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Retrospective dosimetry focussed on the reaction 93nb(n,n')

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An overview is given of practices for “retrospective” dosimetry based on extraction and counting of niobium present in different structural materials in power reactors, mainly the pressure vessel. This neutron influence can be derived from the activity induced in small amounts of the material that has been obtained by scraping or nibbling a few milligrams of reactor material at the location of interest. Main problem of counting of structural materials is the overwhelming presence of the nuclide 60Co in the gamma-ray spectra of irradiated steels. Chemical separation of niobium is needed to determine the induced 93Nbm Mn activities. Introduction of chemical procedures is not common within the reactor dosimetry community. Just the absence of chemical expertise within the reactor dosimetry community made this type of dosimetry long untouched. This challenge was picked up in a joint exercise of NRG, Petten, The Netherlands, SCK-CEN, Belgium and VTT Processes, Espoo, Finland. The aim was to develop a procedure for retrospective dosimetry focussed on the reaction 93Nb (n,n’) 93Nbm be applicable at normally equipped radiochemical laboratories. The project was organised within the EU 5th framework program under contract number FIKS-CT-2000-91 and sponsored by the EU. The partners in the project investigated a chemical separation procedure of niobium in parallel. This happened also with the determination of the amount of niobium in dissolution obtained after the separation. Two different procedures have been applied: Inductively Coupled Plasma - Mass Spectrometry (ICP-MS) and Neutron Activation Analysis. The accurate counting of the very weak X-rays of 93Nbm was tackled also. Different methods as counting of a few drops of a niobium solution deposited on filter paper, counting of niobium in solution or application of liquid scintillator counting (LSC) are tested and reviewed. This report has to be considered as the conclusion of the project and gives an overview of the different procedures that have been addressed. The advantages and disadvantages are reported just as the accuracies that can be realised. Details can be found in the reports produced by each of the partners in the project (1), (2), (3) and (4). Within this, the dosimetry was focused on the reaction 93Nb (n,n’) 93Nbm while the reaction 54Fe(n,p) 54Mn was considered for control reasons only. During the project the combination of the results of both reactions proved to be a powerful tool to collect information about the part of the neutron spectrum above 100 keV, being responsible for neutron damage. Therefore a procedure to separate manganese is included also.