Mapping the scattered family tree of fission neutrons

July 20, 2021, 9:30AMNuclear News
Nicholas Thompson of LANL helps set up the neutron clustering measurements at the Walthousen Reactor Critical Facility at Rensselaer Polytechnic Institute in Schenectady, NY. (Photo: LANL)

A statistically predicted tendency for neutrons produced inside fission reactors to form in clusters can cause asymmetrical energy production that is counterbalanced, at least in part, by the spontaneous fission of radioactive material in the reactor.

This neutron-clustering effect theory has been demonstrated in a nuclear reactor for the first time and was described in an article published by Los Alamos National Laboratory on July 12. The findings of a study supported by the Department of Energy’s Nuclear Criticality Safety Program, funded through the National Nuclear Security Administration, and carried out in collaboration with two French nuclear agencies—the Institute for Radiological Protection and Nuclear Safety (IRSN) and the Atomic Energy Commission (CEA)—could improve reactor safety and lead to more accurate simulations, according to LANL. The team’s conclusions were recently published in the journal Nature Communications Physics in an article titled “Patchy nuclear chain reactions.”

Gambler’s ruin: “The neutron-clustering phenomenon had been theorized for years, but it had never been analyzed in a working reactor,” says Nicholas Thompson, an engineer with LANL’s Advanced Nuclear Technology Group. “The findings indicate that, as neutrons fission and create more neutrons, some go on to form large lineages of clusters while others quickly die off, resulting in so-called power tilts, or asymmetrical energy production.”

A statistical concept known as the gambler’s ruin, believed to have been derived centuries ago by French mathematician Blaise Pascal, suggests that even if the chances of a gambler winning or losing each individual bet are 50 percent, the chance that the gambler will eventually go bankrupt is 100 percent. The concept has been demonstrated repeatedly in the life sciences in contexts including the spread of epidemics and the growth of bacteria on petri dishes.

Each neutron produced through a fission chain reaction in a nuclear reactor can be said to have a similar 50 percent chance of dying or fissioning to create more neutrons, according to LANL. According to the gambler’s ruin concept, the neutrons in a reactor would have the statistical chance of dying off completely in a future generation, stopping the chain reaction and leading to an unexpected reactor shutdown. The risk of asymmetrical energy production from neutron clustering effects leading to an unplanned scram is increased during reactor startup.

Seeking answers at RPI: To understand the extent to which the gambler’s ruin concept holds true for neutrons in nuclear reactors, the researchers collected data over the course of a week in August 2017 at the low-power Walthousen Reactor Critical Facility at Rensselaer Polytechnic Institute in New York. According to LANL, the team used three different neutron detectors, including the Los Alamos–developed Neutron Multiplicity 3He Array Detector (NoMAD), to trace interactions inside the reactor.

“We were able to model the life of each neutron in the nuclear reactor, basically building a family tree for each,” said Thompson. “What we saw is that even if the reactor is perfectly critical, so the number of fissions from one generation to the next is even, there can be bursts of clusters that form and others that quickly die off.”

The team found that a complete die-off was avoided in the small reactor because spontaneous fission—nuclear splitting of radioactive material inside reactors that is not caused by direct impact from a fission neutron—creates more neutrons. The balance of fission chain reactions and spontaneous fission lessens the impact of energy bursts created by clustering neutrons.

Safety implications: “Commercial-sized nuclear reactors don’t depend on the neutron population alone to reach criticality, because they have other interventions like temperature and control rod settings,” according to Jesson Hutchinson, of LANL’s Advanced Nuclear Technology Group. “But this test was interested in answering fundamental questions about neutron behavior in reactors, and the results will have an impact on the math we use to simulate reactors and could even affect future design and safety procedures.”


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