The presence of caustic soda in the spent sodium borohydride (NaBH4) fuel in high concentrations creates environmental problems associated with the transportation and the disposal of the fuel, orits recycling into NaBH4. It is clear that the high level of caustic soda in the spent fuel requires efficient and proper formulation of its industrial recycling applications. The present study regards the conversion process of the caustic soda present in high concentrations in spent NaBH4 fuels. Properties of the caustic used in NaBH4 fuel is characterized using a comprehensive technique. A specific application is presented here, which includes the treatment of the spent fuel solution with anhydrous borax (Na2B4O7) at 90 A degrees C for 3 h followed by calcination after the crystallization, resulting in the conversion of this highly caustic spent NaBH4 fuel into sodium metaborate (NaBO2), which is an environmentally friendlier and an economically valuable material.