ICEM’05: The 10th International Conference on Environmental Remediation and Radioactive Waste Management
September 4-8, 2005, Scottish Exhibition & Conference Centre, Glasgow, Scotland |
S. A. Philips, Canberra Industries, Inc.
S. Croft, Canberra Industries, Inc.
Abstract
The processing of uranium ores generates a waste product rich in radium. The
waste product is mobilized into a liquid slurry form by mixing with water and
is measured while being pumped around a re-circulation loop. In order to prepare
the appropriate mix of slurry and hardening agents for the final disposal product,
the radium concentration in the slurry must be determined in real time. We
have developed an In-Line Monitor to provide density and radium concentration
information for the slurry on a time scale on par with the re-circulation process.
The density of the slurry is determined from a transmission measurement using
the 122 keV g from 57Co. The radium concentration in the slurry is determined
from a direct measurement of the 226Ra 186 keV emission using quantitative
gamma-ray spectrometry. The concentration determination includes a self-attenuation
correction factor obtained from the transmission measurement and uses the ratio
of the mass attenuation coefficients of the slurry at the assay and transmission
energies. The density and concentration results are presented as real-time
rolling averages along with a full uncertainty analysis. Concentration values
for other (daughter) nuclides may also be quantified in the same measurement.
Presented here are the main features of the In-Line Monitor design, results
from initial on-site testing, and a description of the concentration calibration
process.
Introduction
At the Fernald Environmental Management Project outside Cincinnati, Ohio (operated
by the U.S. Department of Energy), a radioactive sludge waste comprised of
uranium ore tailings has been removed from two silos and is being prepared
for final disposal. The waste was generated at a uranium processing plant that
operated from the early 1950’s to the late 1980’s in the process
of transforming raw ores into uranium cast billets. By mixing the sludge waste
with water a liquid slurry is formed and eventually blended with fly ash and
cement in order to create a disposable waste product. The activity of the final
waste product is limited to amounts determined by regulatory limits for transport
and for the final disposal site. The waste is known to contain radium, lead,
and thorium as the primary radio-nuclides and is considered low-level radioactive
waste. The activity in the final waste product is controlled by varying the
mix of solid materials and water in the slurry, and to this end the radium
concentration in the slurry must be monitored continuously. The monitoring
system must therefore be able to provide accurate results on what amounts to
a real-time basis.
Based on the design parameters provided by the end user, a transmission based
in-line densitometer was adopted as the most suitable system for this application.
Although there are several transmission based gamma-ray densitometry techniques
and applications [1-6], the fundamental mathematical principle is the same.
In this specific application, however, an adaptation to the standard algorithms
and method was used. In the density determination a mixing rule is used relating
the density of the dry solid material to the mass attenuation coefficient (MAC)
of the slurry. The density obtained in this way is then incorporated with a
self-attenuation correction factor (CF) based on the MAC’s, and implemented
in the concentration determination. Essential features of the design and algorithms
for the In-Line Monitor (ILM) are described in the ensuing sections.
