Impact of H₂ and Consecutive H₂O₂ Exposures on the Oxidative Dissolution of (U_1–xGd_x)O₂ Pellets Under Deep Repository Conditions for Spent Nuclear Fuel

Abstract

The reactivity of H 2 O 2 towards (U 1-x Gd x )O 2 (x = 0.03, 0.045, 0.08) in the presence of H 2 was studied experimentally to assess the effect of H 2 on oxidative dissolution of Gd-doped UO 2 under deep repository conditions for spent nuclear fuel. Gd-doped UO 2 was chosen as a model substance for spent nuclear fuel due to its chemical similarity but low radiotoxicity. While H 2 O 2 is a radiolysis product capable of driving fuel matrix dissolution, H 2 , which is expected to evolve mainly through the corrosion of cast iron canisters encapsulating the fuel, might 157 Gd have high neutron cross-sections and can be introduced into the reactor core as burnable absorbers by doping standard UO 2 fuel. In addition, Gd builds up as a fission product in minor quantities in nuclear fuel during in-reactor irradiation. Therefore, Gd-doped UO 2 is part of the nuclear fuel cycle and after being discarded from the nuclear reactor it will eventually be stored in a deep geological repository according to the current plans in many countries. [3] After being discarded from a nuclear reactor, used fuel still consists mainly of UO 2 with small fractions (approx. 5 %) of highly radioactive fission products and heavier actinides that are present in different forms within the fuel matrix. [4,5] Among [a] Dr.

X-Ray and ultraviolet photoelectron spectroscopy studies of Uranium(iv),(v) and(vi) exposed to H2O-plasma under UHV conditions

“…15 Electrochemical studies have indicated that catalytic decomposition of H 2 O 2 on UO 2 requires the presence of uranium in higher oxidation state than +IV 16 and recent studies of H 2 O 2 -induced leaching of UO 2 have shown that the redox reactivity of UO 2 (towards H 2 O 2 ) decreases with H 2 O 2 exposure and that this trend is accompanied by a change in the uranium oxidation state even under conditions where dissolution of U(VI) is favoured. 17,18 To better understand these trends, alternative experimental methods are called for. An interesting approach with potential to provide additional mechanistic insights is to expose uranium oxide films of well controlled oxidation state to water plasma in vacuum systems.…”

Time-dependent surface modification of uranium oxides exposed to water plasma

“…1,[18][19][20][21][22] Some studies were made by continuously exposing uranium oxide to low steadystate concentrations of oxidants produced by irradiating suspensions of uranium oxide or pellets immersed in aqueous solution. 8 Other studies focused on the leaching of pellets ( pure or doped with non-radioactive isotopes of fission products) [23][24][25][26][27] and even of thin films. 18,28,29 The impact of dose rate, pH, temperature, oxidant or reductant concentration/pressure, bicarbonate and other groundwater constituents, have been assessed.…”

“…18,28,29 The impact of dose rate, pH, temperature, oxidant or reductant concentration/pressure, bicarbonate and other groundwater constituents, have been assessed. 14,19,25,26,[30][31][32] During the last decade, considerable progress has been made in the use of surface spectroscopy to study thin films, including uranium oxide films under UHV conditions. 18,28,33 Very recently, X-ray Photoelectron Spectroscopy (XPS) and UV Photoelectron Spectroscopy (UPS) were used to study changes in uranium oxide thin films exposed to O 2 , H 2 and H 2 O plasmas.…”