Fusion processes, like combining two deuterons to form a He nucleus are impossible at ordinary temperatures and pressure. The reasons for this can be traced to the fact:

Fusion process like fusion of deuterons to form nucleus is called nuclear fusion, nuclear fusion is a process of combination of 2 or more atomic nuclei into a single or multiple atomic nuclei and subatomic particles. The energy released during the combination is known as binding energy. The reason why nuclear fusion can’t happen at ordinary temperature and pressure is due to Coulomb Barrier. Coulomb Barrier or the electrostatic fore of repulsion is a force that acts between the positive proton particles. To achieve nuclear fusion one has to break or overcome this force or barrier now to simply overcome this barrier, the protons has to be in motion but it can’t be possible in ordinary temperature or pressure, to break the repulsion force or coulomb barrier one must heat the heavy nuclei to a temperature of 100 million kelvin Celsius, which is practically impossible; as the nearest possible temperature to that can be found only at the core of the sun. After reaching 100 million kelvin we will achieve a stable quantum tunnel effect will help in carrying out the fusion reaction for as long as we want. Therefore, properties like positive charge and less nuclear force coverage in an atom are the only reason due to which nuclear fusion is not possible in an ordinary temperature and pressure.