IE Circular No. 81-15, Unnecessary Radiation Exposures to the Public and Workers During Events Involving Thickness and Level Measuring Devices

                                                           SSINS No.: 6830 
                                                           Accession No.:  
                                                           IEC 81-15       

                               UNITED STATES 
                           WASHINGTON, D.C. 20555 
                              December 8, 1981 

                         MEASURING DEVICES 


This circular provides information about radiation hazards associated with 
the possession and use of measuring devices containing radioactive sources. 
These devices are generally considered to present only minimal potential for
radiological problems under normal use conditions. However, two recent 
events involving measuring devices resulted or may have resulted in 
unnecessary radiation exposure to members of the public and to maintenance 
workers. This information should be brought to the attention of all persons 
involved in the administration and operation of facilities and processes 
using measuring devices. Particular attention should be given to the 
environment where measuring devices are located and to procedures to assure 
that the radioactive source is properly shielded and contained. 

Description of Circumstances: 

Case 1: An NRC licensee was closing a facility in Oklahoma City, Oklahoma 
and had sold a trailer containing a mounted measuring device (Tube Wall 
Caliper) for determining pipe wall thickness. Since the device contained a 
1.5 curie cesium-137 source and the new owner had not yet obtained a license 
to possess the radioactive source, the licensee removed the device from the 
trailer prior to the new owner taking possession. During removal of the 
device (which was performed by an unauthorized user), the radioactive source 
was inadvertently released from its shielded position in the device and fell 
to the trailer floor. The dismounting of the device was performed without 
benefit of a survey meter or personnel monitoring equipment. The radiation 
dose to the individual may have been as high as 600 millirems. Subsequently,
the new owner had the trailer towed to Houston, Texas, with an interim stop 
for tow truck engine repair in Norman, Oklahoma. The driver, who was not 
aware of the presence of the radioactive source, waited near the trailer for
approximately four hours. He may have received a radiation dose as high as 
1.4 rems. The next day, the licensee found that the source was missing from 
the measuring device. Local health authorities performed a search using 
radiation detection equipment along the highway route between Oklahoma City 
and Houston. The source was found lodged on a bridge support structure near 
Lewisville, Texas. 

The major causes of the event were that (1) the licensee failed to employ an
authorized user to remove the device, and (2) the unauthorized user failed 
to make a radiation survey. 


                                                        IEC 81-15         
                                                        December 8, 1981  
                                                        Page 2 of 2       

Case 2: A cooler in an iron ore pellet plant was shut down for repairs on 
March 30, 1981. On that day, the shutter mechanism of a level control 
device, which contained a nominal 10-curie cesium-137 sealed source, was 
locked in the closed position. Radiation surveys performed at that time 
indicated that the source appeared to be properly shielded. After a cooldown 
period, workmen entered the cooler on April 3, 1981 to replace refractory 
material on the cooler walls. On April 7, licensee personnel discovered that 
there were radiation levels in excess of 100 millirems per hour within the 
cooler (later determined to be as high as 2.2 rems per hour where the 
radiation beam entered the cooler). It was determined that several 
individuals had been exposed to a radiation beam from the source during the 
working days between April 3 and 7, 1981. The device a source holder was 
removed from its mounting, and licensed personnel found that the lead 
shielding in the shutter had melted and drained from the shielded location. 
This rendered the shielding integrity of the shutter useless. 

Investigation showed that 17 licensee personnel and 14 contractor personnel 
had entered the cooler between April 3 and 7, 1981. The calculated radiation
exposures received ranged from 0.14 to 3 rems. During the repairs, the 
pellet cooler area was considered an unrestricted area. It is estimated that 
14 of the 31 individuals exposed may have received whole-body exposures in 
excess of 0.5 rems. No health effects were observed or would be expected 
from these exposures. 

The event occurred because a hole had been cut in the side of the cooler to 
reduce shielding and allow more effective operation of the cesium-137 source
in the device. During recent efforts to increase production, the pressure of
the air forced into the cooler had been increased as a means of accelerating
the cooling of the pellets. As a result, hot gases may have been forced out 
of the aperture in the cooler wall at the location of the source holder. The
temperature of the pellets entering the cooler is about 2400F, 
considerably higher than the melting point of lead. The heat reaching the 
device was sufficient to melt the aluminum alloy dust cover over the device 
shutter mechanism and the lead in the shutter, thereby allowing a radiation 
beam to escape the device. In addition, the licensee's survey failed to 
determine that the radioactive source was not safely shielded. 

No written response to this circular is required. If you need additional 
information regarding these subjects, contact the regional administrator of 
the appropriate NRC Regional Office. 

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