onsite shallow land-disposal facility known as the Z-Area saltstone 1disposal facility (see Figure 1.1). After emplacement of the grout, the vaults are covered with nonradioactive grout in preparation for closure—burial under an impermeable cap.
A schematic process flowsheet for this option is shown in Figure 7.1 and comprises the following steps:
high-level waste in the tanks is treated with monosodium titanate (MST) to remove strontium and actinides (see Chapter 3 for details of this process);
slurry is then processed through a continuous filter to separate the strontium- and actinide-bearing MST solid phase (slurry) from a cesium-bearing salt supernate solution;
salt supernate solution is transferred to the onsite Z-Area saltstone production facility, where it is blended into a grout mixture and poured into disposal vaults located at the saltstone disposal facility; and
the MST solid phase (slurry) is transferred for immobilization in glass at the DWPF.
In the salt supernate solution to be processed and disposed of in the saltstone production and disposal facilities for the direct grout option, significantly higher concentrations of cesium isotopes are expected than would be handled for the in-tank precipitation (ITP) process. A comparison of concentrations of some materials in the reference salt supernate waste stream used as a basis for saltstone facility operations using the planned ITP process performance parameters (Martin Marietta Energy Systems, Inc., et al., 1992; Stevens, 1999) and anticipated concentrations in a direct grout option supernate stream (Stevens, 1999; Beck et al., 1998) are shown in Table 7.1. A higher concentration of cesium-135 exists in the direct grout waste stream than in the reference salt supernate stream. Cesium-135, with a half-life of 2.3 × 106 years, can be expected to be a contributor to radiation doses for long times following disposal. Cesium-137 also has a similarly higher concentration in the direct grout waste stream than in the reference waste stream. Although having a short half-life of 30 years, cesium-137 is present in such quantities that it will remain a major contributor to the dose for as long as 15 to 20 half-lives (450 to 600 years). Other radioisotopes of concern in long-term safety are not significantly higher in the direct grout waste stream than in the reference waste stream, although actinides in grout will present a long-term problem in meeting safety criteria. The salt supernate may also contain trace levels of hazardous metals (arsenic, barium, cadmium, chromium, lead, mercury, selenium, and silver), but Jones (2000a) notes that these metals will react with the slag components of the saltstone grout mixture to generate a reaction product that is insoluble. Other, more mobile toxic
The term “saltstone” is applied to the grout-salt supernate mixture (a blend of fly ash, slag, and portland cement with the salt supernate) after it is poured into the concrete vaults and cured.