Other Binders - Stabilization and Solidification of Hazardous, Radioactive, and Mixed Wastes - page 132

against the WIPP-WAC, which contain a number of requirements specific to TRU
waste. One restriction prevents accumulation of unsafe concentrations of radiolyti-
cally generated hydrogen gas inside the TRUPACT-II container. This restriction
determines the maximum amount of special nuclear material (SNM) that can be
present in a drum of cemented waste. Most requirements are similar to those for
Class A LLRW forms. No minimum compressive strength is required for a TRU
waste form, and free liquid is restricted to 1 vol%.
Although U.S. Gypsum did not formally market Envirostone as a TRU waste
solidification medium, company representatives conducted limited small-scale tests
at LANL that convinced the LANL technical staff that TRU waste solidification
requirements are within the capability of Envirostone performance. Envirostone
primarily solidified evaporator bottoms generated from nitric acid-based plutonium
processing at TA-55. This waste liquid consisted of high concentrations of nitric
acid (3 to 9 N ) and a saturated nitrate salt solution, 9 including several cations later
regulated by the EPA under the Resource Conservation and Recovery Act (RCRA) 10
as hazardous waste. portland Type I/II cement was used for this waste stream prior
to the conversion to Envirostone. After converting to Envirostone, adjusting the liquid
waste pH to about 3 prior to solidification was no longer required, resulting in no
pretreatment dilution and higher loadings in the final solidified waste. LANL also
began using Envirostone and Envirostone Emulsifier to solidify its non-polar organic
liquid wastes from machining activities, which had previously been treated by
absorption into an inert material. The disadvantage of the conversion to Envirostone
was its relatively short shelf-life of 6 months 1 and a materials cost approximately
10 times that of portland Type I/II cement. 11

6.4.2.1 Free-Liquid Generation from Envirostone™

Concerns with Envirostone performance arose in 1989 when waste forms produced
with Envirostone generated free liquid several weeks after solidification. 12-14 The
free liquid formed from 7 to 44 weeks after solidification and resulted in the
generation of up to 15 l (4 gal) of liquid in a 55-gal waste drum. 15 The phenomenon
was first observed when liquid seeped through a drum lid's venting filter, collected
on the lid, and crystallized from evaporation (see Figure 6.4.1). The crystalline
material consisted primarily of sodium salts, the most soluble cation in the original
waste. The 239 Pu and 241 Am concentrations were ~10 ng per gram of crystalline
material, consistent with their low solubility for the liquid pH of 5. This incident
resulted in removal of WIPP certification for the TA-55 solidification process and a
major effort to remove the free liquid from the drums. 15,16

FIGURE 6.4.1 Drum of Envirostone product with free liquid residue on top.