The shut-down of the Barsebäck 1 BWR: A unique opportunity to measure the power distribution in nuclear fuel rods

Reactor poolside measurements of gamma radiation specific for the fission product 140La (1596 keV) have been used for an experimental determination of axial power distributions in 55 nuclear fuel rods irradiated in the Barsebäck 1 BWR nuclear power plant. The measurements take advantage of the unique situation of a very short last reactor cycle of only three months due to the out-phasing of the reactor unit at November 30 1999. 140La whose decay is controlled by the mother nuclide 140Ba with the half-life 12.75 days reflects an average power distribution, representative for the latest weeks of core operation (in this case basically during November 1999). The measured intensities have been transformed into a 25 nodal representation to allow a precise and direct comparison with the corresponding calculated power distribution. The 55 rods were selected from two different fuel assemblies with average burn-ups of 1.9 and 9.7 MWd/kgU, respectively (that is one fresh bundle and one slightly more than one cycle bundle). The stability and the linearity of the measurement system were evaluated. The linearity was checked using the two-source method. The stability was checked by recurrent measurements on a reference fuel rod. The results have been used in the validation of the pin power reconstruction model of Westinghouse 3D core simulator POLCA-7. The deviation between measured and calculated 140Ba concentration (expressed as radial error) is typically a few percent on rod level. Results indicate that also Gd-rods are properly modelled over a broad range of conditions. It is indicated that predictions for fuel rods in their first month of operation are less accurate than for the rest of the rods.