Pons and Fleischman used a palladium cathode in their cold fusion experiment of 1989 (cathode immersed in heavy water,D20) and they said that excess energy was released from the system. Since 1989 lots of experiments have been done and some researchers have agreed with the findings of Pons and Fleischman. I have seen many explanations of why cold fusion can't take place according to known physics but here is one suggestion I have as to why it can! Palladium metal is very resistant to fracturing.This means that impurity atoms or ions face a high energy barrier to get into the crystal lattice and distort the face-centred cubic arrangement,and that it is difficult for defects to grow larger to cause fracturing. But when an electric current flows through palladium metal (which has hydrogen gas on its surface and inside it - palladium absorbs 900 times its own volume of hydrogen) hydrogen molecules and hydrogen ions (which are present in the heavy water) are given the energy to distort the metal structure and remain in situ.As time passes the number of distortions and hence the potential energy of the palladium cathode increases.At some critical point, the face-centred cubic arrangement of palladium atoms is quickly restored and the hydrogen molecules and ions are expelled from the palladium cathode as potential energy is released like a mini-earthquake.In a cylindrical-shaped cathode the released energy could be focused at its end or in the centre of the rod.In particular,at some locations in the rod where deuterium molecules are located, the focused energy could be sufficient to raise the temperature and pressure and cause helium 4 to form.The formation of helium inside the cathode would explain why nuclear products have rarely been detected in the heavy water, in experiments of this type.And the energy yield from fusion would be low because few deuterium atoms would get to the centre of the cathode by diffusion - most would be at or near the surface - the place where energy is least likely to be concentrated in order to raise the temperature and pressure enough for helium formation.