Metal particles are introduced to reactor beds in a hydrogen generator without introducing contaminating oxygen or nitrogen to the generator. The particles are protected against oxidation under an inert gas atmosphere, are flooded with water prior to injection into the generator, and are further flooded with hydrogen gas drawn from the generator to replace the water prior to injection. The generator includes a receiving hopper for the metal particles and an injector connected to the hopper. Particles introduced to the receiver are flooded with the inert gas and are transferred to the injector (which is flooded with water) by gravity. The water is displaced with gas from the generator prior to injecting the particles into the generator with a reciprocating piston associated with the injector.