Since it is essential to minimise neutron absorption in the region of the moderator tank in order to obtain a high neutron flux, the part of the target unit extending into the moderator tank is filled with heavy water, except for the rod bundle, which is about 50 cm long and is located in the centre of the moderator tank. The target enclosure cooling system prevents overheating and melting of the target window, where it is penetrated by the proton beam. The cylindrical moderator tank of about 2 meters in diameter is surrounded by a light water reflector jacket. It contains inserts for the horizontal extraction of cold and thermal neutrons via neutron guides and beam tubes on the ground floor level. Thermal neutrons are extracted to the experimental areas in 4 sectors of the target block.
Every sector has two beam tubes, leading the thermalised neutrons to different expriments.
Heavy water for moderation and cooling was preferred to light water because of its smaller cross section for neutron capture, resulting in a higher efficiency of neutron thermalisation. The photograph of the open moderator tank with visible inserts for cold source and beam tubes. The moderator is enclosed by 7000 tons of steel bulk shielding , which is cooled by a light water cooling system and surrounded by the outer target block structure of heavy shielding concrete, . In two opposite sectors, cold neutrons are extracted to the experiments by neutron guides from a cold moderator of about 20 liters of liquid deuterium, at the very low temperature of 25 Kelvin. The heat generated by the irradiation of the cold moderator is removed by a thermosyphon cooling loop into a vertical insert, which contains at its top a heat exchanger.