Protecting People and the EnvironmentUNITED STATES NUCLEAR REGULATORY COMMISSION
UNITED STATES
NUCLEAR REGULATORY COMMISSION
OFFICE OF NUCLEAR MATERIAL SAFETY AND SAFEGUARDS
WASHINGTON, D.C. 20555
June 11, 1996
NRC INFORMATION NOTICE 96-35: FAILURE OF SAFETY SYSTEMS ON
SELF-SHIELDED IRRADIATORS BECAUSE OF
INADEQUATE MAINTENANCE AND TRAINING
Addressees
All U.S. Nuclear Regulatory Commission irradiator licensees and vendors.
Purpose
The U.S. Nuclear Regulatory Commission (NRC) is issuing this information notice (IN) to alert
addressees to two incidents where safety interlocks on self-shielded irradiators (Category I)
failed to prevent inadvertent exposure. The causes of these exposures stemmed from a lack of
appropriate maintenance and/or worker training. The incidents include a broken spring --
possibly causing malfunction of the safety interlock -- and a worker who intentionally bypassed
a safety interlock. It is expected that recipients 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 are not NRC requirements; therefore, no
specific action nor written response is required.
Description of Circumstances
The first incident occurred when an operator may have been able to open the shielded door of
an irradiator with the sources in the exposed position. After irradiation of several pocket
dosimeters, the operator opened the shielded door of the irradiator to retrieve the dosimeters,
but did not perform a radiation survey, as required by the facility's internal procedures, before
opening the door. Twice, the operator placed one hand inside the irradiator to retrieve the
dosimeters. Subsequently, the operator observed that the unit timer continued to count,
indicating that the sources remained in the exposed position. The operator checked his
personal pocket dosimeter, but did not note an unusual reading. However, the operator did
not report the incident until questioned by the radiation safety officer, who had noted an
unusually high dosimetry report of 3.55 millisievert (355 mrem) deep dose equivalent for the
worker. The dose to the right hand was calculated to be a maximum of 12.5 millisievert (1.25
rem).
The design of the irradiator includes two interconnected interlock systems, intended to prevent
unshielded exposure of the sources. These include a door interlock system -- designed to allow
opening of the shielded door only after the sources are placed in the fully shielded position --
and a source exposure interlock system -- designed to secure the sources in the
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fully shielded position whenever the shielded door is open or unlocked. The manufacturer of
the irradiator indicated that under normal operations, either system individually would prevent
inadvertent access to the unshielded sources.
Following the incident, the manufacturer of the irradiator was requested to perform an onsite
inspection of the irradiator and facilities. During the inspection, the manufacturer noted:
(1) the irradiator was located in an area that was not climate-controlled;
(2) internal components of the irradiator were in a degraded state;
(3) maintenance of the irradiator had last been performed approximately 10 years ago; and
(4) a return spring, integral to the source exposure safety interlock system, was broken.
The manufacturer indicated that the lack of environmental control may have accelerated
the degradation of the internal components of the irradiator, and that the lack of periodic
maintenance of the irradiator may have contributed to the failure of the return spring.
The broken return spring may have caused the source securing mechanism of the source
exposure interlock system to malfunction, possibly allowing exposure of the sources after the
shielded door was unlocked and opened. However, during the post-incident investigation,
neither the manufacturer nor the licensee were able to identify a failed component of either
interlock system that could have allowed the shielded door to be opened with the sources in the
exposed position. The manufacturer indicated that the design of the source exposure
mechanism -- the operator must manually move the sources from the shielded to the exposed
position with a lever -- would have provided the operator with a positive indication of source
position even if the interlock systems failed. Source position would have been further provided
by a series of green and red source position lights on the irradiator.
The operator's actions indicate either a lack of training on the proper functioning and use of the
irradiator, a lack of understanding of the training provided, and/or a disregard for following the
established operating and safety procedures. The operator indicated that the timer continued
to count when the shielded door was opened. However, the manufacturer reported that the
timer automatically activates whenever the source lever is manually moved to one of the two
source exposed positions and the lever is fully engaged in the source slot, and stops counting
as soon as the lever is moved from the fully engaged position. The fact that the timer continued
to count indicates that the operator had not moved the sources from the fully exposed and
engaged position. The licensee reported that the operator had been trained in the operation of
the irradiator and was listed as an authorized user, but that the irradiator was used infrequently
and that this was only the operator's second use of the irradiator since being trained.
