Neutron cluster effect demonstrated for the first time in nuclear reactors demonstrated

PICTURE: Reactor operator Nicholas Thompson from Los Alamos National Laboratory is helping set up the neutron cluster measurements in the Walthousen Reactor Critical Facility of the Rensselaer Polytechnic Institute in Schenectady, NY. view More

Photo credit: Los Alamos National Laboratory

Los Alamos, NM, July 12, 2021 – For the first time, the long theorized neutron clustering effect has been demonstrated in nuclear reactors, which could improve reactor safety and enable more accurate simulations, according to a new study recently published in. the journal Nature Communications Physics was published.

“The phenomenon of neutron cluster formation has been theorized for years but has never been analyzed in a working reactor,” said Nicholas Thompson, an engineer with the Los Alamos Advanced Nuclear Technology Group. “The results suggest that when neutrons split and produce more neutrons, some form large lines of clusters while others die quickly, resulting in so-called ‘power tilts’ or asymmetrical energy production.”

Understanding these cluster fluctuations is especially important for safety and simulation accuracy, especially when starting nuclear reactors for the first time. The study was a collaboration with the Institute for Radiation Protection and Nuclear Safety (IRSN) and the Atomic Energy Commission (CEA), both in France.

“We were able to model the lifespan of each neutron in the nuclear reactor and basically create a family tree for each neutron,” Thompson said. “What we have seen is that even if the reactor is absolutely critical, that is, the number of divisions is even from one generation to the next, there can be outbreaks of clusters that form and others that die quickly.”

This clustering phenomenon became important to understand because of a statistical concept known as gambler ruin, believed to have been derived from Blaise Pascal. In a betting analogy, the concept says that even if a player’s chances of winning or losing every single bet are 50 percent, the statistical certainty that the player will go bankrupt with enough bets is 100 percent.

In nuclear reactors, from generation to generation, each neutron has a similar 50 percent chance of dying or fissioning to produce more neutrons. According to the player’s concept of ruin, the neutrons in a reactor could then have a statistical chance of completely dying off in a future generation, even though the system is critical.

This concept has been extensively studied in other scientific areas such as biology and epidemiology, where this generation clustering phenomenon is also present. Using this statistical mathematics, the research team was able to analyze whether the gambler’s concept of ruin would apply to neutrons in nuclear reactors.

“You would expect this theory to be correct,” says Jesson Hutchinson, who works with the laboratory’s Advanced Nuclear Technology Group. “You should have a critical system that, while the neutron population varies between generations, has some chance of becoming sub-critical and losing all of the neutrons. But that doesn’t happen.”

To understand why the player’s concept of ruin did not apply, the researchers used a low-powered nuclear reactor in the Walthousen Reactor Critical Facility in New York. A low powered reactor was essential to keeping track of the lifetime of individual neutrons as large reactors can have trillions of interactions at any one time. The team used three different neutron detectors, including the Los Alamos-developed Neutron Multiplicity 3He Array Detector (NoMAD), to track every interaction in the reactor.

The team found that while neutron generations accumulated in large family trees and others died out, the small reactor avoided complete death due to spontaneous fission or the uninduced fission of radioactive material in reactors, resulting in more neutrons. This balance of fission and spontaneous fission prevented complete extinction of the neutron population and also smoothed out the bursts of energy generated by neutron clustering.

“Commercial-size nuclear reactors don’t just depend on the neutron population to become critical as they have other interventions like temperature and control rod adjustments,” said Hutchinson. “But this test was interested in answering basic questions about neutron behavior in reactors, and the results will have an impact on the math we use to simulate reactors and could even influence future design and safety practices.”

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Funding: This work was supported by the DOE Nuclear Criticality Safety Program, which is funded and administered by the National Nuclear Security Administration for the Department of Energy.

Articles: Dumonteil, E., Bahran, R., Cutler, T. et al. Patchy nuclear chain reactions. Nature communication physics. July 01, 2021

DOI: https://doi.org/10.1038/s42005-021-00654-9

Via the Los Alamos National Laboratory

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