Draft White Paper 

REVIEW OF NIOSH WHITE PAPER: 

 NEUTRON EXPOSURES AT AREA IV OF THE SANTA SUSANA FIELD 

LABORATORY AND RELATED SITES 

Contract No. 211-2014-58081

Revision 0 

Prepared by 

Bob Barton 

John Stiver 

S. Cohen & Associates 

1608 Spring Hill Road, Suite 400 

Vienna, VA 22182 

October 2014

Disclaimer 

This document is made available in accordance with the unanimous desire of the Advisory Board on 

Radiation and Worker Health (ABRWH) to maintain all possible openness in its deliberations. However, 

the ABRWH and its contractor, SC&A, caution the reader that at the time of its release, this report is predecisional and has not been reviewed by the Board for factual accuracy or applicability within the 

requirements of 42 CFR 82. This implies that once reviewed by the ABRWH, the Board’s position may 

differ from the report’s conclusions. Thus, the reader should be cautioned that this report is for 

information only and that premature interpretations regarding its conclusions are unwarranted.
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 2 of 20

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. 

S. Cohen & Associates:

Technical Support for the Advisory Board on

Radiation & Worker Health Review of

NIOSH Dose Reconstruction Program

Document No.: Review of NIOSH

White Paper – SSFL Neutron 

Exposures

Effective Date: 

Draft – October 22, 2014

Revision No.:

0 (Draft)

REVIEW OF NIOSH WHITE PAPER: 

NEUTRON EXPOSURES AT AREA IV OF THE 

SANTA SUSANA FIELD LABORATORY AND 

RELATED SITES

Page 2 of 20

Task Manager:

_____________________________ Date: ___________

John Stiver, MS, CHP

Supersedes:

N/A

Project Manager:

_____________________________ Date: ___________

John Stiver, MS, CHP

Reviewers:

 

 John Mauro

Record of Revisions

Revision 

Number

Effective 

Date Description of Revision

0 (Draft) 10/22/2014 Initial issue
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 3 of 20

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 OF CONTENTS 

Abbreviations and Acronyms ..........................................................................................................4

1.0 Statement of Purpose ...........................................................................................................5

2.0 Historical Milestones Leading Up to This Review ..............................................................6

3.0 Overview of NIOSH N-P Coworker Model ........................................................................7

4.0 Detailed Review ...................................................................................................................9

4.1 Sources of Data Utilized in N-P Ratio Development ..............................................9

4.2 Comparison of Rank Ordered N-P Ratios to Lognormally Fit N-P Ratios ...........10

4.3 Comparison of N-P Ratio Data by General Area of Survey Measurement ...........11

4.4 Selection of Neutron Survey Data When Multiple Paired Measurements 

Exist .......................................................................................................................12

4.5 Omission of Data from Three Reactors .................................................................14

4.6 Age of the Reactor Core at Time of Survey ..........................................................15

4.7 Use of Latest ICRP Guidance on Radiation Weighting Factors ............................16

5.0 Other Noteworthy Observations ........................................................................................18

References ......................................................................................................................................19

LIST OF FIGURES 

Figure 1. Rank-Ordered N-P Data by Reactor ............................................................................... 9

Figure 2. Comparison of NIOSH White Paper 50th and 95th Percentile Values with the 

Rank-Ordered Raw Data .............................................................................................. 10

Figure 3: Rank-Ordered N-P Ratios by Measurement Location ................................................. 12

Figure 4: Ratio of Neutron Dose Rates (Eberline PNC-1 Neutron 

Detector/Ludlum Tissue Equivalent) ........................................................................... 13

Figure 5: Comparison of White Paper N-P Data versus the Same Data if the 

Omitted Reactors were Included .................................................................................. 15

Figure 6. Distribution of N-P Samples by the Year of Operation ............................................... 16
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 4 of 20

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. 

ABBREVIATIONS AND ACRONYMS 

Advisory Board Advisory Board on Radiation and Worker Health 

AETR Advanced Epithermal Thorium Reactor 

AI Atomics International 

DOE (U.S.) Department of Energy 

ER Evaluation Report 

ETEC Energy Technology Engineering Center 

GM geometric mean 

GSD geometric standard deviation

ICRP International Commission on Radiological Protection 

IREP Interactive RadioEpidemiological Program 

keV kiloelectron volt 

MeV megaelectron-volt, 1 million electron-volts 

NIOSH National Institute for Occupational Safety and Health 

N-P neutron-to-photon 

NTA Neutron Track Analysis 

OCAS Office of Compensation Analysis and Support 

ORAUT Oak Ridge Associated Universities Team 

POC probability of causation 

rem Roentgen equivalent man 

SC&A S. Cohen and Associates (SC&A, Inc.) 

