Intense neutron generation at a rate of > 108 n/s in cold fusion was achieved when neutron emission “avalanches” were observed as deuterium forcefully penetrated into a large 2-cm-diam × 5-cm-long palladium cathode. A very specific process involving intense charging and discharging of deuterium from the palladium cathode during continuous electrolysis of heavy water, called the “on-off effect,” was discovered. This effect is 10 to 100 times stronger than the ordinary on-off effect of the current. As the palladium absorbed and exhausted the deuterium, the thermal behavior of the palladium was examined in detail. It is concluded that the particular characteristics of palladium and the generation of a huge inner pressure within the palladium are necessary conditions for a cold fusion reaction. Other researchers have used a much smaller palladium cathode than the one used here. They measured only the electrolysis temperature, and not the cathode temperature. Thus, their experiments failed to discover the thermal characteristics of the palladium cathode, the on-off effect, and intense cold fusion. This experiment proves that an unknown nuclear fusion process that generates a large amount of heat, as proposed by others, does not exist. Instead, the heat is actually reaction heat generated by the explosive absorption and exhaustion of the deuterium in the palladium cathode, caused by the on-off effect.