Now on ASCR Discovery: Core Data

Wednesday, May 18, 2016

ASCR Discovery: Core Data

For those concerned with climate change, nuclear energy presents a distinct advantage: It’s free of carbon and other greenhouse gases.

But splitting atoms to run homes and factories still gives pause. For one, there’s the long-lived radioactive waste reactors generally produce. Then there are safety concerns, highlighted by 2011’s Fukushima Daiichi disaster, when an earthquake and tidal wave damaged a nuclear power plant and spread radiation.

More-efficient reactor designs could mitigate these problems. Argonne National Laboratory researchers are applying the power of high-performance computing (HPC), combined with sophisticated experiments, to refine plans for one, the sodium-cooled fast reactor (SFR).

The SFR uses uranium more efficiently and could even outlast the century-long lifespans of today’s water-cooled nuclear reactors, but they have a big problem: If the sodium metal coolant interacts with air or water, it can burn or even explode. That makes it expensive and difficult to test SFRs’ performance and safety.

Computational models and simulations can help fill the gap, providing critical tools to understand how sodium metal coolant behaves in SFR fuel cores, especially how it flows around the complicated geometry of wire-wrapped fuel pins, says Elia Merzari of Argonne National Laboratory.

Teasing apart the details of fluid flow within SFR reactor cores is challenging. SFRs hold thousands of parallel uranium-filled pins or rods, arranged in bunches of 61 to 271. A helical spiral of wire surrounds each pin to keep it from interacting with its neighbors. Liquid sodium coolant is pumped through the core and flows between and around these complex shapes.

Experiments provide essential benchmarks for understanding SFRs’ behavior, Merzari says, but they only fill in part of the picture. Detailed computations help researchers understand sodium’s behavior in reactor regions beyond the reach of experimental probes or where conditions are too extreme to measure experimentally.

It’s incredibly difficult work, Argonne’s Aleksandr Obabko says. “The sodium is in a complex geometry. The wire wraps around the pins, and those structures create a lot of turbulence, a multitude of vortices on different scales.” The vast size range involved also complicates calculating fluid flow’s subtleties. The tiniest swirls around wires can be millionths of a meter in size, but their cascading effect produces physical forces on the scale of meters. And these changes occur in milliseconds.

Calculations that span this range are so demanding that modelers have had to make restrictive assumptions so HPC systems could complete them in a reasonable amount of time.

But access to the huge machines at Argonne’s Leadership Computing Facility lets Merzari and his colleagues simulate these phenomena with far fewer assumptions. “We’re the only ones in the nuclear field who are working on this scale with these sorts of complex geometries.”

The team is using an ASCR Leadership Computing Challenge award of 72 million processor hours on Mira, the lab’s IBM Blue Gene/Q, to get a sharper picture of the turbulent eddies that form as sodium metal flows around fuel assemblies. The DOE Office of Nuclear Energy also supports the work through its Advanced Reactor Technologies and Advanced Modeling and Simulation programs.

Read more at ASCR Discovery, a website highlighting research supported by the Department of Energy’s Advanced Scientific Computing Research program.

Image caption: A detail of the turbulent velocity magnitude in a rod bundle, in a hexagonal arrangement typical of sodium fast reactors. Warmer colors represent a higher velocity, colder colors a lower velocity. The multiple scales of turbulence are of particular interest to researchers. Image courtesy of Argonne National Laboratory.