Actually it is not justbut , butnot X but Y .
X is a phrase: a nuclear reactor (,in which atoms are actively broken apart)
Y is also a phrase: rather a kind of nuclear battery (that uses natural radioactive decay to produce power)
I have put the modifiers (relative clauses) in brackets above to make it clear that the actual phrases area nuclear reactor anda kind of nuclear battery .
In fact, I can't think of any scenario whereNot X but Y would havecomplete clauses in X and Y. Does something
...
X is a phrase: a nuclear reactor (,in which atoms are actively broken apart)
Y is also a phrase: rather a kind of nuclear battery (that uses natural radioactive decay to produce power)
I have put the modifiers (relative clauses) in brackets above to make it clear that the actual phrases area nuclear reactor anda kind of nuclear battery .
In fact, I can't think of any scenario whereNot X but Y would havecomplete clauses in X and Y. Does something
...