Europe’s neutron source

In plain words

A neutron is one of the particles that make up the nucleus of an atom. Set free, it becomes a remarkably useful probe: it travels deep into materials, and, as the ILL puts it, it can behave as a particle, as a wave or as a microscopic magnet. To do experiments with neutrons you need a great many of them, and a research reactor is one way of producing them.

The ILL is such a reactor, built not for energy but for science. The ILL itself draws the contrast: commercial power plants use nuclear reactions to produce energy, while the ILL uses them to extract neutrons from uranium nuclei for research.

The reactor

The ILL’s high-flux reactor has a thermal power of 58.3 megawatts. Its core is a single fuel element containing about 10 kilograms of highly enriched uranium, cooled by heavy water. The ILL states that it gives the most intense continuous neutron flux in the world in its moderator region: 1.5 × 10¹⁵ neutrons per second per square centimetre.

The neutrons released by fission in the core are far too fast to be useful, so they have to be slowed down (moderated). Around twenty beam tubes take them out of the reactor, and neutron guides lead them to the instruments, of which there are around forty, some of them up to 100 metres from the reactor.

The ILL’s pages describe the operating pattern in two ways: one speaks of four 50-day cycles a year, its current reactor overview of about three cycles of roughly 60 days, each followed by a shutdown to change the fuel element. The published schedule for 2026 had three cycles, of 48, 56 and 58 days, the last ending on 29 October (status as of 3 November 2026).

Hot, cold and very slow neutrons

Different experiments need neutrons of different speeds, and the reactor has special sources to provide them. The ILL describes a “hot source”, a graphite sphere kept at 2,600 °C, which speeds neutrons up, and a “cold source”, a sphere holding 20 litres of liquid deuterium at −248 °C, which slows them down. According to the ILL, fast neutrons from fission travel at around 20,000 kilometres per second, thermal neutrons at 2.2 kilometres per second, cold neutrons at about 700 metres per second and ultra-cold neutrons at about 10 metres per second.

Slow neutrons have longer wavelengths, which suits larger structures such as proteins and polymers; thermal neutrons match the spacing of atoms in a crystal.

How it started

The ILL was founded on 19 January 1967 by an agreement between France and the Federal Republic of Germany. The French physicist Louis Néel, who later won the 1970 Nobel Prize for his work on magnetism, and Heinz Maier-Leibnitz, the father of neutron research in Germany and the ILL’s first director (1967–1972), were the driving forces behind the project; the ILL credits their friendship and influence with bringing it to Grenoble. The reactor was started for the first time on the last day of August 1971.

The United Kingdom decided to join in January 1973 and became the third associate country in 1974. Other countries joined later as scientific members, starting with Spain in 1987 and Switzerland in 1988; Poland has been one since 2006.

Why “Laue-Langevin”

The founders named the institute after a German and a French physicist. Max von Laue (1879–1960) won the Nobel Prize in 1914 for showing that crystals diffract X-rays, the discovery behind much of what happens at the ESRF next door. Paul Langevin (1879–1946) worked on magnetism, ultrasonics and relativity, and his study of how neutrons slow down helped shape the design of research reactors.

Renewing an old machine

The reactor is more than fifty years old, but much of it is not. Its vessel was replaced in 1995, which the ILL says extended the reactor’s technical life to around 2060. The instruments have been rebuilt in successive programmes. The Millennium programme, from 2001 to 2018, built or upgraded 25 instruments and replaced neutron guides; the ILL says it raised the average neutron detection rate 25 times. The Endurance programme that followed delivered 30 new and upgraded instruments and infrastructures, with count rates on average ten times higher.

Safety work continues alongside. The 2024–2025 shutdown lasted nearly ten months, during which a new fire sprinkler system was installed in the reactor building and the casemates that house the neutron guides were reinforced. The ILL designs its safety systems to withstand, among other scenarios, a magnitude 5.7 earthquake directly beneath the reactor, and its environmental laboratory takes about 1,500 samples of water, air and other indicators each year. In 2025 the governments of France, Germany and the United Kingdom approved operation until 2033.

The ILL today

The ILL is funded by its three associate countries, France, Germany and the United Kingdom, together with its scientific member countries. Its annual budget is about 100 million euros. It has more than 500 staff of about 30 nationalities. In 2023 more than 1,600 researchers from 40 countries came to carry out 1,062 experiments.

Beam time is scarce. Proposals are judged on scientific merit by subcommittees of the ILL’s Scientific Council, with two main deadlines a year, in February and September. The ILL says it receives almost 1,500 requests a year and can accept about half. What neutrons reveal, and how they differ from X-rays, is explained on Two ways of seeing atoms.