Research Needs for High-Level Waste Stored in Tanks and Bins at U.S. Department of Energy Sites Environmental Management Science Program (2001) / Chapter Skim
Currently Skimming:
5. Immobilization
5. Immobilization
Pages 48-64
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 48... ...
. The DOE vitrification process involves blending HLW with borosilicate glass frit or glass precursor chemicals, such as oxides and carbonates.
|
From page 49... ...
.That committee also advocated further investigation of a number of viable candidate waste forms for the calcified wastes, in addition to borosilicate glass,with the main objective being to increase the waste loading.These candidate waste forms include (1) alternative glass compositions, including high-waste-loading glasses prepared in single-use containers; (2)
|
From page 50... ...
,"is defined by the mechanistic models in PCCS that bounds the range of allowable waste compositions and frit blends. The "target"point indicates maximum waste loading with the minimum amount of frit for a given batch of waste being fed to the melter.
|
From page 51... ...
that serves to trap condensable volatile emissions and recycle them into the melt.The molten glass is then poured through the pour spout into stainless steel canisters and allowed to cool. Successful, continuous, long-term glass production normally requires a constant batch composition, uniform feed rate, and steady-state operating conditions.
|
From page 52... ...
However, it may not be possible to achieve substantial increases in waste loading using borosilicate glasses. Furthermore, these glasses may not be the optimum media for some problematic wastes such as the INEEL calcines, as 3Waste loading is the fraction of waste contained in a glass log or other waste form.
|
From page 53... ...
When the United States decided, at the beginning of 1977, to forego reprocessing of commercial reactor fuel, the primary form of HLW became spent fuel, and the plans and regulations governing repository disposal were changed accordingly. Later, the DOE was authorized to dispose of HLW from the defense program sites in the first HLW repository.The defense wastes are expected to constitute only about 10 percent of the HLW in the first repository, with the balance of about 90 percent being spent fuel from commercial nuclear reactors.The initial defense HLW designated for the first repository will be borosilicate glass from the SRS and from the WVDP.
|
From page 54... ...
Off-gas particulates4 from slurry volatilization represent another potential problem with the use of borosilicate glasses because the particulate compositions will likely differ significantly from that of the original waste feed. Thus, they may have to be blended into the waste feed at relatively low concentrations because they would be highly enriched in volatile species, such as technetium, mercury, iodine, ruthenium, cesium, boron, sodium, and possibly molybdenum.
|
From page 55... ...
in the waste feed of species with limited solubility in borosilicate glass, such as halides, sulfates, phosphates, chromium, and bismuth. Glass-in-glass phase separation and/or crystallization of possibly undesirable phases will occur if the solubility limits of these species are exceeded, producing a waste form that may not meet current acceptance criteria.
|
From page 56... ...
Crystal Content of the BorosiIicate Glass Matrix The current stipulations for the SRS borosilicate glass waste form state that the waste glass should not contain any significant degree of crystallinity or glass-in-glass phase separation Oantzen et al., 1999~. Crystallization can occur during cooling of molten HLW glass if there is sufficient overlap between the temperature ranges for substantial crystal nucleation and crystal growth.
|
From page 57... ...
, which measures the release rates for sodium, lithium, silicon, and boron during water leaching. The deleterious influence on glass durability of certain crystalline phases derives from both chemical and mechanical effects on the surrounding glass: the residual glass composition is altered, and the glass matrix is stressed by the volume mismatch of the crystal and the glass space it replaces.
|
From page 58... ...
tong-Term Research Need Long-term basic research is needed to broaden the envelope of acceptable borosilicate glass compositions to include a level of crystal content that does not adversely affect product durability.
|
From page 59... ...
The models should also i ncl ude consideration of the i nfl uence on waste form du rabi I ity of such factors as groundwater radiolysis and internal radiation damage (Weber et al., 1997; 19981. Use of Unreacted GIass-Forming Chemicals Versus PremeIted Glass Frit Plans to immobilize HLW in borosilicate glass at Hanford include the option of using a melter feed containing "raw" chemicals (mainly oxides and carbonates)
|
From page 60... ...
Results will strengthen the scientific basis for a rational choice between using un reacted glass farmers and using premelted frit in waste feeds for future melter designs (including Hanford melters)
|
From page 61... ...
. Precipitation of Noble Metals and Crystalline Phases in JouIe-Heated Melters Future melting campaigns at the SRS and at the Hanford Site will involve tank wastes with higher concentrations of noble metals (palladium, rhodium, and ruthenium)
|
From page 62... ...
The ultimate goal of this long-term basic research is to increase glass production rates and prolong the operating life of the melter. [imitations of JouIe-Heated Melters in Achieving Higher Processing Temperatures As noted previously, it may be advantageous to develop alternative glass or glass-ceramic waste form materials to the present generation of borosilicate waste glasses in order to achieve higher waste loadings, or to immobilize problematic wastes with unusual compositions.
|
From page 63... ...
. In many cases, these alternative processes can avoid some of the inherent problems with a continuous melter, including refractory corrosion and precipitation of noble metals and crystalline phases, although they would likely introduce other technical issues.
|
From page 64... ...
is required, or where it is important to minimize the initial volume of HLW feed. tong-Term Research Needs Long-term basic research is needed to identify and develop alternative melting techniques, including batch-type processes using concepts other than the continuous melters in current use at DOE sites for preparing waste forms with higher waste loadings.
|
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.