The ILL’s high-flux reactor is devoted exclusively to research. It operates continuously during 50-day cycles.
Its core comprises a single highly enriched uranium fuel element (10 kg) that is cooled by heavy water. The reactor produces the most intense continuous neutron flux in the world, namely 1.5 x 1015 neutrons per second and per cm2. Its thermal power of 58 MW is not reused and is removed by a secondary cooling system supplied with water from the river Drac.
The heavy water vessel that contains the core is situated in a pool filled with demineralised water which provides shielding from the neutron and gamma radiation produced by the core. The reactor is controlled by means of a neutron-absorbing rod, which is gradually withdrawn from the core as the uranium is burned up. It also has 5 safety rods, which are also neutron-absorbing devices and are used to shut down the reactor in the event of an emergency.
The neutrons produced in the reactor by fission are very high-energy neutrons (speed: 20 000 km/s). They are slowed down by the heavy water both to trigger further fission events in order to sustain the chain reaction (thermal neutrons with a speed of 2.2 km/s) and to supply neutrons to the scientific instruments.
Three components located in the immediate vicinity of the core also make it possible to produce hot neutrons (10 km/s), as well as cold and ultra-cold neutrons (700 m/s and 10 m/s, respectively). These components are a hot source, comprising a graphite sphere maintained at a temperature of 2600°C, and two cold sources, the largest of which is a sphere containing 20 litres of deuterium maintained in a liquid state at -248°C, in which the neutrons are slowed down to the desired energy by a succession of collisions with the deuterium atoms.
The neutrons are extracted from inside the reactor vessel by around twenty beamtubes, some of which are directed at the hot source or one of the cold sources. These beamtubes, which extend outside the reactor pool in the form of neutron guides, then supply neutrons to around forty experimental areas equipped with leading-edge instruments, located up to 100 m from the reactor.
