a nuclear reactor puts out more energy than is put into it....

？？
就是啊，为什么是错的？？
You tell your friend that no machine can possibly put out more energy than is put into it, and your friend states that a nuclear reactor puts out more energy than is put into it.
——选自美国的物理教案

这句话实际上要说的是：a nuclear reactor puts out more energy than （energy) is put into it.由于前面有energy 了，就省略了。因此整句话应该是对的。

is put 改成puts
than 前后所接的结构要一样