"transform some of the energy from radioactive decay"
Not so simple. You need a hot source and a cold sink to transform energy. Where's your cold sink? This thing is intended to melt what's around it, and the outside of the probe is not that different in temperature from the inside.
Erm. If it's melting the rock around it, then the rock is your cold sink. By the time the temperature of the rock catches up with the probe, it's already molten anyway.
The rock would not work as a cold sink - the cold rock is too far from the probe. The rock right near the probe is at almost the exact same temperature.
Is it really that simple? You can't have any active electronics - they would melt.
And the tungsten is touching the cobalt, with no air gap (i.e. to opportunity to harvest power from the electrons returning to the cobalt to neutralize charge).
Of course it's not simple, it's a very challenging project. But in terms of providing electrical power, if it's possible to have electronics that work at all then it would be possible to power them via beta decay. This is already an established technology in the form of "betavoltaics", which have powered existing devices (such as pace makers) in the past.
Not so simple. You need a hot source and a cold sink to transform energy. Where's your cold sink? This thing is intended to melt what's around it, and the outside of the probe is not that different in temperature from the inside.