“Once the lunar module was on the Moon, the lunar module pilot removed the plutonium 238 fuel capsule from the cask and inserted it into the SNAP generator.”
SNAP-27 is one of a series of radioisotope thermoelectric generators, or atomic batteries, developed by the U.S. Atomic Energy Commission under its SNAP program. The SNAP (Systems for Nuclear Auxiliary Power) Program was directed at development of generators and reactors for use in space, on land, and in the sea.
The basic SNAP-27 unit is designed to produce at least 63 electrical watts of power. It is a cylindrical generator fueled with the radioisotope plutonium 238. It is about 18 inches high and 16 inches in diameter, including the heat radiating fins. The generator, making maximum use of the lightweight material beryllium, weighs about 28 pounds un-fueled.
The rugged fuel capsule was contained within a graphite fuel cask from launch through lunar landing. The cask is designed to provide reentry heating protection and added containment for the fuel capsule in the unlikely event of an aborted mission. The cylindrical cask with hemispherical ends includes a primary graphite heat shield, a secondary beryllium thermal shield, and a fuel capsule support structure made of titanium and Inconel materials. The cask is 23 inches long and eight inches in diameter and weighs about 24.5 pounds. With the fuel capsule installed, it weighs about 40 pounds. It is mounted on the lunar module descent stage by a titanium support structure.
Once the lunar module was on the Moon, the lunar module pilot removed the fuel capsule from the cask and inserted it into the SNAP- 27 generator. To do this the Astronaut rotates the cask which contains the plutonium fuel element down 90 degrees to a horizontal position. Then a protective dome is removed, and, finally, the element is removed and inserted it into the RTG.
The spontaneous radioactive decay of the plutonium 238 within the fuel capsule generates heat in the generator. An assembly of 442 lead telluride thermoelectric elements converts this heat — 1480 thermal watts — directly into electrical energy — at least 63 watts. There are no moving parts.
SNAP was first used in the Apollo 12 mission to provide electricity for the first Apollo Lunar Surface Experiments Package (ALSEP).
(Above Photo) Fred Haise extracts the fuel element for the SNAP-26 RTG from its cask mounted on the side of the Lunar Module in a training session.
On Apollo 13, the LM was used to get the crew and their crippled Command Module back to Earth and was abandoned just prior to re-entry. The fuel cask survived the journey and currently resides at the bottom of the Tonga Trench in the western Pacific.