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Return to: 2010 Feature Stories

CLIENT: MACTEC, Inc.

July/August 2010 : Radwaste Solutions

HERE’S THE DIRT ON SOIL ASSAY AT PLUM BROOK

Technology to assay radiologically impacted soils is being used to help decommission a NASA reactor facility.  The technology is quickly helping the environment by returning clean soil to the local ecosystem.

During its operations, NASA’s 60-megawatt Plum Brook Reactor Facility (PBRF) in Sandusky, OH conducted over 70 experiments, most of which studied the effects of radiation on various materials. The reactor was closed down in 1973 and was placed into a “safe storage” mode, during which it was monitored by NASA until initial decommissioning work began in 1998.

Orion ScanSort, a soil sorting system developed by MACTEC, deploys advanced scanning spectrometer technology, is now providing soil measurement and sorting services at the facility. The technology has been proven and has been accepted by the U.S. Nuclear Regulatory Commission (NRC) and the U.S. Environmental Protection Agency (EPA).

More than 160 million pounds of potentially contaminated soil material is being sorted. The soil being processed is from the facility’s hot retention basin shielding, an emergency retention basin dike, cold retention basins, storm drains and other buried piping, foundations and underground storage tanks.

As this article went to press, more than 80,000 tons have been processed; soil processing for the project is expected to be completed by June 1, 2010.  The targeted daily production rate is 550 tons/day – in the summer and fall months, with minimal weather delays, typical production rates were nearly 1,500 tons/day; in winter months, about 500 tons/day.

How It Works

The conveyor-based soil handling system surveys and monitors excavated soils at high throughput rates. Multiple detectors are arrayed above material conveying equipment that provides real-time radiological data and material sorting. A variety of material conveying equipment can be used to handle most common waste forms.

The system supports gamma spectrometers of any size, and up to 12 auxiliary detectors (radiological or otherwise) that provide additional material characterization and control. In its current configuration, it detects gamma-emitting radionuclides as the soil on the belt moves past the active area of the detector (belt speed, depth, and density of the material are continuously monitored for measurement accuracy).

The software automates and controls measurement and sorting processes. Some key features:

  • Real time spectral stripping improves the signal-to-noise ratio in selected areas
  • Monitors multiple isotopes simultaneously
  • Evaluates radiological concentrations against multiple volumes simultaneously
  • Auto-generates process summary reports
  • Automated quality assurance measures and reporting
  • Real time system status is available over WiFi

Putting the System to Work

Following the physical setup of the system and supplemental material conveyors, calibration standards material was brought to the system for site-specific calibration. Following the completion of set-up and calibration, two operators remained on site for the duration of the soil sorting field operations. On-site set-up, integration, and calibration took about two weeks.

Here’s a snapshot description on how the process unfolds each day – potentially contaminated material is brought in by heavy equipment such as a front-end loader, sifted to remove large debris, and conveyed to the system’s weather-tight spectroscopy unit housing. As the material passes beneath the housing, the radiation detectors measure the emitted radiation signal and output the data to a computerized monitoring station. The detector outputs are then compared against pre-established radiation thresholds.

If either the radioactivity of surveyed material does not meet specifications, a divert signal is generated and a reversing conveyor quickly switches direction, sending the offending material to the contaminated material conveyor for future off-site disposal. Clean material is sorted down a different conveyor for return to the environment as excavation backfill. Meanwhile the system continues assaying incoming material for its radiological concentration.

Benefits

Other key advantages include:

  • Large volume detectors provide superior sensitivity
  • Isotope specific measurements
  • Spectral stripping improves accuracy
  • Multiple decision criteria

The system also controls the speed of the assay conveyor and the direction of the proprietary reversing conveyor with a unique electric motor – rather than slower hydraulic actuators, the motor enables precise/instantaneous conveyor control.

To date, the system has assayed and segregated in excess of 160 million pounds of radiologically suspect soil from NASA’s Plum Brook Reactor Facility into material that may either stay on site for use as backfill (less than half of the release limit for Cesium-137 in soil) or shipped offsite for permanent disposal as radioactive waste. The system is expected to reduce the volume of radioactive waste by up to 95 percent compared to simply excavating it and disposing of it as radioactive waste. The technology is expected to result in substantial cost avoidance savings for shipping and burial charges and permit NASA to compress its decommissioning schedule.

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Return to: 2010 Feature Stories