The First Law of Thermodynamics explains how the amount of energy gained from something cannot exceed the amount put in.
So it seems to violate the laws of thermodynamics – but in some ways it’s just like burning paper. Energy is released (not destroyed or created).
The trade off is that the thing that is burning ends up with less energy. Just as ash and carbon dioxide has less energy than the paper did, the helium we form has less energy than the fusion fuels, tritium and deuterium.
What’s even more intriguing about this is that the energy that is released is actually mass being converted to energy: the amount of mass lost can be calculated from the energy released from Einstein’s E=mc² equation. In other words the total mass of the tritium and deuterium nuclei before the reaction is greater than the mass of the helium and neutron after fusion has taken place – even though it is the same number of protons and neutrons.