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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 9104170066 . IN 91-30 April 23, 1991 Page 2 of 3 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. . IN 91-30 April 23, 1991 Page 3 of 3 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 . Attachment 1 IN 91-30 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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