Nuclear study provides key insights into plutonium isotope fission

Nuclear study provides key insights into plutonium isotope fission

On March 4 this year, India stepped into the threshold of the second stage of its nuclear power programme when engineers began the core-loading process of the Prototype Fast Breeder Reactor (PFBR) at the Madras Atomic Power Station in Kalpakkam. While the first stage used uranium isotopes as nuclear fuel in reactors with plutonium-239 (Pu-239) and pressurised heavy water to produce energy, the second stage is more about plutonium fission.

When a Pu-239 nucleus captures a neutron, it has a 27–38% chance of becoming Pu-240 rather than undergoing fission. It is therefore present in the fallout of many nuclear reactors and nuclear weapon tests. When Pu-240 captures a neutron, it often transforms into Pu-241. However, if it does undergo fission, there is considerable uncertainty about the energy carried away by its fission products. Researchers currently use models that involve many complex calculations based on theory to estimate the output.

only the second time

A portion of the fission energy carried away by a neutron is called the prompt fission neutron spectrum (PFNS). 'Prompt' means the neutron that can be emitted by a Pu-240 nucleus when it captures a neutron with enough energy to destabilize it, but before the nucleus has reached a stable (or equilibrium) state.

So far there has been only one study that attempted to study the PFNS of induced fission in Pu-240, where the energy of the neutrons bombarding the Pu-240 nuclei was 0.85 mega-electron-volts (MeV). Recently, researchers in the US reported only the second attempt so far to measure the PFNS of induced fission in Pu-240 and the first attempt to use neutrons with energies greater than 0.85 MeV.

Their findings, published in the journal Physical Review C on June 13, note the significant differences between predicted and measured PFNS after induced fission. This information will be useful to researchers in a variety of fields, from reactor designers to nuclear medicine practitioners.

“The PFBR uses plutonium recovered from CANDU's used fuel and hence will contain a considerable amount of Pu-240. And if the used fuel coming out of the PFBR is reprocessed, it will also contain Pu-240,” explained M V Ramana, a professor at the University of British Columbia. Hindu“So any new information about how Pu-240 behaves would be relevant.”

CANDU is a Canadian design for pressurised heavy water reactors, like the one India uses for its first stage.

Introduction of Pu-240

Pu-239 is formed when U-238 is exposed to neutrons of a certain energy in the reactor. Since Pu-239 converts neutrons into Pu-240 at a certain rate, any Pu-239 left in the reactor for a certain period of time will accumulate a predictable amount of Pu-240. The two isotopes are difficult to separate, so as the Pu-240 is formed, the spent fuel is vented out.

Pu-240 undergoes spontaneous fission, i.e. without any 'external' neutrons first striking it, and emits an alpha particle. For these reasons, the isotope is considered a contaminant of weapons-grade plutonium, where its composition is limited to less than 7% by weight. There are a few ways However, much larger quantities of Pu-240 must be used to make a nuclear weapon.

Generally, if plutonium contains more than 19% Pu-240 by mass, it is considered reactor-grade rather than weapons-grade.

Test Setup

The new findings are based on a test carried out by researchers at the Los Alamos Neutron Science Center (LANSCE) in the US. They hit a tungsten disk with pulses of protons, producing neutrons of 0.01-800 MeV energy. Those neutrons, which rotated 15 degrees to the left of the proton beam, were redirected into a chamber containing 99.875% pure Pu-240.

An array of liquid scintillators — substances that emit flashes of light when struck by energetic particles — arranged around the Pu-240 sample tracked its output. The total weight of the Pu-240 was 20 milligrams; the researchers wrote in their paper that they used such a small sample to minimize the amount of alpha particles emitted.

Computer-generated rendering of the liquid scintillator detector system at LANSCE. The Pu-240 sample is located at the center and the neutrons intended to bombard the sample enter from the bottom left.

A computer-generated rendering of the liquid scintillator detector system at LANSCE. The Pu-240 sample is located at the center and neutrons intended to bombard the sample enter from the bottom left. | Photo credit: U.S. Department of Energy

With this setup, the researchers measured the neutrons emitted by the sample as well as the energy of other fission products.

Because they were interested in particles of a specific origin (neutron-induced fission), the researchers had to carefully subtract contributions from spontaneous fission, alpha particles, and other sources to extract the relevant PFNS data from the overall data. Having done this, they reported their analysis for incident neutrons of 1-20 MeV energy.

Updating the Atomic Data Library

In addition to the deviation between the PFNS predicted by the model and the PFNS observed in the test, the researchers also reported a higher-than-expected rate of second-chance fission of Pu-240: when a nucleus is not fissile but becomes so after losing a neutron. They also reported signs of “a small contribution from third-chance fission”, but said it was “difficult to see directly in the data”.

Models that predict the output of nuclear reactions are based on a library of data collected from various experiments, reactor operation records, simulations, and other sources. For example, the application of ENDF Library The library, developed by the U.S. National Nuclear Data Center, is useful in researching nuclear reactors, radiation shielding design, radiation dose calculations in nuclear medicine, investigating the smuggling of nuclear materials, and understanding the origin of elements in the universe. Other such libraries include JEFF-3.3, developed by the OECD's Nuclear Energy Agency, and JENDL-5.0, developed by the Japan Atomic Energy Agency.

The PFNS paper concludes that only the JENDL-5.0 library includes “multiple-probability fission and pre-equilibrium neutron emission processes” and that the ENDF/B-VIII.0 and JEFF-3.3 libraries “include multiple-probability fission but not pre-equilibrium”. “However,” it adds, “there are also clear differences observed within these. [energy levels] “Possible energy shifts in the thresholds relate to the magnitude of the second opportunity fission in the data, as well as to the pre-equilibrium neutron emission process prior to fission.”

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