Neutronics modelling of the BR1 reactor with the Serpent code

BR1 – the first nuclear reactor ever built in Belgium – was critical for the first time on May 11th, 1956. BR1 is air-cooled through a forced convection, moderated by graphite (approximately 500 tons) and fueled with natural metallic uranium. Remarkably, BR1 has used the same original fuel for more than 50 years of operation, and nowadays it has only achieved a burnup of few percent. Years of experience have made this reactor the perfect instrument for fundamental research and training. Existing programs involving instrumentation and data calibration in BR1 require the support of neutronics calculations and an accurate model of the reactor geometry and of the involved particle physics. In the last decade such as support was provided using the MCNP particle transport code. In this work a detailed model of the BR1 is developed using the Monte Carlo transport code Serpent on the basis of the previous work with MCNP. A comparative analysis is carried out to assess the quality of the new model with respect to experimental data and previous calculations. Added values (or drawbacks) have been identified that the Serpent code can bring to the model in terms of state-of-the-art capabilities, simplifications, computer resource optimization and quality assurance.