Non-destructive verification techniques for centrifuge enrichment plants

In the framework of the programme of the Federal Republic of Germany in support of the IAEA under task C14.7, Field Testing of NDA-Equipment, measurements were performed in the gas centrifuge enrichment plants at Almelo and Gronau. These measurements were to investigate the applicability and feasibility of non-destructive assay techniques at uranium hexafluoride piping in the cascade area. Taking account of the results of the Hexapartite Safeguards Project, the measurement method should be capable of giving a go/no go result on current and preceding high enriched uranium (HEU) production. Initial experimental work focused on straightforward passive γ-spectroscopy with high resolution Ge detectors. The results of these measurements showed that this method was not capable of determining the 235U enrichment in the gas phase when the gas pressure is not known. Therefore, a method was proposed to overcome this problem by combining the passive γ-ray measurements with a pressure determination by means of X-ray excitation of the uranium in the gas. In addition, very optimistic assumptions have to be made about the operating conditions of the plants. The paper describes the measurements and, on the basis of the results, discusses to what extent these assumptions are encountered in the Almelo and Gronau plants. Alternatively, passive γ-spectroscopy was tested for one cascade under two different geometries, making use of the differing detection efficiencies of the 185.72 keV γ-radiation for the deposits or for gas when measured with two different collimators. It was proposed theoretically that this method, together with a pressure determination, enables the gas enrichment to be determined unambiguously. The result shows that with a non-optimized system, because of the small differences of the detection efficiencies between gas and deposits, long measurement times and homogeneously distributed uranium deposits are required. (author)