Contaminated Soil at Nuclear Weapons Site

GeoMelt® vitrification technology was selected in 1989 for a decade-long project to stabilize contaminated soil from above-ground nuclear testing dating back to the 1950s and 1960s

Maralinga, South Australia

Situation Overview
The Maralinga site is a former nuclear weapons test range in South Australia used by the British in the 1950s and 1960s for aboveground nuclear testing. Several hundred experiments involving conventional explosives dispersed plutonium (Pu), uranium (U), and beryllium (Be) into the environment. At the Taranaki area of Maralinga, 12 minor tests were performed that involved the explosive dispersal of 22 kg of plutonium, resulting in large amounts of contaminated debris and soil that were placed in burial pits. The Taranaki pits were typically excavated by blasting in the native limestone. Several years later, concrete caps were placed over the tops of the pits.

A lack of pit characterization data and inaccurate historical records presented challenges to the project. The estimated volume of the pits treated was approximately eight times larger than specified in historical records. The pits were ultimately determined to have contained sealed drums and reactive chemical compounds such as hydrocarbon fuels and, probably, explosive materials, even though historical records did not indicate the presence of such materials.

Regulatory Process
Cleanup of the Taranaki burial pits was part of the Maralinga Rehabilitation Program managed by the Commonwealth of Australia. In 1989, GeoMelt® was identified as the preferred technology to remediate the burial pits in a cooperative British/Australian study by the Maralinga Technical Assessment Group. Confirmatory testing and radioactive demonstrations were performed before dedicated equipment was designed and manufactured for remediating the pits. The pits were believed to contain approximately 2 to 4 kg of plutonium; a similar amount of uranium; various toxic metals, including lead (Pb), barium (Ba), and beryllium (Be); and large amounts of debris (e.g., massive steel plates, steel beams, lead bricks, barytes shielding bricks, cable and organic-based materials).

Remediation Approach
The project was conducted in three phases:

  1. Onsite testing, design, and construction of a dedicated GeoMelt® plant
  2. Site preparation
  3. Full-scale remedial operations

After the pits were located, the concrete caps were removed. Soil overburden was removed to locate the surface boundaries of each pit. Sand-filled trenches were installed around the pits to provide a barrier to outward melting. Monitoring instrumentation was placed in the trenches. The pits were probed using a heavy steel rod affixed to a hydraulic rock hammer mounted on a tracked excavator. The rod was driven vertically into each pit in a grid pattern to collapse and fill voids, confirm the lateral pit boundaries, and help determine pit depths.

Finally, a mound of silica sand was placed over each pit, and the GeoMelt® equipment was positioned for treatment. The sand provided a level and uncontaminated base for workers to prepare the system. Melting was initiated in the sand layer. The sand augmented the melt chemistry by providing more glass-forming ions, which improved the chemical and physical characteristics of the vitrified product.

Results
This project was completed in 1999 and achieved its primary objective of converting the loose, friable, radioactive contamination in the pits into dense, hard, intrusion-resistant vitrified masses, eliminating the long-term hazards of subsidence or human disturbance.

The resulting vitrified monoliths of each pit were intrusively sampled and examined to characterize the vitrified product and confirm the completeness of treatment; all the vitrified monoliths were exhumed.

  • The composition of the main core of the monoliths was usually relatively uniform.
  • No partitioning or elevated concentrations of plutonium were found in any of the monoliths.
  • There was no indication that plutonium was present in the melted steel phase, when such a phase was present at the base of the vitrified monoliths.
  • Plutonium retention in the melts was >99.99%, which minimized the degree of equipment contamination and radiological hazard to workers. The hoods and off-gas piping did not require decontamination after any one melt.
  • Electron photomicrograph analyses of samples of un-melted steel in contact with the vitrified product revealed that the melt was corrosive enough to promote dissolution (decontamination) of the steel, thus transferring plutonium from the steel surfaces to the melt.
  • Samples of vitrified product from two of the full-scale melts were subjected to the Product Consistency Test (PCT). The normalized release rates for the major oxides from the 28-day tests were substantially <1 g/m2-day, with most release rates <0.1 g/m2-day. Long-term leach rates (4.5 years) established that the normalized leach rates decline with time. The normalized leach rate for plutonium decreased by over 1,000 times during the leaching period, demonstrating that the vitrified product has outstanding leach resistance.