Memo – Pinellas Matrix Issue #5 1 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.

MEMO 

TO: Pinellas Plant Work Group 

FROM: SC&A, Inc. 

DATE: February 3, 2016 

SUBJECT: Draft Pinellas Issues Matrix Issue #5: Gamma LOD for RTG Pu-238 Generators 

A sub-item of Issue #5 of the Pinellas draft issues matrix update provided to the Pinellas Plant 

Work Group (WG) on January 6, 2016, was the basis for the limit of detection (LOD) of 

10 mrem for the post-1974 period in Table 6-9 of ORAUT-TKBS-0029-6 instead of the SC&Arecommended 20 mrem. For the reader’s convenience, Attachment 1 provides a description of 

Issue #5 as it stood prior to the November 19, 2012, WG meeting. The sub-item was discussed at 

the November 19, 2012, WG meeting (pp. 44–50 of the November 19, 2012, transcript), where 

NIOSH indicated that it was going to look into the basis for an LOD of 10 mrem instead of the 

SC&A-recommended 20 mrem. 

For film dosimetry and photons in the range of 30–250 kiloelectron volts (keV), SC&A believes 

that a 10 mrem LOD is acceptable. However, the issue arose as to whether workers exposed to 

Systems Nuclear Auxiliary Power (SNAP) plutonium-238 (Pu-238) radio-thermal generators 

(RTGs) were in a higher energy gamma field. Because dosimeter film is less sensitive to highenergy photons, a higher LOD would be appropriate for those workers. Recently, a detailed 

National Aeronautics and Space Administration (NASA) (TRW) report came to light that 

investigated the emissions from SNAP generators with a view to the impact on the scientific 

payload of deep space probes (TRW 1971). This memo summarizes the information contained in 

the NASA report as it pertains to the RTG photon emission energy spectrum that Pinellas 

workers would have encountered and to the appropriate LOD for dosimeter films worn by those 

workers.

The TRW (NASA) report was reviewed to ascertain the dose rate spectrum of photon emissions 

from SNAP generators (TRW 1971). This 191-page report summarizes a detailed investigation 

of the emission characteristics of SNAP generators to determine the feasibility of using them and 

the potential long-term impact of ionizing radiation emissions on specific payload instruments. 

The investigation included modeling and physical measurements of actual and simulated SNAP 

sources. This report serves as a good surrogate for an analysis of the gamma spectrum to which 

Pinellas RTG workers would have been exposed. There are a few potential caveats, though they 

turn out to be minor: There is some in-growth of fission and decay products over the multi-year 

lifetime of the generators; thus, “fresh” units did not necessarily exactly represent the concern 

that investigators had for instrument packages once the probe reached the outer planets. Also, 

orientation and partial shielding were very important with regard to exposure of specific 

instruments on the probe, whereas presumably workers were more likely to be exposed from a 

variety of angles around the generators. A detailed review of the report indicates that these issues 

have little bearing on the spectrum of photons that make it out of the very dense core. 
Memo – Pinellas Matrix Issue #5 2 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.

The following extracted graphs and table show the spectrum of radiation that made it out of the 

RTG device and associated thermal and radiation shielding. 

Figure 4.8 from the report (above) describes the gamma flux leaving the cell. It can be seen that 

the majority of the flux will be above 500 keV. 
Memo – Pinellas Matrix Issue #5 3 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.

Figure 4.10 from the report (above) shows the dose rates at various distances from the 

generators. At 20 inches, 25–35 millirem per hour (mrem/hr) is measured. This dose rate will 

vary with geometry and the source/age of the fuel. However, this graph gives an indication that 

personnel exposures could have been significant at typical distances at which a worker would 

manipulate tools and equipment.
Memo – Pinellas Matrix Issue #5 4 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.

Table 4.16 from the report (above) puts the emissions in discrete dose bins. It is readily seen that 

83% of the dose is from >800 keV photons. 

Based on this NASA report, it is clear that the RTG photon emissions are predominantly (83%) 

high-energy gamma. There will be some changes in the spectrum depending on the age of the 

fuel, the mode of fuel production, and the steps taken to remove impurities. (Production methods 

were varied in an attempt to minimize the impact of the generator emissions on science 

experiments in the space probes.) However, the contributions to external exposure will remain 

essentially high energy due to the physical dimensions and density of the source. 

As is described for other work sites, film dosimeters respond well to lower energy photons and 

can readily detect 10 mrem. However, because a film dosimeter is less sensitive to high-energy 

photons, SC&A believes that the LOD for film dosimeters should be 20 mrem when high-energy 

gammas are prevalent. Associated IREP parameters should be modified accordingly. 

References 

ORAUT-TKBS-0029-6. 2011. Pinellas Plant – Occupational External Dose, Revision 01, Oak 

Ridge Associated Universities Team. April 28, 2011. 

TRW 1971. RTG/Science Instrument Radiation Interactions Study for Deep Space Probes: 

Phase II, III, and IV Report, R.A. Kaminskas, J.S. Brunhouse, and K.T. Hartwig, TRW Systems 

Group, Redondo Beach, CA, NASA/AMES Contract# NAS 2-5222. January, 31, 1971. 
Memo – Pinellas Matrix Issue #5 5 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.

Attachment 1 

The following is SC&A’s response to Action Item #5 in the Work Plan proposed by SC&A on 

October 20, 2011, following the Pinellas Work Group meeting on October 13, 2011. 

