The present invention deals with a method to liquefy the viscous oil of oil wells and also to clean the paraffin off the walls of tubing and other production equipment. The method consists in using heat produced by a thermal generator, and also using steam produced by the contact between the thermal generator and water. The thermal generator is a metallic shielded container of cylindrical shape holding individual units of already encapsulated Strontium-90 sources able to generate a temperature of 100 degree Celsius or a combination of encapsulated Strontium-90 sources able to generate 100 degree Celsius each in order to obtain, according to necessities, up to or over 600 degree Celsius. The thermal generator is transported in a metal housing.