The long-theorized neutron-clustering effect in nuclear reactors has been demonstrated. A new study recently published in the journal Nature Communications Physics details the work. The research was carried out by a collaboration of Institute for Radiological Protection and Nuclear Safety (IRSN) and the Atomic Energy Commission (CEA), both located in France.
Nicholas Thompson is an engineer with the Los Alamos Advanced Nuclear Technology Group. He said, “The neutron-clustering phenomenon had been theorized for years, but it had never been analyzed in a working reactor. 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.”
Understanding these clustering fluctuations is very important for safety and simulation accuracy. This is especially critical when nuclear reactors first begin to power up.
Thompson said, “We were able to model the life of each neutron in the nuclear reactor, basically building a family tree for each. 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.”
This clustering phenomena is important to understand because of a statistical concept known as the gambler’s ruin. The concept is attributed to Blaise Pascal. In an analogy to betting, the concept says that even if the changes of a gambler winning or losing each individual bet are fifty percent, over the course of enough bets, there is a statistical certainty that the gambler will lose all of his money.
In nuclear reactors, each neutron can be said to have a similar fifty percent chance of dying or triggering a fission event. According to the gambler’s ruin idea, the neutron in a nuclear reactor might then have a statistical chance of dying off completely after a nuclear fission event, even thought the system is at critical.
This concept has been widely studied in other scientific fields such as biology and epidemiology where this clustering effect is also found. The research team drew on this related statistical math. They were able to analyze whether the gambler’s ruin concept would hold true for neutrons in nuclear reactors.
Jesson Hutchinson works with the Laboratory's Advanced Nuclear Technology Group. He said, “You would expect this theory to hold true. You should have a critical system that, while the neutron population is varying between generations, runs some chance of becoming subcritical and losing all neutrons. But that's not what happens.”
In order to understand why the gambler’s ruin concept did not apply in nuclear reactors, researchers utilized a low-power nuclear reactor at the Walthousen Reactor Critical Facility in New York. The low powered reactor was critical for tacking the lifespans of individual neutrons because large-scale reactors can have trillions of interactions at any moment. The team used three different neutron detectors, including the Los Alamos-developed Neutron Multiplicity 3He Array Detector (NoMAD), to trace every interaction inside the reactor.
The team discovered that while generations of neutrons would cluster in large family trees and others died out, a complete die-off was avoided in the small reactors because of spontaneous fission. This is the non-induced nuclear splitting of radioactive materials inside reactors which creates more neutrons. This balance of neutron-induced fission and spontaneous fission prevented the neutron population from dying out completely. It also tended to smooth out the energy bursts created by clustering neutrons.
Hutchinson said, “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. 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.”