National Institute for Occupational Safety and Health

SEC-00236 Metals and Controls Corporation

NIOSH Update on Thorium and Welding Exposure 

Model

Christine Corwin

Health Physicist

Teleconference of the Metals and Controls Work Group

January 9, 2020
Overview

 Summary of current NIOSH responses to the SC&A Review of the NIOSH 

White Paper: “Metals and Controls Corp. Thorium and Welding Exposure 

Model”
Summary of M&C Welding and Thorium Issues

 2 Findings

• 2 In Progress

• SC&A and NIOSH 

agreement on 

methodology needed

 3 Observations

• 3 Resolved

• SC&A and NIOSH in 

agreement 
Finding 1: Thorium Internal Exposures Model

 NIOSH underestimated the Th-232 concentration in the sediments and 

residues in the pipes under Building 10, leading to an underestimate of Th232 intakes by workers performing subsurface activities. 

– Uranium to thorium ratio

• Sowell data cannot be used (with the exception of 2 samples in 

Table 11) because the Th-232 data is consistent with naturally 

occurring Th-232 in soils.

• The only data that can be used is Sowell Tables 5A and 6A.
Finding 1: Thorium Internal Exposures Model (continued-1)

– NIOSH Response: The ratio provided was intended to be 

supplemental information to support the conservatism in the 

model where NIOSH assumes subsurface sediments contained 

equivalent mass concentrations of natural uranium and Th232.

– New NIOSH bounding method to estimate intakes

• Assuming that the specific activity of the uranium contamination 

in the pipe sediments was that of natural uranium is not valid.

– NIOSH Response: Any adjustment that would result in a higher 

specific activity (i.e., a higher enrichment for uranium) would 

be less favorable to the claimant.
Finding 1: Thorium Internal Exposures Model (continued-2)

– SC&A alternate method

• paired-sample method to determine a uranium to thorium ratio.

– NIOSH Response: the assumption of equivalent mass is the 

most claimant-favorable and defensible approach.

• OCAS-TIB-009 to determine daily ingestion rates.

– NIOSH Response: the use of NUREG/CR-5512 to determine 

ingestion intakes when exposures are characterized by massbased samples is more appropriate than the use of OCAS-TIB009. 

– This issue was addressed in the Board Review System (BRS) 

Overarching Issue #2.
Finding 2: Welding Internal Exposures Model 

 NIOSH understated the resuspension factor related to activities 

accompanying welding. 

– NIOSH Response: The decision to use a resuspension factor of 1 x 10-2

as opposed to 1 x 10-3 is considered a technical basis document (TBD) 

issue. The 1 x 10-2 resuspension value in the reference from OTIB-0070 

is listed in Table 5.11 of the reference (SRDB REF ID 77730 p. 55) and 

footnoted: “Values not used due to unrepresentative conditions”. 

Therefore, NIOSH believes that the use of a 1 x 10-3 resuspension 

factor is claimant appropriate. 
Observation 1: Thorium Internal Exposures Model 

 SC&A Review

– The uranium inventory cited by NIOSH is inconsistent with that in the 

source document.

 NIOSH Response

– The inventory is not used in the proposed dose methodology, NIOSH 

acknowledges that this was a data entry error.

 Action Needed

– The data entry error will be corrected in the TBD.
Observation 2 : Welding Internal Exposures Model 

 SC&A Review

– NIOSH should clarify the source of the 4 hour per month time 

estimate.

 NIOSH Response

– An incorrect source (reference) was used, the correct reference was 

subsequently provided.

 Resolution

– The reference will be corrected in the Evaluation Report (ER).
Observation 3: Welding Internal Exposures Model 

 SC&A Review

– In estimating doses from the welding scenario, NIOSH should assign 

doses using the most claimant-favorable isotope of thorium or 

uranium, selected from isotopes known to have been used at M&C.

 NIOSH Response

– NIOSH agrees with this observation and intends to apply it to the 

exposure model.

 Resolution

– NIOSH will ensure the revised ER’s exposure model reflects the 

observation.
Summary for Path Forward

 Finding 1

– Underestimate of Th-232 intakes by workers performing subsurface 

activities 

• Uranium to thorium ratio

• New NIOSH bounding method to estimate intakes 

• SC&A alternative method

 Finding 2

– Underestimate of resuspension factor related to activities 

accompanying welding

• TBD issue
Summary for Path Forward (continued)

 Observation 1

– inventory data entry error to be corrected in the TBD.

 Observation 2

– the reference for the 4 hour per month time estimate will be corrected 

in the revised ER.

 Observation 3

– the exposure model in the revised ER will assign dose from the most 

claimant-favorable isotope.