Seriously, these things are amazing, and are an effective use of nuclear fuel and radioactive isotopes.
But what are they? Well, lets start with the general idea of ThermoElectric Generators (TEGs). TEGs use the reverse effect that peltier coolers use, called the Seebeck effect. "Well, what is a peltier cooler?!"
A peltier cooler uses a bimetallic alloy, two metals pressed together (certain metals of course, such as copper and iron). This is connected to an electric circuit, and when electric current is sent through the circuit, the bimetallic element becomes a heat pump, with one side becoming "cold" while the other heats up (in reality, it is just transferring heat from one side to the other). The Seebeck effect reverses this, which means if you create a thermal gradient across the bimetallic element, you will generate a voltage drop (and ultimately an electric current) when you complete the circuit.
"But how is this at all cool to use with Radioisotopes?"
Easy. This means if you create a chamber to hold a radioisotope that emits a lot of alpha radiation but little to no beta or gamma radiation, and store the radioisotope sealed inside of that chamber, the chamber will heat up. Surround that with a TEG, and you end up with a battery that produces a constant amount of power up until the half life time of the radioisotope has been reached. If done correctly, you will have a constant power source for a century, and if properly constructed and sealed, it will pose absolutely no threat to anyone. Clean and constant power, which never needs to be recharged, only replaced every century or so.
Why aren't we using these? Surely if we put efforts into researching and developing this technology more, we'll get to the point where we can produce it relatively affordably, right? Unfortunately, that isn't the problem today.
It's the assholes who will crack these things open to get to the nuclear goodies inside, and we all know how much anything involving radioactive elements scares the living hell out of us.
If only, right?