Information Notice No. 92-34: New Exposure Limits for Airborne Uranium and Thorium

                                UNITED STATES
                          WASHINGTON, D.C.  20555 

                                May 6, 1992 

                                   AND THORIUM 


All licensees whose operations can cause airborne concentrations of uranium 
and thorium. 


The U.S. Nuclear Regulatory Commission is issuing this information notice to 
alert addressees of changes, in 10 CFR Part 20, governing airborne uranium 
and thorium exposures that may significantly affect exposure calculations 
and reporting requirements.  It is expected that recipients will review this 
information for applicability to their facilities and consider actions, as 
appropriate, to avoid violations of the new Part 20.  However, suggestions 
contained in this information notice are not new NRC requirements; 
therefore, no specific action or written response is required. 

Description of Circumstances: 

The major revision of 10 CFR Part 20 was published on May 21, 1991 (56 FR 
23360).  This revision adopted the dose-assessment methodology recommended 
by the International Commission on Radiation Protection (ICRP).  The new 
methodology significantly changes the occupational exposure limits for 
airborne uranium and thorium compounds.  The current regulation requires 
licensees to implement the new Part 20 by January 1, 1993.  However, 
licensees should note that the Commission is proposing to extend the 
implementation date to January 1, 1994. 


Because NRC has adopted the dose-assessment methodology of ICRP 26 and 30, 
the new Part 20 contains two changes that can impact greatly on licensees 
that experience airborne concentrations of uranium and thorium compounds.  
These are changes in occupational exposure limits, and equivalence of 
internal and external dose. 



                                                            IN 92-34 
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As shown in the table below, the new Part 20 includes a reduction in 
occupational airborne concentrations of certain thorium and uranium 
compounds.  However, the allowed concentrations of other nuclides have been 
relaxed.  Although most changes have been made to reflect radiological 
concern, 10 CFR 20.1201 (e) limits intakes of soluble uranium to 10 
milligrams in a week, in consideration of chemical toxicity, which is likely 
to be controlling at enrichments of 5 percent or less.  

               Table 1:  Old versus New Occupational Exposure Limits 
� Compound        �         Old Part 20       �        New Part 20        � 
�                 �Appendix B, Table I, Col. 1�Appendix B, Table I, Col. 3� 
� Th-234, class W �       6E-8 uCi/ml         �         8E-8 uCi/ml       � 
� Th-234, class Y �       3E-8 uCi/ml         �         6E-8 uCi/ml       � 
� Th-232, class W �       3E-11 uCi/ml        �         5E-13 uCi/ml      � 
� Th-232, class Y �       3E-11 uCi/ml        �         1E-12 uCi/ml      � 
� Th-231          �       1E-6 uCi/ml         �         3E-6 uCi/ml       � 
� Th-230, class W �       2E-12 uCi/ml        �         3E-12 uCi/ml      � 
� Th-230, class Y �       1E-11 uCi/ml        �         6E-12 uCi/ml      � 
� Th-228, class W �       9E-12 uCi/ml        �         4E-12 uCi/ml      � 
� Th-228, class Y �       6E-12 uCi/ml        �         7E-12 uCi/ml      � 
� Th-227          �       2E-10 uCi/ml        �         1E-10 uCi/ml      � 
� U-238, class D  �       7E-11 uCi/ml        �         6E-10 uCi/ml      � 
� U-238, class Y  �       1E-10 uCi/ml        �         2E-11 uCi/ml      � 
� U-235, class D  �       5E-10 uCi/ml        �         6E-10 uCi/ml      � 
� U-235, class Y  �       1E-10 uCi/ml        �         2E-11 uCi/ml      � 
� U-234, class D  �       6E-10 uCi/ml        �         5E-10 uCi/ml      � 
� U-234, class Y  �       1E-10 uCi/ml        �         2E-11 uCi/ml      � 

The relative equivalence of internal and external dose means that exposures 
to airborne concentrations of radioactive particles must now be maintained 
as part of a worker's total occupational dose limit (10 CFR 20.1202).  Under 
the old Part 20, internal and external doses had separate limits and were 
not combined.  Section 20.1204 states, further, that internal exposure can 
be determined through air sampling in the work place or routine bioassay. 

If air sampling is chosen as the most accurate means of tracking dose, 
compliance with the new limits may be demonstrated through precise 
solubility, isotopic, and particle-size classifications, pursuant to 
Appendix B to 10 CFR 20.1001-20.2401. 


                                                            IN 92-34 
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The new revision to NRC Regulatory Guide 8.25, "Air Sampling In The 
Workplace," gives guidance for setting up an air-sampling program that can 
be used to estimate internal dose from airborne activity.  The associated 
NUREG-1400 explains acceptable implementation of methods described in 
Regulatory Guide 8.25.  

In addition, Table 2 of Appendix B to 10 CFR 20.1001-20.2401 places much 
more stringent limits on effluent concentrations for soluble uranium and 
thorium in water, and for thorium and insoluble uranium in air.  The 
reduction in the allowable effluent concentrations to about a hundredth of 
the former values may necessitate changes in procedures or monitoring 
methods, or may even necessitate installation of new effluent treatment 
systems requiring appreciable lead time to procure.  

This information notice requires no specific action or written response.  If 
you have any questions about the information in this notice, please call the 
technical contact listed below or the appropriate NRC regional office. 

                                   Richard E. Cunningham, Director 
                                   Division of Industrial and 
                                     Medical Nuclear Safety 
                                   Office of Nuclear Material Safety 
                                     and Safeguards 

Technical contact:  Edward Shum, NMSS 
                    (301) 504-2607 

1.  List of Recently Issued NMSS Information Notices
2.  List of Recently Issued NRC Information Notices

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