SEC Special Exposure Cohort 

SGR Sodium Graphite Reactor 

SNAP Systems for Nuclear Auxiliary Power 

SRDB Site Research Database 

SRE Sodium Reactor Experiment 

SSFL Santa Susana Field Laboratory 

TBD Technical Basis Document 

TLD thermoluminescent 

WB whole body 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 5 of 20

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. 

1.0 STATEMENT OF PURPOSE 

The purpose of this review is to examine the proposed National Institute for Occupational Safety 

and Health (NIOSH) model for assigning neutron doses at the Santa Susana Field Laboratory 

(SSFL). NIOSH’s proposed neutron model is presented in the white paper entitled, Neutron 

Exposures at Area IV of the Santa Susana Field Laboratory and Related Sites (Hughes 2010). 

The neutron model and underlying data were evaluated for relevance to potential neutron 

exposures at SSFL and claimant favorability.
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 6 of 20

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. 

2.0 HISTORICAL MILESTONES LEADING UP TO THIS REVIEW 

In August of 2008, SC&A submitted a draft report entitled, Review of the NIOSH Site Profile for 

the Santa Susana Field Laboratory (SC&A 2008). In the review of ORAUT-TKBS-0038-6, 

Technical Basis Document for Atomics International – Occupational External Dosimetry 

(ORAUT 2006, also referred to as TBD-6), SC&A identified a total of seven findings that were 

subsequently summarized by SC&A in the Issues Resolution Matrix for the Santa Susana Site 

Profile Review1and discussed by the SSFL Work Group in August 2008, April 2009, and again 

in April 2010. Included among seven findings pertaining to external monitoring for neutrons 

were Findings 4.6-2 and 4.6-4, which state the following: 

Finding 4.6-2. Workers were unlikely to have been monitored for thermal neutrons

As stated in Section 6.2, “...Both fast and thermal neutrons were measured and 

recorded as whole-body (WB) dose in rem.” This statement is contradicted in 

Section 6.4, where it states, “...It is assumed that the dose recorded was the result of 

fast neutron exposure.”

The second statement is likely to be correct, since the common practice at DOE 

facilities was to assess NTA film for tracks produced by proton recoil. It is unlikely 

that NTA dosimeters were modified and calibrated for track analysis of thermal 

neutrons. [Tracks in emulsions exposed to thermal neutrons may be produced by 

nitrogen in the gelatin that captures a thermal neutron and releases a 0.58 MeV 

proton – N14(n, p) C14.]

Finding 4.6-4. NTA film applicability to neutrons at energies below 500 keV 

This TBD assumes that the NTA film effectively measured all neutron exposure 

received at AI, and does not consider correction factors for the insensitivity of NTA to 

neutrons at energies below 500 keV. Actual neutron energy spectrum data is limited 

to a few facilities (i.e., SRE). There is no discussion of neutron-to-photon ratios in 

the site profile; however, it is mentioned as an option for calculating thermal neutron 

exposure in the ER report.

In response to Findings 4.6-2, 4.6-4, and SSFL Work Group discussions, NIOSH issued the 

white paper Hughes 2010 on June 14, 2010. SC&A was asked by the Advisory Board to review 

Hughes 2010 as an action item arising from the April 2010 Work Group meeting.

 

1 The issues matrix is a “living document” that was originally created by SC&A and has been updated 

several times by both NIOSH and SC&A based on Work Group discussions and any additional guidance or 

information that comes to light over time. 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 7 of 20

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. 

3.0 OVERVIEW OF NIOSH N-P COWORKER MODEL 

NIOSH performed an extensive review of available documentation, records, and site-specific 

interviews that characterize neutron monitoring practices and overall radiological monitoring and 

control at the SSFL and related sites.2 Based on this review, it was determined that, while there 

was a well-documented radiation safety program, there may have been a lack of consistent 

access control to prevent unmonitored workers from entering restricted radiological areas. In 

addition, while neutron monitoring was in place to some extent, the limitations of the monitoring 

technology employed at the time (NTA Film) and inconsistent application of neutron monitoring 

led to the decision that an alternate method needed to be developed to evaluate neutron doses to 

SSFL workers. 

