insulation. Boron nitride can be used as a block or on a suitable substrate, such as thin platelets of gold, copper or alloys of nickel or the like, and similar arrangements.

The electrical interconnections between the thermopile 18 and the connection 32 can be formed by suitable wire conductors made of materials such as gold or gold-plated Alloys can be made or they can be made using an interconnect in the form of a printed circuit and a conductor system can be provided, which, for example, aluminum oxide substrates with chrome-gold or Platinum-gold ladders used. The insulator block 14 and the plug 22 can be made of suitable high temperature thermal insulating material such as blends of 75% silica powder, 9% silica fiber, and 16% titanium oxide opacifier. The outer jacket 14 and the end plate 26 can be made of suitable stainless steels or the like exist that are compatible with the environmental conditions to which the generator 10 is to be exposed.

The heat source 12 shown in detail in Figure 2 comprises a radioisotope-containing heat source material or Fuel mixture 34, which may be enclosed within a sealed liner or container 36, with an additional reinforcing member 38 being provided or not. The container 36 and the reinforcement member 38 may in turn be enclosed in a sealed outer jacket 40 and inserted therein to provide a complete To form heat source 12 capable of keeping the radioactive material for extended periods of time, for example for up to 16 years and more, to contain without the release of radioactive material as a result of the decomposition of the container under normal operating conditions

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or any accident conditions that may arise. The container 36, the reinforcement member 38 and shell 40 may be of the same general shape and form with a generally hollow cylindrical body portion 42 be formed, which at one end by an arcuate or hemispherical end part 44 is closed, which - if desired - can be somewhat flattened, and the other end being closed by a flat end cap which is suitably connected to the cylindrical body part 42 is welded. The shape shown minimizes the number of welds to be provided and allows that the cylindrical part 4 2 and the arcuate or hemispherical parts 44 by relatively simple and cheap hydroforming or deep-drawing processes can be produced with just a few machining operations. Such a shape and the right one Training the same can also minimize the work and mechanical stress points that are normally machined in or otherwise trained limbs and which often act as a point of failure in long-term use in High temperature environments are effective with materials that tend to oxidize the container materials reduce or otherwise attack.

Because of the possible proximity of the heat source 12 to radiation-sensitive Materials, such as electronic circuits, and towards people it is useful to have a To use a radioisotope that requires little, if any, radioactive shielding. An alpha emitting isotope, such as plutonium-238, is due to its low or nonexistent beta emission and gamma radiation, and also because of its relatively high power density and half-life. Plutonium-238 has a half-life of approximately 88.4 years, and in the dioxide form it can have a specific power of approximately 0.4 watts / gram. To check the reactivity of the plutonium-238 radioisotope with protective materials To reduce it, it is preferred that the isotope be in a cermet or ceramic form. It was found that a high power density by using

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of plutonium oxides, mainly - can be obtained at least initially in the form of plutonium dioxides, though Carbides and nitrides provide similar power densities under some operating conditions.

Should the integrity of the protective layers of the heat source 12 be compromised, it is desirable that the isotopic material 34 is in a form that allows for the formation of small particles, i.e. particles of less than about 3.4 microns in size, minimized as these particles are especially dangerous to humans because they can be could be inhaled accidentally. The plutonium oxide fuel in material 34 may be in the form of a solid pressed Pellets or in the form of irregularly shaped particles, the latter as pieces or splinters (Shards) that have random sizes (determined by sieving) of approximately 50 to 500 micrometers. These Bits can be made with much less processing and, as a result, much less cost than other forms of plutonium fuel. It was determined, that these bits or shards, although in particulate rather than solid form, are no longer the ones above The above-mentioned fine particles form as individual pellets or pelletized fuel, and they are also lighter and to be introduced into the heat source 12. Plutonium oxide fuel, which is 80% enriched in the plutonium-238 isotope, can typically do about 14,000 neutrons / gram second and emit gamma at a peak of about 6 rad / year. The neutron flux can be approximately 3500 to 4000 neutrons / gram

Second by sintering the plutonium oxide pieces in a stream of oxygen-16.

