WES developed a grout to be used as a cold (non-radioactive) cap or void-fill material between the solidified low-level waste and the cover blocks of near-surface disposal vaults at the U.S. Department of Energy (DOE) Hanford Facility. The project consisted of formulation and evaluation of candidate grout, followed by a physical scale-model test to verify grout performance under project-specific conditions and provide data to verify numerical models of stresses and isotherms inside the Hanford demonstration vault. Evaluation of unhardened grout included segregation, bleed, flow, and working time. For hardened grout, strength, volume stability, thermal heat rise, and geochemical compatibility with surrogate wasteform grout were examined.

The grout was formulated to accommodate unique environmental boundary conditions (vault temperature = 45 °C) and exacting regulatory requirements (mandating less than 0.1% shrinkage with no expansion and no bleeding); and to remain pumpable for a minimum 2 hr. A grout consisting of API Class H cement, an ASTM C 618 Class F fly ash, sodium bentonite clay, and a natural sand from the Hanford area met all performance requirements in laboratory studies.