<i>&gt; perhaps in this environment the Big Crunch could still occur.</i><p>No, it couldn&#x27;t. The nuclear waste will still have the same average energy density, on the scale of the universe as a whole, as the unexhausted fusion fuel does now. We know that energy density is too small now to make the universe recollapse, so the same must be true any time in the future.<p><i>&gt; what type of &quot;fuel&quot; is needed to create an exothermic fusion reaction comparable to the Big Bang?</i><p>The Big Bang was not an exothermic fusion reaction. Our current best model is that the Big Bang was caused by a very large energy density being transferred from the inflaton field (the field that drove inflation in the very early universe) to the various fields in the Standard Model of particle physics (electrons, quarks, photons, etc.). This &quot;reheating&quot; created the hot, dense, rapidly expanding state that we refer to as the Big Bang.