The second incident occurred when a maintenance worker preparing to perform maintenance
on an irradiator bypassed the irradiator door interlock system to observe movement of the inner
irradiation chamber. The maintenance was being performed to correct previous
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maintenance that resulted in the irradiator not functioning properly. The maintenance worker
was unaware that, although the sources remained shielded during movement of the irradiation
chamber from the load to irradiate position, high levels of radiation scatter would be present.
The maintenance worker, upon hearing the in-room monitor alarm, immediately returned the
radiation chamber to the "load" position (maximum shielding).
Although the maintenance worker was familiar with the operation of the irradiator and had been
responsible for its maintenance for nearly 15 years, the worker apparently had not been given
formal training on radiation safety or the operation and maintenance of the irradiator. The
maintenance worker was not aware of the scatter radiation and assumed that since the sources
were not directly exposed, radiation from the sources would be contained within the device.
During this incident, another worker, hired to perform contract maintenance on the irradiator,
was also in the room near the irradiator. Neither worker wore dosimetry nor had any
documented training in radiation safety. Therefore, their doses could only be calculated based
on their recollection and were estimated to both be approximately 4 microsievert (0.4 mrem)
whole body.
Discussion
Although neither incident resulted in doses in excess of regulatory limits, the doses received in
both incidents were unnecessary and possibly could have been avoided with proper training
and routine equipment maintenance. A similar incident in 1984, where a door interlock failed,
resulted in the operator being exposed to 222 terabequerel (6000 curies) of cesium-137.
The first incident clearly demonstrates the need to perform appropriate maintenance on these
types of units. Even though these units are designed with interlocks and safety features
intended to prevent inadvertent exposures, the components of these systems depend on
adequate maintenance to function properly. Failure to properly maintain these systems and
provide appropriate training could result in unnecessary exposures. Manufacturers of these
types of irradiators frequently provide initial and periodic training on the operation of their units
and, in some cases, training on other manufacturers' units, as well. Initial training is typically a
condition of the license and, therefore, must be provided to all irradiator users and maintenance
personnel. Periodic refresher training is also beneficial as a reminder for working safely around
the irradiator and provides for a means to receive or disseminate additional or updated
information.
In addition, most manufacturers have a recommended schedule of maintenance and/or
recommended preventative/periodic maintenance that should be performed. Users of these
types of irradiators should evaluate their usage to determine the applicability of the
recommended maintenance to their situation and usage. Users who operate their unit more
than usual or who use their units under harsh conditions should consider the need for
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stepped-up maintenance or shortened maintenance intervals. In addition, each manufacturer's
recommended maintenance may vary according to the specific unit or type of use. Therefore,
persons performing maintenance on their unit may require specific maintenance training for
their unit.
Users who are not aware of the required training for their unit, or who wish to receive
information concerning training in general, should consult their license, licensing authority, or
the manufacturer of the unit. Regulatory Guide 10.9, provides additional guidance in this area
and may assist persons who wish to develop a training and maintenance program. Users who
wish to receive additional information concerning recommended maintenance for their unit
should contact the manufacturer of the unit. In addition, third-party service companies may also
be available for training and maintenance services for these types of irradiators.
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
listed below or the appropriate regional office.
Donald A. Cool, Director
Division of Industrial and Medical Nuclear Safety
Office of Nuclear Material Safety and Safeguards
Technical contacts: Douglas Broaddus, NMSS
(301) 415-5847
Internet: dab@nrc . gov
Anthony Kirkwood, NMSS
(301) 415-6140
Internet: ask@nrc . gov
Attachments:
1. List of Recently issued NMSS Information Notices [see WordPerfect file]
2. List of Recently issued NRC Information Notices [see WordPerfect file]