NIOSH has elected to utilize ratios of neutron-to-photon (N-P) dose, based on available survey 

reports from SSFL, so that individual external exposure records could be used to evaluate and 

bound the potential neutron dose received by individual workers. A summary of available 

survey reports is presented in Table 1 (recreated from Table 1 of Hughes 2010). Many of the 

reports were similarly structured and contained important information, such as purpose of the 

survey, measurement location, reactor power level, and important shielding configurations. The 

paired neutron and photon measurements contained in the survey reports were transcribed by 

NIOSH into an Microsoft Excel® file for analysis [the Excel file can be found at (O:\AB 

Document Review\Santa Susana\Neutron white paper files\NP data analysis worksheet_LDH_ 

05252010.xls)]. Table 1 shows a reasonably positive correlation between the photon and neutron 

dose based on the Pearson (r) and Spearman (ρ) coefficients; the only exceptions are the SRE 

and SNAP 8 reactors, the data for which did not show a very strong correlation. 

Table 1. Summary of Available Neutron-Photon Survey Reports for Various SSFL 

Reactor Areas (Recreation of Table 1 of Hughes 2010) 

Reactor

Reactor 

Operating 

Dates

Survey 

Data 

Range

# of 

Data 

Points

Pearson

(r)

Spearman

(ρ)

SRDB Ref IDs for Survey 

Reports*

AE-6/ 

L-85 1955–1980 1959–1974 826 0.87 0.79

74959, 74972, 75034, 75037–

75039, 75042, 75043, 75048, 

75054, 75058, 75064, 75065, 

75066, 75261, 75415, 75416, 

75418, 75050, 75070

SNAP 

CTR 1962–1964 1964 2

Not Enough 

Data

Not Enough 

Data 74987

SRE 1957–1964 1963 186 0.04 0.44 74841–74846, 74849, 74850, 

74852, 74854

STIR 1960–1972 1968 33 0.93 0.92 75032

SGR 1956–1967 1967 18 0.98 0.94 74999

SNAP 8 1968–1969 1969 82 0.02 0.65 77450, 77451, 77561

AETR 1960–1974 1972 21 0.97 0.71 75055

L-77 1960–1976 1964 8 0.998 0.62 77526

 * Incorporated by reference in Hughes 2010. 

 

2

 The SSFL is considered part of the Energy Technology Engineering Center (ETEC), which consisted of 

the Canoga Avenue Facility, Downey Facility, and DeSoto Facility in addition to Area IV of the SSFL. 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 8 of 20

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. 

In selecting an appropriate model for the N-P data, NIOSH found that the data from all reactors 

except the SRE, SNAP 8 and L-77 fit reasonably well to a lognormal distribution. Lognormal 

parameter values for the N-P data from the seven reactors are presented in Table 2. In Table 2, 

GM refers to the geometric mean and GSD to the geometric standard deviation. It was 

determined that data from SRE, SNAP 8, and L-77 would be omitted from the analysis because 

they did not fit the lognormal model. In addition, NIOSH considered it claimant favorable to 

exclude these data, since most N/P values associated with those three reactors were less than 1. 

Table 2. Lognormal Fit of N-P Data for Individual Reactors 

Reactor GM GSD Upper 95th

Percentile

AE-6/ L-85 1.73 2.76 9.22

SRE 0.24 4.91 3.32

STIR 1.07 2.07 3.54

SGR 5.73 1.42 10.19

SNAP 8 0.45 8.92 16.46

AETR 1.23 2.41 5.2

L-77 0.60 6.84 14.29

 Source: Hughes 2010, Table 2 

The remaining data were fit to a lognormal distribution with a GM of 1.73, a GSD of 2.76, and a 

95th percentile value of 9.18. Hughes 2010, page 9, instructs the dose reconstructor to apply this 

distribution in the following manner: 

The neutron to photon ratio should be applied to all workers at SSFL sites who 

worked in or near the reactor areas and who were expected to have been exposed 

to neutron radiation in addition to photon radiation, regardless whether they 

were issued a radiation monitoring badge for neutrons or photons... The 

distribution should be multiplied by the measured and missed photon dose (or 

coworker dose if unmonitored worker [sic] suspected of exposure) using Monte 

Carlo combination methods.

NIOSH indicates that any existing neutron monitoring records for a specific worker 

should be analyzed individually by the dose reconstructor to determine the claimant 

favorability of using existing data over the aforementioned approach.
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 9 of 20

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. 

4.0 DETAILED REVIEW 

This section provides detailed discussions of the following seven areas of focused investigation 

in the SC&A review: 

 Sources of Data Utilized in N-P Ratio Development 

 Comparison of Rank Ordered N-P Ratios to Lognormally Fit N-P Ratios 

 Comparison of N-P Ratio Data by General Area of Survey Measurement 

 Selection of Neutron Survey Data When Multiple Paired Measurements Exist 

 Omission of Data from Three Reactors 

 Age of the Reactor Core at Time of Survey 

 Use of Latest International Commission on Radiological Protection (ICRP) Guidance on 

Radiation Weighting Factors

4.1 SOURCES OF DATA UTILIZED IN N-P RATIO DEVELOPMENT 

Although the NIOSH coworker model uses data from five reactors (comprising 911 data points), 

more than 90% are from the AE-6/L-85 reactor. This would obviously be of concern if the N-P 

ratios for that facility were generally lower than those for the other reactor facilities, as the sitewide N-P distribution parameters that are derived from the data would not be claimant favorable. 

