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Fissile materials such as plutonium require a finely calibrated approach for safe handling. In the aqueous chloride operations in PF-4 (Plutonium Facility), scientists have identified several parameters that inaccurately estimate the neutron flux in these vital operations. On p2 we describe the experimental efforts supported by machine learning designs undertaken in 2021 to improve these calculations and prepare a benchmark sensitive to chlorine-35 (n,γ) for the solution applications.

Criticality calculations are typically performed using the Monte Carlo method, a technique invented at Los Alamos in the 1940s for the purpose of predicting neutron diffusion in the hydrogen bomb. This method has had an enormous impact on innumerable disciplines both within and outside of science and technology— including economics, finance, transport, health, manufacturing, and virtually every profession that must measure risks. On p35 we present an account of the fascinating history of how the idea was conceived and realized with early computers.

Americium-241 is an important isotope for industry, yet until recently the US was dependent on foreign sources for its supply. Today, Los Alamos is proud to be the only manufacturing site of americium-241 in the US, having delivered its first shipment to customers in 2020. Recovery of americium from aged plutonium residues not only mitigated our dependence on foreign countries for sensitive materials but also reduced the waste disposal footprint, transforming a radioactive contaminant into a valuable commercial and research product (p8).

In the face of accelerating climate change, most experts agree that the only way to achieve rapid decarbonization of the energy grid is to embrace nuclear power. Specifically, development of advanced nuclear reactors, which promise improved safety, reduced cost, and a smaller waste footprint compared with conventional light-water reactors. To develop and deploy this technology rapidly, the federal government is working with private industry, providing support in the form of funding and licensing. Recently, the Department of

Energy Advanced Research Projects Agency– Energy (DOE ARPA-E) has launched a suite of funding programs with a total of $165 million to support advanced reactors. Two of these programs are highlighted in our article on p28.

— Owen Summerscales, Editor