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From heat sources to heart sources:

Los Alamos made material for plutonium-powered pumper

Be still my beating heart. In a novel program that started in the 1960s, the Laboratory began a project to help those faint of heart.In 1967 Los Alamos Scientific Laboratory explored a new mission: developing a self-contained energy source that would last for decades to power a conceptual artificial heart. The energy source would be powered by the same material developed in 1963 for the space program-plutonium-238. The heat from the radioactive decay of the plutonium-238 can readily be used directly or used to produce electric power for space probes, etc.

In a joint effort between the National Heart and Lung Institute (NHLI) and the Atomic Energy Commission (AEC), the Department of Energy's predecessor agency, Los Alamos researchers began the endeavor to balance the hazards of plutonium-238 with the benefits. Researchers believed that they could minimize the element's neutron radiation effects while supplying future artificial heart recipients with a long-life power source.

Art Beaumont compares a mockup welded capsule and the gauge tube into which a heat source capsule had to fit to meet size specifications.

Beaumont measures the outside diameter of a gauge tube with a micrometer. The gauge tube was designed to ensure that encapsulated heat sources would fit inside the available space in the artificial heart unit.

Members of Chemistry-Metallurgy "Baker" Division's CMB-11 fabricated the source with 50 watts of energy-enough to drive the artificial heart-and focused on reducing the radiation while George Matlack and Joe Bubernak of CMB-1 analyzed the radiation properties.

Researchers considered two or three different isotopes but finally chose plutonium-238 because of its half-life of eighty-seven and a half years, which was long enough to provide power with no significant loss of energy during the lifetime of the mechanical heart. The idea came from the Lab's active space program. More than thirty years ago, NHLI concluded that an external power source needed to run the implanted device would be as large as a telephone booth, somewhat impractical for artificial heart recipients.

Scientists at Los Alamos believed that they could make a plutonium-238 heat source small enough to implant into the human body. The heat source would power a stirling-cycle engine that would pump the blood. But the conundrum was if the plutonium-238 was powerful and long-lasting enough to save the patient, would the radiation effects end up killing the heart recipient?

Another drawback to using plutonium-238 metal was that it has a relatively low melting point, so if a deceased patient was cremated, the crematorium might become a radiological cleanup site. The research team considered using an alternative form with 3 percent gallium, which raised the melting point, but it still didn't prevent the melt that could occur in a crematorium.

Larry Mullins records data during a direct-oxide reduction (DOR) experiment.

Researchers experimented with plutonium-238 oxide from Savannah River, but the radiation levels were hard to measure at the time, and several light-element impurities produced an alpha-neutron reaction.

Plutonium-238 is relatively easy to shield because its gamma rays are mostly of low energy and it produces a low amount of spontaneous neutrons. However, in the presence of other light-element isotopes such as nitrogen-14, oxygen-17, oxygen-18, carbon-12, or fluorine-19, the neutron emission is much higher because of the interaction of alpha particles (produced by plutonium-238 radioactive decay) with the other elements. A heavy but energetic alpha particle (helium ion), which can be shielded with a piece of paper, would hit an atom like fluorine and cause a neutron to escape. Neutrons are very difficult to attenuate and cause significant potential radiological doses to employees and candidate artificial heart recipients.

Scientists reduced this threat of the induced neutrons by two steps. In the original process, the first step was to convert the plutonium oxide to metal, which was historically done by means of a plutonium-fluoride intermediate with excessive alpha-induced neutrons. From the health-physics standpoint, this was quite undesirable for workers in the immediate area.

Carl Peterson transfers plutonium-238 oxygen-16 oxide inside an inert-atmosphere glove box from a storage container in preparation for pressing a fuel pellet. The furnace in front of Peterson was used for sintering pressed pellets at high temperature.

Researchers therefore came up with the direct-oxide reduction process. In this process the plutonium oxide is mixed with calcium chloride-calcium fluoride or only calcium chloride, along with calcium metal, and heated to produce plutonium metal. A variation of the direct-oxide reduction (DOR) process today is the mainstay of producing plutonium-239 for the weapons program.

Researchers prepared the metal and removed nonradioactive light elements from it through electrorefining. The pure metal ingot was then converted to finely divided metal particles by reacting it with hydrogen and subsequently removing the hydrogen three times-a hydride-dehydride cycle. The purified finely divided metal was then ready for reaction with oxygen-16 water vapor, in the second step.

Normal water contains three oxygen isotopes: -16, -17, and -18. The oxygen-17 and -18 isotopes are susceptible to alpha-neutron reactions, but the oxygen-16 isotope is not. High-purity water vapor enriched at Los Alamos in the oxygen-16 isotope was reacted with the finely divided plutonium-238 metal to prepare plutonium oxide that had minimal alpha-induced neutron radiation. The techniques for the hydride-dehydride cycle and preparation of the plutonium-238 oxygen-16 oxide fuel were developed by Robert Nance.

The process of reacting oxygen-16 water with plutonium oxide to produce isotopic exchange is still used today to reduce the alpha-neutron reactions in heat sources for the space program. The recently highly successful Cassini space probe to Saturn and its moon, Titan, is a good example. A number of earlier, spectacular, landmark deep-space explorations such as Voyagers I and II were powered by specially processed plutonium-238.

Because Congress was funding dual-track research programs, it decided to proceed with NHLI research, so the AEC's program was dropped and the project ended in 1977.

Jim Foxx (below left) and Larry Mullins discuss a helium-release experiment with a vented plutonium-238 oxide heat source. The encapsulated heat source was welded into a container like the one shown and connected to a helium leak detector by the tube joined to the top of the capsule to measure helium released by the source. The helium was generated by the alpha decay of the plutonium. The experimental apparatus can be seen in the hood behind the experimenters.

The heat source capsule had to be handled inside a glove box with forceps because of the high temperature of the unit. The heat was generated by the alpha radioactive decay of the plutonium-238.

Even if the project had continued, there were some real challenges to overcome. Insurance companies surely would have balked at the $200,000 price tag. The plutonium was encapsulated with three layers of metals: first tantalum, then tantalum-10 tungsten, and finally an outermost capsule of platinum-20 rhodium. These metals provided shielding and protected the heat source from oxygen. Three of the "D" cell sized 50-watt plutonium heat sources were made for testing.

Another drawback was discovered when rigorous tests were conducted. Researchers found that the encapsulation would not survive a 30.06 gunshot, which could cause a radioactive contamination threat if a recipient was shot through the heart. Finally, there was a problem at that time identifying pump materials that would not cause coagulation during long-term contact with blood.

While the program demonstrated many firsts at the Lab, some of which are still used in actinide processing today, there was also a successful spinoff. The Lab produced 63 grams of high-purity plutonium-238 metal for the pacemaker program. Medtronics made about 250 of the plutonium-powered pacemakers, and about twnty-two were still stimulating human heart more than twenty-five years after they were manufactured, a feat that no battery-powered pacemaker could match.

The Nuclear Regulatory Commission maintains strict guidance for hospitals conducting patient monitoring and how to dispose of the pacemaker when the patient no longer needs it. The pacemaker is clearly stamped with the radioactive symbol and is labeled as containing plutonium-238. Other successful technologies that are still used today as a result of the program include the minimization of neutron radiation and the methods of accurately measuring the radiation from plutonium-238 materials.

This article was contributed by Kathy DeLucas of the Public Affairs Office; Jim Foxx of the Nuclear Materials Technology Division; and Robert Nance, formerly with the Chemistry-Metallurgy "Baker" Division and now retired.


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