Reducing measurement uncertainty in Ni-63 measurements in reactor coolant water with high Co-60 activities

In methods for quantification of Ni-63, in e. g. reactor coolant water, a chemical separation is required due to Ni-63 being a pure beta emitter with limited means of quantification. Co-60, a common radionuclide in reactor coolant water, is not completely separated with the commonly used separation procedure, and it is not resolved from Ni-63 in the beta spectrum. The separation method discussed in this work consists of TRU resin (Eichrom) and Ni resin (Eichrom). After running the separation procedure, depending on the initial activity of Co-60, there may still remain enough Co-60 to interfere in the measurement of Ni-63. The Co-60 interference is corrected for via a gamma spectrometric measurement. This correction may, depending on the Ni-63/Co-60 ratio, introduce a large contribution to the measurement uncertainty. The aim of this work was to evaluate the possibility to reduce the measurement uncertainty of Ni-63 measurements by adding a second Ni separation to the method. Double Ni separations were performed on reactor coolant water having a Co-60 activity much higher than the Ni-63 activity (Ni-63/Co-60 = 0.01), in order to decrease the radioactivity of Co-60 in the sample. The measurement uncertainty of the Ni-63 measurement result was reduced by a factor of about three