Re-characterization of irradiation facilities for k0-NAA at RPI after conversion to LEU fuel and re-arrangement of core configuration

The 1 MW pool-type Portuguese research reactor (RPI) was converted from using highly enriched uranium (HEU) to using low enriched uranium (LEU) fuel in September 2007. Although the new LEU core has the same number of fuel assemblies as the first HEU core, it has three assemblies less than the last HEU core and also has a different arrangement of the beryllium reflectors. Therefore, the primary irradiation positions that are used with the k0-based neutron activation analysis (k0-NAA) at the RPI were redefined and characterized. The neutron spectrum parameters of the deviation of the epithermal neutron spectrum distribution from the 1/E shape (α), the thermal to epithermal neutron flux ratio (f), the thermal to fast neutron flux ratio (fF) and the neutron temperature (Tn) were determined by using a combination of Au, Lu, Ni and Zr monitors. The experimental results of the neutron spectrum parameters obtained in this study are compared to the results were obtained prior to the refuel conversion in the applicable irradiation positions. The synthetic multi-element standard SMELS was used to verify the accuracy of k0-NAA applied to practical samples using the newly measured neutron spectrum parameters.