In addition to the plutonium oxides in material 34, a getter material can be mixed therewith, for example Yttrium or hafnium metal particles, which are the most stable oxide

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forming agents are compatible with the materials used for container 36 and reinforcing member 38 are. The plutonium oxides can be used as plutonium dioxide between about 96.8 and 92.9 percent by weight and between about 3.2 and 7.1 percent by weight for the getter material. Yttrium is preferred as the getter material because it is simply in Air is to be handled, and since it acts as a getter for hydrogen, carbon and nitrogen, what in small amounts is called organic There may be impurities in the fuel material 34, is more effective. The getter material should preferably be in particulate or Be in chunk shape and the dimensions should be approximately 3.0 3.0 0.25 mm or smaller. Bigger pieces or metal foils from the getter material may not be able to sufficiently reduce the plutonium oxide fuel cause.

Materials for the container 36, reinforcement member 38 and jacket 40 must take into account the corrosion and oxidation resistance of the materials in the particular environments in which they will be used, including mechanical properties such as tensile strength at high temperatures ( above 1200 ⁰ K) and impact resistance, manufacturability and weldability must be taken into account. The container and reinforcement member 38 must be resistant to corrosion and oxidation caused by the heat source material and the contaminants trapped therein, whereas the jacket 40 should be resistant to corrosion or other attack coming from materials or environments outside of the Heat source 12 are located, such as against fresh water and sea water, air, etc. In view of this, the container materials must be able to remain operational for the life of the heat source, namely at the operating temperatures of the heat source and also at temperatures in excess of that caused by fire, reentry from space and the like, and in addition, a high impact

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resistance should be present in the event that the heat source 12 hits an object at high speed even after use over its lifetime under normal operating conditions and in addition to the fact that abnormal or accident conditions have occurred .

The container 36 can serve as a "sacrificial" layer, the one has sufficient strength to react with any contaminants or other materials contained within the Heat source may be included in the radioisotope fuel material 34 for the life of the heat source 12 away, so that only the container 36 is adversely affected. The container 36 is preferably made of the same material as the reinforcing member 38, to simplify compatibility, or can be formed in a single container (container). It was determined, that a material which has the durability properties desired for the heat source is based on tantalum solid solution based alloys, particularly the tantalum-tungsten-hafnium alloy of approximately 90 percent by weight Tantalum, 8 percent by weight tungsten and 2 percent by weight hafnium. Other similar alloys can be used in heat sources, but with somewhat lower strengths, lifetimes and the like. It was recognized that although such tantalum alloys have the tendency to absorb oxygen at elevated temperatures, thus embrittling, the difficulty is then overcome when the training is carried out according to the present invention, and then over extended Can be used for periods of time without significant degradation by the fuel mixture. Of the Container (liner) 36 of this material is typically down to a thickness of about 0.5 mm in the illustrated Form deep drawn and comes with a lid. 36a of similar thickness and made of the same material, the Lid is welded to it. The reinforcing member 38 can be brought into a similar shape in a comparable manner,

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in which the container 36 is inserted, but the thickness is about 1 to about 1.5 now and that Reinforcing member 38 in turn closed by an end cap 38a of similar thickness and made of the same material is, namely the cap 38a is welded to the member 38.

The layer or lining material for the jacket or the lining 40 can be one which is corrosion-resistant at high temperatures Material or an alloy, such as certain nickel-based alloys, which have the desired formability and weldability. It was found that a suitable alloy with these desired properties would be approximately 14.5 to 16.5 chromium, 15 to 17% molybdenum, 3 to 4.5% tungsten, 4 to 7% iron, 2.5% cobalt with small amounts of Mn, Si, V, C, P and S. with the balance of about 55% being nickel. The jacket can be cylindrical and hollow throughout having a closed arcuate hemispherical end Form are brought by deep drawing, in which the reinforcing member 38 can be used and the open end is sealingly closed by an end cap 40a which is made of a material similar to the jacket 40 exists. The jacket thickness can typically be from about 0.5 to 1.5 mm.

