Fernald SEC Petition Review

Focused SC&A Status Update: SEC Issue 6B –

Use of Chest Counts to Reconstruct Th-232 Intakes 

(1968–1978) 

Presented by John Stiver, SC&A

Presented to the Advisory Board on Radiation and 

Worker Health

Full Board Meeting Teleconference 

April 26, 2012
Description of Issue 6B - Review

• No DWE data after MIVRML introduced in 1968 – therefore completely 

dependent on integrity of chest count data thereafter

• 1968–1978 – results reported in mass units (milligrams thorium) with no raw 

data 

• 1979–1988 – results reported in activity of progeny (nCi Pb-212 and Ac-228)

Status of issue 

• White papers exchanged; issue discussed in detail at April 19, 2011; August 

11, 2011; and February 9, 2012, WG meetings. 

• Full Board meeting – February 2012

• SC&A Position: 1968–1978 data reported in milligrams thorium likely 

inadequate for DR (SEC)

1. Large uncertainties related to how the mg thorium data were derived 

(potentially underestimated actual by factor of 100; overestimated 

background level by 1,000)

2. “Technical shortfall” issue (deferred: program-wide issue of “sufficient 

accuracy” under review by SEC Issues Subcommittee)

Open Issue (#6B) Use of Chest Counts to Reconstruct 

Th-232 Intakes (1968–1978) 

2
• February 2012 (before Board meeting) – NIOSH posts set of 

documents claimed to be “relevant references related to the 

estimation of thorium-232 [Th-232] intakes for Fernald workers using 

in vivo data from the Y-12 mobile counter (MIVRML)” 

• NIOSH white papers and other documents that describe different 

approaches that could have been used to calculate Th-232 lung for 

1968–1978

• SC&A response delivered April 6, 2012, “Summary of SC&A 

Concerns Regarding the Latest Documents Posted by NIOSH to 

Complement their White Papers on In-vivo Thorium Bioassay”

• Conclude – NIOSH white papers based on unsupported assumption that 

Pb-212 was measured and Th-232 burdens in mg were calculated using 

those measurement results 

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

3
• Inconsistencies between mg Th-232 and nCi Pb-212 for period of 

overlap (1978, 1979) suggest that Pb-212 was not used to derive mg 

Th-232 (next slide) 

• Three highest mg Th-232 correspond to negative Pb-212 results

• Nine values in 1979 reported as 2.1 mg but Pb-212 ranged from 0.19 to 

0.40 nCi and Ac-228 from 0.33 to 0.7 nCi. These data are suspect. 

Only 1 Pb-212 result was less than the stated MDL of 0.23 nCi (1987). 

• Expect proportionality between Th-232 and progeny in 1979 data 

(divided by plant work and by dates) because the source was 

probably the same. 

• Results highlighted green: workers co-located and measured in June, 

yet results of Pb-212 varied from 0.25 to 0.40 nCi with the same result in 

mg of Th-232. 

• Results highlighted pink: workers in Plant 4 monitored in October, yet 

results varied from 0.19 nCi of Pb to 0.40 nCi, with the same result in 

mg of Th-232. 

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

4
Issue #6B: Use of Chest Counts to Reconstruct 

Th-232 Intakes (1968–1978)

5

Reported Thorium 

Result (mg)

Reported Pb-212 

Activity (nCi)

Reported Ac-228

Activity (nCi) Monitoring Date Location or Plant #

-5.00 -0.04 -0.02 8/29/1974 7 or Pilot

-0.60 -0.08 0.03 06/08/88 Maintenance

-0.54 -0.18 -0.01 06/12/86 5

-0.16 -0.16 -0.09 06/23/87 5

-0.12 0.06 0.01 07/10/73 6

-0.05 -0.05 0.02 05/02/87 Maintenance

-0.01 -0.01 0.05 12/09/86 No Information

0.01 -0.06 -0.08 05/02/77 Mech

0.30 0.15 0.06 04/06/77 6

1.81 -0.10 0.04 08/09/85 5

2.10 0.25 0.35 06/02/79 7 or Pilot

2.10 0.30 0.5 06/09/79 7 or Pilot

2.10 0.40 0.7 06/12/79 7 or Pilot

2.10 0.40 0.65 06/19/79 7 or Pilot

2.10 0.40 0.5 10/08/79 4

2.10 0.19 0.33 10/22/79 4

2.10 0.27 0.33 10/29/79 4

2.10 0.28 0.41 10/17/79 4

2.10 0.29 0.39 10/15/79 4

2.20 -0.10 -0.1 04/13/77 Mech

4.30 -0.04 0.05 04/26/71 Inspection

5.10 -0.04 0.01 06/04/80 Mech

Source: Table 1. SC&A Final Position on the Th-232 In-vivo Data Quality and Adequacy for FEMP Workers 
• April 9, 2012 – NIOSH posts PowerPoint presentation titled, 

