The results of the study show that lightweight radioisotope heat sources can be considered for use undersea with little shielding necessary for electronics packages in close proximity to the heat source. Acceptable integrated electronic dose levels of 10 to the 7th power rads for gamma irradiation and 10,000 reps for neutron bombardment were obtained for extended mission times for the three isotopes considered. It is unnecessary to shield the isotope 210Po for electronic protection. However, use would be restricted because of its short half-life and this isotope could present special heat management problems not encountered with either 60Co or 244Cm. For 244Cm and 60Co it was found that the shield around the electronics package becomes important due to the strong back scattering effect of neutrons and gammas from the surrounding seawater. The 244Cm heat source appears to require less shielding to electronics level and would therefore take up less volume in the undersea system than the 60Co heat source.