Excess energy production, well above the background and in amounts of the same order of magnitude as the input energy, has been measured that has been caused by sparking in hydrogen isotopes between electrodes made of metallic hydride-forming metals (palladium and stainless steel). This excess energy production is stable over long periods (several weeks) and is observed with both hydrogen and deuterium. Only extremely low levels of neutrons and tritium have been detected, many orders of magnitude below what would be expected from the excess energy production measured. On the contrary, copious emission of low-energy radiation (likely to be beta rays) has been observed. A class of hypothetical nuclear reactions, based on the action of the weak electronuclear force, is proposed that accounts f or all the experimental f acts observed.