Organic Polymers for Stabilization/Solidification - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 91

FIGURE 5.5 Production-scale single-screw extrusion process system installed at Brookhaven
National Laboratory.
and 16.3 MPa (1020 to 2360 psi), but typically is in excess of 13.8 MPa (2000
psi). 18-20 No loss in mechanical integrity was observed following thermal cycling in
accordance with ASTM B-553 or immersion in water for 90 days. Irradiation of
polyethylene waste forms at a dose of 3.6 × 10 6 rad/h to a total dose of 10 8 rad
resulted in an increase in compressive strength of about 20%, due to cross-linking
of the polymer chains, which occurs on exposure to high radiation doses. Polyeth-
ylene is known to be resistant to biodegradation due to its branched structure,
relatively high molecular weight, and hydrophobic properties. 21,22 Testing of waste
form samples according to ASTM G-21 and ASTM G-22 resulted in no fungal or
bacterial growth. 18,23,24 Polyethylene is insoluble in virtually all organic solvents and
is resistant to many acids and alkaline solutions. 25
Leachability of polyethylene-encapsulated waste forms has been extensively
examined. In general, releases from polyethylene are controlled by diffusion, and
its low permeability results in relatively low leach rates. Leaching of soluble salts
(e.g., nitrates) and other wastes from polyethylene is inversely proportional to the
quantity of waste encapsulated. In one study, the diffusion coefficient ranged from
3 × 10 -9 cm 2 /sec for waste forms containing 50 dry wt% salts to 5 × 10 -8 cm 2 /sec
for wastes containing 70 dry wt% salts. 19 Using the ASTM D-1308 Accelerated
Leach Test, diffusion was determined to be the predominant leaching mechanism
for microencapsulated polyethylene waste forms. 26,27 Based on these laboratory
results, 200-l (55-gal) LDPE waste forms containing 50 to 70 dry wt% nitrate salt
waste were projected to leach 3.7 to 9.5 wt% of the original contaminants at ambient
temperature in 300 years. In contrast, similar cement grout waste forms were pro-
jected to release 17 wt% of the contaminants in just 11 years, approximately 25
times more rapidly. 28 In another study, the ANSI/ANS-16.1-2003 Leachability Index