Determination of Trace 55Fe and 63Ni in Steel Samples via Liquid Scintillation Counting

In the decommissioning of nuclear facilities, activated steel often contains radionuclides such as 55Fe and 63Ni, which are categorized as hard-to-measure due to their emission of only low-energy beta particles or X-rays. In samples exhibiting very low radioactivity, close to background levels, a large quantity of steel must undergo extensive physical and chemical processing to achieve the Minimum Detectable Activity Concentration (MDC) necessary for clearance, recycling, or reuse. Italian regulations set particularly stringent clearance levels for these radionuclides (1 Bq/g for both 55Fe and 63Ni), significantly lower than those specified in the EU Directive 2013/59 (1000 Bq/g for 55Fe and 100 Bq/g for 63Ni). Additionally, Italian authorities may enforce even stricter limits depending on specific circumstances. The analytical challenge is compounded by the presence of large amounts of non-radioactive Fe and Ni, which can cause color quenching, further extending analysis times. This study presents a reliable and optimized method for the quantitative determination of 55Fe and 63Ni in steel samples with activity levels approaching regulatory thresholds. The methodology was specifically developed and applied to steel from the Frascati Tokamak Upgrade (FTU) facility, under decommissioning by ENEA. The optimization process demonstrated that achieving the required MDCs necessitates acquisition times of approximately 5 days for 55Fe and 6 h for 63Ni, ensuring compliance with stringent regulatory requirements and supporting efficient laboratory workflows.