80 Years After the Discovery of Fission, Refinements on the Neutron Energy of U-238 Fission.

[NNadir]

Uranium-238 is generally not thought of as a nuclear fuel, since in the thermal spectrum that dominates the world nuclear fleet, its fission cross section is too small to matter. It is the heaviest nuclide that cannot sustain a critical mass, whereas lighter nuclides such as U-233, U-235, and Np-237 all can do so. In fast reactors, including many designs of the "breed and burn" types of small modular reactors being developed around the world, U-238 can fission to a limited extent, directly, without transmutation into plutonium. Thus the properties of neutrons emitted by the direct fission of U-238 are important to understand.

I came across this news item on a Los Alamos News Feed:

Chi-Nu experiment concludes with data to support nuclear security, energy reactors

Subtitle:

Experiment measures the energy spectrum of neutrons emitted from neutron-induced fission

A picture of the apparatus:



The caption:

Physicist Keegan Kelly installs a fission-counting target containing approximately 100 milligrams of an actinide of interest for a Chi-Nu experiment. The apparatus includes 54 liquid scintillation neutron detectors and 22 lithium-glass detectors to measure neutrons in different energy ranges.

An excerpt:

The results of the Chi-Nu physics experiment at Los Alamos National Laboratory have contributed essential, never-before-observed data for enhancing nuclear security applications, understanding criticality safety and designing fast-neutron energy reactors. The Chi-Nu project, a years-long experiment measuring the energy spectrum of neutrons emitted from neutron-induced fission, recently concluded the most detailed and extensive uncertainty analysis of the three major actinide elements — uranium-238, uranium-235 and plutonium-239.

“Nuclear fission and related nuclear chain reactions were only discovered a little more than 80 years ago, and experimenters are still working to provide the full picture of fission processes for the major actinides,” said Keegan Kelly, a physicist at Los Alamos National Laboratory. “Throughout the course of this project, we have observed clear signatures of fission processes that in many cases were never observed in any previous experiment.”

The Los Alamos team’s final Chi-Nu study, on the isotope uranium-238, was recently published in Physical Review C. The experiment measured uranium-238’s prompt fission neutron spectrum: the energy of the neutron inducing the fission — the neutron that crashes into a nucleus and splits it — and the potentially wide-ranging energy distribution (the spectrum) of the neutrons released as a result. Chi-Nu focuses on “fast-neutron-induced” fission, with incident neutron energies in millions of electron volts, where there have typically been very few measurements...

...Actinide elements, and the chain reactions they can undergo, are important for nuclear weapons and energy reactors. (Actinides are the 15 elements, all radioactive, with an atomic number from 89 to 103.) When a nucleus undergoes fission, or splits, several neutrons are released, potentially inducing fission in neighboring nuclei to create the chain reaction. The probability of subsequent reactions in the chain depends on the energy of the fission neutrons...

The full paper to which this news release refers is this one:

Measurement of the 238 U (n,f) prompt fission neutron spectrum from 10 keV to 10 MeV induced by neutrons with 1.5–20 MeV energy, K. J. Kelly, M. Devlin, J. M. O'Donnell, D. Neudecker, A. E. Lovell, R. C. Haight, C. Y. Wu, R. Henderson, E. A. Bennett, T. Kawano, J. L. Ullmann, N. Fotiades, J. Henderson, S. M. Mosby, T. N. Taddeucci, P. Talou, M. C. White, J. A. Gomez, and H. Y. Lee Phys. Rev. C 108, 024603 – Published 14 August 2023

An excerpt from the introduction to that paper:

Monte Carlo simulations are utilized for performance and safety calculations of new nuclear reactor designs (see, e.g., Refs. [1,2]). Light water (i.e., lower neutron energy) reactors are largely insensitive to the neutron-induced fission of 238U owing to the fission threshold of approximately 1.5 MeV [3]. However, the recent emergence of fast (i.e., higher neutron energy) sodium-cooled reactors has brought deficiencies in nuclear data related to 238U near the top of lists of current nuclear data needs (see Refs. [1,4] and references therein). Thus, the energy spectrum of neutrons emitted from neutron-induced fission [i.e., the prompt fission neutron spectrum (PFNS)] of 238U is of fundamental importance for understanding the distribution of neutron energies available for reactions in these fast reactor systems. Along with the average number of emitted neutrons from fission and the fission cross section, the PFNS is one of the major fission quantities required for criticality calculations, and of these three is has by far the highest uncertainty.

Compared with other actinides, 238U has a reasonably broad coverage of incident ( Eincn) and outgoing ( Eoutn) neutron energies from historic experimental data sets. See Table I for a list of all experiments and the incident energies measured. However, upon further investigation two important features of these data become clear: (1) As stated in Table VI of Ref. [5], the results of Refs. [6–15] were all reported with incomplete uncertainty quantification, and (2) the measurements of Refs. [8–13] are all highly correlated with each other in that the author lists are usually almost identical, the analysis techniques applied are generally consistent, and the experimental facility and equipment are similar if not identical. Thus, despite this wide Eincn coverage range, it is possible that there exists a systematic bias in the measurements of Refs. [8–13], which make up the vast majority of literature data on the 238U PFNS...

Fission in U238 is a driver of thermonuclear weapons, since fusion neutrons, with an energy of 14.1 MeV can fission it. To a limited extent, fast neutrons in a reactor (typically on the order of 1 to 2 MeV before thermalization) can fission U-238 directly, but the moderation by U-238, although not dramatic is enough to bring the average neutron energy below the region in which the cross section is high enough to cause fission. Neutrons are captured instead, and two β decays lead to fissionable plutonium.