Bulk and surface controlled diffusion of fission gas atoms

Technical Report · Thu Aug 09 00:00:00 EDT 2012

DOI:https://doi.org/10.2172/1048823· OSTI ID:1048823

Andersson, Anders D. ^[1]

---

1. Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)

Fission gas retention and release impact nuclear fuel performance by, e.g., causing fuel swelling leading to mechanical interaction with the clad, increasing the plenum pressure and reducing the gap thermal conductivity. All of these processes are important to understand in order to optimize operating conditions of nuclear reactors and to simulate accident scenarios. Most fission gases have low solubility in the fuel matrix, which is especially pronounced for large fission gas atoms such as Xe and Kr, and as a result there is a significant driving force for segregation of gas atoms to extended defects such as grain boundaries or dislocations and subsequently for nucleation of gas bubbles at these sinks. Several empirical or semi-empirical models have been developed for fission gas release in nuclear fuels, e.g. . One of the most commonly used models in fuel performance codes was published by Massih and Forsberg. This model is similar to the early Booth model in that it applies an equivalent sphere to separate bulk UO₂ from grain boundaries represented by the sphere circumference. Compared to the Booth model, it also captures trapping at grain boundaries, fission gas resolution and it describes release from the boundary by applying timedependent boundary conditions to the circumference. In this work we focus on the step where fission gas atoms diffuse from the grain interior to the grain boundaries. The original Massih-Forsberg model describes this process by applying an effective diffusivity divided into three temperature regimes. In this report we present results from density functional theory calculations (DFT) that are relevant for the high (D₃) and intermediate (D₂) temperature diffusivities of fission gases. The results are validated by making a quantitative comparison to Turnbull's and Matzke's data. For the intrinsic or high temperature regime we report activation energies for both Xe and Kr diffusion in UO_2±x, which compare favorably to available experiments. This is an extension of previous work. In particular, it applies improved chemistry models for the UO_2±x nonstoichiometry and its impact on the fission gas activation energies. The derivation of these models follows the approach that used in our recent study of uranium vacancy diffusion in UO₂. Also, based on the calculated DFT data we analyze vacancy enhanced diffusion mechanisms in the intermediate temperature regime. In addition to vacancy enhanced diffusion we investigate species transport on the (111) UO₂ surface. This is motivated by the formation of small voids partially filled with fission gas atoms (bubbles) in UO₂ under irradiation, for which surface diffusion could be the rate-limiting transport step. Diffusion of such bubbles constitutes an alternative mechanism for mass transport in these materials.

• Research Organization: Los Alamos National Laboratory (LANL), Los Alamos, NM (United States)
• Sponsoring Organization: USDOE National Nuclear Security Administration (NNSA)
• DOE Contract Number: AC52-06NA25396
• OSTI ID: 1048823
• Report Number(s): LA-UR--12-24002
• Country of Publication: United States
• Language: English

Related Subjects

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS
36 MATERIALS SCIENCE
ATOMS
BOUNDARY CONDITIONS
BUBBLES
CHEMISTRY
DEFECTS
DIFFUSION
DISLOCATIONS
FISSION
FUNCTIONALS
GASES
GRAIN BOUNDARIES
IRRADIATION
Materials Science(36)
NUCLEAR FUELS
NUCLEATION
REACTORS
RESOLUTION
RETENTION
SEGREGATION
SOLUBILITY
THERMAL CONDUCTIVITY
TRAPPING
URANIUM