Neutron beam tubes will be used to supply those instruments which are located in the reactor building around the core. By tangential arrangement of the tubes in relation to the core axis, direct view from the instrument back to the core is avoided. As a result, contamination by fast neutrons and gammas as an outcome of the fission process is significantly reduced. Each beam tube is designed to deliver two neutron beams with typical cross sections of 120mm x 80mm. With an angle of 8° between these beams, two instruments can be placed at the beam exit without geometrical constraints. FRM II's beam tubes are arranged at tangents to the reactor core, i.e. they do not exhibit a direct line-of-sight to the core. This markedly reduces the background of fast neutrons and gamma radiation to which experiments are exposed and further improves the signal-to-background ratio.

Twelve beam tubes (10 horizontal, 2 inclined) are tangentially arranged around the core with their noses in close vicinity to the maximum flux of thermal neutrons or to one of the secondary sources. There will be one through-going beam tube to house the target of a new fission fragment accelerator. Two beam tubes are inclined at about 45° to open additional areas i the experimental hall above the ground floor instruments. The spectrum of neutrons delivered by these tubes is determined by the characteristic source temperature in front of the nose, which will be thermal in respect of D2O (300K), cold in respect of tubes facing the cold D2 source, or hot in respect of the graphite source (2900K). The beam tubes are made out of aluminium alloys to minimize neutron losses from passage through the walls. For safety reasons, all guide tubes are filled with Helium gas.