The three-layer capsule or heat source arrangement described is not only desirable for safety reasons, but also for reasons of manufacture and handling. With a three-layer heat source, there are two layers in between the external environment and the contaminant-removing external surface of the container (liner) 36. This may be preferable, although the said materials have a high level of resistance to contamination or have attack by other materials and the environment to which the heat source 12 may be exposed if it is treated in the manner described above. When the materials themselves at a decreased or low rate be attacked over the life of the heating source, so

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this can be compensated for by a sufficient thickness of the container materials (container materials) to accommodate all survive potential environments to which the container may be exposed. Generate the strengths given above a more than sufficient amount of protective materials to allow the heating source to operate for a 16-year lifetime.

When arranging the heating source 12, after each of the container elements and the end caps are suitably formed, the desired mixture of pieces of getter material or particles and plutonium oxides mixed in the amounts and in the form indicated above. Then the Fuel mixture 34 poured into container (liner) 36 or introduced in another way and a disk 4 made of the same material as the container 36 is used as a cover the fuel mixture 38 is used during welding. The insertion of the disc 48 into the body of the container 36 has the tendency to part of the inner surface of the container 36 to wipe off any remaining fuel material, which will aid weld integrity and create a space 50 am open end of the container 36 is generated. The end cap 36a can then be in place over the open end of the container be welded on. The space 50 creates a zone in which the radioactive decay of plutonium fuel or helium other gases can be collected over the life of the heating source. This volume should be for a heat source with about 12 grams of plutonium are about 2.7 ecm. Then the container 36 and its end cap 36a be cleaned by suitable means to its outer surface thoroughly from contamination of any fuel material 34 or other impurities. The sealed container 36 can then be inserted into the reinforcement member 38 and sealed therein by end cap 38a and the container 40 and end cap 40a can be placed around it in any suitable manner. The different End caps can be put in place in any suitable manner

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Creation of a gas-tight seal using tungsten gas arc welding.

It has been found that when the heat source 12 manufactured in the above manner is at a temperature of about 1170 ° K is used for 60 days, the container 36 at the container-fuel interface starts oxygen to absorb into the container material and increases in its hardness, as indicated by the curves 52 and 54 in FIG is. As the use of the heat source continues, the absorption of oxygen into the container material proceeds away through the container, which progressively diminishes its drawability. This oxygen is released from the fuel material 34 and any contaminants that may have remained therein.

However, it was found that this oxygen absorption and the container material drawability reduction can be drastically reduced by a pretreatment which includes the Heating the brought together fuel material 34 with the container 36 and / or the reinforcement member 38 at a Provides a temperature of about 1570 K to about 1720 K for about 1 hour. The desired effects of this Pretreatment begins to degrade at temperatures below about 1570K and above about 1720K. The pretreatment below 1570 K requires unduly long periods of time, resulting in excessive and undesirable grain growth the container (container) materials can result. The temperature fluctuation could be above 1720 K. lead to the melting of the getter. The improved operation is shown in Fig. 4, where curve 5 6 shows the hardness of the Representing lining or container material across its cross-section, namely after the heat source at 1670 K was heat treated for one hour. The curve 58 illustrates the hardness of a similar liner that is heat treated at 1720K for one hour and then 60 Operated at 1170K for days. The curve 60 causes

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shows another lining, which at 1570 K a Was heat-treated for one hour, and then operated for 60 days at 1170 ° K. Curve 62 shows a liner (Container) treated at 1670K for one hour and then operated at 1170K for 60 days. This pretreatment effectively reduces - as shown - the oxygen reactivity within the capsule before the Oxygen has time to diffuse into the liner 36 and other container materials Curve 56 illustrates that little or no oxygen absorption results from the pretreatment itself results. -

A heat source with a thermal output of approximately 6 watts from 10.4 grams of plutonium-238 can produce approximately 25 electrical milliwatts per channel or 50 milliwatts of total output over a 16-year operating life with the heat source operating at a nominal temperature of approximately 700K. The heat source 12 can be about 2.3 cm in diameter, about 2.3 cm long, and weigh about 70 grams. The plutonium-238 isotopic fuel is initially PuO in the form of irregular ceramic shards that range in size from approximately 50 to 500 micrometers. Using the recited materials for the liner 36 and the reinforcing member 38, operating temperatures as high as 900 ^° K can be used for similar lifetimes with some modifications or other generator materials and shapes.

