An elevation plot of the highest energy neutron flux distributions from an axial slice of a nuclear reactor core is shown superimposed over the same slice of the underlying geometry. This figure shows the rapid spatial variation in the high energy neutron distribution between within each plate along with the more slowly varying, global distribution. UNIC allows researchers to capture both of these effects simultaneously. (Courtesy: Argonne National Lab/Flickr)

U. CHICAGO (US)—A new computer algorithm allows scientists to view nuclear fission in much finer detail than ever before.

A team of nuclear engineers and computer scientists at the U.S. Department of Energy’s (DOE) Argonne National Laboratory are developing the neutron transport code UNIC, which could prove crucial in the development of nuclear reactors that are safe, affordable, and environmentally friendly.

To model the complex geometry of a reactor core requires billions of spatial elements, hundreds of angles, and thousands of energy groups—all of which lead to problem sizes with quadrillions of possible solutions.

Such calculations exhaust computer memory of the largest machines, and therefore reactor modeling codes typically rely on various approximations. But approximations limit the predictive capability of computer simulations and leave considerable uncertainty in crucial reactor design and operational parameters.

“The UNIC code is intended to reduce the uncertainties and biases in reactor design calculations by progressively replacing existing multilevel averaging techniques with more direct solution methods based on explicit reactor geometries,” says Andrew Siegel, a computational scientist at Argonne and leader of Argonne’s reactor simulation group.UNIC has run successfully at DOE leadership computing facilities, home to some of the world’s fastest supercomputers, including the energy-efficient IBM Blue Gene/P at Argonne and the Cray XT5 at Oak Ridge National Laboratory. Although still under development, the code has already produced new results.

In particular, the Argonne team has carried out highly detailed simulations of the Zero Power Reactor experiments on up to 163,840 processor cores of the Blue Gene/P and 222,912 processor cores of the Cray XT5, as well as on 294,912 processors cores of a Blue Gene/P at the Jülich Supercomputing Center in Germany.

With UNIC, the researchers have successfully represented the details of the full reactor geometry for the first time and have been able to compare the results directly to the experimental data.

Development of the UNIC code is funded principally by DOE’s Office of Nuclear Energy through the Nuclear Energy Advanced Modeling and Simulation program.

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