The general purpose heat source (GPHS) module is the building block for both the GPHS-Radioisotope Thermoelectric Generator. These modules contain the radioisotope plutonium-238 (Pu-238) used to produce heat that is subsequently converted into electricity by thermoelectrics. Pu-238 is fabricated into pellets and encapsulated in an iridium cladding forming a fueled clad. Fueled clads are encased within nested layers of carbon based material and placed within an aeroshell housing to comprise the complete GPHS-module.
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GPHS modules stand approximately two inches tall and have a base that is almost square with sides less than four inches in length. Each GPHS module is designed to weigh no more than 1.44 kilograms and produces a nominal thermal power of 250 watts at beginning of mission. A total of 18 modules are stacked together to provide the heat source for each of the radioisotope thermoelectric generators used on the Cassini mission. The multi-mission radioisotope thermoelectric generator uses eight GPHS modules, and the advanced radioisotope stirling generator uses two GPHS modules. The mass of the GPHS module is 1.6 kg.
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