Results of column dissolution experiments designed to evaluate the physical and chemical processes inherent to saltcake dissolution are presented along with model chemical equilibrium calculations. Two different compositions representing saltcakes in Hanford tanks were characterized, and porosities and permeabilities for a third composition based upon the saltcake waste in Tank 41H at the Savannah River Site (SRS) were also evaluated. Whereas the surrogates are all chemically similar, the presence of high phosphate loadings for the Hanford (HNF) simulants was noted as significantly affecting draining. The permeability was higher for the SRS saltcake, and the sodium nitrate loading in this saltcake was roughly 80% by weight compared to less than 60% by weight for the HNF compositions. Average values of the permeability and porosity were reduced for the surrogates based on Hanford Tanks S-112 and S-101. Here a secondary layer formed above the saltcake bed and was found to contain a large amount of gibbsite, Al(OH)3. Experiments with 3 molal (m) NaOH as a diluent, compared to water, did not result in additional layer formation that has been attributed to a change in local pH thereby altering the solid liquid equilibrium. Chemical analysis of the two HNF saltcakes indicated solids re-precipitation as a function of diluent added. The events were signified by large decreases in the nitrate and carbonate anion concentrations and were confined to low % dilution by weight values. Solids re-precipitation is noted as arising from the mixing of the dissolved saltcake stream with pockets of saturated interstitial liquor.

It is possible to retrieve a large fraction of soluble waste from the Hanford single-shell waste tanks (SSTs) by dissolving it with water. This retrieval method will be demonstrated in Tanks U-107 and S-112 in the next few years. If saltcake dissolution proves practical and effective, many of the saltcake SSTs may be retrieved by this method. Many of the SSTs retain flammable gas that will be released into the tank headspace as the waste dissolves. This report describes the physical processes that control dissolution and gas release. Calculation results are shown and describe how the headspace hydrogen concentration evolves during dissolution. The observed spontaneous and induced gas releases from SSTs are summarized, and the dissolution of the crust layer in SY-101 is discussed as a recent example of full-scale dissolution of saltcake containing a large volume of retained gas. The report concludes that the dissolution rate is self-limiting and that gas release rates are relatively low.

It is possible to retrieve a large fraction of soluble waste from the Hanford single-shell waste tanks (SST) by dissolving it with water. This retrieval method will be demonstrated in U-107 and S-112 in the next few years. If saltcake dissolution proves practical and effective, many of the saltcake SSTs may be retrieved by this method. Many of the SSTs retain a large volume of flammable gas that will be released into the tank headspace as the waste dissolves. This report describes the physical processes that control dissolution and gas release. Calculation results are shown describing the headspace hydrogen concentration transient during dissolution. The observed spontaneous and induced gas releases from SSTs is summarized and the dissolution of the crust layer in SY-101 is discussed as a recent example of full-scale dissolution of saltcake containing a very large volume of retained gas. The report concludes that the dissolution rate is self limiting and gas release rates are relatively low.

This document describes the process controls for the tank 241-U-107 (U-107) saltcake dissolution proof-of-concept operations. Saltcake dissolution is defined as a method by which water-soluble salts will be retrieved from the Hanford Site radioactive waste tanks utilizing dissolution as the mobilizing mechanism. The proof-of-concept operations will monitor the retrieval process and transfer at least 100 kgal of fluid from tank U-107 to the double-shell tank (DST) system during the performance period. Tank U-107 has been identified as posing the highest long-term risk to the Columbia River of all single shell tanks (SSTs). This is because of the high content of mobile, long-lived radionuclides mostly in the saltcake waste in the tank. To meet current contractual and consent decree commitments, tank U-107 is being prepared for interim stabilization in August 2001. It is currently scheduled for saltcake retrieval in 2023, near the end of the SST retrieval campaign because of a lack of infrastructure in U-Farm. The proof-of-concept test will install a system to dissolve and retrieve a portion of the saltcake as part of, and operating in parallel with, the standard interim stabilization system to be installed on tank U-107. This proof-of-concept should provide key information on spray nozzle selection and effective spray patterns, leak detection, monitoring, and mitigation (LDMM) and in-tank saltcake solubility data that will help in the design of a full-tank retrieval demonstration system.

DOE Tank Waste: How clean is clean enough? The U.S. Congress asked the National Academies to evaluate the Department of Energy's (DOE's) plans for cleaning up defense-related radioactive wastes stored in underground tanks at three sites: the Hanford Site in Washington State, the Savannah River Site in South Carolina, and the Idaho National Laboratory. DOE plans to remove the waste from the tanks, separate out high-level radioactive waste to be shipped to an off-site geological repository, and dispose of the remaining lower-activity waste onsite. The report concludes that DOE's overall plan is workable, but some important challenges must be overcomeâ€"including the removal of residual waste from some tanks, especially at Hanford and Savannah River. The report recommends that DOE pursue a more risk-informed, consistent, participatory, and transparent for making decisions about how much waste to retrieve from tanks and how much to dispose of onsite. The report offers several other detailed recommendations to improve the technical soundness of DOE's tank cleanup plans.