From “The Trouble with Fusion”: The goal of the fusion program is to produce a reactor fueled by deuterium and tritium, isotopes of hydrogen containing one and two extra neutrons. This choice of fuel eases the problem of achieving an energy-producing reaction, but also has features that make it more difficult to turn that energy source into a useful power plant. The most serious difficulty concerns the high energy neutrons released in the deuterium-tritium reaction. These particles damage the reactor structure and make it radioactive. A chain of undesirable effects ensures that any reactor employing D-T fusion will be a large, expensive, and unreliable source of power.
When these drawbacks become more widely realized, disillusionment with the fusion program will weaken the prospects for other fusion programs, no matter how wisely redirected, for decades to come.