Date of Award

5-2009

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Legacy Department

Environmental Engineering and Earth Science

Advisor

DeVol, Timothy A

Committee Member

Creager , Stephen E

Committee Member

Fjeld , Robert A

Committee Member

Lee , Cindy M

Abstract

Tritium is naturally present in very small concentrations in the environment, principally in the form HTO. Thermonuclear detonations, leaks from waste tanks at the Department of Energy (DOE) sites and accidental releases from nuclear power plants have introduced significant quantities of tritium into the environment. Even though tritium is the least toxic of the known beta emitters, its presence in the environment, primarily in the aqueous form, poses a radiological threat because of its easy accessibility. Tritium removal from the environment is technologically impractical. Thus tritium contamination is generally contained, decays in place and is monitored to protect the public and regulatory compliance. Research is needed for the design of new selective monitoring systems to detect current and changing conditions of tritium contamination in the subsurface. In-situ sensors, which respond to this criterion, avoid expensive sampling operations as well as laboratory analysis. They also facilitate real time measurement, and decrease the risk to health and cost of long term monitoring.
Our research project, developed in three parts, consisted in building a laboratory prototype for a field instrument designed for continuous, long-term monitoring of tritium in groundwater. The tritium monitor was constructed with a compact Polymer Electrolyte Membrane (PEM) Pt/Ir electrolyzer mounted in series with a gas proportional counter. This instrument was designed for measurements of tritium concentrations at a level down to 740 Bq/L (20,000 pCi/L), since groundwater aquifers may be used as drinking water sources for public water systems. This maximum concentration corresponds to the Safe Drinking Water Act (SDWA) maximum contaminant level of 4 mrem/year.
The first part of the research consisted in studying the Pt/Ir PEM enrichment parameters. The parameters were compared to those of a classical tritium enrichment system, like the one operated at the Miami Tritium Laboratory used in the analysis of water samples with very low tritium concentration levels. Aqueous tritium enrichment parameters E (tritium aqueous enrichment), F (Evolved tritium activity fraction), β (tritium fractionation factor) and βe (electrolytic fractionation factor) were determined from PEM electrolysis of tritium aqueous standards. Lower aqueous enrichment was observed in the Ir/Pt PEM electrolyzer in comparison to the conventional Ni/Fe electrolytic cell. This was explained by the values found for the PEM cell fractionation factor βIr/Pt and electrolytic fractionation factor βeIr/Pt which were determined to be 4.7±0.3 (βNi/Fe=26), and 6.6±0.7 (βNi/Fe=37), respectively. A direct consequence of the Ir/Pt βe value was the richer tritium gas phase produced relative to the conventional cell, which was advantageous for direct reduction of HTO to HT gas.
The second part of the project consisted of quantifying the tritium gas generated by the PEM electrolyzer in a proportional counter mounted in series with the PEM cell. Counting conditions as well as the possibility of using enrichment before counting to reach the Environmental Protection Agency (EPA) primary drinking water standard (740 Bq/L) detection limits were studied. The detector operating voltage, efficiency, and background count rate of the passively shielded counter were measured in order to calculate the minimum detectable concentration of the detection system. The optimum operating voltage was found to be 2250 V for a high purity mixture hydrogen/propane of 94/6 by volume, at atmospheric pressure. The efficiency of the counter determined with a tritium gas standard diluted with the optimized high purity hydrogen /propane gas mixture was 49±5%. The background for the 1 L detector passively shielded with 5 cm of low-activity lead was 0.52±0.03 C/s for the optimized tritium region-of-interest. The electrolytic fractionation factor of the PEM electrolyzer was determined by gas phase tritium measurement to be 6.6± 0.6 and identical to that obtained from the prior aqueous enrichment experiments. The minimum detectable concentration of the detection system was calculated to be 530 Bq/L for a four hour count time without isotopic enrichment. The system was used to quantify an aqueous phase solution of 740 Bq/L with a four hour count and was in good agreement with conventional liquid scintillation analyses. Aqueous enrichment of the sample by a factor of five before gas phase collection and counting showed the precision can be significantly improved. Finally, analysis of tritiated water standards of concentrations above 3000 Bq/L by this detection system was in good agreement with conventional analyses.
The third part of the study consisted in determining a simple and efficient sample pretreatment method to be used before electrolysis for direct measurement in a groundwater well. This critical part to the analysis of actual tritiated water samples was developed as the electrolytic cell can only receive pure water to perform the electro-chemical reduction of tritiated water. A groundwater sample from the Savannah River Site and a surface water sample collected downstream of a nuclear power station were treated before analysis of their tritium content by both, liquid scintillation counting and our tritium gas detection system. In order to process the samples, columns analogous to the Eichrom Tritium columns® were prepared in our laboratory. For deionization of the water sample, Diphonix® resin in the H+ form was used as the cationic exchange-complexation resin and the Dowex® resin 1X4 in the OH- form was used as the anionic exchange resin. A polymethacrylate resin was placed after the deionizing segment in the column to remove naturally occurring organic matter including organically bound tritium carbon-14. The breakthrough capacity of the deionizing 'segment' of the column was determined with a 0.01 N KCl solution by conductivity measurement of the column effluent and confirmed with ICP-AES. The breakthrough volume was used to estimate the quantity of resin to be used to treat different samples. Conductivity measurements after the deionizing step were equal to conductivity measurements of DDI water, confirming the effectiveness of the deionizing treatment. However the total organic carbon (TOC) measurement of the sample effluent, after the naturally occurring organic matter removal, was found identical the measurements performed on the raw samples, revealing a probable leaching of monomers from the polymethacrylate resin. Average tritium recoveries for the groundwater and surface water samples were determined to be 99±5% and 96±16%, respectively. The average concentrations measured by LSC and our electrolysis/proportional detection system were not different within associated experimental error for both the GW and the SW samples.

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