Heavyweight aggregate, mortarless block equals lean radiation shield

Targeting subatomic particle containment, engineers have turned to a modular concrete design for an accelerator research facility at Uppsala University, Sweden: precast blocks bearing MagnaDense, a high magnetite content aggregate derived from a deposit near the Arctic Circle.

Crews placed the modular, high density blocks in two layers, each 40 cm thick; the 80 cm cross section is a significant reduction in thickness compared to the standard concrete mass required to contain radiation. All joints between modules are placed to avoid alignment, adding to the containment feature.
Uppsala University staff designed the Facility for Research Instrumentation and Accelerator Development’s radiation shielding scheme around three bunkers enclosed with precast concrete block bearing magnetite-rich aggregate. Researchers aim to house the world’s most powerful neutron source.

The modular radiation shielding design was developed by Uppsala’s Ångström Laboratory, which is overseeing construction of the large-scale Facility for Research Instrumentation and Accelerator Development (FREIA). The facility is charged with engineering and testing components for the European Spallation Source project in Lund, Sweden, which will include a linear proton accelerator that will become the world’s most powerful neutron source.

As with all research facilities where excess radiation is generated, effective subatomic particle shielding is critical for health and safety reasons. The precast concrete blocks perform accordingly.
Their modular aspect “is almost like Lego, you build it and can take it down and rebuild in another place or configuration,” says Bengt Johansson, quality manager at Strängbetong’s Långviksmon, Sweden, plant. The Uppsala laboratory contract required tight quality control and innovation, he adds, noting, “The request from the FREIA Laboratory was that we produce high density concrete modules with extreme precision and a density of 3.9 t/m3. The modules were to be mounted using only gravity for jointing, which is why they demanded a precision of plus/minus 1 mm.”

NS-LKAB-400-IISträngbetong is part of Consolis Group, a key concrete element source in Europe, and proved equal to the project’s strict quality parameters. The producer secured four precise steel forms to cast the shapes of various sizes from 80 cm to 5.6 m, and a width of 40 cm. In all, Strängbetong fabricated 514 modules in a time-consuming production process.

“To keep costs down, as the steel forms were very expensive, we could only cast four modules per day. The advance planning of our customer made this approach possible,” affirms Johansson. Meeting specifications with workable mixes was not too difficult, he adds, as “We ended up with densities in the range of 3.9 to 4.0 t/m3. Thanks to the MagnaDense used in the mix, only small variations would occur depending on the amount of water.”

Strängbetong designed a mix with two MagnaDense gradations of 8 mm and 20 mm top sizes. The aggregate bears iron ore mined and processed at an LKAB Minerals AB operation in northern Sweden. The company dispatched two trucks every other week to Strängbetong, whose contract required nearly 1,500 tons of MagnaDense. LKAB Minerals tailored the logistic setup to match customer requirements as they could not store the full volume at their production site.

Approved for Radiation Shielding Concrete DIN 6847-2, the aggregate exhibits these physical properties: 4.8–5.2 particle density (dry); water absorption < 0.3 percent; angular particle shape; and, rough surface texture. — Adapted from a report by David Högnelid of LKAB Minerals AB, Lulea, Sweden; www.lkabminerals.com. North American contact: Mats Drugge, president, LKAB Minerals Inc., Cincinnati, Ohio, 513/322-5530; [email protected].