United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 91-30: Inadequate Calibration of Thermoluminescent Dosimeters Utilized to Monitor Extremity Dose at Uranium Processing and Fabrication Facilities

                                UNITED STATES
                        NUCLEAR REGULATORY COMMISSION
              OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
                           WASHINGTON, D.C. 20555

                               April 23, 1991 


Information Notice No. 91-30:  INADEQUATE CALIBRATION OF 
                                   THERMOLUMINESCENT DOSIMETERS UTILIZED TO 
                                   MONITOR EXTREMITY DOSE AT URANIUM 
                                   PROCESSING AND FABRICATION FACILITIES


Addressees:

All fuel cycle licensees and other licensees routinely handling unshielded 
uranium materials.

Purpose:

This information notice is being provided to alert addressees to potential 
problems resulting from inadequate calibration of thermoluminescent 
dosimetry (TLD) utilized to monitor extremity dose.  It is expected that 
licensees will review the information for applicability to their facilities, 
distribute it to radiation safety personnel, and consider actions, as 
appropriate, to avoid similar problems.  However, suggestions contained in 
this Information Notice do not constitute new U.S. Nuclear Regulatory 
Commission (NRC) requirements; therefore, no specific action or written 
response is required.

Description of Circumstances: 

During August-September 1990, an NRC licensee conducted evaluations of 
extremity exposures to beta particles for selected personnel handling 
unshielded uranium materials.  The evaluation included both extremity 
monitoring for workers handling the materials and a determination of the 
dose rate from unshielded uranium material (pellets).  Monitoring was 
conducted using single chip TLDs mounted in plastic finger rings.  Pellet 
dose rates were determined by exposing TLDs to the unclad material for a 
known period.  TLDs were supplied and subsequently processed by a vendor 
laboratory.  The licensee had verbally discussed with the vendor that the 
monitoring with the finger ring TLDs was conducted to determine employee 
beta extremity dose from the unshielded uranium materials.  In addition, the 
licensee included instructions stating the type of radioactive materials 
which were to be monitored with their purchase agreement accompanying the 
TLDs to the processing vendor laboratory.  

Based on vendor reports for TLDs affixed to uranium pellets for a set amount 
of time, the licensee calculated dose rates of 38 and 40 mrem/hr through an 
absorber thickness of 7 milligrams per square centimeter (mg/cm�).  
Initially, the licensee assumed the pellet dose reports to be accurate and 
no additional calculations were made to support the vendor measurements.  
However, in response to NRC inquiries, independent calculations were made 
which indicated that the 

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minimum dose rate from the pellet material exceeded the values derived from 
the vendor reports by a factor of approximately 2. 

During discussion of the dose measurements with the TLD vendor, the licensee 
was informed that the TLDs were calibrated with a cesium-137 (Cs-137) 
source.  The vendor had a calibration factor for TLDs exposed to a slab of 
natural uranium and noted that a correction factor was required to convert 
from a cesium-based to uranium-based shallow (skin) dose.  Review of the 
licensee dose data indicated that the appropriate correction factor was not 
applied to the original results provided by the vendor.  A beta-correction 
factor of approximately 2.0 was required, and higher dose values were 
subsequently assigned to personnel handling unclad uranium materials.  

Subsequently, NRC contacted four other fuel fabrication facilities regarding 
their evaluations of extremity dose from depleted or low enriched unshielded 
uranium.  These facilities used single chip TLDs processed by vendor 
laboratories.  Among the facilities, three different vendors were 
represented in providing and processing the TLDs.  During discussions with 
these vendors, it was determined that a correction factor of approximately 
2.0 should have been applied to the reported results.  In all cases, this 
value had not been applied to any of the actual TLD measurements.  As a 
result, extremity exposures had been underestimated by a factor of 
approximately 2. 

Discussion: 

At fuel fabrication facilities, as well as at facilities handling natural 
and depleted uranium, selected operational processes potentially require 
extensive handling of unshielded uranium materials by employees.  Depending 
on the facility processes, dose rates to the skin of the extremity from 
unclad uranium materials can increase significantly during operations.  At 
fuel fabrication facilities following introduction of uranium hexafluoride 
(UF-) into the process, the major contributor to the skin dose results from 
ingrowth of the beta-emitting metastable protactinium-234 (Pa-234m) isotope.  
Dose rates through 6-7 mg/cm� absorbers of approximately 200 millirem per 
hour (mrem/hr) for exposure to depleted, natural, or low-enriched unshielded 
uranium materials in equilibrium with short-lived daughter isotopes have 
been reported (References 1-2).  It is the licensee's responsibility to 
properly evaluate the potential dose to the skin of the extremity to 
determine the need for extremity monitoring, and to verify that no 
individual exceeds the exposure limits specified in 10 CFR Part 20.  In 
using TLD's for extremity monitoring, licensees should verify that the 
appropriate correction factor for the different TLD response between uranium 
and the calibration source is used. 

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No specific action or written response is required by this information 
notice.  If you have questions about this matter, please contact the 
technical contacts listed below or the appropriate regional office. 




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


Technical Contacts:  George B. Kuzo, Region II
                     (404) 331-2560

                     John Potter, Region II
                     (404) 331-5571 


Attachments: 
1.  References           
2.  List of Recently Issued NRC Information Notices
3.  List of Recently Issued NMSS Information Notices
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                                                            Attachment 1
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                                                            April 23, 1991 
                                                            Page 1 of 1


                                 REFERENCES


1.  Coleman, R. L., C. G. Hudson, and P. A. Plato, 1983.  "Depth-dose Curves 
    for Sr-90 and Natural and Depleted Uranium in Mylar."  Health Phys. 
    44(4):395-402.

2.  U. S. Department of Health Education and Welfare (DHEW).  1970.  
    Radiological Health Handbook.  Public Health Service Publication, 
    Rockville, Maryland.


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