ML20024G044
| ML20024G044 | |
| Person / Time | |
|---|---|
| Issue date: | 12/07/1990 |
| From: | Dan Collins NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION II) |
| To: | Jim Hickey NRC OFFICE OF NUCLEAR MATERIAL SAFETY & SAFEGUARDS (NMSS) |
| References | |
| NUDOCS 9012270274 | |
| Download: ML20024G044 (5) | |
Text
c#C/a l 0
DEC 0 71990 l
MEMORANDUM FOR:
John W. N. Hickey, Chief Operations Branch Division of Industrial and Medical Nuclear Safety FROM:
Douglas M. Collins, Chief Emergency Preparedness and Radiologica',
Protection Branch Division of Radiation Safety and Safegaards
SUBJECT:
TRANSMITTAL OF DRAFT INFORMA'il0N NOTICE ON BETA DOSE EXTREMITY MONITORING AT FUEL FABRICATION FACILITIES This memorandum transmits for your information and possible use the subject draft Information Notice.
As a result of potential skin dose assessment inadequacies identified at several Region II fuel fabrication facilities, we feel this Notice may help negate possible existing dose evaluation concerns at other facilities.
If you have any questions, please call me at FTS 841-5586.
0(19192lS iM nj D.M. Coll Douglas M. Collins
Enclosure:
Draft Information Notice cc w/ encl:
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UNITED STATES NUCLEAR REGULATORY COMMISSION OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS-WASHINGTON, D.C. 20555 December,_, 1990 NRC INFORMATION NOTICE NO. 90-INADEAVATE PROCESSING 0F THERM 0LIMINESCENT DOSIMETERS UTILIZED TO MONITOR-~ EXTREMITY DOSE AT URANIUM PROCESSING AND FABRICATION FACILITIES Addressees:
All-fuel Lcycle licensees and other licensees routinely handling unclad uranium materials.
. Purpos_e :
This information ' notice is being provided to alert addressees to potential problems resulting from inadequate vendor processing of thermoluminescent
- dosimetry : utilized to monitor extremity skin dose at fuel f abrication facilities.
It is expected that licensees will review the information for applicability to their facilities and consider actions, as appropriate, to avoid similar ' problems.
However, suggestions contained in this Information Notice do.not constitute U.S.
Nuclear Regulatory Commissio_n-(NRC) requirements; therefore, no specific action
~~
or written response is required, i
Description of Ci_rcumstances:
- Selected processes such as grinding, press and inspection--operations at fuel
. fabrication facilities -potentially require extensive contact (handling) of
, unshielded uranium materials by. employees.
Following introduction of uranium hexafluoride (UF6) into the fabrication process, the major contributor to the o
skin dosetresults froni wrowth of the beta-emitting metastable protactinium-234 (Pa-234m) isotope.
The ingrowth of the relatively short-lived Pa-234m radioisotope (1.17 ' minute - physical half-life) follows i.re. decay of f the longer-lived (24.1 day physical half-life) thorium-234 (Th-234) parent radionuclide.
Dose rates through 617 mi_lligrams per square centimeter (mg/cm2) absorbers _ ranging from approximately 100 to 200 millirem per hour (mrem /hr) for exposure to depleted, natural, or low-enriched unshieldded -uranium materials in equilibrium with short-lived daughter isotopes have been. reported
-(References 1-3).'
Proper evaluation of the dose to the skin of the extremity is required to determine the threshold for extremity monitoring relquirements:and to verify -that no individual exceeds the exposure limits specified.i n 10 CFR Part 20.
During August - September 1990, an NRC licensee conducted evaluations of extremity skin exposures for selected personnel handling unshielded uranium materials.
The evaluation included both extremity monitoring-for workers handling the materials and, in addition, a determination of the dose rate from unshielded uranium material.
Monitoring was conducted using single chip TLDs mounted in plastic finger rings which were. supplied and subsequently processed.by a vendor laboratory.
