The heat produced by the fission in the Generator generates electricity is according to the Seebeck effect.

Seebeck effect

The seebeck effect occurs when any two members of the thermoelectric series (metals or semiconductors) and connect wires made of them to form a circuit with two junctions, such pairs of junctions are called thermoelectric couples (or thermocouples). Heated up the junctions, in the presence of a temperature difference between the junctions a voltage is produced from the diffusion of electrons across the joint hence a small current flows around the circuit. The power output is a function of the temperature at each junction and thermoelectric materials properties. Conversion of heat directly into electricity is not a new principle. It was discovered 150 years ago by a German scientist named Thomas Johann Seebeck. He observed that an electric voltage is produced when two dissimilar, electrically conductive materials were heated up.

Advantage, Disadvantage & Safety

It should be noted that RTGs use a different process of heat generation to that used by nuclear power stations. Nuclear power stations generate power by a chain reaction in which the nuclear fission of an atom releases neutrons which cause other atoms to undergo fission. This allows the rapid reaction of large numbers of atoms, thereby producing large amounts of heat for electricity generation. However, if the reaction is not carefully controlled the number of atoms undergoing fission (and the heat production) can grow exponentially, very rapidly becoming hot enough to destroy the reactor.

Chain reactions do not occur inside RTGs, so that such a nuclear meltdown is impossible. In fact, fission itself does not normally occur inside an RTG; forms of radioactive decay which cannot trigger other radioactive decays are used instead. As a result, the fuel in an RTG is consumed much more slowly and much less power is produced.

But RTGs are still a potential source of radioactive contamination: if the container holding the fuel leaks, the radioactive material will contaminate the environment. The main concern is that if an accident were to occur during launch or a subsequent passage of a spacecraft close to Earth, harmful material could be released into the atmosphere.

There have been five known accidents involving RTG powered spacecraft. In order to minimise the risk of the radioactive material being released, the fuel is stored in individual modular units with their own heat shielding. They are surrounded by a layer of iridium metal and encased in high-strength graphite blocks. These two materials are corrosion and heat resistant. The plutonium fuel is also stored in a ceramic form that is heat resistant, minimising the risk of vaporization and aerosolization. The ceramic is also highly insoluble.