A Cosmic View of Soviet Estonia’s Nuclear Legacy

The two reactors once used to train the Soviet Union’s elite nuclear submariners on the Estonian coast are going to stick around a lot longer than the October revolution.

And to make sure they’re safe, Estonia’s government—which after years of independence, likes to think of itself as forward-looking—turned to the latest in nuclear-inspection gear: muon scanners. Estonian agency officials hired an engineering startup to inspect the defunct reactors using scanners that use muons spawned by particles from interstellar space to image them.

The reactors are entombed in concrete just outside Paldiski, a small town on a small peninsula in the blustery Baltic not far from the capital Tallinn. It’s a lush, green site, quiet and wooded, dotted with rabbits and wind turbines spinning on the horizon. This is set to be the resting place and safe storage area for all of the country’s radioactive byproducts. For decades Paldiski was a closed military area, says Pärtel Saluvere, environmental technology specialist at A.L.A.R.A. (As Low As Reasonably Achievable), Estonia’s national agency responsible for handling and storing radioactive waste and decontaminating it.

Paldiski and the surrounding rocky coast are open now; local points of interest include the home of a sculptor from the early 20th century, Amandus Adamson (renowned enough to have a street named after him), and a tavern decked with maritime bric-a-brac touting local lore about how Peter the Great built Paldiski’s 18th-century harbor. The former Soviet sub base is on the edge of some woods behind a modular, cream-colored wall with a distinct mid-century modern vibe.

“That’s where they took the scanner in,” Saluvere says, pointing to a bright yellow steel hatch in the main room of a hangar-shaped engine hall. The door is at the bottom of a huge steel disc-shaped superstructure, nine meters across, three stories high: this is a cross-sectioned slice of submarine hull, one of two dominating the building. The hall is the length of two football pitches. A stray can of WD-40 sits in the corner.

The Paldiski base was a training center. The Soviet navy built simulators, full-sized submarine hulls as close to real life as possible, mimicking first the Yankee and then the Delta-class silent runners. Here, submariners could practice so that in the event something went wrong with a reactor, they could shut down systems without the added pressure of being under water. The two reactors dominated the site: one was designated VM-A and the other, VM-4. They’re both uranium-fueled pressurized-water reactors.

When the Soviet Union collapsed, newly-independent Estonia cut a deal: the fuel rods were shipped back across the new frontier; what was left was decommissioned and encased in concrete.

Prepping A Sub Simulator to Become a Waste Storage Site

Now that the Paldiski site is being prepared as a long-term nuclear-waste storage site, those huge, dense reactors need inspecting, to make sure that they are structurally and radiologically stable. To peer inside, GScan, an Estonian startup headed by a science team and a former customs official, used an alternate subatomic particle: muons.

Muons are close cousins to electrons, but 200 times heavier, with origins in intergalactic space. Big plasma jets ripping out of binary stars act as particle accelerators, zooming cosmic rays through the void. These rays eventually bombard our upper atmosphere: one of the byproducts of these collisions are muons, and they pepper every square meter of the planet surface. Muons pass through objects (including us) and can reach a kilometer into the ground. Six hundred muons cross your body every minute, says CERN.

“We’re lucky we don’t have too many, because we’d all be dead. But we have enough so that we can measure things,” says the nuclear physicist Christopher Morris, a fellow at Los Alamos National Laboratory, where over many decades he pioneered the development of muon tomography. This is a way of probing the composition of dense materials and creating two- and three-dimensional images of the interior.

One such technique measures muon transmission through an object, resolving the contrast in the resulting image according to the different densities contained within. Another observes how muons angle through an object. A detector placed above and below can track the changing trajectory of the particles as they pass through and use this scattering to create an image.

GScan makes mats of thin scintillated plastic fibers at a small fab outside the university town of Tartu: the mats are dotted at one end with tiny arrays of photon sensors. They signal when muons excite them. Last summer the GScanners eased one of these trackers underneath the sub reactors in Paldiski. Section by section, over the course of eight weeks, they observed muon transmission through the concrete. From those sessions emerged images of the reactors—indicating the integrity of the vessels once exposed to neutrons, radioactive cobalt, and plutonium. GScan’s physicist and chief scientific officer Madis Kiisk says they presented these images last month to the International Atomic Energy Agency in Vienna.

Sarah Barnes, group leader at the German Aerospace Center’s Institute for Protection of Maritime Infrastructure, is keen on the application of novel machine-learning methods to deliver better resolution and faster scanning times for muon devices. Scanning speed remains a challenge, says Ralf Kaiser, program head at ICTP Trieste (ICTP was not involved in the Paldiski work). Kaiser, a former head of the IAEA’s physics section, has consulted on the international atomic agency’s reporting about state-of-the-art muon imaging. For stationary objects, imaging speeds are not much of an issue. France’s atomic energy commission, for example, is experimenting with muon tomography instruments in its large legacy civil- and weapons- reactor dismantling program.

For applications like these, muon scanners are becoming a better option. Morris has worked repeatedly over the last decade with teams experimenting with muon scanners to aid the still-ongoing cleanup from the Fukushima meltdown after the terrible 2011 tsunami. He and his Los Alamos colleagues are working now with Toshiba to develop a small muon detector to assay the melted fuel at the Fukushima core.

Now, with nuclear power apparently poised for resurgence as an energy source feeding the enormous demands of AI data centers, and with AI titans going so far as to plan to reopen old nuclear plants, the need to verify the integrity of old reactors could mean more business for muon scanning.

“The applications for muon tomography are growing,” Morris says. “There are leftover nuclear problems around the world.”