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Two Sample Preparation Methods for Measuring 3 H and 14 C in Incinerator Ash and Spent Lime

Duke University Medical Center Division of Radiation Protection. Two Sample Preparation Methods for Measuring 3 H and 14 C in Incinerator Ash and Spent Lime. Ben Edwards , Le-Xuan Thai and Dan Sprau Master's Project - East Carolina University

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Two Sample Preparation Methods for Measuring 3 H and 14 C in Incinerator Ash and Spent Lime

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  1. Duke University Medical Center Division of Radiation Protection Two Sample Preparation Methods for Measuring 3H and 14C in Incinerator Ash and Spent Lime Ben Edwards, Le-Xuan Thai and Dan Sprau Master's Project - East Carolina University in partial fulfillment of the requirements for the degree of MS in Occupational Safety 46th Annual HPS Mtg; MPM-B.3

  2. Problem: • Analyze two selected methodologies for measuring 3H and 14C in ash and spent lime from the incineration of low level radioactive biomedical research waste. • Assess the analytical performance of each method. 46th Annual HPS Mtg; MPM-B.3

  3. Relevance • Radioactive material was used in the development of every major drug discovered since 1946, and in millions of analytical, diagnostic, and therapeutic medical procedures each year in the US. • Radioactive waste from this biomedical use is highly regulated. Disposal is expensive, particularly for 3H and 14C. Radioactive waste management dissipates biomedical research institutions' financial resources. 46th Annual HPS Mtg; MPM-B.3

  4. Relevance (cont’d) • Incineration generates ash and spent lime waste. Disposal of this waste as non-radioactive requires demonstrating that the radioactive concentration does not exceed specified regulatory limits. Disposal of the ash and lime as radioactive waste is prohibitively expensive. • A reliable analytical method, capable of achieving the required sensitivity, can reduce waste disposal costs of academic, industrial and government biomedical research facilities. 46th Annual HPS Mtg; MPM-B.3

  5. Performance Criteria • Count time t needed, for each nuclide and material, to achieve a specified "minimum detectable concentration" [MDC] based on the regulatory constraints 46th Annual HPS Mtg; MPM-B.3

  6. MDC Formula MDC = [2.71+4.65(RBxt)½]x[60xExMxYxt]-1 • RB = Background count rate in counts minute-1 (cpm) • t = Background & gross count time (minutes) • E = Counter efficiency (counts/disintegration) • M = Sample mass (g) • 60 = disintegrations minute–1 [dpm] per Bq of activity • Y = fraction of chemical yield, if applicable Gollnick (1994) 46th Annual HPS Mtg; MPM-B.3

  7. Target MDC Specified regulatory limits: • 37 Bq g-1for 3H • 1.1 Bq g-1 for 14C [10 CFR 20 App. B Table 2 Column 2; PG 8-10 (1997)] Per Fong and Alvarez (1997), set target MDC at 1/10 of regulatory limit; target MDC: • 3.7 Bq g-1for 3H • 0.11 Bq g-1 for 14C 46th Annual HPS Mtg; MPM-B.3

  8. Specific Activity [C] Formula C = (S - RB)Y (E M)-1 • S = sample [gross] count rate (cpm) • RB = Background count rate (cpm) • Y = sample yield • E = counting efficiency (dpm/cpm) • M = sample mass (g) 46th Annual HPS Mtg; MPM-B.3

  9. Available Methods • Oxidation - combustion of the solid sample in an oxygen-rich environment; drives off the 3H as HTO vapor and the 14C as 14CO2. These gaseous combustion products are then captured in separate collection vials for liquid scintillation counting. • Gel Suspension - the powdered solid sample material is suspended in a gel-forming liquid scintillation counting solution. 46th Annual HPS Mtg; MPM-B.3

  10. Measurement Results 46th Annual HPS Mtg; MPM-B.3

  11. Time to Achieve MDC a 3H MDC = 3.7 Bq g-1 b 14C MDC = 0.11 Bq g-1 46th Annual HPS Mtg; MPM-B.3

  12. Conclusions • Both methods easily achieve 3H MDC • Only oxidizer achieves 14C MDC in less than 2 hours; gel takes 126 hours for ash & 26 hours for lime • Gel method fails to detect 95+% of 3H in ash 46th Annual HPS Mtg; MPM-B.3

  13. Sampling Campaign (n=30) 46th Annual HPS Mtg; MPM-B.3

  14. 3H MDC vs count time MDC (Bq g-1) Desired MDC (3.7 Bq g-1) Count time (minutes) 46th Annual HPS Mtg; MPM-B.3

  15. 14C MDC vs count time MDC (Bq g-1) Desired MDC (0.11 Bq g-1) Count time (minutes) 46th Annual HPS Mtg; MPM-B.3

  16. Error Propagation If x, y, z, … are directly measured variables for which we know the standard deviations x, y, z,…, then the standard deviation for any quantity u derived from these counts can be calculated from: u² = (u/x)²x² + (u/y)²y² + (u/z)²z² + … where u = u(x, y, z, …) is the derived quantity. Knoll (1989) 46th Annual HPS Mtg; MPM-B.3

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