Processes leading to the formation of hydrogen inside the reactor WWER in case of large failures with loss of heat-carrier have been considered: radiolysis of water, radiolysis of water steam, steam-zirconium reaction and thermal dissociation of water. The model of failure consisting of :1) depressurization of contour and loss of pressure, 2) boil-off water from contour, 3) boil-off water from active zone and constructional elements of active zone is accepted. The contribution of various processes to hydrogen formation at various stages of failure has been discussed.