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Strontium-90 Sensor Project - Hanford
Background
Strontium-90 contamination of ground water is an important issue for the
Department of Energy (DOE), especially at Washington State's Hanford
Site where N-Reactor operations have resulted in a plume of 90Sr in the
near surface groundwater that is continually released into the Columbia
River. DOE is planning to remediate this plume with the objective of
reducing the amount of 90Sr entering the Columbia River. The planned
cleanup method involves a permeable barrier to absorb 90Sr and a line of
trees/shrubs along the river's edge to take up 90Sr down gradient from
the proposed barrier. In order to assess the effectiveness of the
remediation effort and to verify the reduction of the 90Sr levels
entering the river, it is imperative to compare 90Sr levels before and
after remediation begins. Moreover, continuous 90Sr monitoring will be
required throughout the remediation effort, since 90Sr concentrations
vary seasonally and as the river level rises and falls. The
effectiveness of bioremediation also will vary, depending on
temperature, time of year, and plant growth. These factors necessitate
obtaining data more frequently (once or twice a week) than is practical
using conventional sampling and laboratory analysis methods, which are
prohibitively expensive and time consuming.
Technical Approach
The measurement approach is based on the direct detection of the
characteristic Cherenkov light produced in the water sample by the
high-energy beta particles from the decay of the 90Y daughter of 90Sr.
This approach is particularly selective for measuring 90Sr(90Y), since
it is the only isotope with a long enough half life and high enough beta
decay energy to be of radiological concern in groundwater. The objective
will be to measure the concentrations of 90Sr in groundwater samples via
quantification of the Cherenkov radiation generated by its daughter
isotope, 90Y. Strontium-90 has a beta decay endpoint energy of only 546
keV, while the equilibrium daughter has a beta endpoint energy of 2281
keV. Since it takes more than a 250-keV electron to generate Cherenkov
radiation, and since the intensity of the Cherenkov photons is
proportional to the electron energy, the 90Sr is a relatively poor
source of Cherenkov light, while the 90Y is a prolific producer.
Benefits
The major universal benefit from field deployment of this monitoring
system is the ability to acquire up to 100 times as many measurements as
the baseline technology in any given period of time, and at virtually no
cost. Assuming that the investment in this proposed field deployed
system is comparable to the investment cost in the baseline laboratory
counting system, the cost per analysis for the proposed system is at
least $1000 less than the baseline technology. Perhaps even more
important than the cost savings is the availability of virtually
continuous data. In the case of the Hanford 100-N Area plume remediation
efforts, these data will be extremely important to understand the
efficacy of the remediation technologies and the effects of
environmental conditions on those technologies.
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