Multicomponent multiphase diffusion studies for fuel-clad compatibility in metallic nuclear fuels

Dennis Dale Keiser, Purdue University

Abstract

As part of studies relevant to fuel-clad compatibility in the Integral Fast Reactors, isothermal interdiffusion experiments were carried out at 700$\sp\circ$C with solid-solid diffusion couples assembled with a U-23 at.% Zr alloy and two series of clad alloys of selected compositions. Series I clad materials included pure Fe, pure Ni, binary Fe-20.1 at.% Cr, Ni-16.4 at.% Cr, and Fe-10.1 at.% Ni alloys and an Fe-16.4Ni-9.4 at.% Cr ternary alloy. Series II alloys included actual clad materials used in-reactor, namely D9, HT9 and 316SS. The development of diffusion layers and their variation with compositional changes of the clad alloys are discussed in the light of phase diagrams, intermetallic formation, the relative diffusion behavior of the various elements in the fuel/clad assembly, and the experimental diffusion paths. Types of phases identified in the diffusion structures of Ni-containing diffusion couples include: U$\sb6$Ni, UNi$\sb5$, UNi$\sb{3-4}$, U$\sb5$Ni$\sb7$, (U, Zr)Ni$\sb2$, (Zr, U)$\sb7$Ni$\sb{10}$, (Zr,U)$\sb2$Ni, and U(Fe, Ni)$\sb2$. Diffusion couples which do not contain Ni have diffusion structures which accommodate the phases: U(Fe, Ni, Cr)$\sb2$, a U matrix, and Zr-rich precipitates. For diffusion couples not containing Ni, the penetration of Fe and Cr into the fuel is limited, and the overall width of the diffusion structure is small. Cr seems to affect the activity of U and Zr in the $\delta$ precipitates of the fuel used for the diffusion couples, and as a result Zr-rich layers have been observed to form in the diffusion structure of diffusion couples not containing Ni. The development of a Zr layer has been linked to narrow diffusion structures. On the basis of the calculated diffusion data, the elemental interactions play a major role in the interdiffusion process and diffusion coefficients are larger in the presence of Ni. In the context of fuel-clad compatibility, HT9 is the most expedient cladding since it contains negligibly small amounts of Ni. As a result, the Cr contained in the cladding affects the activity of the U and Zr in the $\delta$ precipitates which results in the development of a Zr-rich layer. (Abstract shortened with permission of author.)

Degree

Ph.D.

Advisors

Dayananda, Purdue University.

Subject Area

Materials science

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