Reactions between U-Pu-Zr Alloys and Fe at 923 K

Abstract

In metallic U-Pu-Zr fuel, metallurgical reactions occur between the fuel slug and the cladding, and a phase of which the melting point is relatively low is formed in the reaction zone. If a liquid phase is formed, it can degrade cladding integrity. The potential for liquid phase formation near the cladding, therefore, should be excluded during normal reactor operation. In order to clarify the mechanism of liquefaction, the authors have conducted diffusion experiments at 923 K using two couples: U-13 at%Pu-22 at%Zr/Fe and U-22at%Pu- 22at%Zr/Fe, and examined the influence of the Pu content in the fuel alloy on the phases formed in the reaction zones. The liquid phase has been observed in the U-22 at%Pu-22 at%Zr/Fe couple. An assessment of the diffusion paths for these couples has indicated that the Pu content in the (U, Pu)₆Fe-type phase in the reaction zone is a crucial factor in determining the conditions that lead to liquefaction. The Pu content in the (U, Pu)₆Fe-type phase increases with that in the initial U-Pu-Zr alloy. The fuel design specifications to exclude the potential for liquefaction will be clarified by further examination of the Pu content in the (U, Pu) ₆Fe-type phase for various U-Pu-Zr/Fe couples annealed at different temperatures.

KEYWORDS:

• diffusion
• diffusion couple
• uranium alloys
• plutonium alloys
• U-Pu-Zr
• metallic fuels
• fast reactors
• cladding
• phase diagram
• electron probe microanalysis