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Retrospective dosimetry of fast neutrons focussed on the reaction {93}Nb(n,n¿){93}Nb{m} (RETROSPEC)

Project ID: FIKS-CT-2000-00091
Funded under: FP5-EAECTP C


Neutron irradiation affects the mechanical properties of the materials in a nuclear power plant (NPP) used for internal structures and for the Reactor Pressure Vessel (RPV). Accurate data on the neutron fluence combined with information from databases with material properties for the various structural materials will give information on the condition of the various components of a NPP, including the RPV, indicating for instance the need for annealing. This neutron information contributes also to the determination of the end-of-life of a NPP.
The neutron dose induced in reactor structural materials obtained from transport calculations can only in limited cases be checked with measured data. Monitor sets are irradiated in surveillance capsules only and sometimes outside the reactor, in the cavity close to the reactor vessel. An extra obstacle is that monitor sets irradiated in surveillance capsules do not always give accurate information due to lack of precise information about the position of the set during irradiation.
Retrospective Dosimetry could give a valuable solution for the lack of fluence monitor sets at critical positions. Retrospective dosimetry refers to the use of structural materials in reactors, not originally intended for dosimetric purposes. In principle the neutron fluence can be derived from the activity induced in small amounts of material that has been obtained by scraping, drilling or nibbling of a few milligrams of reactor material at the location of interest. Particularly material from the RPV as one of the most critical reactor components can be analysed in this way. Retrospective dosimetry can be applied to both, operating ("biopsy") and shut-down facilities ("autopsy"). This has been demonstrated before [6->10], but none of the references gives detailed information about neither the technique that has been applied nor the accuracy that can be achieved.

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