Implementation of new integrated evaporation equipment for the preparation of 238U targets and improvement of the deposition process

摘要

Measurement of neutron cross section data is a core activity of the JRC-Directorate G for Nuclear Safety and Security in Geel. After a period of reduced activity and in line with a renewed interest for nuclear data required for GenIV reactors and waste minimization, the demand for high quality actinide targets increased. Physical vapour deposition by thermal evaporation is a key technique to prepare homogeneous thin actinide layers, but due to ageing effects the earlier in-house developed equipment can no longer provide the required quality. Because of a current lack of experience and human resources cooperation with private companies is required for the development of new deposition equipment directly integrated in a glove box. In this paper we describe the design, implementation and validation of the first commercial actinide evaporator in a glove box as well as the optimization of the deposition process. Highly enriched ~(238)U_3O_8 was converted to ~(238)UF_4 powder and several deposition runs were performed on different substrates. The deposition parameters were varied and defined in order to guarantee physical and chemical stable homogeneous UF_4 layers, even on polished substrates which was not longer feasible with the older equipment. The stability problem is discussed in view of the thin layer growth by physical vapour deposition and the influence of the deposition parameters on the layer quality. The deposits were characterized for the total mass by means of substitution weighing and for the areal density of ~(238)U by means of alpha particle counting and thermal ionization mass spectrometry (TIMS). The quality of the layer was visually evaluated and by means of stereo microscopy and auto radiography.