Isotope powered Stirling generator for terrestrial applications Page: 4 of 8
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and design to the final RSG except heated with an electrical heater equipped with power input leads.
SELECTION OF RADIOISOTOPE FOR HEAT SOURCE
Of the many sources of energy for production of electric power, the use of heat producing radioisotopes
is uniquely suited for small, long-lived, portable power generators. Therefore, an evaluation of the most
suitable radioisotope was performed. The evaluation of suitability considered the following:
" Decay half-life,
" Energy released/unit weight,
" Shielding requirements,
" Source volume and weight,
" Stability of compound,
" Compatibility with encapsulating material,
" Heat transfer properties,
" Toxicity and safety,
" Regulatory controls,
" Isotope availability,
" Infrastructure for production,
" Possible heat source delivery schedule, and
" Overall heat source cost.
Although these criteria are clearly interrelated, they provided a structure on which to base a selection.
For example, the heat source cost is directly affected by the available infrastructure for production of the
isotope as well as production of the heat source. For the initial assessment, only those radioisotopes with
a half-life between 100 days and 1000 years were considered. From the 103 isotopes in this category,
31 were considered for more careful evaluation. Eight of the 31 radioisotopes (with half lives less than
2 years) were eliminated because of the large heat rejection required at beginning of life to have sufficient
energy at the end of 5 years. Another 10 were eliminated because of the shielding requirements due to
the production of gamma rays or bremsstrahlung. An additional five were eliminated from consideration
because of the requirement for isotope separation to obtain a suitable density of the radioisotope in the
heat source. The remaining eight (3H, 63Ni, 147Pm, 71Tm, 227Ac, 238PU, "Am, and 2"Cm) were
considered to be suitable and were thus evaluated primarily on the basis of availability and cost.
Plutonium-238 was desirable based on the ultimate source size and weight. Of even greater importance
is availability of the heat sources. The infrastructure is well established for producing the 238Pu and
cladding for the General Purpose Heat Sources (GPHS) for space applications. One iridium clad, 2"Pu
fuel pellet generates the proper amount of heat for a 10 We RSG. Thus, sources are taken directly from
the fabrication line for the GPHS and further encapsulated as necessary for our application. The design
and fabrication of these Isotopic Heat Sources (IHS) are the subject of another paper presented herein.
RADIOISOTOPE STIRLING GENERATOR DESIGN
The conceptual design of the converter unit for the RSG was presented in some detail earlier
(Ross et al. 1991). The proposed Stirling generator employs only proven technologies. The engine
subsystem technology, using radioisotope energy in the power range of interest, was developed and
successfully demonstrated by the STC of Richland, Washington in the 1970s for the National Institutes
of Health. The technology has undergone continuous refinement in the intervening years.
A simplified schematic of the converter design is shown in Figure 1. The piston and displacer are
arranged in a single cylinder (beta configuration) to minimize dead volume and for simplicity. The
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Tingey, G.L.; Sorensen, G.C. & Ross, B.A. Isotope powered Stirling generator for terrestrial applications, article, January 1, 1995; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc677432/m1/4/: accessed April 20, 2019), University of North Texas Libraries, Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.