United States Nuclear Regulatory Commission - Protecting People and the Environment

Information Notice No. 94-15: Radiation Exposures During An Event Involving a Fixed Nuclear Gauge


March 2, 1994



All U.S. Nuclear Regulatory Commission licensees authorized to possess, use,
manufacture, or distribute industrial nuclear gauges.


NRC is issuing this information notice to alert addressees to events,
involving industrial gauges, that resulted, or may have resulted, in
unnecessary radiation exposure to members of the public and licensee
personnel.  It is expected that recipients will review the information for
applicability to their operation and consider action, as appropriate, to avoid
similar problems.  However, suggestions contained in this information notice
are not new NRC requirements; therefore, no specific action nor written
response is required.

Description of Circumstances

A recent incident occurred at a glass factory where a level gauge with
approximately 185 gigabecquerels (5 curies) of cesium-137 was subjected to a
severe heat environment that resulted in the loss of lead shielding, producing
a high radiation dose rate near the source housing.  Licensees were alerted to
similar incidents in Information Notice No. 81-37 (see Attachment 1).

The glass manufacturing company (an NRC licensee) informed NRC on August 23,
1993, that lead shielding melted from one of the level gauges that was mounted
on the exterior surface of a glass furnace.  The licensee believed that the
damage occurred when it lost electrical power for approximately 3 hours on
Saturday, August 21, 1993, and the glass furnaces were operated using natural
gas, which caused higher than normal temperatures.  During the outage, an
employee noticed that some lead had melted and accumulated on a mounting
bracket adjacent to a level-measuring nuclear gauge; however, the employee did
not notify the facility Radiation Safety Officer (RSO) at that time.  The
following Monday, the same employee noticed more lead on the mounting bracket
and other adjacent areas.  The RSO was then notified, who restricted access to
the area.

The lead melt was apparently caused by intense heat emanating through
refractory board covering an opening in the furnace wall adjacent to the
source housing.  It was later determined that this opening had been in the
furnace wall for some time, to be used for another type of measuring device.
However, on this occasion when the device was not in use, the opening was

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March 2, 1994
Page 2 of 3

covered with refractory board instead of being closed with the original
refractory brick.  Radiation surveys performed by the licensee and a
manufacturer's representative revealed an exposure rate of 1.29 X 10-5
C/(kg-hr) at 2.44 meters (50 mR/hr at 8 feet) and 5.16 X 10-5 C/(kg-hr) at
1.22 meters (200 mR/hr at 4 feet) from the source housing.   The
representative estimated the highest exposure rate at the surface of the
source housing to be 7.74 X 10-4 C/(kg-hr) (3000 mR/hr).  He indicated that
the shutter block of this device might be melted, and that half of the lead
might no longer be contained in the source housing.  This particular device
contained a nominal 185 gigabecquerels (5 curies) of cesium-137, and between
86 and 91 kilograms (190 and 200 pounds) of lead shielding. Although there
was no facility or personnel contamination, since the cesium-137 source did
not leak, dose equivalent calculations indicated that one individual may have
received approximately 2 mSv (200 mrem) to the whole body.  Other individuals
working in the vicinity (about 34 people) received lower doses.

The major causes of the incident were: 1) the licensee did not take into
consideration the effect of extreme heat on the source housing before removing
the refractory brick; and 2) the licensee failed to follow its emergency
procedures by not immediately notifying the RSO when the leaking lead was
first discovered.  This contributed to delays in establishing appropriate
radiological controls to minimize radiation exposure to personnel.


Under normal working conditions, these gauges are designed and can be operated
without problems related to radiological safety.  Operational history has
shown that many devices will also survive severe conditions such as fires and
explosions, but there are limitations on the source housing.  A common factor
in incidents described in this information notice and within IN 81-37 is the
impact that modification of a gauge's environment has on its safety and
integrity.  It is important that licensees consider the effects on nuclear
gauges when changes are made to the gauge's environment.  Individuals working
near a nuclear gauge should be aware of the potential hazard.  Any changes in
gauge surroundings, or the gauge itself, need to be reviewed by radiation
safety personnel, and compared with the manufacturer's design criteria, so
that inadvertent exposure can be avoided.

.IN 94-15
March 2, 1994
Page 3 of 3

This information notice requires no specific action nor written response.  If
you have any questions about the information in this notice, please contact
one of the technical contacts below, or the appropriate regional office.

/s/'d by Carl J. Paperiello

                        Carl J. Paperiello, Director
                        Division of Industrial and
                          Medical Nuclear Safety
                        Office of Nuclear Material Safety
                          and Safeguards

Technical contacts:  Judith A. Joustra, RI
                     215) 337-5257

                     Joseph E. DeCicco, NMSS
                     (301) 504-2067

1.  Excerpts from Information Notice No. 81-37
2.  List of Recently Issued NMSS Information Notices
3.  List of Recently Issued NRC Information Notices
Excerpts from Information Notice 81-37:  UNNECESSARY RADIATION EXPOSURES TO
DEVICES, dated December 15, 1981.

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
55.5-gigabecquerel (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 before the new owner took 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 6 mSv (600 mrem).
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 4 hours.  He may have received a radiation dose as
high as 0.014 Sv (1.4 rem).  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: (1) the licensee failed to employ an
authorized user to remove the device; and (2) the unauthorized user failed
to make a radiation survey.

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 370-gigabecquerel (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 millirem per hour
within the cooler (later determined to be as high as 0.022 Sv (2.2 rem) 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 source holder
was removed from its mounting, and licensee 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 1.4 to 30 mSv (140 to 3000 mrem).  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 .
dose equivalent in excess of 5 mSv (500 mrem).  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 1300 C (2400 F),
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.
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