SC&A rank-ordered the N-P ratios for the AE-6/L-85 reactor area, as well as the other reactor 

areas used in the NIOSH analysis, to determine how the AE-6/L-85 data might bias the overall 

results. The rank-ordered survey data are shown in Figure 1. 

Figure 1. Rank-Ordered N-P Data by Reactor 

As seen in Figure 1, the AE-6/L-85 reactor N-P ratios are generally limiting (i.e., higher) when 

compared to three of the four other reactors. Although the SGR reactor had higher N-P ratios 

than the AE-6/L-85, that survey was based on a set of 18 measurements taken on a single day in 

1967 at various locations outside the reactor building and distant from normal work locations. In 

addition, the SGR reactor ceased operating in 1967. Thus, its N-P data may not represent typical 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 10 of 20

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. 

operations and neutron exposure potential through the relevant portion of the post-SEC period 

(1965 through 1987). It is also worth noting that the SGR data were taken with the more 

conservative Eberline PNC-1 neutron detector, while most of the AE-6/L-85 measurements were 

taken with the less conservative Ludlum Tissue Equivalent detector. The differences in 

measured neutron dose rates between these two types of detectors are further discussed in 

Section 4.4. 

When comparing the AE-6/L-85 data to the data for the four other reactors combined, the 

AE-6/L-85 is limiting from about the 20th percentile to the 80th percentile. Therefore, it does not 

appear that the AE-6/L-85 data unduly bias the derived site-wide N-P ratio distribution. 

4.2 COMPARISON OF RANK ORDERED N-P RATIOS TO LOGNORMALLY FIT 

N-P RATIOS 

SC&A performed a simple rank order of the raw data to see how the lognormally fit values 

compare at the 50th and 95th percentile. The resulting comparison is shown in Figure 2. As seen

in Figure 2, the rank ordered 50th percentile N-P ratio is higher (2.07) than the lognormally fit 

50th percentile (1.73), which represents approximately a 20% increase over the white paper 

value. Conversely, the rank ordered 95th percentile is lower (6.25) than the lognormally fit 95th

percentile (9.16), which represents a 33% decrease over the white paper value. Therefore, when 

comparing the data to the lognormal model, the simple rank ordering of the data appears to be 

more claimant favorable at the 50th percentile, while the fitted values are more claimant 

favorable at the upper end of the distribution. 

Figure 2. Comparison of NIOSH White Paper 50th and 95th Percentile Values with the 

Rank-Ordered Raw Data 

In summary, it appears that for the NIOSH model, a slightly lower fitted median value is 

compensated by the higher 95th percentile, and results in a generally claimant-favorable neutron 

dose distribution for the purposes of assessing probability of causation (POC) in the Interactive 

RadioEpidemiological Program (IREP).
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 11 of 20

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. 

4.3 COMPARISON OF N-P RATIO DATA BY GENERAL AREA OF SURVEY 

MEASUREMENT 

The location where survey measurements were taken is an important factor to consider when 

analyzing and developing realistic or bounding neutron exposures. For example, if the survey 

measurements were taken directly on the reactor face, then the resulting N-P ratio is likely to be 

smaller because of the large unattenuated gamma fluence that close to the reactor. Additionally, 

this would not be consistent with a normal working configuration for workers. Conversely, if the 

measurements were taken outside of the reactor building, such as around an exclusion fence, the 

N-P ratio is likely to be higher due to increased attenuation of the gamma component from 

shielding from the reactor building walls and other obstructions. Ideally, the survey 

measurements would be taken in established working areas, such as the reactor control room or 

other surrounding work areas of the building. 

In order to gain insight into where the survey measurements were taken and what implications 

that might have on a site-wide N-P ratio, SC&A compiled general location data from the original 

survey reports. Survey locations were classified in three general categories: outside the reactor 

building, within a structure adjacent to the reactor building,3

and normal working locations 

within the reactor building. 

A summary of the number of measurements in each category is shown in Table 3. Of the 911 

data points used in the coworker model, approximately76% were measured outside the reactor 

building, about 9% were taken in an adjacent structure and only about 13% were taken in a 

typical working location of the reactor building. It is not surprising that a comparison of the 

magnitude of N-P ratios by location show that the measurements taken outside the reactor 

building are limiting. 

