Graphite model validation process

The BR1 reactor contains approximately 492 tons of graphite, which is converted to i-graphite during reactor operation and therefore classified as radioactive waste. The neutron irradiation converts stable elements, some of which being present as impurities in the virgin graphite, into radionuclides in the i-graphite. The type of radionuclides, their half-lives and their activity levels dictate the appropriate route for the waste disposal and therefore the importance of having an accurate inventory of the radionuclide activities in the BR1 i-graphite. The radionuclide inventory present in the i-graphite is determined by combining modelling (the activation calculations using Monte Carlo N-Particle code (MCNP)-based on the virgin graphite impurity levels and the reactor operation history) and the radionuclide activity measured in the i-graphite (for validation and model adjustment). SCK CEN has already developed the model for predicting the radionuclides activities of the BR1 graphite at the end of field life and during decommissioning. However, a complete validation of the model was not possible because some important information on the impurity and beta-emitting radionuclides concentration were not available. In this document, the process for the model validation is presented to support the assessment of graphite characterisation and inventory at the end-of-life of the BR1 for the correct waste category classification.