[thedoc (OP)]

Do radioisotope Thermo-Electric Generators use a safer radioative material, like thorium?
Asked by RF Axel, SecondLife


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[thedoc (OP)]

We answered this question on the show...



Richard -   Traditionally, it’s been plutonium-238 which has been used for powering radioisotope Thermo-Electric Generators.  Thorium is not used for such systems.

[CliffordK]

I assume it all depends on energy density, and longevity of the device.

RTGs use nuclear decay energy rather than the fission energy used in nuclear plants. 

Pu-238 has a half-life of 88 years.  Not too long, not too short.  But, generally active enough that it generates sufficient heat, without burning out in a few hours.

There are a number of different Thorium isotopes.  Thorium 230 has a halflife of 75,000 years.  And Thorium 232 has a halflife of over 14 billion years.  So, while one's RTG device would last longer, less of the element is undergoing radioactive decay, and thus, it would be generating less heat.

One has to, of course, look at the entire decay chain, but if the first decay is the rate limiting factor, or what determines the initial heat of the object, then that may be sufficient.

The reason RTGs are used in deep space probes is that the further from the sun, the less energy the probes receive from the sun.  But, likewise, the longer time they are in use, the less energy they RTGs emit. 

So, with a halflife of 88 years, then over a century, the power output more than drops in half.  After a few centuries, it would reach a small fraction.

If we choose to build probes with an expected lifespan of several thousand years, then a different isotope for RTGs would need to be selected.

[syhprum]

Radium with a half life of 1600 years might be useful for a long life unit but the output would be pretty poor