Table 3: Locations of Measurement Samples Used in White Paper 

Location Number of 

Samples

Percentage of 

Total

Outside and Away from Reactor Building 541 59.4%

Directly Outside Reactor Building 153 16.8%

In Adjacent Structure 78 8.6%

Work Area in Reactor Building 117 12.8%

Other/Unknown Location 22 2.4%

The samples in each category were also rank ordered to determine the magnitude of the N-P 

ratios in each location category; the results are shown in Figure 3. 

 

3

 Some AE-6/L-85 surveys contained data labeled for the ‘KEWB Office,’ the KEWB reactor was located 

in a building adjacent to the AE-6/L-85 and operated up until 1966. It has been assumed that the KEWB Office was 

located within the KEWB reactor building. Reports containing survey data for the KEWB reactor prior to its 

permanent shutdown in 1966 contain the heading, “Weekly Doses from the AE-6 in the KEWB Office,” so it is 

unclear if the KEWB reactor operations contributed to the listed doses in AE-6/L-85 work spaces. 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 12 of 20

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 3: Rank-Ordered N-P Ratios by Measurement Location 

It is evident from Figure 3 that the measurements that were taken outside and away from the 

reactor building resulted in the highest N-P ratios, for the reasons cited above. While locations 

outside of the reactor building are not considered to be a realistic or consistent work location, the 

fact that the majority of samples under consideration are taken in such areas will bias the 

resulting site-wide N-P ratio in the claimants’ favor.

4.4 SELECTION OF NEUTRON SURVEY DATA WHEN MULTIPLE PAIRED 

MEASUREMENTS EXIST 

It was found in at least two reports (Owen 1971and SSFL 1968) that neutron dose rates were 

measured using two different survey meter types (Ludlum Tissue Equivalent Neutron Meter and 

the Eberline PNC-1 Neutron Meter) at the same time and location. The Eberline PNC-1 meter 

provided doses for both thermal and fast neutrons, while the Ludlum meter provided only a 

single neutron dose rate. In these cases, it appears that only the results from the Ludlum were 

used in the development of the N-P ratio, though the Eberline PNC-1 dose rates were 

consistently higher. There were 50 such paired measurements from the two identified reports 

and the neutron dose rate from the Eberline PNC-1 meter was, on average, approximately 3 

times higher than the Ludlum Tissue Equivalent meter. In only one instance did the Ludlum 

neutron result surpass the Eberline measurements. 

Figure 4 shows the magnitude of the ratio of the Eberline to Ludlum results, ranked from lowest 

to highest, for all 50 neutron measurement pairings. 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 13 of 20

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. 

0

1

2

3

4

5

6

7

0 5 10 15 20 25 30 35 40 45 50

Paired Neutron Sample Number Ratio of Eberline PNC-1/ Ludlum Tissue Equivalent Neutron Dose Results

Figure 4: Ratio of Neutron Dose Rates (Eberline PNC-1 Neutron 

Detector/Ludlum Tissue Equivalent) 

Because the Eberline PNC-1 neutron measurements are typically higher than the Ludlum Tissue 

Equivalent results and there is no apparent technical reason to reject the Eberline measurements, 

it would be claimant favorable to choose the Eberline measurements over the Ludlum 

measurements. NIOSH provides no justification as to why the Ludlum Tissue Equivalent results 

were used while the Eberline PNC-1 results were ignored. It would seem prudent to at least 

include both sets of measurements when developing a site-wide neutron/photon ratio. It is worth 

noting that results from an Eberline PNC-1 meter were accepted in the analysis when there were 

no available Ludlum neutron measurements. An example of where Eberline readings were used 

in the analysis is found in Klostermann 1967for the SGR reactor which, not surprisingly, is the 

limiting reactor out of those used in the analysis (see Figure 1, Section 4.1). 

Finding 

In summary, when measurements from two different types of neutron survey instruments were 

available, NIOSH chose to use measurements from the less conservative Ludlum Tissue 

Equivalent neutron meter in developing the neutron exposure model. SC&A believes that 

NIOSH should provide justification as to why the less claimant-favorable neutron measurement 

technique was deemed more appropriate for these data. 

Observation 

SC&A believes it is noteworthy to comment on the issue of intrinsic bias and uncertainty in the 

individual neutron and gamma data pairs. Hughes 2010 does not address the intrinsic bias and 

uncertainty in the N-P ratios that arise from factors such as instrument response characteristics 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 14 of 20

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. 

and variations in measurement technique among operators. A review of ORAUT-TKBS-0038-6 

(ORAUT 2006) and OCAS-IG-001 (OCAS 2006) Section 3.2.2.3, “Neutron Dose 

Reconstruction – Survey Data - Uncertainty,” revealed that both documents are silent on intrinsic 

bias and uncertainty in survey data, though the subject is covered in detail for film badge and 

thermoluminescent (TLD) dosimeters. Given NIOSH’s recommendation to use Monte Carlo 

methods, bias and uncertainty factors unique to this dataset could be derived and incorporated in 

the algorithm for neutron dose calculation. The introduction of a new uncertainty term would 

increase the GSD to some extent, and result in a more credible and claimant-favorable neutron 

dose distribution. 