Action Item #5 was a result of the original Finding #5, “Problems with Personnel Dosimetry,” 

documented in SCA-TR- TASK1-0015, “Review of the NIOSH Site Profile for the Pinellas Plant 

Site,” performed by SC&A in September 2006. Finding # 5 was expressed as follows:

Section 6.2.2 of ORAUT-TKBS-0029-6 states the following:

“This analysis was unable to locate specific designs of the film dosimeters used 

for approximately the first 20 years (1957 to 1974) at the Pinellas Plant, and 

there is limited documentation that indicates there was an early relationship with 

Nuclear-Chicago”. 

Table 6-5 on page 16 of ORAUT-TKBS-0029-6 assigns a missed dose of 0.24 rem 

for beta/photons (monthly) for badges used during this time period. This 

assignment of missing dose evidently assumes that the badges used during this 

time period were equivalent to those provided by Nuclear-Chicago. Additional 

discussion is needed on the uncertainty associated with the assumed missing dose, 

given that the origin of the dosimetry is not clearly established.

During the Pinellas Work Group meeting of October 13, 2011, NIOSH offered the following 

resolution to the finding: 

Changes were made to the Dosimetry Technology and Missed Dose sections to 

address SC&A Finding #5. Updated information on the film badge characteristics 

post-1974 is contained in Tables 6-9 through 6-11 of the revised TBD-6. Pre1974 performance characteristics are not available, so NIOSH uses the 

maximizing approach in “A Standard Complex-Wide Methodology for 

Overestimating External Doses Measured with Film Badge Dosimeters” 

(ORAUT-OTIB-0010). 

Subsequently, SC&A was authorized to review the performance characteristics identified by 

NIOSH and tabulated in ORAUT-TKBS-0029-6, Rev. 01, for dosimeters used in the post-June 

1974 time period. 

The three relevant tables were reviewed (Tables 6-9, 6-10, and 6-11) and portions of relevant 

references were examined, including technical information bulletins (OTIBs) and the Mound site 

profile, as Mound dosimetry was utilized at the site during the period in question. Generally, the 

assumptions in the revised tables under consideration are sound, except as noted in the 

observation below: 

The original Table 6-5 in ORAUT-TKBS-0029-6 (September 15, 2005) used 0.02 rem as the 

minimum detection limit (MDL) for photons for Landauer film dosimetry. Table 6-9 in the 
Memo – Pinellas Matrix Issue #5 6 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.

current version (April 28, 2011) shows 0.01 rem for the limit of detection (LOD). No reason for 

the change is evident in the site profile document. 

For film utilized in a high-energy photon environment, 0.02 rem is more realistic and claimant 

favorable as the LOD. The issue of film emulsion response to high-energy photons is well known 

and discussed in ORAUT-OTIB-0010. 

The issue was also spelled out in the Idaho National Laboratory (INL) site profile review of 

December 2008 (Idaho National Laboratory (INL) Site Profile Review Update):

[3.2 ISSUE 26: MINIMUM DETECTION LIMIT 

The selection of 10 mrem as the MDL [minimum detection limit] for high energy 

gamma is questionable. Even for modern densitometers and film, it is a challenge 

to achieve this level, as a single density “click” can correspond to greater than 

10 mrem for high-energy gamma radiation; this is not a problem, however, for 

intermediate and low-energy x-rays. Rather, one click of the densitometry system 

may correspond to 15 or 20 mrem for 660 keV or 1.2 MeV gammas, for example. 

If the claim is made that 10 mrem is a valid choice for the MDL, then supporting 

materials should be provided, such as film dose-to-density curves and 

densitometer calibration data. Other sites (e.g., Savannah River Site - SRS) have 

adopted 40 mrem as the high-energy gamma MDL for early film.] 

For doses for the “pre-1974 period,” the tables in the revised Pinellas site profile show a 

generous 0.04 rem as the LOD for photons and electrons detected with photographic film. This is 

based on ORAUT-OTIB-0010. Thus, for non-Landauer film, a 40 mrem LOD is being 

recommended, but 10 mrem is being recommended for Landauer film. The revised tables do not 

specifically state why the lower 10 mrem LOD for Landauer film was chosen. Speculating, it 

might be based on a broad-brush claim by Landauer of an LOD of 10 mrem in older dosimetry 

reports and client literature. However, for Landauer or any other film dosimeter from the era in 

question, 10 mrem was more correctly the LOD for lower energy photons. LOD is dependent on 

a host of issues, including film emulsion age (background fog), processing, densitometer set up, 

calibration, etc. Thus, although a 10 mrem high-energy photon detection threshold might have 

been attainable on a “good day,” it should not be adopted as the LOD for establishing missed 

dose.

Based on the above, it is recommended that the photon LOD in Table 6-9 should be 

returned to the original 0.02 rem for Landauer beta/gamma film. 

All other assumptions that were examined appear reasonable and claimant favorable.
Memo – Pinellas Matrix Issue #5 7 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.

Attachment 2 – Copies of the First Two Cover Pages of the TRW 1971 (NASA) Report: 
Memo – Pinellas Matrix Issue #5 8 SC&A – February 3, 2016 

NOTICE: This report has been reviewed for Privacy Act information and has been cleared for distribution. 

However, this report is pre-decisional and has not been reviewed by the Advisory Board on Radiation and 

Worker Health for factual accuracy or applicability within the requirements of 42 CFR 82.