# Structural and chemical insights on the incorporation of americium into zircaloy-derived monoclinic zirconia

## TL;DR

The study reveals how americium is incorporated into zirconia in nuclear fuel cladding, affecting its structure and phase transformations.

## Contribution

The paper provides new insights into the structural and chemical behavior of americium in monoclinic zirconia.

## Key findings

- Americium enters monoclinic zirconia tetravalently with a solubility of approximately 1.0 mol%.
- Excess americium forms a C-type phase with a specific crystal structure.
- Americium incorporation lowers the phase transformation temperature of monoclinic zirconia from 1150°C to 1050°C.

## Abstract

Monoclinic zirconia (m-ZrO2) forms on the internal surface of nuclear fuel Zircaloy cladding, acting as a critical barrier against radionuclide release at the fuel-cladding interface. However, the incorporation of minor actinide elements like americium in m-ZrO₂ and resultant structural chemistry remains poorly understood. Using a combination of diffraction and high-resolution X-ray spectroscopic techniques, we have examined m-ZrO2 with 5 mol% Am doping. We show Am enters m-ZrO2 tetravalently, where its solubility is approximately 1.0 mol%, m-(Am4+0.011(7)Zr4+0.989(7))O2, attributed to the large Am4+ cation, where excess Am, that is predominantly trivalent, adopts a C-type (Am4+/3+1-xZr4+x)2O3+x phase in space group Ia-3. The known reversible high temperature phase transformation of m-ZrO2 to tetragonal is further shown to be reduced from 1150 oC to 1050 oC via Am4+ incorporation. The investigation provides critical insight into the chemical reactivity and speciation of minor actinide elements with nuclear fuel cladding related m-ZrO₂.

Monoclinic zirconia (m-ZrO2) serves as a crucial barrier against radionuclide release in nuclear fuel Zircaloy cladding, yet the effects of minor actinide incorporation and chemical reactivity remain unclear. Here, the authors reveal that americium enters m-ZrO2 tetravalently with limited solubility and influences phase transformations, offering insights into behavior of minor actinides in spent nuclear fuel materials.