Not as far fetched as you might think.
From the link:
Using this back-of-an-envelope calculation, Wilson worked out that a single 1000 GeV proton could lead to the release of 12,000 GeV of fission energy. Of course, this neglects all the messy fine details in which large amounts of energy can be lost. For example, it takes some 20MW of power to produce an 0.2MW beam in the Energy Doubler.
But even with those kinds of losses, it certainly seems worthwhile to study the process in more detail to see if overall energy production is possible.
Wilson’s conclusion is this: “There are probably better ways of producing plutonium, but it does appear that it would be feasible to construct an intense proton accelerator that would produce more energy than it consumes.”