Abstract: A hydrogen generator and a fuel cell system including the hydrogen generator are disclosed. The hydrogen generator includes a reactant that undergoes a thermal decomposition reaction to produce hydrogen when heated. A laser is used to initiate the reaction. The reactant is contained in a reactant composition in a user-replaceable disc-shaped fuel unit. The reactant composition can be segregated into individual quantities. The fuel unit and the laser beam are periodically realigned by incrementally rotating the fuel unit and/or incrementally redirect the laser beam.