Radiochemical characterisation of graphite from Juelich experimental reactor (AVR)

Graphite built-in nuclear reactors may receive a high neutron dose for a long period. Depending on its chemical composition a lot of activation products are produced. In addition, there is more or less fission product contamination depending on the location. The migration of fission products may be supported by high temperatures which occur in high temperature reactors. At the Juelich 15 MWe High Temperature Gas-cooled experimental Reactor AVR (Arbeitsgemeinschaft Versuchsreaktor) two different types of nuclear graphite had been in use. High-purity graphite was used as basic material for core structures of the AVR. Insulation layers from carbon bricks (graphite with larger amounts of impurities) surrounding the graphite reflector were used to protect the metallic structures from high temperatures. For many reasons it is important to know the amount of contamination of graphite and carbon bricks with activation products and fission products. The head end of nuclear graphite analytics must be the incineration. Volatile activities (14C, 3H, 36Cl ...) must be caught for determination. In case of handling dustlike samples the incineration furnace must be small enough to be operated in a glove box. The resulting ashes can be used for determining all non volatile nuclides with different radiochemical methods. In early 1999 some graphite and carbon brick samples from AVR-reactor had been taken by drilling. The samples had been analysed in our laboratories at Juelich research centre. For incineration we used a vertical quartz-tube which dips at the bottom into a small electric furnace. Tritium, 14C and 36Cl are caught in washing bottles. After further preparation, they are determined by LSC. After dissolving the ashes, the elements are separated by ion exchange, extraction methods and HPLC. The radionuclides are then determined by a-spectrometry, LSC, low level g-spectrometry and x-ray spectrometry. (author)