RUST Geotech, Incorporated
Description
The Prompt Fission Neutron (PFN) Logging System is also termed Integrated Borehole Geophysical System for Contaminant Identification. This technology is applied for in situ detection of fissile materials, principally U-235 and Pu-239, in soil and rock media surrounding the borehole. It has been reported that PFN probes can also be used as porosity devices or moisture gages. This technology addresses the need for better methods to characterize subsurface geohydraulic features, for in situ methods of characterizing contaminants, and for understanding subsurface contaminant behavior.
The logging system is self-contained and operates as a stand-alone instrument. During field operations, the probe is lowered into a borehole and data are collected. These data are stored digitally, processed rudimentarily, and displayed to permit quality assurance and initial interpretation. At a later time, the data are processed in detail and interpreted. To generate PFN data, the instrument generates a short burst of neutrons using a linear accelerator in a sealed tube within the probe. The neutrons penetrate the soil and rock surrounding the borehole but are slowed down and eventually captured by other atoms. The atoms that capture neutrons, U-235 and Pu-239 (and other elements with comparable fission, cross sections), spontaneously fission producing additional neutrons. These additional neutrons are counted by a detector that is shielded so that it detects only the energetic neutrons from fission. The observed count-rate, which varies as a function of time after each neutron burst, is related to the partial density of fissionable elements in the soil. Epithermal neutrons detected after 200 microseconds are from fission reactions.
An advantage of PFN technology is that it provides a near-continuous profile of contaminants as a function of position along the borehole. Another advantage is that it analyzes some three orders of magnitude larger volume of material than an individual borehole sample. Furthermore, PFN logging provides the opportunity to repeat measurements in the same borehole, year after year, for monitoring purposes. PFN logging can produce in situ assay data in a fraction of the time it takes to submit all of the samples from a borehole to an analytical laboratory and obtain results. With the PFN Logging System, the time needed to log the hole is a few hours, and results are available instantly. On the other hand, the PFN system is not as sensitive in providing assays for waste concentrations as is possible in the laboratory.
Technical Performance
Design. The PFN probe is approximately 11 ft long and is designed to bombard a 2-ft radius immediately around the borehole with a burst of 14 MeV neutrons at a repetition rate of 100/s. A 100-ft borehole could be logged in about 3 hours. PFN logging requires a cased borehole with a minimum inner diameter of 5 in. Vertical direction data point spacings are typically 0.1-0.2 ft. PFN would analyze 1000 times greater volume than borehole sampling. The lower detection limit of the system is about 1 nCi/g Pu-239. (However, some regulatory limits are in the range of pCi/g.)
Cost . It is estimated that a cased, 100-ft deep borehole with an uncontaminated interior could be logged for $1,000 as part of a multiple-hole program. (This is compared to $100,000 for borehole sampling and laboratory analysis of a 100-ft borehole with 20 samples taken at 5-ft intervals.)
Projected Performance
The field demonstration scheduled for the Hanford Site in late FY93 will prove or disprove the applicability of this technique. If PFN logging is successful, it should be possible to reduce the required borehole sampling and laboratory analysis costs by 25%, as a conservative estimate, leading to a savings of $24,000 per 100-ft borehole.
Waste Applicability
This measurement technique is applicable for in situ detection and quantification of fissile materials, principally U-235 and Pu-239, in soil and rock surrounding a cased borehole.
Status
The PFN logger prototype has been constructed and will be demonstrated at the Hanford Site in late FY93. The prototype will be further enhanced in FY94 according to lessons learned and should be available by the end of that fiscal year.
Regulatory Considerations
Of concern is the potential for human exposure to neutron bursts. Appropriate field procedures eliminate this risk because no radiation is generated until power is applied to the probe. The use of this tool in and around aquifers would be of concern if the tool with the neutron source were lost in the borehole and abandoned.
Potential Commercial Applications
This technology could be used in mining and exploration activities to log and assay potential uranium deposits.
Baseline Technology
The baseline technology is borehole sampling and laboratory analysis. While the laboratory analysis could detect fissile materials present in smaller concentrations than the PFN lower detection limit and give a full radiologic spectrum assay, laboratory analysis is much more expensive per data point and analyzes a smaller volume per borehole. PFN logging is not intended to replace requisite sampling and analysis but to reduce the amount of sampling needed. PFN provides an in situ technique that would be useful to provide more data point details to a borehole sampling program. It also provides a relatively inexpensive method for borehole monitoring.
Intellectual Property Rights
The Department of Energy owns the intellectual property.
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References
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