Time-dependent surface modification of uraniumoxides exposed to water plasma

• Thin UO2 films exposed to water plasma under UHV conditions have been shown to be interestingmodels for radiation induced oxidative dissolution of spent nuclear fuel. This is partly attributed to the factthat several of the reactive oxidizing and reducing species in a water plasma are also identified as productsof radiolysis of water. Exposure of UO2 films to water plasma has previously been shown to lead to oxidationfrom U(IV) to U(V) and (VI). In this work we have studied the dynamics of water plasma inducedredox changes in UO2 films by monitoring UO2 films using X-Ray photoelectron Photoemission (XPS) andUltra-Violet Photoemission (UPS) spectroscopy as a function of exposure time. The surface compositionin terms of oxidation states obtained from U4f7/2 peak deconvolution could be retraced along theexposure time, and compared to the valence band. The spectral analysis showed that U(IV) is initially oxidizedto U(V) which is subsequently oxidized to U(VI). For extended exposure times it was shown that U(VI)is slowly reduced back to U(V). UPS data show that, unlike the U(V) formed on the surface upon oxidationof U(IV), the U(V) formed upon reduction of U(VI) is localized in the bulk of the film. It also displays adifferent reactivity than the initially formed U(V). The experiments can be reproduced using a simplekinetic model describing the redox processes involved.

Ämnesord

NATURVETENSKAP  -- Kemi -- Oorganisk kemi (hsv//swe)

NATURAL SCIENCES  -- Chemical Sciences -- Inorganic Chemistry (hsv//eng)

Nyckelord

Kemi

Chemistry

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