4.5 OMISSION OF DATA FROM THREE REACTORS 

As described in Section 3, data for three reactors (SRE, L-77, and SNAP 8) were omitted in 

developing the site-wide N-P ratio distribution. The rationale provided was that the data did not 

fit the lognormal distribution, which was the basis for the site-wide N-P distribution. SC&A 

examined the data from these three omitted reactors to determine how the N-P ratios compared 

to the derived values presented in the NIOSH white paper. 

All surveys for the SRE reactor were taken directly on the core face and therefore do not 

represent a normal working configuration for the purposes of dose assessment. Because the 

surveys were taken so close to the actual reactor core, the resulting N-P ratios are all very small,

with only 4 of the 186 N-P ratios above the NIOSH-derived 50th percentile of 1.73. One of these 

four measurements appears to be the result of data being entered into the wrong fields and is 

likely a typo; the other three N-P ratios ranged from 2-2.5, which is well below the NIOSHderived 95th percentile of 9.18. 

The L-77 reactor had only eight N-P ratio data points, all of which were below the NIOSH 95th

percentile. The rank-ordered 50th percentile N-P ratio for the L-77 reactor was only 0.16 and 

only three N-P ratios were greater than the NIOSH-derived 50th percentile. Of those three 

values, two were taken at locations labeled ‘N. Port’ and ‘S. Port’ which do not appear to be 

routine work locations, though the third value was taken in a doorway that is assumed to be a 

normal work area. 

When reviewing the survey report for the L-77 (Wildanger 1964), it was found that neutron dose 

data were provided for three separate energies: ‘thermal,’ ‘1 MeV,’ and ‘2.5 MeV.’ When 

NIOSH compiled data from this report, the ‘2.5 MeV’ dose rate data were considered for the fast 

neutron component and the ‘1 MeV’ dose rate data were ignored.

Finding 

SC&A believes that NIOSH should provide justification as to why the ‘1 MeV’ neutron dose 

was not considered when analyzing the N-P ratios for this reactor. For example, NIOSH could 

have added the ‘1 MeV’ and ‘2.5 MeV’ dose rates together to obtain the combined “fast neutron”

component. If the two fast neutron components are added, this results in an 89% increase in total 

neutron dose on average, which almost doubles the resulting N-P ratios for the L-77. 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 15 of 20

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 SNAP 8 reactor had 82 N-P ratio data points associated with it. Of the 82 values, 26 (or 

about 32%) were zero, in that there was a positive gamma dose, but no associated neutron dose. 

There were also about eight measurements that indicated a positive neutron dose with no 

associated gamma component (these were incorrectly entered as an N-P ratio of ‘0’ in the 

NIOSH compilation). If both of these sets of ‘zero’ measurements are excluded, the rankordered 50th percentile N-P ratio for the remaining data is 0.96, which is well below the derived 

NIOSH median of 1.73. If the correctly transcribed ‘zero’ N-P ratios are included, the 50th

percentile drops to 0.051, though this omits the ‘infinite’ N-P ratios where there was a positive 

neutron result with no gamma component. 

Figure 5 shows the rank-ordered N-P ratios if the data from the three reactors omitted from the 

analysis are included with the main data. While the two curves closely match one another for 

both datasets, one can see that the N-P ratios are slightly higher (up until about the 95th

percentile) if those three reactors are omitted from the analysis. Thus, it appears that omitting 

the data from these three reactors has negligible impact on the overall modeled N-P distribution. 

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

0 2 4 6 8 10 12 14

N-P Ratio Rank Ordered Percentile

Data for NIOSH Selected Reactors

Data for All Reactors with Available Data

Figure 5: Comparison of White Paper N-P Data versus the Same Data if the 

Omitted Reactors were Included 

4.6 AGE OF THE REACTOR CORE AT TIME OF SURVEY 

An important factor to consider when evaluating survey reports and developing a site-wide N-P 

ratio from neutron and photon dose measurements is the age of the reactor core. It is logical to 

assume that a reactor core being operated at the start of its lifecycle will have a higher N-P ratio 

than that same core operating later in its lifecycle. This owes to the fission products that build up 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 16 of 20

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. 

over time and give rise to an increased gamma component, while the neutron component remains 

largely unchanged. Unfortunately, the currently available survey data are not sufficient to 

quantify this effect because of the large variations in survey locations, operating power and 

shielding configurations in the available survey reports. That is, it was not possible to identify a 

sufficient subset of data that used the same survey location, shielding configuration and had 

multiple samples across a significant period of time. 

