Nuclear materials aging

Overview

Aging of nuclear materials caused by alpha decay poses a critical challenge to the long-term safety and reliability of the nation’s nuclear deterrent. Using ion implantation technology at CAMS, our scientists produce specimens of plutonium and other materials that contain the equivalent of hundreds of years of radioactive decay products. In addition, we collaborate with materials scientists who characterize the material’s behavior under dynamic thermal and pressure conditions. Results from these experiments help us predict the stockpile’s long-term behavior and inform decisions regarding life-extension programs.

At CAMS, researchers inject helium atoms into the material, which creates bubbles in its structure, causing swelling and instability. This process simulates how radioactive materials such as plutonium age over hundreds of years, although we can alter the material in hours or days. In addition, we collaborate with scientists who use powerful transmission electron microscopes to conduct a post-irradiation analysis of the material, including features that are less than one nanometer in size. These research findings provide insight regarding how nuclear materials continuously evolve in response to radioactive processes and their environment.

The impact of temperature changes on nuclear materials

Diagram showing the bubbles that form when helium atoms are injected into material. — At CAMS, we inject helium atoms into material, which creates bubbles and causes swelling and instability. These experiments enable researchers to better understand how harsh radiation environments alter material.

LLNL scientists and their collaborators are exploring how alpha-radiation damage degrades materials, producing swelling and embrittlement. For example, we studied swelling caused by nano-scale helium bubbles in aluminum metals, which have characteristics that are similar to plutonium (e.g., bubble morphology and melting point). The aim of this research was to more accurately predict how nuclear materials respond to temperature variations.

We studied the connection between temperature changes and the growth of inert-gas bubbles in the aluminum, which damage the material’s crystal lattice structure. At CAMS, we implanted helium ions into the aluminum, and then we used various imaging tools to analyze the number of bubbles that formed, their size and the speed at which they grew, as well as how they migrate and coalesce at varying temperatures. We also analyzed changes to the bubbles as the temperature increased and after cooling, exploring whether bubbles grow at higher temperatures and shrink in size at lower temperatures.

Findings help scientists better understand how temperature changes affect bubble growth, providing insight regarding strategies that can optimize material so that it is more tolerant to temperature variations.

Key collaborator: Argonne National Laboratory

Contact:

The impact of pressure on nuclear materials

Bubbles caused when gold foil is implanted with helium ions. — Gold foil implanted with helium ions, captured just before a high-pressure experiment. The bubbles can be seen as circular areas of lighter contrast, approximately 5 nm in diameter.

LLNL scientists and their collaborators are exploring how inert-gas bubbles that form in solid nuclear materials respond to varying pressures. For example, we studied how helium bubbles form and evolve in metallic gold material, which has a high-electron density that is similar to fissile metals, including interatomic spacing and melting temperature.

At CAMS, we implanted helium ions into the gold foil, and then we used diamond anvil cells to apply pressure to the material. We then analyzed the behavior of the gas bubbles in the gold foil using imaging tools, such as transmission electron microscopy and small-angle x-ray scattering.

These experiments demonstrated that equation of state (EOS) of a two-component system could be determined via independent measurements of the individual components. This result will help materials scientists more accurately calculate EOS in complex systems.