Radionuclide Thermal Generators Blake Owens

Topics to be Covered

• Basics of RTG’s Basics of RTG’s • Health Physics Concerns • Safety and Design • Examples

Generating Electricity Spacecraft Power

• RTG’s use heat generated from Plutonium Pellet • RTG’s are commonly used as radioactive decay to produce the power source for electrical energy. spacecrafts.

• Thermoelectricity was discovered • Radioisotope power in 1822 by a German scientist, applications have been used Thomas Johann Seebeck. since 1956. Wow… That’s Hot! • RTG’s are the most desired power • The SNAP-3 device was sources for unmanned or developed in 1959, it was the unmaintained environments. size of a grape fruit, and weighed about 4 lbs. Thinking Like an HP

• From a Health Physics standpoint, there are Health Physics Concerns many benefits and consequences for RTG use in certain situations.

• ALARA goals are always a concern when using radioactive material as a power source.

• Shielding can become very complex when trying to protect workers and the public.

RTG Sources

Plutonium-238 Strontium-90

• This alpha emitting source has a • This beta emitter has a half-life of half-life of 87 years 28.1 years

• This source can produce a • Temperatures provided by this temperature of 1050°C source are generally between 700°C Safety and Design to 800°C • Generates 85% of its initial power in its first 20 years • Generates 61% of its initial power in its first 20 years • Extremely expensive and is difficult to manufacture • Strontium-90 is a fission product, meaning that it is easily obtainable • The cost to produce a 50W RTG from many reactors power supply would be about one million dollars • The Strontium-90 source is about 50 to 100 percent heavier

Thermopiles Protecting the Source

• A thermopile coverts thermal • Graphite is used in the outer • Aluminum/Iridium/Beryllium energy into electrical energy. covering to provide protection cladding is used to protect the against eroding in the case of actual fuel source from impact • A thermopile is made up of reentry into Earth’s atmosphere. damage. many thermocouples. • The weight of the devices must • Thermocouples are made of be minimal for space two dissimilar metals which applications, which can affect are joined in a closed circuit. the cost. The two junctions are kept at different temperatures and an electrical voltage is produced. • To avoid heat loss problems, a spherical design is used, since a sphere has the minimum surface • This is known as the Seebeck area per volume of any shape. Effect. Advanced Stirling Radioisotope Generator (ASRG)

Examples

ASRG Other RTG Applications • The Stirling radioisotope generator (SRG) is based on a Stirling engine powered by a large • There are approximately 1000 radioisotope heater unit. The hot radioisotope thermoelectric end of the Stirling converter generators in Russia which reaches high temperature and provide power to lighthouses heated helium drives the piston, and navigation beacons. heat being rejected at the cold end of the engine. A generator or alternator converts the motion • They have exhausted their 10 into electricity. This Stirling year lifetime design. converter produces about four times as much electric power from the plutonium fuel than a • Some of the RTG’s have been radioisotope thermoelectric replaced with solar panels. generator (RTG). The Stirling generators have been extensively tested but have not yet been • Ex. Woodcutters, Vandalism, deployed on actual missions. Striped for Metal

How far have we come? The Future

SNAP-3 ASRG • Eventually, the need will arise for a nuclear reactor system to • 1959 • Scheduled for use in 2012 provide power in order to execute longer missions. • 2.5 W • 224 W • Isotopes alone cannot provide enough power to execute a • 280 days • 14 years mission of this caliber.

• As technology grows, the need for more power will also grow.