Analysis of 36Cl in Savannah River Site radioactive waste

Abstract

Radioactive waste cleanup and subsequent closure of waste storage tanks is currently underway at the Savannah River Site, prompting the need to characterize the residual contents (heels) of the tanks. Each tank heel has variable chemical constituents and radiological distributions that pose unique analyte characterization and quantification challenges. For example, similar target detection limits for each tank heel are often difficult to meet as a result of complex matrices and methods must be tailored to characterize each new tank heel to the level of detection required. Furthermore, continuous programmatic evaluations of allowable closed-tank radiological inventories result in steadily decreasing detection limit targets for many of the isotopes required for characterization. Some of the target isotopes required for characterization need to be quantified to levels approaching 10 orders of magnitude below the radioactive concentrations of interfering isotopes. One particularly troublesome target analyte is 36Cl. As a pure beta-emitting radionuclide, 36Cl (T1/2 = 3.01 × 105 ± 3,000 years) must be isolated from interfering beta-emitting radionuclides such as 90Sr, 90Y, 137Cs, 99Tc, and 14C, as well as actinides and lanthanides prior to quantification. Due to its half-life, the target detection limit for 36Cl is extremely low compared to the concentrations of other beta-emitting radionuclides typically present in residual tank solids. A method of analysis for 36Cl was developed which utilizes a combination of traced acid digestion of radioactive waste tank samples followed by aggressive decontamination of interfering beta-emitting radionuclides via repeated resin-strikes, distillation, precipitation as AgCl, and analysis by gas flow proportional counting for the 36Cl—containing precipitate. The separation is traced by neutron activation analysis of stable chloride imparted during the initial digestion, and results are corrected for the separation yield. Recent campaigns have successfully achieved interference-free AgCl precipitates and, ultimately, detection limits of 0.37 Bq/g from matrices containing ~1E7 Bq/g of other beta-emitting radionuclides.