Salt Disposal of Heat-Generating Nuclear Waste

materials would also have an impact on the pH, and one might consider adding a
chemical buffer if the alteration-phase assemblage does not sufficiently constrain
the system.
The gas introduced during the construction of a repository is primarily
atmosphere, with nitrogen and oxygen as the major components. If brine reacts
with steel waste packages, the oxygen is consumed, and the closed repository
becomes anoxic. In the absence of oxygen, water could react with steel or other
metallic components of the waste packages and produce hydrogen (H 2 ) gas via
anoxic corrosion. Anoxic corrosion is the corrosion of steel or other metals by the
oxygen in H 2 O instead of by free molecular O 2 in the aqueous or gaseous phase
(Haberman and Frydrych 1988; Simpson and Schenk 1989; Grauer, Knecht, and
Simpson 1991).
If HLW waste form(s), canisters, and/or backfill materials interact with brines,
substantial changes could occur. For example, alteration of oxide wastes or
backfill materials, degradation of glass and/or ceramic waste, and corrosion of
waste metals or metal canister materials might alter the oxidation state and pH of
the near field. These processes may also change the nature of the solubility
controlling phases for radionuclide solubilities.
If UNF (~95 % UO 2 ) reacts with brines under reducing conditions in salt
repositories, the dominant alteration phase is likely to be coffinite (USiO 4 •nH 2 O).
The formation of coffinite is favored in the presence of silica. The source of silica
required for the formation of coffinite can be from dissolution of clay minerals
and quartz in salt formations and from dissolution of borosilicate glass in the
waste. Typical borosilicate glass consists of 20% of fission product oxides, 50%
of silica, and 30% of boric acid. Borosilicate glass is inherently
thermodynamically metastable solid, and therefore will convert to a more stable
assemblage of crystalline phases upon reaction with brines. In natural uranium
deposits, coffinite is observed as a major alteration phase of uraninite (UO 2+x )
under reducing conditions. In addition, niobium and rare earth elements are
present in spent nuclear fuel as fission products. When they are dissolved in
brines, they could combine with dissolved uranium(IV) to form brannerite as an
additional alteration phase.
The solubility controlled dissolved concentrations of radioelements under
hypothesized salt repository conditions define the source term for releases (e.g.,
any sort of direct brine release or advective transport to the biosphere). The key
factors that establish the concentrations of dissolved radionuclides are the
following:
• redox chemistry
• pH, aqueous complexes, and speciation
• intrinsic colloid and pseudocolloid formation
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