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POSIVA Report 1998-6

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Name:

Dissolution of Unirradiated U02 Fuel in Synthetic Groundwater - Progress Report '97

Writer:

Kaija Ollila

Language:

English

Page count:

26

ISBN:

951-652-044-8; 1239-3096

Summary:

Working report: POSIVA REPORT 98-06, 17 PAGES

DISSOLUTION OF UNIRRADIATED UO2 FUEL IN
SYNTHETIC GROUNDWATER – PROGRESS REPORT ‘97

Abstract
This report presents the results obtained from dissolution experiments with unirradiated UO2
pellets and UO2 powder in synthetic groundwaters. The results are partly preliminary, because
the dissolution experiments of UO2 typically need long contact time periods to reach steady
state. The purpose is to study the dissolution mechanisms and solubilities of UO2 in synthetic
groundwaters which simulate saline and fresh groundwater environments under the disposal
conditions of spent fuel. The redox conditions varied from oxidizing (air-saturated), anaerobic
to reducing (N2, low Eh). The Eh was maintained at a low level (-300 ... -200 mV) by the
addition of redox species (S2-, Fe2+) to the aqueous phase. The ratio of UO2 surface area to
water volume was varied. The study is a part of the research project ‘Source term for
performance assessment of spent fuel as a waste form’ within the European Commission
Nuclear Fission Safety Program (1994-1998).

Under anoxic conditions (N2), the control of the Eh (-300 ... -200 mV) by the addition of redox
species to the aqueous phase seems to lower the solution concentrations of uranium. The
solubilities are at the level of the theoretical solubility of the well-crystallized U(IV)oxide, UO2
(10-9 M). In earlier experiments in synthetic groundwater (N2) without the Eh control, the
measured solubilities were one order of magnitude higher. They were close to the solubility of
the mixed valence U(IV)(VI)oxide, U4O9. The composition of groundwater (saline, fresh) has a
minor effect.

Under oxidizing (air-saturated) conditions, the experiments with UO2 powder (S/V= 1000 m-1)
and UO2 pellets (S/V= 0.66...19.8 m-1) have not reached steady state. A preliminary comparison
with theoretical solubility data (EQ3/6, NEA and composite databases) was made assuming
secondary phases of uranium to limit its solubilities. As the experiments proceed, the solid
phases in solution will be analysed after different contact time periods in order to get
information on secondary alteration phases that possibly control the uranium solubility.




Keywords: UO2, solubility, dissolution mechanisms, batch experiments,
synthetic groundwaters, oxidizing, reducing conditions




POSIVA REPORT 98-06, 17 PAGES

DISSOLUTION OF UNIRRADIATED UO2 FUEL IN
SYNTHETIC GROUNDWATER – PROGRESS REPORT ‘97

Abstract
This report presents the results obtained from dissolution experiments with unirradiated UO2
pellets and UO2 powder in synthetic groundwaters. The results are partly preliminary, because
the dissolution experiments of UO2 typically need long contact time periods to reach steady
state. The purpose is to study the dissolution mechanisms and solubilities of UO2 in synthetic
groundwaters which simulate saline and fresh groundwater environments under the disposal
conditions of spent fuel. The redox conditions varied from oxidizing (air-saturated), anaerobic
to reducing (N2, low Eh). The Eh was maintained at a low level (-300 ... -200 mV) by the
addition of redox species (S2-, Fe2+) to the aqueous phase. The ratio of UO2 surface area to
water volume was varied. The study is a part of the research project ‘Source term for
performance assessment of spent fuel as a waste form’ within the European Commission
Nuclear Fission Safety Program (1994-1998).

Under anoxic conditions (N2), the control of the Eh (-300 ... -200 mV) by the addition of redox
species to the aqueous phase seems to lower the solution concentrations of uranium. The
solubilities are at the level of the theoretical solubility of the well-crystallized U(IV)oxide, UO2
(10-9 M). In earlier experiments in synthetic groundwater (N2) without the Eh control, the
measured solubilities were one order of magnitude higher. They were close to the solubility of
the mixed valence U(IV)(VI)oxide, U4O9. The composition of groundwater (saline, fresh) has a
minor effect.

Under oxidizing (air-saturated) conditions, the experiments with UO2 powder (S/V= 1000 m-1)
and UO2 pellets (S/V= 0.66...19.8 m-1) have not reached steady state. A preliminary comparison
with theoretical solubility data (EQ3/6, NEA and composite databases) was made assuming
secondary phases of uranium to limit its solubilities. As the experiments proceed, the solid
phases in solution will be analysed after different contact time periods in order to get
information on secondary alteration phases that possibly control the uranium solubility.




Keywords: UO2, solubility, dissolution mechanisms, batch experiments,
synthetic groundwaters, oxidizing, reducing conditions




Keywords:

U02; solubility; dissolution mechanisms; batch experiments; synthetic groundwaters; oxidizing; reducing conditions

File(s):

Dissolution of Unirradiated U02 Fuel in Synthetic Groundwater - Progress Report '97 (pdf) (432.1 KB)


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