[¦thər·mō·i′lek·trik ′nü·klē·ər ′bad·ə·rē] (nucleonics) A low-voltage battery in which a heat source, consisting of a radioactive isotope such as polonium-210, is hermetically sealed in a strong, dense capsule, and a series of thermocouples are alternately connected thermally, but not electrically, to the heat source and the outer surface of the capsule.