“Bounding Thorium-232 Intakes Using MIVRML Data,” and 

reference summary to support their position

• March 15, 2012 – NIOSH Interviewed L. Max Scott, PhD., Principal 

designer and developer of the MIVRML (Board and SC&A not notified)

• MIVRML patterned after fixed Y-12 system and calibrated and operated in 

exactly the same manner as the fixed system

• Same calibration standard used for both systems: Th-232/Ra-228 = 1.67 

(60% equilibrium); Th-232/Th-228 = 1.27 (80% equilibrium)

• Note : these ratios are not possible for a single purified thorium source. 

Indicates Ra-228 contamination.

• REMAB phantom 

• 20 minute counts

• Used empirical “sum of ratios” method to derive mg values (next slide)

• NIOSH position unchanged

• Thorium mass reporting methodology is not an SEC issue

• Thorium intakes estimated from MIVRML measurements are plausible, 

claimant favorable, and bounding

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

6
MIVRML Empirical Equation for Calculating Mg Thorium Values

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

7

• Based on Y-12 method and “Rule of Thumb” document (letter from L. Max 

Scott to C.M. West dated November 21, 1961)

• Where “ROI” is the total count in a region of interest. For example:

• ROI0.208-0.248 is the count in the portion of the spectrum between 0.208 MeV 

and 0.248 MeV (from Pb-212) and

• ROI0.249-0.295 is the “background” count in the adjacent higher energy portion 

of the spectrum between 0.249 MeV and 0.295 MeV

• 3.23 (3.23 ± 0.7) is an average value of summed ratios of counts in the 

three ROIs obtained for about 1,100 non-exposed persons (Table 1, 

Scott 1966)

• Variation (3.23 ± 0.7) represents 95th percentile confidence interval

• 8.84 is a coefficient to convert the summed ratios to units of mass 

(milligrams thorium)

• Specific to the calibration source and conditions at Y-12
8

Issue #6B: Use of Chest Counts to Reconstruct 

Th-232 Intakes (1968–1978)

Sample spectra for exposed and non-exposed workers showing ROIs and adjacent higher 

energy “background” regions (West 1965)
April 17, 2012 – SC&A responds to NIOSH presentation with a memo titled, 

“SC&A Comments on Slide 7 of NIOSH Presentation”

• “Thorium coefficient” of 8.84 is an empirical value that converts the dimensionless sum 

of ratios to thorium mass for the sources and conditions of calibration at Y-12, as 

indicated in the “Rule of Thumb” document (letter from L. Max Scott to C.M. West 

dated November 21, 1961)

• Health Physics Considerations Associated with Thorium Processing (West 1965) states that a 

rise in the ratio of 1 is equivalent to 33% of the lung burden of the listed mixture. Indicates 

applicability to the particular mixture of daughters of the source he was using.

• “Rule of Thumb” document does not explain how the coefficient was derived or its limitations 

as a screening tool

• Coefficient varies depending on the equilibrium rates from Th-232 and daughters for the 

sources comprising the lung burden in any given worker

• MDA issue: mean sum of ratios of 3.23 for non-exposed persons has a variation of 0.7

• Used to derive the “MDA” for Th-232 in the lung (6 mg). All results between -0.7 and 0.7 * 

8.84 (-6 mg to 6 mg), produce values within the variation of the population, with a mean of 0

• Stated MDA of 6 mg is an empirical value - not based on the counting statistics of the MIV 

system

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

9
April 17, 2012 – SC&A response (continued)

• SC&A hypothetical example using “Rule of Thumb” method: Worker exposed to Type M thorium 

source for 60 days then monitored on the MIVRML. Assume he was monitored in the year he was 

working with thorium, on one of these dates:

