move residual salt solutions and are transferred to the Defense Waste Processing Facility (DWPF) for additional processing and eventual immobilization in borosilicate glass. The MST step is performed in a separate reaction vessel for the ion exchange, solvent extraction, and direct grouting options. For the small-tank TPB option, the MST step is carried out concurrently with the cesium removal step in a single reaction vessel.

To operate successfully, the baseline process must meet the following requirements:

  • After treatment with MST, the “decontaminated” salt solutions must meet the strontium and actinide limits shown in Table 3.1 to be acceptable for disposal in the onsite saltstone facility.

  • The process must deliver sufficient feed of MST solids to the DWPF to provide for continuous operation of the glass melter. To this end, MST sorption kinetics for strontium and actinides, MST solids filtering, and MST solids washing must be rapid enough to support the required process cycle times.

  • The MST solids feed to the DWPF must have a composition that is compatible with the DWPF glass. In particular, the concentration of titanium (Ti), must be less than the limits established for the DWPF, or else the feed will have to be diluted, resulting in the production of additional glass canisters.

TABLE 3.1 Saltstone Waste Acceptance Criteria for Decontaminated Salt Solutions

Radionuclide

Limit (nanocuries per gram of saltstone)

Strontium-90

40

Plutonium-241

200

Neptunium-237

0.03

Total Alpha

20

SOURCE: Jones (2000b).

During its information-gathering sessions, the committee received written information and briefings on all of these issues, several of which are reviewed below.

STRONTIUM AND ACTINIDE REMOVAL

The mechanisms for strontium and actinide removal by MST are not well understood. Presumably, the removal mechanism involves an ion-exchange reaction of the sodium ions in the MST, primarily with cations in



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