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.