Addressing Uncertainty in Radiochemistry Measurements M. C. Nichols, CHP [email protected] 404-819-5118 RETS-REMP meeting

Addressing Uncertainty in Radiochemistry Measurements

M. C. Nichols, [email protected]

404-819-5118

RETS-REMP meeting

June, 2009 RETS-REMP meeting

Guidelines for Using Quality Control and Performance Testing (PT) Data

• USNRC regulatory guide 4.15 revision 1, performance testing– Specification in 6.3.2 for performance

testing– Acceptance criteria– Uncertainty estimates from PT data

• Revision 2, based on MARLAP– Total propagated uncertainty


USNRC Regulatory Guide 4.15, Rev. 1

• Performance testing - 6.3.2– Measure accuracy (normalized deviation)– Measure precision (normalized range)

• Basis: “Environmental Radioactivity Laboratory Intercomparison Program”, EPA 600/4-07-001, January 1977


Acceptance Criteria

• Confidence interval estimates

• Normalized deviation and normalized ranged in the EPA cross check program

• Ratio and resolution based on US NRC Inspection Procedure-84750


Confidence Limit

2 2

2known

2measured

average of the reported values

= known value

k = constant (coverage factor)

variance in the known value

variance in the reported values

known measuredX k

where

X


EPA Cross Check - Accuracy

ND =

X = measured average

known activity

standard error of the mean= / N

= expected lab precision of one determination

number of replicate analyses

X

where

N


EPA Cross Check - Precision

3 single

3

single

Range 4

4

Mean Range R D ( )

where

D 1.693 for three replicate measurements

R maximum result - minimum result

expected lab precision of one determination

(D R R) / N

where

D 2.575 for 3 repl

icate analyses


NRC IP-84750

• Resolution - divide each NRC result by its associated uncertainty

• Uncertainty defined as the relative uncertainty, 1S, of NRC results calculated from counting statistics

• Ratio - licensee result / NRC result• Criteria - gives a range of acceptable

ratios as a function of resolution


Single Laboratory Precision

• Counting uncertainty

• Providers specification

• Use performance testing history

• Construct an uncertainty budget– May be available for specific procedures

(gamma spectroscopy)– Build on existing quality control data


Strontium-90

-3-2-10123

0 10 20 30

Order

No

rmali

zed

D

evia

tio

n


Performance Testing - Strontium-90

0

5

10

15

20

25

30

35

0 5 10 15 20 25 30 35

Known Concentration (pCi/ Liter)

Rep

orte

d C

on

cen

trati

on

(p

Ci/

Lit

er)


2i

2x -x1 new2S Y =MSE 1+ +

new 2n (x -x)i

where

MSE = means square error from a linear regression

(x -x) = sum of squared deviations from the mean known value

n = number of measurements

Variance - Future Prediction


Sr-90 Concentration

(pCi/Liter)

Prediction Interval (1S)

Coefficient of Variation

5 1.8 37%

10 1.8 18%

15 1.8 12%

20 1.8 9%

25 1.8 7%

30 1.9 6%

Prediction Intervals for Sr-90


Suggestions for Performance Testing Data

• Two components to the evaluation of performance data – Uncertainty of the known value– Uncertainty of the measured value

• Regression - use replicate measurements for each performance testing round

• Uncertainties from performance testing results are limited to the range of testing


Uncertainty Budget

• Define the process including components of the activity estimate

• Identify uncertainty estimates for each component

• Use software tools to create uncertainty budgets (e.g. Gumcalc)

Efficiency

( )

( )1000( )

( )

R El

netR

f p fTR R

c c

RE x

W WC V e

V C

ΕR(xp) = expected count rate per disintegration rate as a function of areal

thickness for carbon-14 or strontium-89

Rnet = net count rate for the source (min-1) (see equation 3-2)

CR = concentration of the source solution (min-1 mL-1) at a specified date and

time

VR = volume of the source solution (mL)

λR = decay constant for strontium-89 or carbon-14 (s-1)

TEl= elapsed time (s) from the source date to the mid-point of the count time.

Wf+p = mass of filter plus precipitate (g)

Wf = mass of filter (g)

1000 = conversion factor, mg g-1

Vc = Volume of carrier added (mL)

Cc = Concentration of carrier (mg mL-1)


Example - Uncertainty for Balance Measurement

2 2 2 2( ) )(L

I envaru m Ms s

where:

Sr = standard uncertainty due to repeatability – obtained from balance checks ;

Sal = standard uncertainty due to balance linearity – manufacturers specification;

MI2 = balance indication (variable); and

Φenv2 = relative standard uncertainty due to environmental factors,

estimated from linear regression



Strontium-89 Uncertainty at 18.5 mg cm-2, Efficiency 0.453 ± 0.012 (95% CL).

x[i] u[i] ER(xρ) Units % of variance Concentration of radionuclide (CR) 0.00283 min-1 mL-1 20.6 % Volume of source solution (VA) 0.00269 mL 18.7 % Mass of filter plus precipitate (Wf+p) 0.00184 g 8.72 % Mass of filter (Wf) 0.00184 g 8.72 % Volume of carrier (Vc) 0.00148 mL 5.64 % Gross count rate (Rg) 0.00202 min-1 10.5 % Background count rate (Rb) 8.12E-05 min-1 0.017% Concentration of carrier (Cc) 0.00317 mg mL-1 25.9 % Decay constant (λR) 0.000652 s-1 1.10 %


Summary

• All performance testing evaluation is based on an estimate of measurement uncertainty

• Performance testing data will provide an estimate of total measurement uncertainty

• Measurements outside the range of PT data and new program requirements will require assessment of measurement uncertainty

• Laboratories currently have sufficient data to construct these estimates


References– Ellison, S.L.R., M. Rosslein, and A. Williams, 2000.

EURACHEM/CITAC guide CG 4: quantifying uncertainty in analytical measurement, 2nd edition

– MARLAP, 2004. Multi-agency radiological laboratory analytical protocols manual (final) volume i (epa 402-b-04-001a), volume ii (epa 402-b-04-001b), volume iii (epa 402-b-04-001c)

– Mccroan, K. D., 2005. Gumcalc v 1.0. Freeware developed for the MARLAP project. Http://mccroan.Com/gumcalc.Htm

– USEPA, 1977. Environmental radioactivity laboratory intercomparison program”, EPA 600/4-07-001

– USNRC, 2007 Quality assurance for radiological monitoring programs (inception through normal operations to license termination) -- effluent streams and the environment, revision 2, ML071790506, July, 2007

– USNRC, 1994. NRC inspection manual procedure 84750 for radioactive waste treatment, effluent, and environmental monitoring

http://www.epa.gov/rpdweb00/marlap/manual.html



http://adamswebsearch.nrc.gov/idmws/ViewDocByAccession.asp?AccessionNumber=ML071790506

Documents

Addressing Uncertainty in Radiochemistry Measurements M. C. Nichols, CHP [email protected] 404-819-5118 RETS-REMP meeting