Use of the FBSR process to produce a highly leach resistant mineralized waste form from Hanford low activity waste (LAW) has been investigated since 2001 (see page 217). Initial studies focused on producing and testing the granular mineral product created by processing high sodium waste feeds with clays at ~720°C to produce nepheline (NaAlSiO4) and nepheline-based minerals such as the sodalites to host I, F, Cl, and nosean to host sulfate and sulfide. Numerous studies (74-80) have shown that it is possible to produce a mineral waste form that effectively immobilizes both radionuclides and hazardous constituents.

To be accepted for near-surface disposal, the waste form is required to meet an acceptance criterion for compressive strength of 500 psi. This requirement is derived from a Nuclear Regulatory Commission Branch Technical Position on low level waste (LLW) forms in the US, which some­what arbitrarily specifies 500 psi to preclude subsidence in the waste dis­posal system. It is also noted that a monolithic waste form reduces the impact to human health for the intruder scenario in the waste site perform­ance assessments. While a monolith is desirable, there are other means by which this requirement can be met, e. g. waste stabilization in high integrity containers (HICs).

In 2005-2006 the Savannah River National Laboratory (SRNL) per­formed a monolith feasibility study for granular FBSR product [157]. Mon­oliths were made out of ordinary Portland cement (OPC) at 80-87 wt% FBSR loading, out of ceramicrete (a blend of MgO and monopotassium

Na2O

6.4 Formulation region for geopolymers compared to hydroceramics in the Na2O-SiO2-Al2O3 (mol%) ternary. Note that the fourth dimension is water content and not shown on the ternary mol% diagram. The geopolymer region labeled as G1 is the target range. Optimum formulations are designated as A and B and a 1" x 2" cylindrical monolith made with composition A is shown in the photograph.

phosphate (KH2PO4)) at an FBSR loading of 35.7 wt%, and out of hydroce­ramics (aluminosilicate zeolite phases formed from metakaolin plus NaOH) at FBSR loadings of 50-80 wt%. The hydroceramics had the best durability as they had a similar chemical makeup to the FBSR product (see Fig. 6.4) but the hydroceramics required hydrothermal processing. Therefore, geopolymers were used to bind the granular mineral waste form due to the similarity of the chemical makeup (see Fig. 6.4) to the FBSR product and the fact that the geopolymers did not require hydrothermal processing. Up to 70 wt% granular product was stabilized in the geopolymer. The granular mineral stabilized geopolymer was shown to be more durable than the granular product alone [158].