ML20154R892
| ML20154R892 | |
| Person / Time | |
|---|---|
| Site: | Portsmouth Gaseous Diffusion Plant |
| Issue date: | 10/16/1998 |
| From: | Jonathan Brown UNITED STATES ENRICHMENT CORP. (USEC) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GDP-98-2046, NUDOCS 9810270219 | |
| Download: ML20154R892 (5) | |
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USEC A Global Energy Company October 16,1998 GDP 98-2046 United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001 Portsmouth Gaseous Diffusion Plant (PORTS)
Docket No. 70-7002 Event Report 97-22, Revision 3 The subject event report revision is being submitted to correct an administrative error. Enclosed is the revised 30 day written Event Report 97-22, Revision 3. This event report is being revised to change the completion date of corrective action #4. Due to the administrative error, the date February 4,1998, was inadvertently reported as the date of completion. The actual completion date was February 27,1998. The change to this report is marked with a vertical line in the right margin.
Should you require additional information regarding this events, please contact Scott Scholl at (740) 897-2373.
Sincerely, M
M
. Morris Brown General Manager Portsmouth Gaseous Diffusion Plant Ericlosures: As Stated T
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NRC Region III Office
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NRC Resident Inspectors -PORTS 2
c- ::s 9810270219 981016 PDR ADOCK 07007002 C
PDR 110. Box 800, lurtsmouth, Oli 45661 Telephone 740-897-2255 Fax 740-897-2644 http://www.usec.com Offwes in Livermore, CA Paducah, KY Portsmouth, OH Washington, DC
1 6
GDP 98-2046 Page1of4 Event Report 97-22 Revision 3 Description of Event On October 25,1997, at 1800 hours0.0208 days <br />0.5 hours <br />0.00298 weeks <br />6.849e-4 months <br />, X-344 Autoclave (AC) #3 was in Mode 11, heating a Uranium llexafluoride (UF ) cylinder. The cylinder had been heating for approximately 25 minutes when the 6
operator noted that pressure indicator PI-165 indicated an internal UF cylinder pressure of-50 psia.
6 Since a reading of-50 psia indicated the instrument had malfunctioned, the operator immediately initiated steam shutdown manually, utilizing the local steam isolation controls to place the autoclave in a shutdown condition until the cause of the -50 psia instrument indication could be determined.
After the faulty PI-165 reading was first observed on October 25,1997, Maintenance personnel initially observed that PI-165 was reading the expected pressure when they arrived to investigate.
Maintenance personnel checked the instrument loop and determined that the "as-found" instrument readings for the loop were within tolerance. While Maintenance personnel were performing closcout activities they noticed that the PI-165 cylinder pressure, PI-169 sample loop pressure and PI-186 manifold pressure were all reading -50 psia. Further investigation indicated that the power cord to the transducer cabinet, which supplies power to all three systems, was loose. The plug was tightened and all PI readings returned to normal. Maintenance personnel believed that the loose power cord was the cause for the faulty PI readings. Operations then perfonned the Cylinder Iligh Pressure Test and Cylinder Low Pressure Test successfully and returned the autoclave to service.
On October 27,1997, Operations Management recognized that the condition that caused the erroneous reading on PI-165 may have also caused the UF Cylinder High Pressure Autoclave Steam 6
Shutoff (CIIPASS) safety system to be inoperable. A similar pressure indicator failure at X-343 AC
- 7 had occurred on October 22,1997, and caused the CHPASS to be inoperable (reference Event Report 97-21 Rev 2). Since investigation activities were unable to determine if the CHPASS would J
still function with the identified deficiencies, it was determined that the system may not have been able to perform its design function as a result of this event. The failure of the CilPASS safety system is reportable in accordance with 10 CFR 76.120(c)(2).
The iailure of the CHPASS system is also reportable in accordance with NRC Bulletin 91-01. The CHPASS system provides one of two criticality controls relied on to maintain double contingency where the potential to introduce excess moderation into the autoclave from an incoming cylinder exists. In the unlikely event that the first control on UF,, impurity content is not maintained during cylinder filling, excess Hydrogen Fluoride (HF), a moderator, could be present in the cylinder. The CHPASS system provides a second control because the additional cylinder pressure caused by the presence of excess llF will cause the CHPASS to interrupt the heating cycle before a cylinder rupture occurs. In this event, the first criticality control was maintained. The cylinder cold pressure check, which was done prior to cylinder heating, provided assurance that there were no excess
t GDP 98-2046 Page 2 of 4 Event Report 97-22 Revision 3 impurities in the cylinder which could cause cylinder over pressurization during heating.
The CIIPASS system is a single channel system. The heating of a UF cylinder containing an 6
excessive amount of' light' gases at normal heating temperatures could result in the intemal cylinder pressure exceeding the hydrostatic test pressure and possibly create a UF release in the autoclave.
