EH-89-7 Cesium 137 Release
ENVIRONMENT, SAFETY & HEALTH
BULLETIN
Assistant Secretary for U.S. Department of Energy
Environment, Safety, & Health Washington, D.C. 20585
DOE/EH-0118 Issue No. 89-7 November 1989
Cesium 137 Release
A release of Cesium 137 occurred at a DOE facility because the differential
pressure instruments on a HEPA filter bank were not failsafe and the stack CAM
(Constant Air Monitor) that had been damaged by moisture, failed to function
and alert staff to a problem. The release, which was not confined to the
immediate area, was estimated at 10 microcuries of Cesium 137, which is less
than the prorated monthly guide of 50 microcuries for the area. Following the
discovery, the area was surveyed and barricades were erected to isolate any
contaminated surfaces in the vicinity of the release. All personnel were
monitored and three were found with contamination of hair and personal
clothing. There was no assimilation and the employees were successfully
decontaminated.
Normal Operating Procedures
Normally a filtered vessel vent stack on a concentrate transfer tank handles
the hot discharge liquid from a cesium solution evaporator. Under normal
operation, the tank is controlled at a constant temperature by throttling the
steam supply to the evaporator. In the event of high temperature excursions,
cooling water is added to the system water jackets as directed by written
procedure. The vessel vent system is equipped with demisters, condenser,
reheater, dual HEPA filters, and exhaust fans to remove entrained moisture and
cesium 137 and to maintain the vessels at a slight vacuum relative to the
surrounding transfer pit. The exhaust stream is monitored by a CAM and the
HEPA's are equipped with differential pressure instrumentation to detect
blinding by moisture or excessive particulate loads.
Precipitating Events
A series of events lead up to the cesium release.
I. Initial Start-Up Problems
A. During an initial start-up of a 1H Evaporator, vapors were observed
being discharged from the concentrate transfer pit ventilation stack.
The evaporator was shut down and the source of the vapors was found to
be failed steam bellows on a transfer gang valve.
B. Further investigation revealed saturated pit HEPA filters and a CAM
probe for the pit which had also failed. These items were repaired
and replaced. During replacement the area was monitored (per Health
Physics radiation control procedure) to ensure that no contamination
had occurred on the "clean" side of the HEPA filters.
C. It was also noticed that the tank ventilation system temperature
recorder had failed, but that all operating parameters had been within
limits. The temperature recorder/controller was repaired, but not
recalibrated, and a restart was initiated.
II. Restart: Over Heating of Concentrate
A. During the restart, the concentrate transfer tank contents went above
boiling. The temperature was reduced using cooling water. The
resulting increase in vent stream moisture exceeded the system's
capacity to remove saturated vapors, allowing the HEPA filters to
become saturated.
B. Furthermore, differential pressure instruments that were not failsafe
allowed the saturation to go undetected.
C. Water damage to the CAM probe caused it to fail and allowed the
release of Cesium 137 to go undetected.
Corrective Actions Taken
The following corrective actions were taken to preclude a future
occurrence of this type:
o Replacing the failed temperature recorder with a failsafe recorder.
Additionally, the start-up procedure was modified to require temperature
recorder calibration prior to start-up after any repairs to the instrument
as well as on an annual basis. Note, the immediate cause of this incident
was the temperature recorder/controller failing to maintain a constant
tank temperature, resulting in moisture overloading. The root cause was
an inadequate risk assessment which resulted in the selection of a
temperature/controller which could fail in an unsafe configuration.
o Installing a conductivity probe to detect moisture prior to any HEPA
filter and an in-line secondary HEPA filter for back-up.
o Water-proofing CAM probes.
o Installing failsafe differential pressure transmitters across the HEPA
filters.
o Installing interlocks to shut down Concentrate Transfer Tank exhaust fans
on hi-hi delta P on the HEPA filter, hi-radiation on the CAM or moisture
upstream of primary HEPA.
This incident points out the need to conduct adequate risk assessments in the
designs of systems and get appropriate personnel/supervisor(s) involved in
restarts. Special attention needs to be paid to the proper operation of
safety equipment before proceeding to the next step. Someone with a thorough
understanding of the system and process should have been involved.
Recommendations
Similar actions should be taken for your facilities where moisture may cause
failure of a HEPA filter and/or the associated monitoring equipment. With
particular respect to the installation of interlocks to shut down exhaust
fans, you may want to evaluate whether this is appropriate for your site.
Another action which you may want to consider is the use of cooling water.
Engineered safeguards could be added to control cooling rate (and filter
moisture loading) or the procedures could be modified to reduce or delete the
use of cooling water as allowable.
In addition to the specific actions taken in this particular and site-specific
incident, the broader based root causes would indicate the following general
recommendations, as appropriate and applicable:
o Conduct periodic preventive maintenance on equipment to preclude their
failure, especially safety equipment.
o Require start-up procedures to include: integrity inspection and
verification of calibration of safety equipment; operators notifying
supervisors of abnormal events so they can convene a review group to
determine probable cause and impact of such events; and, probable and
previous abnormalities and approved corrective actions taken in the past.
o Assure that training programs emphasize operator authority, abnormal
occurrence response procedures, and emergency response procedures.
Each facility may have site-specific needs and an evaluation on a case-by-case
basis may be necessary but proper failsafe controls and instruments are
required.
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This publication is one of several series of bulletins published so that DOE
program managers and contractors can share information about potential
occupational safety problems relevant to DOE operations. For more information
or additional copies, contact Eleanor Crampton, Performance Evaluation
Division, Office of Safety Compliance, Assistant Secretary for Environment,
Safety & Health, U.S. Department of Energy, Washington, DC 20545; telephone
FTS 233-3732, Commercial 301-353-3732.
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