Addressing Uncertainty in Radiochemistry Measurements M. C. Nichols, CHP...
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Transcript of Addressing Uncertainty in Radiochemistry Measurements M. C. Nichols, CHP...
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
Strontium-90
-3-2-10123
0 10 20 30
Order
No
rmali
zed
D
evia
tio
n
June, 2009 RETS-REMP meeting
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)
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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)
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
June, 2009 RETS-REMP meeting
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 %
June, 2009 RETS-REMP meeting
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
June, 2009 RETS-REMP meeting
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