6 The safety system function of the CllPASS ensures the pressure in the cylinder does not exceed the maximum allowable working pressure of the lowest rated cylinder that could be heated in the autoclave. The UF cylinder pressure instrument loop is required to alarm if the cylinder pressure at any time reaches 115 psia.
Cause of Event The direct cause of the event was inadequate electrical connections between the pressure transducer and the CIIPASS safety system circuitry. During investigation activities conducted by Maintenance and Engineering,it was observed that movement of the wiring and terminal strip that connects the pressure transducer to PI-165 caused the PI to read -50 psia. This terminal strip also provides the connections between the pressure transducer and the CI1 PASS circuitry. Although it is not known for certain that the loose electrical connections affected the CllPASS circuitry, the investigation was not able to prove that the CIIPASS was operable when this condition was discovered. As a result, it was conservatively determined that the loose wiring could have prevented the CIIPASS from performing its design function.
PI-165 is the pressure indicator for cylinder pressure on X-344 AC #3. The cylinder pressure loop contains a pressure transmitter, a pressure transducer, a pressure indicator, and high and low pressure switches. The pressure transmitter is located on the UF drain line just outside the head of the autoclave. The pressure transmitter converts UF pressure to a mV signal. The pressure transducer 6
converts the mV signal to a mA signal and consist of two cards. One card converts the mV signal to 0-5 Volts DC. The other card converts 0-5 Volts DC to 4-20 mA and contains the high and low pressure switches. The pressure indicator converts the mA signal to a pressure reading in psia.
Engineering determined that a -50 psia reading on PI-165 was a result of a signal loss to the Pl. The PI design is such that it interprets a O mA reading as -50 psia. The signal loss can result from inadequate electrical connections from the transducer to the PI, signal loss within the transducer, a loss of signal within the pressure transmitter or a power loss to the pressure transducer.
The loss of power to the transducer was eliminated as a possible cause because the CIIPASS did not alarm when the faulty PI-165 reading was first observed. The design of the system will cause an alarm if power is lost. The pressure transmitter was also eliminated as a cause because the
GDP 98-2046 Page 3 of 4 Event Report 97-22 Revision 3 transmitter performed as designed through all pressure ranges during testing. The transmitter is a mechanical device and any failure of the transmitter would be repeatable.
During the investigation Engineering and Maintenance observed thatjiggling the wiring and terminal strips that connect the transducer to PI-165 would cause the readings to change from -50 psia to the expected reading. The associated wiring was re-terminated and the two transducer boards were replaced as a precautionary measure. ARer calibration was perfomied on the equipment, PI-165 was again observed to read -50 psia. Since replacement of the transducer cards did not correct the problem, it was suspected that the problem was caused by faulty electrical connections.
The root cause for the event was determined to be inadequate design of the transducer housing mounting which resulted in faulty electrical connections. During the investigation it was noted that the transducer housing was placed on a small pedestal without any method of attachment. To access the terminal strips on the transducer, the housing is turned or rotated on the pedestal. It was observed that when the transducer was rotated, erroneous readings were obtained on PI-165.
The lack of pemianent mounting also allowed stress to be placed on the terminal strips. The wires exit from the bottom of the housing such that the weight of the housing is allowed to exert force on the wires. This creates stress on the wires that can degrade the electrical connection. The transducer cards also plug into the terminal strip and could be affected by movement of the wires. With this type ot' arrangement, any movement of the transducer housing can affect the integrity of the electrical connections. Since the transducer connections are part of the CIIPASS safety system. it was concluded that the CIIPASS may have been affected by this deGeiency.
An engineering walkdown was conducted to determine if this mounting deficiency exists at other autoclave installations. No additional mounting problems were noted.
Corrective Actions 1.
The autoclave cylinder pressure transducer cards and terminal strip connecting the transducer to PI-165 were replaced. Pressure indicator PI-165 and its associated wiring were replaced.
These actions were completed by October 30,1997.
2.
On October 30,1997, the pressure transducer housing was temporarily mounted to the pedestal table to prevent inadvertent movement and to prevent the housing from exerting force on the wires and terminal connections.
3.
On February 4,1998, a design modification was completed to provide a permanent mounting of the UF6 cylinder pressure transducer housing.
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i GDP 98-2046 Page 4 of 4 Event Report 97-22 Revision 3 4.
By March 26,1998, a design modification will be installed to provide permanent mounting i
of the UF. cylinder pressure transducer housing. This action was completed on February 27, i
i 1998.-
i Extent of Exposure ofIndividuals to Radiation or Radioactive Materials There were no exposures to individuals from this incident to radiation or radioactive materials.
Lessons Learned
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Safety systems must be properly mounted and secured to ensure they will remain capable of performing their design function.
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