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Milliwatt Surveillance Program Ensures RTG Safety and Reliability

The U.S. military's need for a rugged, reliable power source with a long life was met with the radioisotope thermoelectric generator, or RTG. The battery-like RTGs provide power for permissive-action links, a family of devices that reduce the possibility of obtaining detonation from a nuclear warhead unless a controlled numerical code is used.

Often referred to as "nuclear batteries" by the media, RTGs convert heat to electricity by harnessing the thermal energy produced by decay of radioactive isotopes-usually plutonium-238. A silicon-germanium thermopile inside the RTG converts the heat energy into electricity.

Because the milliwatt RTG operates from a nuclear heat source rather than from chemical action as in regular batteries, the power sources provide a minimum of 25-30 years of service life for tens of milliwatts of electrical power without needing replacement. Theoretically, the devices are capable of producing electrical power much longer, but helium, a byproduct of isotope decay, can build up and eventually rupture the casing.

These RTGs are relatively compact-4.9 cm in diameter and 8.8 cm long for the 4.5-watt model and 6.8 cm by 9.3 cm for the 4.0-watt version.

The General Electric Neutron Devices Department in Largo, Florida, manufactured RTGs from 1975 to 1990, when all production of the devices ended. NMT-9 began its involvement with the Milliwatt RTG Program in 1980, when the Group began manufacturing heat sources for RTGs. Today NMT-9 performs a broad range of RTG activities, including surveillance testing of shelf-life and stockpile returns, shelf-life heat source inventory, and destruction of excess RTGs and heat sources. All RTG testing and plutonium reclamation activities are done by NMT-9 except vibration and mechanical shock testing, which is performed by ESA-MT. The heat sources for milliwatt RTGs are identical except for the amount of plutonium oxide granules contained in the 4.0-watt and 4.5-watt models. EG&G Mound Applied Technologies qualified initially all lots of the alloy used to encase heat sources with pressure-burst tests. Production lot testing of T-111 has been consistent with the desired probability of failure being less than 0.005 for the conditions of a 25-year-old, 4-watt heat source exposed to a 1010-degree-Centigrade fire for two hours. Plutonium heat sources are enclosed first in a 0.51-mm-thick liner of alloy T-111 (90% Ta, 8% W, and 2% Hf), then in a 1.02-mm-thick strength member of T-111, and finally in a 0.51-mm-thick clad of oxidation-resistant Hastelloy C (55% Ni, 17% Mo, 16% Cr, 5% Fe, 4% W).

NMT-9 performs electrical tests on all shelf-life and stockpile-return RTGs. These tests include thermopile impedance, RTG open circuit voltage, RTG loaded (130 ohm) voltage, and RTG base isolation resistance. A thermopile tester in PF-3 can also test eight characteristics of thermopile operation. Some RTGs are also selected for vibration and mechanical shock testing by ESA-MT personnel. RTG output voltages before, during, and after the tests can then be compared.

Trent Latimer is Project Leader of the Milliwatt Surveillance Program.

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