Quarterly report on the ferrocyanide safety program for the period ending December 31, 1994 Page: 33 of 79
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WHC-EP-0474-15
penetrometer, are being evaluated by other programs. A report examining moisture
monitoring'technologies was completed in April 1993 (Meacham et al. 1993).
Neutron Diffusion. Well-logging techniques, coupled with computer modeling, were
developed and applied to an existing neutron probe to determine information about moisture
levels, material interfaces, and other waste characteristics in the ferrocyanide tanks. Using
the knowledge gained from computer modeling, in situ measurements, and experimental
calibration data with the current in-tank neutron probe (Watson 1993), prototype moisture
measurement neutron probes were developed. This system consists of three neutron probes:
a near-field thermal neutron probe, a far-field thermal neutron probe, and a far-field
epithermal neutron probe. This improved system would primarily be used to determine the
axial moisture concentration profile within the ferrocyanide tanks.
Moisture measurement using neutron diffusion is an established technology. The technique
uses a neutron source and one or more neutron detectors. The thermal neutrons reaching a
detector originate as fast neutrons from the source and are slowed or absorbed by the
medium. Because hydrogen atoms are effective at slowing down neutrons, the detector
response is a strong function of the surrounding moisture concentration.
Two methods are generally used in the measurement of moisture concentration around wells
using neutron diffusion. The first method, the moisture gauge, has a short source-to-detector
spacing (near-field) on the order of 0 to 6 cm. The response of a moisture gauge is
characterized by an increase in detector response with increasing moisture concentration of
the surrounding medium. The second method, the neutron log, often has two detectors with
longer source-to-detector spacings: 20 to 50 cm (far-field). The detectors in a neutron log
arrangement exhibit a decreased response to increased moisture concentrations. The detector
placed at the shorter spacing is used to correct the response of the longer-spaced detector for
borehole effects.
The tank moisture measurements are taken from within an LOW. The LOWs are
permanently installed sealed pipes that extend from the riser top through the tank waste to
near the tank bottom. The LOWs allow axial information about the surrounding waste
materials to be obtained using certain detectors.
" Progress During Reporting Period. In-situ tank measurements have been obtained
in nearly all ferrocyanide Watch List tanks containing an LOW using the prototype
neutron moisture probes. The results of these tests indicate the system is functional in
the waste tank environment. Complete analyses of the tank scan data to produce best-
estimate moisture concentration profiles of the tank waste will be delayed until the
system can be fully calibrated. Scans were taken with each probe without the neutron
source attached to confirm that no gamma interference was encountered in the data.
A computer code is being written to automatically read in the available data from
moisture measurement probes, to correlate the data to responses expected from
different possible material anomalies and moisture contents, and to provide a best
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Meacham, J. E.; Cash, R. J. & Dukelow, G. T. Quarterly report on the ferrocyanide safety program for the period ending December 31, 1994, report, January 1, 1995; Richland, Washington. (https://digital.library.unt.edu/ark:/67531/metadc692188/m1/33/: accessed April 24, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.