Patent Number: 040015882
Section: summary

This invention relates to radioactive heat sources for use in nuclear batteries and the like, and more particularly to the providing protection for radioisotope heat sources for use in thermoelectric batteries which are designed for implantation within human beings in order to prevent destruction of the radioisotope containment vessel under usual cremation conditions. More and more individuals are having pacemakers implanted within their bodies which are powered by small nuclear batteries. Moreover, as medical science progresses, there will likely be additional implantable devices which will utilize such nuclear batteries. These batteries generally utilize thermoelectric materials, such as bismuth telluride, which generates electricity as a result of the application of heat to one end thereof. The source of the heat is commonly a radioactive isotope which produces heat as a product of its constant radioactive decay and generally plutonium-238 is employed because of its availability. Pu-238 is encapsulated within a strong primary containment vessel of a suitable metal alloy. Commonly, a containment vessel made of an alloy of tantalum and tungsten may be used, which alloy exhibits sufficient strength to retain the pressure build-up that occurs as a result of the creation of helium as a part of the decay process of plutonium-238. One example of such an implantable battery incorporating a containment vessel and a thermoelectric unit is described in U.S. Pat. No 3,781,176, issued Dec. 25, 1973 to Alan Penn et al. Obviously, it is important that the radioactive plutonium and any radioactive fission products of the plutonium decay be confined. There is generally no problem in maintaining effective containment under normal conditions; however, it is becoming more common for bodies to be cremated after death. Cremation conditions (for example, heating to about 1300.degree. C. under oxidizing conditions for 2 hours) may result in the rupture and/or destruction of the containment vessel and in consequent radioactive contamination from the plutonium isotope source and the decay products thereof. Solutions to this problem of potential destruction during cremation are needed inasmuch as conditions may arise in which a nuclear battery is not removed from a body prior to cremation. It is an object of the present invention to provide an improved radioactive heat source for use in a nuclear battery which will resist destruction when subjected to cremation conditions. It is another object of the invention to provide a method for making a radioactive heat source for use in a nuclear battery which will be resistant to destruction upon exposure to cremation conditions. A further object of the invention is to provide a nuclear battery for implantation in the human body which is constructed so as to avoid spreading radioactive contamination upon cremation of the corpse in which it is implanted. Still another object of the invention is to provide a nuclear battery which includes a thermoelectric generator, plus a heat source in the form of a radioactive isotope, which heat source is protected so as to maintain its integrity although the battery is subjected to cremation conditions. These and other objects of the invention will be apparent from the following description of the methods for making radioactive heat sources for use in nuclear batteries embodying various features of the invention. For purposes of this application, the terminology "nuclear battery" should be understood to mean the assemblage of a thermoelectric unit, plus a radioactive heat source, having an overall size suitable for implantation in the human body which assemblage is designed to provide a relatively constant output of electric power. It has been found that the usual primary containment vessel in which a radioisotope heat source is encapsulated can be protected from breach and/or destruction when subjected to cremation conditions by providing a secondary containment vessel of refractory metal which is in turn itself suitably protected. In this respect, the refractory metal is provided with an exterior diffusion coating, as by using a pack diffusion process or the like, of the refractory metal silicide which is high temperature stable. On the outer surface of the diffusion-coated refractory metal, there is disposed a layer of ceramic-oxide-containing material which will provide a relatively inert protective film under cremation conditions.