Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 141

reacted 150,000 gallons of liquid metal reactor coolant (sodium) with water, pro-
ducing enough NaOH to convert its entire inventory of calcined reprocessing waste
into HC waste forms, a scenario that treats both wastes for off-site disposal.

6.5.3.2 Mixing

Because HC grouts tend to set rapidly, mixing should be done quickly with equip-
ment that is capable of forcefully ejecting the product, e.g., a mixer-extruder. The
mold (canister) should be situated on a vibrating table to float out air bubbles before
the grout sets. The fresh grout will set, or harden, within a few hours at room
temperature.

6.5.3.3 Curing

The curing rate is governed by the size of the metakaolin particles, the amount of
hydroxide in the formulation, and temperature. If the metakaolin particles are too
large, salt molecules will not diffuse into them before their surfaces are effectively
sealed with reaction products. If they are too small, the grout is apt to exhibit
excessive water demand, set too rapidly, and produce a physically weak and overly
porous product.
Figures 6.5.1 and 6.5.2 illustrate the effects of different curing regimens on the
fractions of sodium and nitrate leached from a typical HC by the Product Consistency
Test (PCT). HC waste forms could be fully cured within a few hours at a temperature
(140°C) corresponding to a steam pressure of 0.23 MPa (35 psi). Domestic water
heater tanks are built to withstand pressures up to ~1 MPa (150 psig,) and represent
a cheap source of grout canisters. An alternative is to store canisters at atmospheric
pressure for several months in an insulated storage shed at a temperature approxi-
mating that of boiling water. Although ~100°C may produce a different mineral
structure than higher temperatures and pressures, the lower temperatures still produce
waste forms capable of passing current standard leach tests.

12

10

8

6

4

2

0

1 10 100 1000 10000 100000

Cure time (minutes)
90 C

118 C 140 C 190 C

FIGURE 6.5.1 Curing Effect on PCT Sodium Leachability.