Is the control metal that powers the Guyver possibly a nuclear radioactive power source? 

The Control Medal doesn't power the Guyver, its a processor/control system and data storage device.

 

If I remember Correctly the unit siphons a small amount of energy from Boost Space via the Gravity Orb.

The control metal does have a dimensional interlink, which means it can siphon energy from the boost dimension, but as you said the gravity globe does that to a larger extent.

actually, we don't know what form that energy takes, but it is probably fairly efficient and the manga or anime has never shown any indications of harmful radioactive effects.  it all seems to be pretty safe to be around.

since the guyver organism can seemingly change some substances into completely different substances, there is a good chance this organism is capable of spontaneous fission/fusion. loose ions, cosmic rays, etc. would cause damage to the structures and material that it creates, so I imagine it must have a method to manage any excess.