The licensee had verbal'iy discussed with the -
vendor that the monitoring with the finger ring TLDs was conducted to determine employee skin dose from exposure to the Pa-234m beta particle, 2.29 MeV maximum energy, in the unshielded uranium materials.
In addition, the licensee included instructions indicating the type of radioactive materials to be monitored with their purchase agreement accompanying the TLDs to the processing vendor laboratory.
For the TLDs utilized to monitor dose rates, the vendor reported pellet exposure rates of 28 and 40 mrem /hr as determined through a density absorber thickness of 7 mg/cm2 Initially, the licensee assumed the measurements to be accurate and no additional calculations were made to support. the vendor measurements. However, in response to NRC inquiries, calculations indicated dose rates were expected to be approximately 65 percent of equilibrium values based on the 35 to 40 day-interval between the introduction of UF6 into the fabrication process and when the actual measurements of the unshielded materials were conducted.
Assuming the range of reported uranium material ex through a density absorber thickness of 7 mg/cm2 (References 1-3)posure rates
, the calculated minimum dose rate from the pellet material was expected to exceed 65 mrem /hr.
~
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 which results in_ a-uniform energy deposition within_ the TLD.
Errors result when the energy deporited is nonuniform as -a result of attenuation, such as for beta
-particle irradiation from uranium.
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 exposure data ' indicated that the appropriate correction factor was-not applied to the original results provided by the vendor.
For the 1990' data, a beta-correction factor of approximately 1.89 was required.- Applying the correction factor resulted in dose rates of -72 and 76 mrem /hr for the monitored unshielded re,ults.
Further discussion with the TLD vendor laboratory indicated that in addition. to the revisions for the 1990 study, a required correction factor was not applied to similar measurements conducted by the licensee in 1983 regarding unshielded uranium material dose rate studies.
For those data, a separate correction factor of approximately 2.21 resulted from differences.in the finger ring absorber materials used for the 1983 and 1990 measurements.
The licensee upgraded their procedures to verify that the appropriate beta correction i' actors were included with processing of TLDs utilized to measure skin dose.
In addition, the licensee proposed to conduct systematic TLD dose measurements of unshielded uranium mater al through time to determine the expected dose rates during ingrowth and at equilibrium for the beta-emitt.ing
Pa-234m. The equilibrium values could be used to evaluate potential exposure and
_ corroborate actual measurements.
Subsequently,- a -NRC Region II representative discussed with representatives from four additional fuel f abrication facilities, TLD evaluations of skin exposure
.from depleted or low enriched unshielded uranium material.
The facilities utilized single chip TLD devices processed by vendor laboratories.
Among the facilities, three different vendors were represented in providing the TLDs and their subsequent processing.
Licensee representatives were informed that
. correction factors, approximately two times the reported values, were required to calculate the accurate skin dose from exposure to the uranium material.
- However, correction factors were not applied to any of the actual TLD measurements conducted.
Licensee representatives took appropriate immediate action to verify that all exposures were below 10 CFR Part 20 limits.
Selected licensees were conducting 'and documenting detailed studies - to determine accurate unshielded uranium dose rates'for future evaluations.
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 Regional Administrator of the appropriate regional office.
Richard E. Cunningham, Director Division of Industrial and Medical Nuclear Safety Office of Nuclear-Material Safety and Safeguards Technical Contacts: George B. Kuzo, Region II (404) 331-2560 Attachments:
1.
Rererences 2.
List-of Recently Issued NRC Information Notices
IN 90-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.
Reece, W.
D., R. Harty, L. W. Brackenbush, and P. L. Roberson,1985.
Extremity Monitoring:
Considerations for Use, Dosimeter Placement and Ev'alIia'tio~n'.~~NGE6TCk-4297 ~TNL76664,"0:'S:~ fEcis5F R'e-@latory commission, Washington, D.C.
3.
U.
S.
Department of Health Education and Welfare (DHEW).
1970.
g.d.io.l.ogic.a..l. _H,eal th Handbook.
Public Health Service Publication, Rockville, Ra q
1
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