Essentially all RTGs that have been produced have been designed for long term unattended operation. An isotope needs several rather specialized attributes to be useful in such an RTG:
- Half life of several decades
- High energy alpha or beta decay
- Low associated gamma radiation
The two isotopes that have been most frequently used are Pu-238 and Sr-90. Each has their advantages and disadvantages that make them preferable for certain types of applications.
Plutonium 238 is a non-fissile, alpha emitting isotope with a half life of 87 years. A sample of pure material would produce approximately 0.54 kilowatts/kilogram of thermal power. In some configurations, the surface temperature of a Pu-238 fuel element can reach 1050 degrees C.
These characteristics make Pu-238 the most capable heat generating isotope. It will outlast most customers; even after 20 years a Pu-238 based power source will produce 85% of its initial power output. It has a high energy density, allowing power system mass and volume to be minimized. It is also easy to shield and its emissions will not interfere with sensitive instrumentation.
Unfortunately, Pu-238 is difficult to manufacture, making it extremely expensive. An accurate price is difficult to determine because of the lack of an open market, but the recent estimates by experts in the field indicate that the material costs several thousand dollars per gram in kilogram sized lots – if it is available at all. Since RTG conversion efficiency is on the order of six to eight percent, this puts the price of a 50 W power supply at close to a million dollars.
There is also the public relations problem associated with the word plutonium. Frequent readers of Atomic Energy Insights might understand that plutonium is not as dangerous as Ralph Nader says it is, but that realization has not yet permeated the general public’s consciousness. Most political decision makers are also not knowledgeable enough about nuclear physics to understand that Pu-238 cannot be used to produce a nuclear weapon; it has the wrong number of nucleons to be a fissile isotope.
Sr-90 is a beta emitter with a 28.1 year half life. A pure sample will supply 0.46 kilowatts/kilogram of thermal power when new, or about 15 percent less than a similar mass of Pu-238. Additionally, an Sr-90 based RTG will deteriorate about three times as fast as one based on Pu-238; a 20 year old power supply will produce only 61 percent of the initial power output.
Because of the lower energy density, a Sr-90 fuel rod will not get as hot as a Pu-238 rod. A new rod, depending on configuration, might be able to achieve a surface temperature of only 700 to 800 degrees C. This is important because a lower temperature available to the hot junction of a thermocouple will reduce the thermoelectric conversion efficiency of the RTG. Because of these characteristics, a Sr-90 RTG will be about 50 to 100 percent heavier than a Pu-238 RTG of the same power output. For space based applications, where every payload gram is carefully controlled, this mass difference makes it uneconomical to consider Sr-90.
Strontium, however, has some advantages over plutonium. It is a fission product with a high yield; about five percent of all fission reactions produce Sr-90. Since Sr-90 has a long half life compared to the time that reactor fuel spends in a core, it is quite feasible to mine Sr-90 from spent nuclear fuel. Sr-90 is considered by most of its current owners to be an expensive waste problem; perhaps some of them would pay to get rid of it.
Strontium is not associated with nuclear weapons and has never been called the most deadly element known to man. There is a precedence in the United States for widely licensing small quantities of sealed Sr-90; it is used in some aircraft ice detection systems.
There is also a precendent for its use in earth based RTGs; most of the Soviet ocean bottom and Arctic devices used SR-90 heat sources. (Chmielewski)