An International Spent Nuclear Fuel Storage Facility Exploring a Russian Site as a Prototype: Proceedings of an International Workshop (2005) / Chapter Skim
Currently Skimming:
Appendix C: Geochemistry of Actinides During the Long-Term Storage and Disposal of Spent Nuclear Fuel
Appendix C: Geochemistry of Actinides During the Long-Term Storage and Disposal of Spent Nuclear Fuel
Pages 260-290
The Chapter Skim interface presents what we've algorithmically identified as the most significant single chunk of text within every page in the chapter.
Select key terms on the right to highlight them within pages of the chapter.
From page 260... ...
is the spent nuclear fuel (SNF) as a product of atomic power stations, as well as naval-propulsion and research reactors.
|
From page 261... ...
The barriers comprise concrete tanks, corrosion-resistant containers, envelopes for nuclear fuel, and bentonite backfill. The SNF can interact with groundwater only after the engineered barriers lose their insulative properties.
|
From page 262... ...
. SPENT NUCLEAR FUEL PROPERTIES DEFINING THE CONDITIONS OF LONG-TERM STORAGE AND DISPOSAL The principal constituent of the nuclear fuel is a heat-generating element (fuel rod)
|
From page 263... ...
. When the geological medium becomes the only barrier retaining the radionuclide release, the repository safety in most cases will be determined by the intensity of the Pu and Am escape from the SNF and the specific conditions of their migration in groundwater.
|
From page 264... ...
The study of U deposits shows that uranium oxides can be highly stable at certain conditions in the geological medium. Ores of U deposits are generally composed of uranium oxides.
|
From page 265... ...
The equilibrium uranium concentration in groundwater at the reducing hydrogeochemical conditions is very low, less than or equal to 108 mol/L (see Fig FIGURE 3 A scheme of the SNF location in an underground mining working.
|
From page 266... ...
The Fe(Ti) -bearing minerals and particularly their alteration products serve as sorbents for U in crystalline rocks.
|
From page 267... ...
. The study of these ore bodies located at different depths from the surface described the behavior of uraninite and associated products of nuclear reactions both in the reducing and oxidizing hydrogeochemical conditions; for example, the uraninite in an ore body at a depth of 250 m under reducing conditions is not affected by secondary processes (Pourcelot and Gauthier-Lafaye, 1998)
|
From page 268... ...
EXPERIMENTAL STUDY OF SPENT NUCLEAR FUEL DISSOLUTION IN GROUNDWATER Many experiments on interaction of natural and synthesized UO2, as well as SNF and its imitators, and groundwater were performed under oxidized conditions and are related to the U.S. project on long-term HLW storage in the Yucca Mountain repository in Nevada (LLNL, 1998)
|
From page 269... ...
. Generally, because of the oxidizing conditions, some experts believe that the long-term safe storage of SNF in the Yucca Mountain repository is not possible (Ewing, 2002)
|
From page 270... ...
Thus, the experiments show that the reducing conditions maintain high SNF stability in the geological media. THE EFFECT OF RADIOLYSIS ON SPENT NUCLEAR FUEL SOLUBILITY The SNF radiation does not necessarily result in reducing conditions.
|
From page 271... ...
The effect of radiolysis is best pronounced in experiments in an oxygen-free atmosphere of nitrogen and argon. The role of a radiolysis of groundwater in the formation of the oxidizing conditions on the SNF surface in an underground repository was proved to be insignificant (Forsyth and Werme, 1986)
|
From page 272... ...
estimated the effect of radiolysis on SNF stability from the experimental data on UO2 solubility, theoretical calculation of the amount of the oxidizing agents produced by SNF interaction with water, and the study of natural uraninite. Admitting that precise estimation was impossible, the authors concluded that the presence of reducing agents in host rocks and canister corrosion products strongly limits the radiolysis effect on the SNF solubility.
|
From page 273... ...
This is generally caused by the fact that the uraninite oxidation is accompanied by the formation of microfractures, thereby increasing the water penetration and interaction area. During the dissolution of the uraninite matrix, actinides can pass into water in dissolved and colloidal forms, precipitate near SNF as isomorphous impurities in the secondary uranyl-ion minerals or as individual mineral phases, and accumulate in sorbed forms in host rocks.
|
From page 274... ...
