Forschungsreaktor Diorit, Nuclear research reactor in Würenlingen, Switzerland

The Forschungsreaktor Diorit was a heavy-water research reactor at the Swiss Federal Institute for Reactor Research (EIR) in Würenlingen, Canton Aargau. Heavy water served as both moderator and coolant, and the initial reactor had a thermal power of 20 MW. Its 2 m long fuel rods, aluminium-clad and nickel-sheathed, were made by AMF Atomics Canada Ltd; natural uranium was used first, later enriched uranium.

Diorit was operated by the Swiss Federal Institute for Reactor Research (EIR) in Würenlingen from 1960 to 1977. After a 1967 incident in which a fuel rod partially melted, the reactor vessel was replaced and the fuel switched from natural uranium to enriched uranium oxide, a configuration known as Diorit II. Operation ceased in 1977; dismantling was first planned in the early 1980s and decided in 1994.

The reactor's name, Diorit, refers to the rock diorite. Heavy water served as both moderator and coolant, a combination that gives good neutron economy and theoretically suits weapons-grade plutonium production. Diorit was used only for civil research, and no weapons plutonium was ever extracted from it. Historian Jürg Stüssi-Lauterburg showed, from formerly classified meeting notes, that the Swiss military nonetheless treated the reactor as grounds for seeing Switzerland as a nuclear threshold power.

The site is located in Würenlingen and is no longer accessible to visitors since the facility has been completely dismantled. Those interested in learning about its history can consult local museums or historical records in the region.

The 1967 incident was a partially melted fuel rod that contaminated the reactor hall and raised radioactivity levels in the River Aare. The entire primary heavy-water cooling system then had to be decontaminated by chemical milling. The accident led to the reactor vessel being replaced and the switch from natural to enriched uranium fuel, the rebuilt unit being known as Diorit II.