Why does Grenoble have a nuclear reactor that makes no electricity?

One process, two different jobs

In a reactor, uranium nuclei split, or fission. Each fission releases energy, mostly as heat, and a few free neutrons. Some of those neutrons go on to split further nuclei and keep the chain reaction going.

A power plant cares about the heat. It turns it into steam, the steam drives turbines, and the turbines generate electricity. The ILL describes the contrast in its own FAQ: commercial power plants exploit nuclear reactions to produce energy, while the ILL uses its reactor to extract neutrons for research. That, it adds, is why its reactor is small and compact.

Why go to such lengths for neutrons? Because neutrons are a uniquely useful probe of matter. They penetrate deep into materials, see light atoms such as hydrogen that X-rays barely notice, and behave like tiny magnets. How they compare with X-rays is explained on Two ways of seeing atoms.

How big is it, really?

Research reactors are usually tiny by power-plant standards. In an article in its Bulletin, the IAEA notes that most research reactors have an output of under 1 megawatt, in contrast to the roughly 3,000 megawatts of heat (about 1,000 megawatts of electricity) of a large nuclear power unit. It also says they have a simpler design, need far less fuel, produce much less waste and run at lower temperatures. According to the same article, 224 research reactors were operating in 53 countries.

The ILL reactor sits between those worlds. At 58 megawatts it is far larger than a typical research reactor, but a large power reactor produces about fifty times as much heat. Its core is a single fuel element containing almost 10 kilograms of highly enriched uranium. That one element is the whole core.

What happens inside

The neutrons released by fission are very fast: the ILL gives their speed as about 20,000 kilometres per second. For experiments they have to be slowed down, or moderated. In Grenoble that job is done by heavy water, water in which ordinary hydrogen is replaced by deuterium. The heavy water both cools the core and slows the neutrons, so that they can trigger further fissions and also be sent to the instruments.

The vessel of heavy water stands in a pool of demineralised water, which shields the surroundings from the neutron and gamma radiation produced by the core. The chain reaction is controlled by a single neutron-absorbing rod, withdrawn little by little as the fuel is used up; five safety rods, also neutron absorbers, can shut the reactor down in an emergency.

Around twenty beam tubes lead neutrons out of the reactor. Neutron guides then carry them to about forty experimental areas, some as far as 100 metres away. Special sources tune their speed: a graphite sphere at 2,600 °C produces “hot” neutrons, and 20 litres of liquid deuterium at −248 °C produce “cold” and “ultra-cold” ones.

And the heat?

Fission inevitably produces heat, and 58 megawatts of it has to go somewhere. The ILL states that this heat is removed by a secondary cooling system supplied with water from the river Drac and is not reused. In a power plant, heat is the product; here, it is a by-product.

The reactor does not even run at full power all the time. Its published cycle table for 2026 lists three cycles, of 48, 56 and 58 days, at 56, 47.9 and 46.2 megawatts respectively; the last one ended on 29 October 2026 (status as of 3 November 2026). The ILL’s pages describe the usual rhythm in two slightly different ways, as 50-day cycles or as about three cycles of roughly 60 days a year. Each cycle uses up one fuel element.

Why a reactor and not something else?

There is another way to free neutrons: hit heavy nuclei, such as mercury, with high-energy protons, a process called spallation. The ILL acknowledges that spallation sources produce pulsed beams with advantages of their own, but says they do not match its time-averaged flux. The figure the ILL gives for its reactor is 1.5 × 10¹⁵ neutrons per second per square centimetre, which it describes as the most intense continuous flux in the world.

That intensity is the whole point. The more neutrons arrive at a sample, the smaller the sample can be and the faster a measurement can be made. It is also why scientists queue for time: the ILL says it receives about 1,500 beam-time requests a year and can accept about half.

Safety in an Alpine valley

A reactor in a city at the foot of the Alps has to be designed for earthquakes and floods. The ILL designs its safety systems to withstand, among other scenarios, an earthquake of magnitude 5.7 at a depth of 7 kilometres directly beneath the reactor. After the Fukushima accident the authorities asked French operators, the ILL among them, to base their calculations on an even more severe earthquake.

The ILL also runs an environmental laboratory that belongs to the French national network for monitoring radioactivity in the environment, run by the nuclear safety authority. It takes about 1,500 samples a year, among them water from the Drac and the Isère, giving some 5,000 separate analyses.

The reactor has been rebuilt in stages. Its vessel was replaced in 1995, which the ILL says extended the reactor’s technical life to around 2060, and in 2025 the governments of France, Germany and the United Kingdom approved its operation until 2033. The institute’s history is told on Europe’s neutron source. Who can use the reactor’s neutrons, including scientists from Poland, is explained in Can scientists from Poland use the ESRF and the ILL?