Characterisation of the uranium tetrafluoride unburnt materials using non-destructive assay techniques for safeguards purposes

Abstract

Inspectors use non-destructive assaying (NDA) techniques for verification measurements of safeguarded nuclear materials. The NDA techniques help the International Atomic Energy Agency to verify all declared nuclear materials and undeclared nuclear materials and activities. In Situ Object Counting System (ISOCS) and Multi-Group Analysis for Uranium (MGAU) as NDA instruments, play a pivotal role in safeguarding special nuclear materials. In this work, NDA techniques, in particular ISOCS and MGAU, were used to characterize and quantify the uranium tetrafluoride (UF₄) unburnts. The natural uranium, in uranium tetrafluoride (UF₄) (unburnts form), produced from the South African former conversion plant is the investigated material. The UF₄(unburnts) were produced from ammonium diurinate (ADU) through calcination, reduction and hydrofluorination reactions. Therefore, the unburnts are as a result of the UF₄ that did not burn during the fluorination process to form uranium hexafluoride and are regarded as early nuclear fuel–cycle waste. Uranium products before the enrichments stage in the nuclear fuel cycle were not subjected to nuclear safeguards implementation. According to the International Atomic Energy Agency (IAEA) Policy Paper 18 of 2009, the starting point of implementation of nuclear safeguards was redefined from the enrichment step to conversion step in the nuclear fuel cycle. The purpose of this study was to characterise the UF4 (unburnts) from the conversion step in terms of the ²³⁵U, ²³⁸U and ²³²Th isotopic mass content and enrichment of ²³⁵U using ISOCS and MGAU. A set of 15 waste drums of the UF4 unburnts were assayed using the broad energy germanium (BeGe) detector, the ISOCS and MGAU softwares. The isotopic mass content of ²³⁵U, ²³⁸U and ²³²Th plus ²³⁵U enrichment were measured by ISOCS while MGAU measured only ²³⁵U enrichment. The samples’ fill heights through the use of peak 356 keV from the ¹³³Ba source were measured effectively for all the 15 UF₄ (unburnts) drums and ranged between 52.80 cm to 85.00 cm for 200 litre metal drums. UF₄ (unburnts) fill heights enabled the density of individual drums to be calculated and it ranged between 1.22 g/cm³ to 3,41 g/cm³ and the density measurements depend on the drums’ net weight. The 15 drums analysed by ISOCS had isotopic mass of ²³⁵U ranging between 0.007±0.003 kg to 0.024±0.003 kg while ²³⁸U mass ranged between 0.742±0.044 to 1.900±0.105 kg with one drum not detected. The enrichment values measured by ISOCS ranged between 0.436±0.044 and 1.358±0.027 weight percentage (wt.%). For MGAU v4.2 results, ²³⁵U enrichment fluctuated between 0.00 and 1.438±0.115 wt.%; and 0,000 to 1.635±0.121 wt.% for MGAU v4.3 results. ISOCS and MGAU software measured successfully the masses and the enrichment level of some uranium isotopes in UF₄ (unburnts). The uncertainties reported were within the accepted sigma of one.