Reassessment of Internal 

Radiation Dose at the Rocky Flats 

Plant Critical Mass Laboratory

SEC – 0192

LaVon B. Rutherford, CHP

SEC Health Physics Team Leader

National Institute for Occupational Safety and Health

Division of Compensation Analysis and Support
Purpose

 Reevaluates prior assumptions used to 

assess upper bounds on personnel dose 

from mixed fission products and activation 

products (MFAP) at the Rocky Flats Plant 

(RFP) Critical Mass Laboratory (CML)

 The report was reassessed because of 

concerns identified by the RFP CML Lead 

Physicist and a former Radiological Control 

Supervisor

Division of Compensation Analysis and Support
Background

 NIOSH issued original white paper on CML 

exposures June 9, 2015

 Assessment of Sealed Radioactive Sources, and 

Fission and Activation Products as Radiological 

Exposure Sources in the RFP CML 

 During the work group meeting on July 15, 

2015 SC&A responded in general 

agreement with NIOSH’s findings.

Division of Compensation Analysis and Support
Background

 The former CML Associate Research Scientist spoke 

during the work group meeting indicating:

 That the neutron flux for a CML experiment could not be 

bounded, and

 That the best one could say is that power level was 

probably less than 50 kW 

 Based on this statement NIOSH committed to do 

further evaluation to ensure the assumptions made 

in the white paper were appropriate.

Division of Compensation Analysis and Support
Background

 Additional interviews were conducted with the former CML 

Associate Research Scientist and with a former Radiological 

Control Supervisor who was identified by the petitioner as a 

person who may have information on the CML.

 The interviews were conducted with work group members, 

SC&A, and the petitioners online as much as possible.

 The former CML Associate Research Scientist reiterated his 

concerns and indicated that he sent 50 boxes of documents 

concerning CML to LANL

 Additional concern identified by the Radiological Control 

Supervisor was a lack of air sampling for Bldg. 886 (CML).

Division of Compensation Analysis and Support
Background

 In Feb. 2016, NIOSH and SC&A were able to review the 

documents at LANL. A number of documents were 

identified for capture. The documents were not released 

until late summer 2016

 Additional data captures were conducted in search of air 

sampling data and surface contamination surveys. NIOSH 

did not receive all of these documents until fall 2016

 Late November 2016 NIOSH issues a new white paper titled 

Reassessment of Internal Radiation Dose from Sources at 

the RFP CML

Division of Compensation Analysis and Support
Power Levels

 In NIOSH’s initial calculations NIOSH used 10 mW

for 1 hour (US DOE public document)

 From the CML documents captured at LANL 

NIOSH found a more accurate estimate of thermal 

power of 3.6 mW averaged over 70.5 minutes

 CML staff however, reported to ERDA an average 

thermal power of 6.7 mW over 70.5 min based on 

the same experiment.

 NIOSH concluded they would use the value 

reported to ERDA for the revised calculation

Division of Compensation Analysis and Support
CML Surface Contamination

 In NIOSH’s original assessment an estimate of surface 

contamination was used to develop air concentrations for 

the dose model

 DOE Surface Contamination Limit (1000 dpm/100cm2)

 NIOSH captured surface contamination and Airborne 

radioactivity surveys for Bldg. 886 from Jan. 1981 – December 1990

 Surveys were conducted regularly

 Values in uncontrolled areas were rarely above the limit

 Evidence indicates spills were promptly cleaned up

 Use of the Contamination limit is bounding

Division of Compensation Analysis and Support
CML Air Monitoring

 The original dose model airborne concentrations 

were based on applying a resuspension factor to 

the surface contamination limit

 Since then NIOSH captured:

 Plant-wide procedures describing air monitoring 

program for alpha particulate emissions

 Air sample locations for Bldg. 875 and 886 (Fig. 8)

 Air sample results (Table 6)

Division of Compensation Analysis and Support
CML Air Monitoring cont.

 NIOSH concludes a well defined air monitoring 

program was required by RFP procedures

 Air samples for bldg. 886 and 875 appear to have 

been routinely collected and analyzed

 Sample results were evaluated against the RCG of 

70 dpm/m3

 Sample results were reviewed and initialed

Division of Compensation Analysis and Support
CML Air Monitoring cont.

 Based on the air sample data NIOSH determined a 

bounding air concentration can be calculated as a 

weighted average concentration by

 Using the three recorded values in excess of the RCG 

 Assuming samples between 10%-100% of RCG were 100% (70 

dpm/m3)

 Samples less than 10% of RCG were 10% (7 dpm/m3)

 No respiratory protection

 This results in a weighted average concentration 

of 19.2 % of RCG or 13.5 dpm/m3

Division of Compensation Analysis and Support
Unmonitored Exposure to MFAP

 Concern with potential internal exposure 

from MFAP associated with numerous spills 

of enriched uranyl nitrate

 No indication of confirmatory bioassay 

being performed for persons involved in 

cleanup of spills

 No indication of routine bioassay for MFAP 

Division of Compensation Analysis and Support
Bounding Estimate MFAP Dose

 The approach used for Bounding MFAP dose is 

the same approach used in the previous white 

paper

 Maximum MFAP internal dose modeled by

 Using a representative UNH experiment

 Average thermal power and duration 

 Average air concentration from CML air monitoring 

results

 ICRP 68 dose conversion factors

 ORAUT-OTIB-0054 (Dosimetrically significant nuclides)

 ORIGEN-S code

Division of Compensation Analysis and Support
Bounding Estimate MFAP Dose

 The bounding values are significantly reduced 

from the previous calculation.

 Soluble (Type F) 2.5 x 10-9 Sv or 2.5 10-4 mrem Bone Surface (BS)

 Moderately Soluble (Type M) 2.3 x 10-9 Sv or 2.3 10-4 mrem (BS)

 Insoluble (Type S) 2.4 x 10-9 Sv or 2.4 10-4 mrem Lung

 The orders of magnitude difference is driven by a 

calculation error in the previous calculation

 The other factors in the lower dose are reduced 

power level and airborne activity

Division of Compensation Analysis and Support
Questions?

Division of Compensation Analysis and Support