Speaker
Description
Characterisation of spent nuclear fuel (SNF) is required to ensure the safe operation of the different facilities involved in Spent Fuel management – for handling, storage, conditioning, reprocessing or final disposal – as well as for transport activities between those facilities. Several quantities are of importance: neutron and γ-ray emission rates and spectra, decay heat and the inventory of fissile nuclides. The activation and fission reactions affecting the fuel are highly dependent on reactor and assembly design, materials, and reactor operation parameters; in turn, the in-pile neutron spectrum is also affected by changes in the fuel composition, which makes the inventory determination a complex process. Yet, the inventory and its evolution over the next thousands of years can only be obtained by theoretical calculations. Evidently, the theoretical calculations need to be validated and realistic confidence limits have to be determined. Both the validation and estimation of confidence limits require high quality experimental data. At present such data are primarily based on a combination of destructive chemical and radiochemical analysis methods involving a series of steps including the dissolution of the sampled material. Data obtained by Non-Destructive Analysis (NDA) are rather scarce.
In this work we present an innovative NDA method to determine in an absolute way the neutron emission rate of a SNF sample in standard controlled area conditions. The procedure relies on a transfer procedure that is adapted to the hot-cell facilities of the SCK CEN in Mol (BE). The procedure can be applied to any neutron counter that is optimised for neutron correlation measurements. Measurements of a segment of a SNF rod, which was irradiated in the Tihange 1 PWR reactor to a burnup of approximately 50 GWd/t, were carried out. The neutron production rate was determined with an uncertainty of about 1.5%. The composition and design specifications of the fuel rod and the irradiation conditions were well documented. The SNF segment sample used for the neutron measurements was characterised for its net fuel weight. Additional representative samples of the same fuel rod were characterised by destructive analysis for different types of investigations, amongst others radiochemical analysis of selected actinides and fission products, and, derived from those concentrations, a burnup assessment.
Due to the strong link between the neutron emission rate and the 244Cm inventory, the results can be used to validate theoretical estimates of the 244Cm abundance in SNF with confidence limits that are superior as those reached by radiochemical analysis. The experimental emission rate was used to validate theoretical calculations obtained with the ALEPH2, SCALE and SERPENT codes using different evaluated nuclear data libraries. Since the 244Cm inventory is very sensitive to the 242Pu(n,g) and 243Am(n,g) reaction cross sections such experimental data are valuable to validate these cross sections.