Improving the radionuclide Inventory Determination of the Irradiated Graphite from BR1 in Mol

Abstract

The Belgian Reactor 1 (BR1) operational since 1956 at SCK'CEN in Mol, is the oldest research reactor in Belgium. It is a graphite-moderated and air-cooled reactor fuelled with natural metallic uranium. The active core consists of a 6.66 x 6.84 x 6.84 m3 graphite matrix, built by stacking squared-base prismatic graphite blocks (~14500), yielding a total mass of 492 tons. The BR1 is supposed to continue its operation for several decades; however it is necessary to already make studies about the activity level of the graphite which is related to the disposal scenario. The objective of this study was to determine the radionuclide inventory to be expected at the dismantling due date in the irradiated graphite. Those radionuclides have formed by neutron activation process following nuclear reactions between the neutrons and stable nuclides present in the graphite as main component or as impurities. The radionuclide build-up rate is a function of the initial atom content (which is different for graphite A and B quality), of the neutron flux, of the activation reaction cross sections, of the position inside the reactor and on irradiation conditions and history. In this work a numerical simulation of the irradiation of the BR1 full core including the fuel elements and many graphite samples selected in various neutron field positions was carried out. The Monte Carlo burn-up code ALEPH was used for the calculation. In 2011, SCK'CEN performed a few gamma spectrometry measurements on the irradiated graphite A test sample collected in