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Title: The effect of fuel chemistry on UO2 dissolution

Abstract

The dissolution rate of both unirradiated UO2 and used nuclear fuel has been studied by numerous countries as part of the performance assessment of proposed geologic repositories. In the scenario of waste package failure and groundwater infiltration into the fuel, the effects of variables such as temperature, dissolved oxygen, and water and fuel chemistry on the dissolution rates of the fuel are necessary to provide a quantitative estimate of the potential release over geologic time frames. The primary objective of this research was to determine the influence these parameters have on the dissolution rate of unirradiated UO2 under repository conditions and compare them to the rates predicted by current dissolution models. Both unirradiated UO2 and UO2 doped with varying concentrations of Gd2O3, to simulate used fuel composition after long time periods where radiolysis has minor contributions to dissolution, were examined. In general, a rise in temperature increased the dissolution rate of UO2 and had a larger effect on pure UO2 than on those doped with Gd2O3. Oxygen dependence was observed in the UO2 samples with no dopant and increased as the temperature rose; in the doped fuels less dependence was observed. The addition of gadolinia into the UO2 matrix showedmore » a significant decrease in the dissolution rate. The matrix stabilization effect resulting from the dopant proved even more beneficial in lowering the dissolution rate at higher temperatures and dissolved O2 concentrations in the leachate where the rates would typically be elevated.« less

Authors:
; ; ORCiD logo
Publication Date:
Research Org.:
Pacific Northwest National Lab. (PNNL), Richland, WA (United States)
Sponsoring Org.:
USDOE
OSTI Identifier:
1290372
Report Number(s):
PNNL-SA-105625
Journal ID: ISSN 0022-3115; DF0961000
DOE Contract Number:
AC05-76RL01830
Resource Type:
Journal Article
Resource Relation:
Journal Name: Journal of Nuclear Materials; Journal Volume: 476
Country of Publication:
United States
Language:
English
Subject:
UO2 dissolution; fuel matrix effects; single pass flow through; nuclear fuel dissolution

Citation Formats

Casella, Amanda, Hanson, Brady, and Miller, William. The effect of fuel chemistry on UO2 dissolution. United States: N. p., 2016. Web. doi:10.1016/j.jnucmat.2016.04.025.
Casella, Amanda, Hanson, Brady, & Miller, William. The effect of fuel chemistry on UO2 dissolution. United States. doi:10.1016/j.jnucmat.2016.04.025.
Casella, Amanda, Hanson, Brady, and Miller, William. 2016. "The effect of fuel chemistry on UO2 dissolution". United States. doi:10.1016/j.jnucmat.2016.04.025.
@article{osti_1290372,
title = {The effect of fuel chemistry on UO2 dissolution},
author = {Casella, Amanda and Hanson, Brady and Miller, William},
abstractNote = {The dissolution rate of both unirradiated UO2 and used nuclear fuel has been studied by numerous countries as part of the performance assessment of proposed geologic repositories. In the scenario of waste package failure and groundwater infiltration into the fuel, the effects of variables such as temperature, dissolved oxygen, and water and fuel chemistry on the dissolution rates of the fuel are necessary to provide a quantitative estimate of the potential release over geologic time frames. The primary objective of this research was to determine the influence these parameters have on the dissolution rate of unirradiated UO2 under repository conditions and compare them to the rates predicted by current dissolution models. Both unirradiated UO2 and UO2 doped with varying concentrations of Gd2O3, to simulate used fuel composition after long time periods where radiolysis has minor contributions to dissolution, were examined. In general, a rise in temperature increased the dissolution rate of UO2 and had a larger effect on pure UO2 than on those doped with Gd2O3. Oxygen dependence was observed in the UO2 samples with no dopant and increased as the temperature rose; in the doped fuels less dependence was observed. The addition of gadolinia into the UO2 matrix showed a significant decrease in the dissolution rate. The matrix stabilization effect resulting from the dopant proved even more beneficial in lowering the dissolution rate at higher temperatures and dissolved O2 concentrations in the leachate where the rates would typically be elevated.},
doi = {10.1016/j.jnucmat.2016.04.025},
journal = {Journal of Nuclear Materials},
number = ,
volume = 476,
place = {United States},
year = 2016,
month = 8
}
  • Description of results from single pass flowthrough tests showing the effect of dissolved oxygen and temperature on the dissolution of pure UO2 and UO2 with 8 wt% Gd2O3 doping.
  • The short-circuit phenomenon caused by dissolution of the NiO cathode in the molten carbonate fuel cell was experimentally investigated. Monitoring CO{sub 2} concentration in the anode exhaust gas can be an effective way to detect cell short circuit. The effects of matrix thickness and cathode CO{sub 2} partial pressure on shorting were elucidated. The time-to-initial-short-circuit (shorting time) is approximately proportional to the second power of the matrix thickness and the reciprocal of the cathode CO{sub 2} partial pressure. This can be explained by the relationship between conductance of the short circuit and the Ni content of the matrix. A simplemore » model to correlate the conductance with the shorting time was developed. It is concluded that part of the deposited Ni exists as lithiated NiO.« less
  • When released out of a canister, the radionuclides originally incorporated in the spent fuel can still deposit radiation energy (even more efficiently) into the pore water, cause water radiolysis, and produce oxidants in the buffering material. This phenomenon is termed secondary water radiolysis. The oxidants thus produced can possibly diffuse back to oxidize the spent fuel and to increase the oxidative dissolution rate of the fuel.The effect of the secondary water radiolysis has been identified and preliminarily addressed by a mass-balance model. To explore whether the effect is significant on spent-fuel dissolution, the upper-boundary limit of the effect has beenmore » set up by considering a scenario that is very unlikely to occur. Several extreme assumptions have been made: First, the canister fails completely 10{sup 3} yr after deposition; second, the spent fuel is oxidized instantaneously; and third, the radionuclides considered are those that dominantly contribute to radiolysis between 10{sup 3} to 10{sup 5} yr. With these assumptions, the spent-fuel dissolution rate can be increased dramatically if 10% or more of the oxidants produced by the secondary water radiolysis diffuse back to oxidize the spent fuel. It thus indicates that the effect of the secondary water radiolysis could be significant with some extreme assumptions. With more realistic assumptions, the effect could possibly become minimal. The subject is worth further investigation.« less
  • An earlier model for the oxidative dissolution of spent fuel is further developed by including the reductive effect of H{sub 2}, which is formed by both the radiolysis of groundwater and the anoxic corrosion of the cast iron insert of the canister. The kinetics of reduction of dissolved uranium species by dissolved hydrogen is derived from a series of previously published experimental studies. The simulation results suggest that the effect of autocatalytic reduction of hexavalent uranium by hydrogen may play an important role in controlling the dissolution of the fuel matrix within a canister. Further experimental studies are required tomore » firmly verify these findings.« less