Heat sources of the type described were left on hardened tool steel surfaces at angles of 45 ° or more and at speeds of 100 m per second to 300 m per second, with no release of fuel from inside the heat sources. In addition, heat source reinforcement members were used for a 1270 K fire Exposed for 1 hour without any breakage, even when the inside was under the pressure of helium as it results from concentrations at the end of the service life.

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Claims (10)

PATENT CLAIMS (jly method of generating a radioisotopic heat source characterized by mixing about 96.8 to 92.9 weight percent plutonia chips of about 50 to 500 micrometers in size with from about 3.2 to. 7.1 weight percent yttrium getter material, followed by sealing the mixture in a first container made of approximately 90 percent by weight Ta, 8 weight percent W and 2 weight percent Hf alloy, then this first sealed container and heating the mixture to a temperature of from about 1570 K to about 172O ° K for about 1 hour, whereupon the first container into a second container made of an alloy of approximately 55% Ni, 16% Cr, 16% Mo, 4% W, 5% Fe and 2.5% Co alloy is used and sealed. 2. The method according to claim 1, characterized in that the getter material has pieces of approximately 3 3 0.25 mm in size. 3. The method according to claim 1, characterized in that the first container in a further container of the same Alloy as the first container is used, and that the two, the first and the additional container of the mentioned Are exposed to heating, and that this additional container is used together with the first container and is sealingly closed. 4 .. The method according to claim 1, characterized in that according to placing the fuel mixture in the first container an interposed holding member made of the first container alloy is inserted into the first container to the Wipe fuel mixture from the interior surfaces of the first container and retain the retaining member therein to create a void above the support member. 509851/0768 5. Radioisotopic heat source, characterized by an outer sealed container of approximately 55% Ni, 16% Cr, 16% Mo, 4% W, 5% Fe, 2.5% Co alloy, and one inner sealed container of approximately 90 weight percent Ta, 8 weight percent W, 2 weight percent Hf Alloy, the latter container being inserted into the outer container, and being a mixture of approximately 96.8 to 92.9 weight percent plutonium dioxide chips with a yttrium getter material at least partially fills the inner container, and wherein the inner walls of the inner container have the improved properties have, which result from the fact that the inner container and the mixture of heating on exposing to a temperature of about 1570 K to about 1720 K for a period of about 1 hour. 6. Heat source according to claim 5, characterized in that the containers have a hollow cylindrical part, which ends in an arcuate end and a flat end which is welded to the cylindrical part is. 7. Heat source according to claim 5, characterized by an additional container made of a 90 percent by weight Ta, 8 weight percent W, 2 weight percent Hf alloy that is inside the outer container between the inner ones and outer containers is arranged. 8. Heat source according to claim 7, characterized in that said mixture the entire arcuate end and a part of the cylinder part of the inner container fills and a retaining member made of a 90 weight percent Ta, 8 weight percent W, 2 weight percent Hf alloy that fits between the barrel portion and against the mix. 509851/0768 9. Heat source according to claim 8 for use in one thermoelectric generator with a service life of about 16 years old, the mixture being about 12 grams Contains plutonium, and the outer container has a wall thickness of approximately 0.5 mm, the intermediate container a wall thickness of approximately 1.0 mm and the inner container mentioned has a wall thickness of approximately 0.5 mm. 10. Heat source according to claim 5, characterized that the getter material has bits of approximately 3 mm 3 mm 0.25 mm. 509851/0768