The Berlin Research Reactor BER I1 started operation in 1973 at a power of 5 MW. It was shut down in August 1985 for a general physical and technical improvement. The aim of the upgrading program was to increase the usable reflector-flux density by a factor of 10 as a result of:•increasing the power from 5 to 10 MW•increasing the fission density by reduction of the core size from 64 to 42 MTR fuel-element positions and•increasing the reflection efficiency by installation of a 30-cm thick beryllium reflector.