A hydrogen generator (30; 50) comprising: a holder having a cavity (34; 54) therein; a packaged fuel unit (10; 110; 210) replaceably contained within the cavity (34; 54) and comprising: a plurality of segregated quantities of fuel (26; 126; 226), each comprising a reactant that can undergo a thermal decomposition reaction that produces hydrogen gas when heated to at least a minimum initiation temperature; a package comprising a container in which the plurality of segregated quantities of fuel (26; 126; 226) is disposed, wherein the container is made from a material with a poor thermal conductivity of less than 5 watts/meter*Kelvin and is covered by a lid, the lid comprising material with a poor thermal conductivity of less than 5 watts/meter*Kelvin and a plurality of apertures therethrough, wherein the apertures are covered by thermal conductor sections (18; 118; 218), and each thermal conductor section is in contact with at least one of the plurality of segregated quantities of fuel; and at least one hydrogen outlet of said fuel unit through which hydrogen gas can escape; and a heating system (38; 58) comprising a plurality of heaters (40; 60), each heater (40; 60) in thermal communication with at least one of the plurality of thermal conductor sections (18; 118). a holder having a cavity (34; 54) therein; a packaged fuel unit (10; 110; 210) replaceably contained within the cavity (34; 54) and comprising: a plurality of segregated quantities of fuel (26; 126; 226), each comprising a reactant that can undergo a thermal decomposition reaction that produces hydrogen gas when heated to at least a minimum initiation temperature; a package comprising a container in which the plurality of segregated quantities of fuel (26; 126; 226) is disposed, wherein the container is made from a material with a poor thermal conductivity of less than 5 watts/meter*Kelvin and is covered by a lid, the lid comprising material with a poor thermal conductivity of less than 5 watts/meter*Kelvin and a plurality of apertures therethrough, wherein the apertures are covered by thermal conductor sections (18; 118; 218), and each thermal conductor section is in contact with at least one of the plurality of segregated quantities of fuel; and at least one hydrogen outlet of said fuel unit through which hydrogen gas can escape; and a plurality of segregated quantities of fuel (26; 126; 226), each comprising a reactant that can undergo a thermal decomposition reaction that produces hydrogen gas when heated to at least a minimum initiation temperature; a package comprising a container in which the plurality of segregated quantities of fuel (26; 126; 226) is disposed, wherein the container is made from a material with a poor thermal conductivity of less than 5 watts/meter*Kelvin and is covered by a lid, the lid comprising material with a poor thermal conductivity of less than 5 watts/meter*Kelvin and a plurality of apertures therethrough, wherein the apertures are covered by thermal conductor sections (18; 118; 218), and each thermal conductor section is in contact with at least one of the plurality of segregated quantities of fuel; and at least one hydrogen outlet of said fuel unit through which hydrogen gas can escape; and a heating system (38; 58) comprising a plurality of heaters (40; 60), each heater (40; 60) in thermal communication with at least one of the plurality of thermal conductor sections (18; 118).