Structural concrete used as biological shield in nuclear power plants (NPP) become radioactive after exposure to neutron radiation. Due to radiative capture interactions, artificial radionuclides are generated to high enough concentrations causing such a concrete to become classified as low-level radioactive waste at the time of NPP's decommissioning. Disposal of this concrete adds to the expense of financing and constructing of a nuclear power plant. Three such radionuclides, namely Co-60, Eu-152, and Eu-154 are shown to account for 99% of total residual radioactivity of decommissioned concrete. The IAEA document RS-G-1.7 Application of the Concepts of Exclusion, Exemption, and Clearance, specifies clearance levels in terms of radionuclides' specific activities. The specific activity of 0.1 Bq/g for Co-60 and Eu-152, and Eu-154 defines criteria for low-activation concrete that can be recycled after decommissioning of the plant. In this paper we present the methodology and the challenges associated with the detection of trace amounts of cobalt and europium in concrete aggregates and how these detection limits affect the accuracy of application of the IAEA regulations on the clearance levels. We used the neutron activation analysis (NAA) as a method to test the detection limits on trace elements in samples of cement, coarse, and fine aggregates from the local suppliers in the State of Utah. These samples were irradiated at different reactor power levels ranging from 10kW to 90kW for time periods of 1, 3, 30, 60, and 120 minutes, with the goal to find if there is a threshold set of the NAA parameters in detecting trace amounts of these isotopes. Each of the samples is counted on a Canberra BEGe High Purity Germanium detector. Cement samples were concurrently irradiated with a NIST coal fly ash standard reference material and coarse and fine aggregates with Montana soil standard reference material in order to accurately quantify the mass concentration of the isotopes. The final results are showing that the reactor power, irradiation and detector measurement times, are heavily correlated in finding the optimum combination of these NAA parameters for detection of trace contents of cobalt and europium. This research represents the first such study in the USA. We have been able to develop the methodology with the goal to identify what sources of cement, fly ash, sand, and coarse aggregate are having cobalt and europium concentrations below the established IAEA clearance levels. Similar studies were firstly developed in Japan a few years ago, in identifying cobalt and europium to be the elements causing concrete activation at the time of power plant decommissioning.
