You tell your friend that no machine can possibly put out more energy that is put into it. Your friend states that a nuclear reactor puts out more energy than is put into it. Can you argue with this or is it true? We know that reactor energy is PE from uranium fuel. Can you help me with a better explanation?
You should ask you friend how does he think the uranium nucleus came about.
After all the uranium nucleus has a lot of positive charges in very close proximity. Why don't they repel each other as the should?
We had to invest energy in bringing all these charges together. However, the do not like to be that close. Other forces are keeping them together ("surface tension", strong force interactions etc), but any tiny disruption to these forces (such as neutron bombardment) ...
This solution discusses energy with respect to nuclear reactors. Concepts covered include potential energy and stable/unstable equilibrium.