The possibility of reducing the concentration of 14 C produced in irradiation of reduced activation ferritic steels (RAFS) intended as structural materials in first wall and blanket of fusion power plants was investigated. During RAFS irradiation 14 C is mainly produced from the more abundant of the two isotopes of nitrogen present in the steel, namely 14 N. The method proposed consists in increasing the enrichment of the other isotope ( 15 N). One-dimension transport calculations show that for a typical DEMO blanket configuration an 15 N enrichment from the natural value (0.37%) to 95% was sufficient to keep the end of life 14 C concentration in the RAFS below the limit (3.7×10 7 Bq/kg) fixed by the Japanese Nuclear Safety Commission for qualifying it as a low level material (LLM) which can be disposed by shallow land burial.