Information is provided in the NIOSH white paper, as well as the TBD, as to what years the 

given reactors were operating. However, specific information on any potential reactor core 

change-outs has not been uncovered. Figure 6 presents the distribution of N-P sampling dates 

based on the year of operation for each particular facility. As seen in the figure, the majority of 

sampling (over 78%) occurred in the 15th year of the reactor facility’s operational history or later. 

It would be beneficial to obtain documentation that describes the reactor operations and lists core 

lifecycles/change-outs for the reactors used in the analysis (particularly the L-85/AE-6 reactor, as 

this represents the majority of data used). 

0%

5%

10%

15%

20%

25%

30%

35%

40%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20

Year of Operation

Percentage of Total N-P Samples

Figure 6. Distribution of N-P Samples by the Year of Operation 

4.7 USE OF LATEST ICRP GUIDANCE ON RADIATION WEIGHTING FACTORS

It is not clear from Hughes 2010 or from TBD-6 (ORAUT 2006) what quality factors were 

applied to the historic neutron dose measurements used in the N-P modeling effort. OCAS-IG001 (OCAS 2006) states that, “Generally, since the 1950s, a quality factor of 10 has been applied 

to fast neutron exposures, however this has varied from 5 to 20 across facilities and time 

frames.”
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 17 of 20

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 provides historical information on quality factors, based on early radiation protection 

guidance documents and updated radiation weighting factors recommended in the most recent 

ICRP guidance. The neutron dose data presented in the N-P reports do not appear to have been 

adjusted for updated radiation weighting factor during model development. It is most likely that 

the data used in N-P ratio modeling included a quality factor of 10. As is evident from Table 4, 

current methods, per ICRP Publications 60 (ICRP 1991) and 103 (ICRP 2008), assign a higher 

weighting factor of 20 for many fast neutron energies. 

Table 4: Neutron Quality Factor and Radiation Weighting Factors

Guidance Document Energy Range Q, WR

Neutrons – (10 CFR Part 20, ICRP 26) Thermal 2

0.01 MeV 2.5

0.1 MeV 7.5

0.5 MeV 11

1 MeV 11

10 MeV 6.5

Unknown 10

Neutrons (ICRP 60, 103) < 10 keV 5

10 to 100 keV 10

> 100 keV to 2 MeV 20

> 2 MeV to 20 MeV 10

> 20 MeV 5

Finding 

SC&A believes that NIOSH should provide guidance to the dose reconstructors on the use of 

radiation weighting factors.
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 18 of 20

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. 

5.0 OTHER NOTEWORTHY OBSERVATIONS 

This section presents some commentary on other observations identified by SC&A during our 

review of the NIOSH white paper that were not directly addressed in Section 4. 

 Measurements that yielded no positive values for either gamma or neutron in a given 

survey were omitted from the analysis, which is logical when attempting to characterize 

potential neutron exposures, and favorable to the claimant (Examples: Swanson 1960 

and 1961). 

 The L-85/AE-6 reactor had 19 measurements entered into the database as an N-P ratio of 

‘0.’ Eighteen of the 19 are correctly labeled, because there was a positive gamma result 

coupled with a neutron result that was zero. However, 1 of the 19 results input into the 

NIOSH database as ‘0’ contained a positive neutron result with a 0 gamma result, which 

is incorrect. Similarly, the SNAP-8 reactor had 34 N-P ratios input into the NIOSH 

database as ‘0;’ however, 8 of the 34 were incorrectly labeled as ‘0’ when there was a 

positive neutron component and a 0 gamma component. Note, however, that the SNAP-8 

reactor was omitted from the development of a site-wide N-P ratio in Hughes 2010. 

(Examples: Swanson 1960, Miller 1960, Owen 1970.) 

 Three survey measurements for the AE-6 reactor on February 7, 1969, could not be 

traced back to their listed reference (Johnson 1970). It is not clear from where these data 

points originated. 

 Three of the available survey reports contained additional neutron and gamma monitoring 

data obtained from film badges placed in working locations within the reactor building. 

These data were not compiled or considered in developing the site-wide N-P ratios. 

SC&A recognizes that the NTA film would not have been used to develop N-P ratios due 

to its limitations, as acknowledged in Hughes 2010. However, the badge data might 

prove useful in corroborating some gamma survey data and, in comparison to neutron 

survey data, could help estimate the impact of NTA film insensitivity to low energy 

neutrons. 