• The middle of his exposure period (30 days after the first day of exposure) 

• The last day of exposure 

• 90 days, 120 days, 180 days or 360 days after first day of exposure 

• Assume 0.23 nCi Pb-212 and 0.24 nCi Ac-228 (post-1978 values)

• If 10 mg measured, daily intake ranges from 17 to 137 Bq

• If source in equilibrium, Pb-212 and Ac-228 = 1.1 nCi on all dates and sum of the counts in all three 

regions of the Pb-212 and the Ac-228 peaks would be higher than background

• Triple purified (NIOSH worst-case assumption)  large disequilibrium ratio of Th-232 to Th-228 

(1:0.19): activities of Pb-212 and Ac-228 corresponding to 10 mg lung burden NOT distinguishable from 

background

• Single purification: Activities of Pb-212 in the lung corresponding to 10 mg Th-232 lung burdens 

would vary from 0.81 to 1.0 nCi. The activities of Ac-228 in the lung corresponding to 10 mg Th-232 

lung burdens vary from 0.003 to 0.0674 nCi, all below detection limits.

• Only peak region that has higher counting than from non-exposed people is the Pb-212 peak

• In reality, there is no hook back to Pb-212 activity. We don’t know the count ratios, and the thorium 

conversion factor (8.84) depends on the conditions of system calibration and cannot be adjusted for 

other sources.

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

10
April 17, 2012 – SC&A response (concluded)

• Three main issues remain concerns to SC&A:

• The sum of ratios can potentially miss very large intakes, depending on the number of 

purifications and the age of the source since separation.

• The thorium coefficient of 8.84 is narrowly defined for a set of conditions unique to Y-12 and is not 

transferrable to FMPC or any other facility.

• Th-232 was present at FMPC in both soluble Type M (TNT) and insoluble Type S (oxide and 

metal) materials. Ra-228 produced in a Type S matrix leaves the lung more rapidly than thorium 

and exhibits properties closer to Type M. These physico-chemical disparities are not captured by 

the sum of ratios screening method.

• Lack of Coherence between Slide 6 and Slide 7 

• Slide 6 of the presentation proposes the use of a correction factor of 5.25 to be applied to 

measured results. This factor of 5.25 was derived assuming that Pb-212 was measured, and that 

the Pb-212 result was used to derive the mg of Th-232 results. The same correction factor is not 

applicable to the empirical method described in slide 7.

• In summary, if the empirical equation in slide 7 of the NIOSH presentation was applied without 

modifications to account for the source terms encountered at FMPC, then the mg Th-232 results were 

not derived correctly and carry huge uncertainties that cannot be reconciled. 

• SC&A believes that the thorium lung burdens reported in units of mg from 1968–1978 cannot be 

reconstructed or associated with meaningful intakes and, therefore, it does not appear possible to place 

a scientifically sound and plausible upper bound on the thorium body burdens for some workers.

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

11
April 19, 2012 – Work Group Teleconference

• NIOSH presents position (slides and references) and SC&A responds (as stated 

herein)

• Work Group discussion

• Issues with “Rule of Thumb” sum of ratios method

• Only mg value reported; counts and/or ratios not reported or available.

• Coefficient for converting dimensionless sum of ratios to mg value varies for every potential 

source. Therefore, 8.84 cannot be applied to sources at FMPC.

• Unknowns in the empirical equation as applied to FMPC sources include: ratios and sum of 

ratios for exposed workers, source-specific conversion coefficient(s), background sum of ratio 

distribution likely different for MIVRML (higher background) – cannot “deconvolute.”

• Lung burdens in the 10’s of mg could have been missed altogether (SC&A example).

• SC&A Table 1 showing Pb-212 and Ac-228 values >MDA are not proportional and are 

not proportional to stated mg values (Slides 4 and 5)

• High values followed by very low values do not comport with known biokinetic 

processes

• Given the current state of knowledge regarding the methods employed and the lack of 

available raw data (ROI counts, efficiencies, source characteristics), the Work Group 

does not believe that a plausible upper bound can be applied to the mg thorium 

data for the period 1968–1978

Issue #6B: Use of Chest Counts to Reconstruct Th-232 

Intakes (1968–1978)

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Questions?