Not only is the verb missing from the second clause, so is the Subject.
They are deleted by the rule of Conjunction Reduction, which has been applied to two conjoined clauses, deleting the (repeated) subject and auxiliary be.
Stripped of modificational bells and whistles, the original compound sentence is
- The power source is not a nuclear reactor
the power source is a nuclear battery.
and the second occurrence of the power source is gets deleted because it's predictable.
producing the compound Verb Phrase
- is not a nuclear reactor but a nuclear battery