The postwar progress of nuclear technology has concentrated on the fission process as the source of useable energy from the nucleus. This emphasis on fission has always been accompanied by a less spectacular interest in the diverse utilization of radionuclides,* uses that include tracing body functions and plant growth, sterilizing food for storage, and controlling the quality of goods in certain continuous production lines. Thus, it has been the delicate and deft probing capability of radionuclides that has usually been pressed into service.
The production of economically useful amounts of power from radionuclides has until lately been largely a matter of laboratory interest. With the introduction of a radionuclide-fueled thermoelectric generator by the Atomic Energy Commission in January of this year, however, serious attention must be given to the many interesting and important tasks that are feasible because of the simplicity and efficiency of this type of power source. In demonstrating this feasibility the Commission’s proof-of-principle generator is a significant achievement, and its demonstrated conversion efficiency has given impetus to the development of radionuclide-fueled devices capable of producing up to several hundred watts of electrical power. As power generators in this relatively low power domain these units will complement rather than compete with reactors.
The significance to the military and particularly to the Navy of several hundred watts of electrical power in such a convenient form must not be overlooked. Uses of radionuclide power units become legion when one appreciates the economic service that can be rendered by devices that furnish several thousand watt-hours of electrical energy per pound of generator weight. Radionuclide power supplies are eminently suited to run a whole gamut of remotely located devices such as radio beacons, fall-out and fire warning alarms, and meteorological observation stations. The sorely needed empirical investigation of the oceans and ocean bottoms of the world should generate an immediate need for radionuclide power for submerged instrument packages. The long life obtainable should be particularly attractive to oceanographers, for these power sources will permit integrated looks at the many ocean environment parameters now only checked intermittently. In but one of several possible ASW applications, radionuclide devices could maintain passive listening and interrogating stations at harbor entrances and river mouths. These generators could power underwater “highway” markers for both navigation and missile-launching purposes.
Radionuclide power can perform many existing tasks better than is now done conventionally and it makes possible many new feats heretofore thought impossible or impractical. It is important that the operating forces of all the services be made aware of the essential characteristics of this type of power so that, as fleet and field commanders participate in the formulation of requirements that establish future weapons systems, the full potential of radionuclide power will be utilized.
An inherent peculiarity of a radionuclide l power source is that it does not provide a constant source of power unless specific allowances and compensations are incorporated into the design of its generator system. The process of nuclear decay dictates that the power level must drop off in an exponential fashion at a rate characterized by the half-life of the particular radionuclide under consideration. Thus, three particularly attractive power producing radionuclides, strontium-90, cerium-144, and polonium-210, have half-lives of 28 years, 290 days, and 138 days, respectively. To provide a constant source of power a radionuclide must generate a higher level of power than required at the beginning of its intended period of service so that it will be producing the required amount at the end of this prescribed period. A heat sink must then be provided to dissipate the extra heat generated during the life of the device. How long a useful life can be expected from a given radionuclide is determined by how fast it decays. Each radionuclide has its own distinct rate.
To be useful power producers radionuclides should have values of half-lives ranging from several months to several decades. In addition to having a useful half-life, a promising radionuclide must produce radiation of sufficient intensity and energy relative to the weight of the radionuclide involved. This intensity and energy of radiation determine the heat producing ability of the substance, an ability termed the specific power of the radionuclide which is measured in watts of heat produced per gram of material. A knowledge of the characteristic radiations of a radionuclide is important, too, in order to indicate the degree of radiological safety and care that will be involved in handling and employing a device containing that particular radionuclide. When practical limits are set on the necessary specific powers, half- lives, and radiological safety considerations required of a good power producing radionuclide, only a half dozen or so stand the test and appear to be useable.
Although the characteristic power decay of a radionuclide power source may seem at first to be a serious disadvantage, standard engineering compensations can be incorporated to produce a lightweight, rugged, constant power source. A radionuclide power source can be built to produce such a long useful life that the limit of operation of the over-all system of which it would be only the prime mover is usually dictated by the reliability of the electronic and mechanical devices which would be driven by the radionuclide source.
Radionuclides produce heat but the conversion of this heat to electricity is a consideration independent of the radionuclide being used. Conventional thermodynamic rotating equipment (turbines, pumps, and condensers) can be used, utilizing phase changes in a fluid. The AEC demonstration device used thermoelectric materials such as those used in home furnace thermostats to achieve its noteworthy conversion efficiency. The well known principles of thermionic conversion (flow of electrons in a vacuum) hold considerable promise, although this method is not as ready for practical application as are the thermodynamic and thermoelectric conversion methods.
Cost is a very important consideration in attempting to estimate the future applicability of radionuclide power sources. The biggest single determining factor of the ultimate cost of these units will be the extent of “consumer demand.” The more units built, the cheaper they will become.
Importantly, a distinction must be made between the costs of the non-nuclear components of the power source, the container for the radionuclide and the conversion system chosen, and the cost of the radionuclide itself. The non-nuclear costs will be nominal, depending mainly on the cost of the conversion equipment. The cost of the radionuclide is another matter. This cost will reflect the nature of the actual production process for producing the radionuclide, depending on whether it had to be recovered from the fission product waste stream of a reactor or whether it had to be produced by special irradiation. Some radionuclides are already being produced quite cheaply; others that seem to have power producing futures must await the development of large scale production or separation facilities before they become economically feasible. Assurance of the availability of presently scarce radionuclides would stimulate the demand for radionuclide power devices. Conversely, numerous firm requirements for these generators would stimulate interest in making these scarce radionuclides more plentiful. It would be unrealistic to attempt to define precisely the costs of radionuclide power sources until radionuclide production costs can be pegged to predictable power levels and uses that are in demand. Costs can be as low as a few cents per watt-hour.
In assessing the comparative costs of doing a particular task by using a radionuclide system and by conventional means, a higher initial cost for the radionuclide may be indicated but the logistic support often required for a conventional system sometimes quickly overtakes and exceeds the cost of the radionuclide system.
In terrestrial applications radionuclide power must still compete with oxygenburning devices. In a space environment it must compete with solar battery power. But in the dark depths of the ocean, nuclear auxiliary power has the unique capability of providing long term electrical power for a multitude of applications. The development of radionuclide power sources by the Atomic Energy Commission has been inspired by the requirement for an auxiliary power unit to run the instrument package in space vehicles. The preorbital, mission, and post-orbital requirements of any subsystem of a space vehicle impose severe demands of ruggedness and integrity on a radionuclide power source. The successful satisfaction of these demands by radionuclide devices that have been built has a considerable immediate transfer value to undersea use.
Although far removed from the realm of panacea and “breakthrough” initially thrust upon it by the uninformed, the recent demonstration of the Atomic Energy Commission’s “SNAP-3” radionuclide-fueled thermoelectric generator has a twofold significance for the Navy and its sister services. Most immediately, the unique utility and limitations of such power sources must be more widely appreciated so that their full potential can be exploited in future weapons and weapons systems. Secondly, as an achievement typical of the accelerated technology of our times, the radionuclide power source is an example of the technical progress of which truly responsive and effective operational leadership must be aware in order to provide the depth and resourcefulness necessary for an unremitting defense of liberty.
* The term radionuclide has been coined by physicists as a noun or adjective referring to the nucleus of a radioactive atom.