National lab scientists complete critical plutonium experiment at test site

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By Jill McLaughlin

The first of 10 critical experiments using plutonium was successfully completed at the Nevada Test Site this year by researchers from Las Alamos and Lawrence Livermore national laboratories, LLNL announced this week.

This was the first successful criticality experiment in 40 years. Another nine total configurations are planned at the site before March 2018, according to Nolan O’Brien, a public information officer at Lawrence Livermore National Laboratory.

“The series is ongoing,” O’Brien said Thursday. “We will continue to test new configurations and bring them to critical.”

For the test, researchers layered stacks of plutonium and brought them to "critical," the point at which fissile material can sustain a nuclear chain reaction.

“We were right at critical, and we kept it together for an hour,” Catherine Percher, a nuclear engineer at LLNL who is leading the experimental series, said. “It was amazing to see the constant, slow increase in the neutron population. If we had too much mass, we would have to correct for that in our models. But we achieved a near-perfect benchmark.”

The experiment was one in a series that aims to help ensure plutonium operations continue to be conducted safely, according to O’Brien.

Tens of thousands of critical configurations have been measured over the last 70 years, but only a few are considered "benchmark quality," data that codes can be judged against by the nuclear criticality safety community. The more exact the data that underpins the safety codes, the more confidence researchers have in their codes predicting when an assembly will go critical, according to O'Brien.

The tests took five years of planning and calculations by the laboratories.

“It’s very exciting for us, and will help us get better benchmark data, a well,” O’Brien said.

The researchers plan to publish the results of the test sometime after March.