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Radioisotope thermal electric generators (RTG's) and radiation heater units (RHU's) are presently being used or considered for sources of electrical power and heat for the Apollo, Pioneer and Viking Projects. Both RTG's and RHU's have considerable merit if proper precautions are taken to compensate for overall system response to the nuclear radiation sources. This paper summarizes the nuclear characteristics of plutonium fuel which gives rise to the radiation from both RTG's and RHU's, gives Monte Carlo estimates of the neutron and gamma isodose profiles from a 2200 watt (th) HELIPAK** thermoelectric generator and concludes with a presentation summary of shield thicknesses required for several sensitive radiation experiments due to combined RTG-RHU fields. This study is a part of a continuing program of analysis at the Jet Propulsion Laboratory on the evaluation of RTG radiation fields on an outerplanet spacecraft.