Does PSI have a nuclear reactor? Where its neutrons really come from

Why the question comes up

The question is not unreasonable. PSI was formed on 1 January 1988 by merging two institutes, one of which was the Swiss Federal Institute for Reactor Research (EIR). EIR itself grew out of Reaktor AG, a company founded in Würenlingen in March 1955 and later taken over by the Confederation. PSI still does research on the safety of nuclear power plants, runs the only laboratory in Switzerland for examining highly radioactive materials, and hosts the federal interim store for radioactive waste.

On top of that, PSI produces neutrons for research on a large scale, and neutrons are what most people associate with reactors. So the answer needs two parts: what happened to the reactors, and where the neutrons come from instead.

Nuclear research itself did not end with the reactors. Under an agreement concluded in 1988, PSI provides expert support to ENSI, and its programme now focuses on nuclear and fuel safety. Since 2011 PSI has calculated the core loading of every Swiss nuclear power plant each year, as an independent check; according to PSI, its calculations have so far always confirmed those of the plant operators. That work runs on computers and in laboratories, not on a reactor of PSI’s own.

Three research reactors, all shut down

Three research reactors stood on what is now the PSI site. DIORIT was finally shut down in 1977 and its fuel removed. SAPHIR was closed down in 1994. PROTEUS, a zero-power reactor with a thermal output of about one kilowatt, cooled by natural air convection, was used from February 1968 to April 2011 for experimental reactor physics; PSI decided for strategic reasons to shut it down and not to refurbish it.

All three are now decommissioning projects supervised by ENSI. For PROTEUS, the second dismantling phase was completed in 2025 and PSI is preparing the final phase. SAPHIR’s dismantling is described by ENSI as well advanced. DIORIT is largely dismantled; in March 2026 ENSI found that PSI’s supplementary project to remove the whole installation, so that nuclear energy legislation no longer applies to the site, meets the protection objectives, subject to 14 stipulations (status as of 3 November 2026).

What nuclear facilities PSI does run

PSI is still a nuclear site, just not a reactor site. ENSI lists two kinds of facilities in operation. One is the Hotlab, where highly radioactive materials such as fuel rods from power stations are examined in hot cells with 1.5-metre concrete walls, handled by remote grippers; PSI calls it the only facility of its kind in Switzerland. The other is radioactive waste management, including the federal interim storage facilities BZL and BZL 2, the second of which went into operation in the summer of 2025. They take in radioactive waste from medicine, industry and research across Switzerland.

Spallation: neutrons knocked out by protons

The neutrons used in experiments come from SINQ, the Swiss Spallation Neutron Source, built from 1989 and inaugurated in 1996. It is driven by PSI’s large proton accelerator, which brings protons to about 80 per cent of the speed of light. The beam is steered upwards from below into a heavy-metal target. When a fast proton hits a lead nucleus, the nucleus heats up and ejects 10 to 20 neutrons. This process is called spallation.

These neutrons are far too fast for experiments, so the target sits in a tank of heavy water that slows them down. For studies of larger structures, a further tank of extremely cold heavy hydrogen (deuterium), at minus 250 °C, slows some of them even more, producing cold neutrons. Neutron guides then carry them to the instruments, as described on Neutrons & Muons.

Fission versus spallation

A research reactor makes its neutrons by fission. Uranium nuclei in the fuel split and release neutrons, some of which split further nuclei. When the arrangement is right, the process sustains itself – the reactor is said to be critical, the condition for a self-sustaining chain reaction.

A spallation source needs no such chain. Each burst of neutrons is caused by an incoming proton, so the source depends entirely on the accelerator. The Oak Ridge National Laboratory, describing its own spallation source in the United States, puts the consequence plainly: neutron production stops when the proton beam is turned off.

For the people doing experiments, the difference is less dramatic than it sounds. PSI notes that, except for the way the neutrons are released, SINQ closely resembles a medium-flux research reactor for most of its users. Some spallation sources work in pulses – the one at Oak Ridge fires its target 60 times a second – whereas SINQ delivers a continuous stream of neutrons; PSI describes it as the only continuous spallation neutron source in the world.

Poland’s neutrons come from a reactor

In Poland the situation is the reverse. The country’s only nuclear reactor, MARIA, at the National Centre for Nuclear Research (NCBJ) in Świerk near Otwock, first reached criticality on the night of 17 to 18 December 1974. It is a research reactor with a nominal thermal power of 30 megawatts, and it produces not electricity but neutrons: for radioisotopes used in medicine, such as molybdenum-99 and iodine-131, for irradiating materials and for neutron beams that reveal the structure of matter.

So both countries use neutrons for similar research, but they get them in different ways: Poland from a fission reactor, the PSI campus from an accelerator. How neutrons are used in experiments, and why they see things X-rays cannot, is explained on Neutrons & Muons. The same proton beam also makes muons, which led to one of PSI’s best-known results: How big is a proton?