The amount of colloidal particles can strongly increase because of tectonic activity, water sampling, mining, or some other processes disturbing the hydrodynamic regime. The main mechanism restraining the migration of colloidal particles in geological medium is their adhesion and precipitation on the fracture walls as loose aggregates.
|
From page 275... ...
Thus, the predominant actinide migration in colloidal form suggested from experiments was verified by natural observations. PLUTONIUM MIGRATION IN GEOLOGICAL MEDIUM Plutonium contents in uranium ores are negligible (Pu/U is approximately 1012)
|
From page 276... ...
the SNF actinides passed into groundwater are retained within the restricted volume of the geological medium, and only refined water suitable for domestic use reaches the zone of active water exchange. To estimate the possibility of realization of these conditions, we used the available information on the properties of synthesized actinides and their natural geochemical analogues, experimental results on SNF interaction with groundwater, data on actinide solubility in aqueous solutions and interaction of actinide-bearing solutions with rocks, experimental results obtained in underground laboratories on velocities of migration of elements that can and cannot be sorbed in fractured rocks, and information on the colloidal form of radioisotope migration with groundwater.
|
From page 277... ...
The above data indicate that the Pu concentration in groundwater interacting with SNF under oxidizing conditions can be as high as n × 108 mol/L for the dissolved form and up to 106 mol/L for totally dissolved and colloidal forms. These values are two to four orders of magnitude higher than the maximum concentration limit for drinking water.
|
From page 278... ...
278 AN INTERNATIONAL SPENT NUCLEAR FUEL STORAGE FACILITY A 1.2 1.0 0.8 0.6 0.4 0.2 0 0.2 0.4 0.6 0.8 0 2 4 6 8 10 12 14 FIGURE 5 Eh-pH diagrams for the Pu-C-O-H (a)
|
From page 279... ...
The Th concentration in groundwater below the zone of extensive hypergenesis never exceeds 109 mol/L, even in blocks with Th ore mineralization. Thorium in ground, river, and sea water generally occurs in suspended and colloidal forms.
|
From page 280... ...
. Kd values are higher under reducing conditions than under oxidizing conditions (Brookins, 1984)
|
From page 281... ...
The above data indicate that the geological medium can prevent the Pu transition to the biosphere in dissolved form. The Pu migration in colloidal form seems to be more hazardous for the environment.
|
From page 282... ...
The repository safety with respect to Pu can be maintained generally by the factors preventing colloid migration, that is, low water permeability and water exchange, the absence of wide fractures, sealing of the fractures by secondary minerals, and the presence of bentonite backfill. Additional factors increasing the insulative properties of geological medium are the mafic composition of crystalline rocks and the distant location of the repository from the site of groundwater discharge (Laverov et al., 2001)
|
From page 283... ...
; for example, a rhyolite alteration with strong redistribution of major elements does not change REE concentrations. Rare earth elements were leached from the rock-forming minerals during the rhyolite alteration and were sorbed in the secondary clay minerals and zeolites.
|
From page 284... ...
Am(OH) 3 solubility is very low in the nearly neutral reducing conditions typical of the deep zones with low water exchange.
|
From page 285... ...
In the Oklo deposit uraninite was stable because of the reducing conditions, and Am, like Pu, was retained at the site of the mineral formation until its complete fission. The experimental data help us to suggest possible Am concentrations in groundwater under the oxidizing conditions induced by radiolysis.
|
From page 286... ...
CONCLUSIONS 1. The spent nuclear fuel in an underground repository can interact with groundwater only after the engineered barriers fail, that is, in 5001000 years.
|
From page 287... ...
The distance and scale of Pu and Am migration in colloidal form can be estimated only for a certain geological medium by experiments in underground laboratories. The absence of wide fractures, low water permeability, and isolation with bentonite backfill are efficient factors retaining the actinide migration in colloidal form.
|
From page 288... ...
Symposium on Scientific Basis for Nuclear Waste Management XXII 556:471478. Cowan, G
|
From page 289... ...
Proceedings of the Symposium on Scientific Basis for Nuclear Waste Management XXIII 608:545550. Pagel, M., F
|
From page 290... ...
Proceedings of the Symposium on Scientific Basis for Nuclear Waste Management XXIV 663:391398. Rondinella, V
|
Key Terms
This material may be derived from roughly machine-read images, and so is provided only to facilitate research.
More
information on Chapter Skim is available.