 Owen 1972 contains 11 additional data points not compiled in the NIOSH database for 

the AETR facility that were handwritten on a map of the facility (last page of document). 

If the handwritten data are included for the AETR, the rank ordered 50th percentile N-P 

ratio for the AETR facility increases by 40%. 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 19 of 20

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. 

REFERENCES 

10 CFR 20, 1991. Title 10, Part 20: Standards for Protection against Radiation. May 21, 1991.

Hughes 2010. Neutron Exposures at Area IV of the Santa Susana Field Laboratory and Related 

Sites, Lara Hughes, National Institute for Occupational Safety and Health. June 14, 2010.

ICRP (International Commission on Radiological Protection) 1977. Recommendations of the 

International Commission on Radiological Protection, ICRP Publication 26, Annals of the ICRP 

(1/3), Oxford: Pergamon Press. January 1977. 

ICRP (International Commission on Radiological Protection) 1991. 1990 Recommendations of 

the International Radiological of Commission Protection, ICRP Publication 60 [Ann. ICRP 21 

(1-3)] Oxford: Pergamon. 

ICRP (International Commission on Radiological Protection) 2008. The 2007 Recommendations 

of the International Commission on Radiological Protection, ICRP Publication 103, Annals of 

the ICRP 37 (2-4), Elsevier. February 2008. 

Johnson 1970. Radiation Survey of the AE-6 Reactor, Building 093. Internal Letter to W.F. 

Heine. North American Rockwell. August 13, 1970. SRDB 75038. 

Klostermann 1967. Radiation Survey of Bldg. 009 (SGR). Internal Letter to W.F. Heine. North 

American Rockwell Corporation – Aerospace and Systems Group. March 15, 1967. SRDB 

74999.

Miller 1960. AE-6 Reactor Operation for the Four Week Period of May 1, 1960 to May 31, 

1960. Inter-Office Letter. Atomics International – A Division of North American Aviation, Inc. 

May 6, 1960. SRDB 74972 

OCAS 2006. External Dose Reconstruction Implementation Guideline, OCAS-IG-001, Rev. 2, 

National Institute for Occupational Safety and Health, Office of Compensation Analysis and 

Support, Cincinnati, Ohio. August 25, 2006. 

ORAUT 2006. Technical Basis Document for Atomics International – Occupational External 

Dosimetry, ORAUT-TKBS-0038-6, Rev. 01. Oak Ridge Associated Universities Team, 

Cincinnati, Ohio. November 6, 2006. 

Owen 1970. Neutron-Gamma Radiation Surveys at T093 (L-85 Reactor Facility) and 

Surrounding Areas. Internal Letter to W.F. Heine. North American Rockwell Corporation –

Aerospace and Systems Group. November 30, 1970. SRDB 75037 

Owen 1971. Neutron-Gamma Radiation Surveys at T093 (L-85 Reactor Facility) and 

Surrounding Areas. Internal Letter to W.F. Heine. North American Rockwell Corporation –

Aerospace and Systems Group. January 29, 1971. SRDB 75048. 
Effective Date:

October 22, 2014

Revision No.

0 – Draft

Document Description: Review of NIOSH

White Paper – SSFL Neutron Exposures

Page No.

Page 20 of 20

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.

Owen 1972. Neutron/Gamma Radiation Survey at T100. Internal Letter to W.F. Heine. North 

American Rockwell Corporation – Aerospace and Systems Group. May 3, 1972. SRDB 75055.

SC&A (S. Cohen and Associates) 2008. Review of the NIOSH Site Profile for the Santa Susana 

Field Laboratory, SCA-TR-TASK1-0027, Rev. 0. SC&A, Inc., Vienna, Virginia, and Saliant, 

Inc., Jefferson, Maryland. August 1, 2008.

SSFL 1968. Method of Calculating Dose Rates from Previous Data. Santa Susana Field 

Laboratory. 1968. SRDB 75034.

Swanson 1960. AE-6 Reactor Operations for Period Nov.1 to Nov. 30, 1960. Inter-Office 

Letter. Atomics International – A Division of North American Aviation, Inc. November 30, 

1960. SRDB 74959.

Swanson 1961. AE-6 Reactor Operations for Period March 1 thru March 31, 1961. Inter-Office 

Letter. Atomics International – A Division of North American Aviation, Inc. April 12, 1961. 

SRDB 75418.

Wildanger 1964. Special Preliminary Radiological Survey – L77 Reactor. Internal Letter to 

A.S. Gibson. North American Rockwell Corporation – Aerospace and Systems Group. January 

31, 1964. SRDB 77526.