Between 1956 and 1964, SCK CEN - then known by the unwieldy Dutch acronym STK - brought four nuclear research reactors online. BR1 came first, then BR2, then BR3, which in 1962 became the first pressurized water reactor in Europe. VENUS followed in 1964. By 1963 the center employed 1,600 people. The campus in Mol grew its own ecology: offices, laboratories, medical buildings, and an entire residential district called the Atoomwijk, the Atomic Quarter, where the scientists and technicians lived alongside the reactors that paid their salaries. The Atoomwijk is still mostly owned by the study center, and most of the people who walk its streets today still work behind the security fences a few hundred meters away.

Most reactors are critical assemblies - tuned to sustain their own chain reaction, regulated by control rods that absorb just enough neutrons to keep the fission steady. MYRRHA, the Multi-purpose hYbrid Research Reactor for High-tech Applications, is being designed to do something different. It is meant to be subcritical, deliberately starved of just enough neutrons that it cannot sustain a reaction on its own. A particle accelerator will fire protons into a lead-bismuth target inside the core, spalling extra neutrons into the fuel and keeping it just-barely-burning. Switch off the accelerator and MYRRHA goes quiet within microseconds. The point is not the trick itself but what the trick allows: a reactor that can incinerate long-lived nuclear waste by transmuting it into shorter-lived isotopes. The smaller experimental version - GUINEVERE, built into VENUS in 2011 - has been proving the concept. The full MYRRHA is being built in stages on the Mol site, the most ambitious nuclear research investment in Europe.

BR2, the second of the original quartet, is still running. Its strange flower-shaped core, with channels arranged in a tilted helix, makes it one of the highest-flux research reactors on earth, and one of the world's main producers of medical radioisotopes. Technetium-99m, lutetium-177, iodine-131 - the radiopharmaceuticals used in millions of cancer diagnoses and treatments every year come, in significant part, from this single reactor in the woods at Mol. When BR2 goes down for maintenance, hospitals across Europe begin to ration scans. In May 2019 a small leak in a BR2 hot cell released selenium-75, an isotope used in industrial radiography. Monitoring stations as far away as Lille picked it up. Belgium's nuclear regulator FANC rated the incident a Level 1 anomaly on the international INES scale - the lowest grade above normal - and the doses involved were tiny. But the story tells you how finely watched this site is, and how connected it is to the air far beyond the Campine.

Not every page of the institute's history is clean. In the 1980s, SCK CEN employees were bribed by a West German firm, Transnuklear, to receive and store high-level nuclear waste they should not have accepted. The scandal made international headlines and contributed to Belgium's eventual split of nuclear and non-nuclear research, with VITO being spun off in 1991. A 2017-2020 Belgian epidemiological study, Nucabel 2, found cancer rates within five kilometers of the Mol-Dessel nuclear complex about three times the national average - a statistically significant figure, though the small number of cases and the study's narrow design mean no causal link to the reactors has been established. The institute notes that its maximum effective dose to neighbors is roughly two microsieverts per year, around five hundred times less than the natural background radiation people in the Campine already absorb.

Outside one of the laboratories at Mol, a 121-metre guyed mast rises above the pines - tall enough to read weather above the treetops. Nearby, a sampling system called Snow White draws huge volumes of air through filters each week, hunting trace radioactivity. It is sensitive enough that when wildfires tore through the Chornobyl Exclusion Zone in 2020, Snow White detected the released cesium-137 from across a continent. SCK CEN was named one of only four IAEA-designated International Centres based on Research Reactor in 2017, putting it in the same global handful as facilities in the United States and Russia. From a sandy clearing in the Kempen, a country smaller than Maryland has built itself one of the most consequential nuclear research outposts in the world.