Information Notice No. 84-55: Seal Table Leaks at PWRs
NUCLEAR REGULATORY COMMISSION
OFFICE OF INSPECTION AND ENFORCEMENT
WASHINGTON, DC 20555
July 6, 1984
Information Notice No. 84-55: SEAL TABLE LEAKS AT PWRs
All power reactor facilities holding an operating license (OL) or
construction permit (CP).
This information notice is provided as notification of a potentially generic
problem involving reactor coolant leaks from incore probe seal tables. The
leaks have occurred during maintenance on these systems while the reactor
coolant system was at an elevated pressure and temperature. The events
described have resulted in personnel hazards and also degraded plant safety.
It is expected that recipients will review the information for applicability
to their facilities and consider actions, if appropriate, to preclude a
similar problem occurring at their facilities. However, suggestions
contained in this information notice do not constitute requirements and,
therefore, no specific action or written response is required.
Description of Circumstances:
On January 20, 1984 (reported by LER No. 50-295/1984-005), a reactor coolant
leak was observed in the seal table room at Zion Generating Station Unit 1.
The unit was in hot shutdown with a plant heatup in progress. The reactor
coolant system temperature was 445F and pressure was 2,235 psi.
Inspection of the seal table by plant personnel revealed that a leak was
located at a point where the high-pressure seal mates to the conduit for
incore thimble E-11. An attempt to repair the leak was made when the system
pressure was reduced to 1,000 psi. These efforts reduced but did not stop
the leak. The system pres sure and temperature were reduced to 400 psi and
370F and another attempt to repair the leak was made. The repairmen
noticed a slight bowing between the high-pressure seal and the thimble
isolation valve. It was believed that this bowing caused the Swagelok
fitting to be improperly seated, thus causing the leak. To correct the
problem, two bolts holding the isolation valve to the valve bracket were
removed to allow straightening of the thimble tube. However, the two bolts
and bracket were the primary support devices holding the fitting in place.
When they were removed, the fitting broke loose causing an unisolatable
reactor coolant leak of approximately 18 gpm to the containment. The area
was immediately evacuated. Later, upon examination of the fittings, it was
found that the ferrules on all but seven of the thimbles had moved 1/32" to
3/8" up from their original position toward the edge of the conduits.
July 6, 1984
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Review of the procedure for assembly of the high-pressure and low-pressure
seals within the Swagelok fittings revealed that the low-pressure fittings
could pull up the ferrules causing improper fitting of the high-pressure
seals. This is believed to have caused the initial leak. Overtorquing of the
fittings during the initial attempt to correct the leak probably
overstressed the ferrule and allowed it to break loose when the restraint
Another event occurred at the Sequoyah Nuclear Plant Unit 1 (reported in LER
SQRD-50-327/84030). On April 19, 1984 workmen were in the seal table room
brush cleaning the incore probe thimble guide tubes. The reactor was
critical at 30% power and normal operating temperature and pressure.
The thimble guide cleaning procedure had made it necessary to disconnect the
thimble tubes leading from the seal table to the 10 path selection device
such that the high-pressure Swagelok fitting at the Seal Table was the only
device restraining the thimble. Because it was not known if the cleaning
brush was being fully inserted during the cleaning process, an unblocked
tube was used to obtain information on brush travel. The cleaning assembly
was installed and inserted 15 ft. into this tube before a shift change
occurred. Following the shift change personnel again began inserting the
brush. On the 78th turn (10 inch insertion per turn) the tool handler noted
that an increased pressure was needed to turn the crank. On the 79th turn
(approximately 80.8 ft. into the tube) the water was noticed on the seal
Water was noticed on the seal table around the Swagelok fitting holding the
guide tube that was being cleaned. This prompted an immediate evacuation of
the seal table room. Subsequently, the fitting broke loose ejecting the
entire thimble tube and cleaning equipment from the core. An unisolatable
reactor coolant leak ensued; the leak rate during this time was
approximately 30 gpm. The leak continued for approximately 11 hours until
the reactor was cooled down with the pressure reduced to nitrogen blanket
pressure and the reactor water level was reduced to a level below the seal
table. A total of approximately 16,000 gallons of reactor coolant leaked
into the containment.
Radiation surveys conducted the following day indicated 2-3 rem at the
entrance to the seal table room, 200-300 rem at the end of the tube near the
seal table and greater than 1,000 rem in the center of the ejected tube. The
tip of the thimble tube was reading approximately 4,000 rem.
A very difficult cleanup operation followed as well as an elaborate failure
evaluation and analysis which included mock-up tests, with identical
fittings, tubing, pressures and forces applied to the thimble tubing. The
tests simulated as near as practical the events at the time of the incident.
Based on the results of these tests, TVA concluded that the separation of
the failed assembly appears to be the result of extraordinary and
unanticipated loads on the assembly, caused by the cleaning fixture that was
being used when the separation occurred. The assembly mock-up testing
demonstrated that strains of considerable magnitude resulted from applied
forces on the manually operated fixture crank handle.
As a result of the tests, TVA recommends that the use of this tube cleaning
fixture in its present form be discontinued. If cleaning is required,
July 6, 1984
Page 3 of 3
modifications to this fixture or other techniques should be employed that do
not transmit bending forces through the fittings.
Even though the above incidents appear to be caused by different
circumstances, both events point out the need for adequate controls and
precautions to ensure personnel and plant safety while performing
maintenance on high-pressure systems, especially activities involving the
seal table. Both of these events occurred with the reactor at elevated
pressures and temperatures, and in the case of Sequoyah the reactor was at
30% reactor power. In both cases maintenance was conducted on a
high-pressure system with what was equivalent to single valve protection.
For both personnel and plant safety considerations, maintenance is not
normally conducted on high-pressure systems while at high-pressure and
temperature and with only single valve protection. To preclude the type of
events just described from occurring, every effort should be made to
schedule seal table maintenance during cold shutdown conditions. Also, the
need for maintenance of any system under hot, pressurized conditions should
be thoroughly evaluated before allowing personnel to perform the work.
Licensees are urged to review their maintenance procedures to ensure that
maintenance under these conditions is minimized.
No one was injured during these events and the operators brought the plants
to cold shutdown without undue problems. However, both of these events have
caused problems associated with the radiological cleanup efforts. In the
case of Sequoyah, a highly radioactive piece of equipment was ejected from
the core. This required that extraordinary measures be taken during the
decontamination of the room and the removal of the thimble. In both events,
decontamination of the room was required. Increased personnel exposure and
down time of the plant due to the cleanup and repair efforts provide
additional incentives for precautions against maintenance under similar
No written response to this notice is required; however, licensees should
review the information contained in this notice for applicability to their
facilities, especially if plants have performed maintenance on the incore
probe guide tubes under similar plant conditions.
If you have any questions regarding this matter, please contact the Regional
Administrator of the appropriate NRC regional office or the Technical
Contact listed in this notice.
Edward L. Jordan Director
Division of Emergency Preparedness
and Engineering Response
Office of Inspection and Enforcement
Technical Contact: D. R. Powell, IE
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