46th annual hps mtg; mpm-b.31 two sample preparation methods for measuring 3 h and 14 c in...

46th Annual HPS Mtg; MPM-B.3

1

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

Duke University Medical CenterDivision of Radiation Protection


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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.


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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.


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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.


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Performance Criteria

Count time t needed, for each nuclide and material, to achieve a specified "minimum detectable concentration" [MDC] based on the regulatory constraints


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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)


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Target MDC

Specified regulatory limits: 37 Bq g-1 for 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-1 for 3H 0.11 Bq g-1 for 14C


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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)


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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.


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Measurement Results

3H (Bq g-1) 14C (Bq g-1)

Material/Method

Mean Std. Dev.

Mean Std. Dev.

Ash/Oxidizer 32.4 13.3 0.74 0.47

Ash/Gel 0.6 0.3 0.80 0.07

Lime/Oxidizer 13.3 0.7 0.24 0.04

Lime/Gel 8.1 0.5 0.57 0.07


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Time to Achieve MDC

3H Count Timea

(minutes) 14C Count Timeb

(minutes)

Ash/Oxidizer < 1 79

Ash/Gel 22 7559Lime/Oxidizer

<1 77

Lime/Gel 22 1175a 3H MDC = 3.7 Bq g-1

b 14C MDC = 0.11 Bq g-1


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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


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Sampling Campaign

(n=30)

Nuclide

Material

Method Sample

(g)

Count t(minutes)

3H Ash Oxidation

0.5 20

14C Ash Oxidation

0.5 180

3H Lime Oxidation

0.5 20

14C Lime Oxidation

0.5 180

3H Ash Gel 0.1 18014C Ash Gel 0.1 1803H Lime Gel 0.1 18014C Lime Gel 0.1 180


14

0.0

5.0

10.0

15.0

20.0

25.0

0 5 10 15 20 25 30 35

Ox-Ash

Ox-LimeGel-Ash

Gel-Lime

3H MDC vs count time

Count time (minutes)

Desired MDC

(3.7 Bq g-1)

MDC(Bq g-

1)


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14C MDC vs count time

0.00.20.40.60.81.01.21.41.61.82.0

0 1000 2000 3000 4000

Ox-Ash

Ox-Lime

Gel-Ash

Gel-Lime

MDC(Bq g-1)

Desired MDC

(0.11 Bq g-1)

Count time (minutes)


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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)²x² + (u/y)²y² + (u/z)²z² + …where u = u(x, y, z, …) is the derived quantity.

Knoll (1989)

46th annual hps mtg; mpm-b.31 two sample preparation methods for measuring 3 h and 14 c in...

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