ML20195G957
| ML20195G957 | |
| Person / Time | |
|---|---|
| Site: | Paducah Gaseous Diffusion Plant |
| Issue date: | 11/16/1998 |
| From: | Pulley H UNITED STATES ENRICHMENT CORP. (USEC) |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| GDP-98-1074, NUDOCS 9811230199 | |
| Download: ML20195G957 (7) | |
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USEC.
A Global Energy Company November 16,1998 GDP 98-1074 United States Nuclear Regulatory Commission Attention: Document Control Desk Washington, D.C. 20555-0001 Paducah Gaseous Diffusion Plant (PGDP)
Docket No. 70-7001 Event Report ER-98-26 Pursuant to 10CFR76.120(d)(2), attached is a voluntary 30-day written report for the safety limit exceedance on the No. I Normetex Pump in the C-315 Tails Withdrawal Facility. This was initially reported on October 18,1998 (NRC No. 34926). Investigation activities are continuing. The target date for a supplemental report to be issued is January 25,1999.
Any questions regarding this matter should be directed to Larry Jackson at (502) 441-6796.
Sincerely,
/
ow ulley
{
General Manager Paducah Gaseous Diffusion Plant
Enclosure:
As Stated cc: NRC Region III Office i
NRC Resident Inspector - PGDP l
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K 001 C
110. Box 1410, Paducah, KY 42001 1
Telephone 502-441-5803 Fax 502-441-5801 http://ww.asec.com j
OfTices in 1.ivermore, CA Paducah, KY Portsmouth, OH Washington, DC l
i Docket No. 70-7001 GDP 98-1074 Page 1 of 6 EVENT REPORT ER-98-26 DESCRIPTION OF EVENT On October 17, 1998, at approximately 1730 hours0.02 days <br />0.481 hours <br />0.00286 weeks <br />6.58265e-4 months <br />, a stlenoid valve failed which caused the discharge block valve to close resulting in the unplanned shutdown of the on-stream No.1 Normetex pump in the C-315 Tails Withdrawal Facility. This resulted in an automatic shutdown of the pump motors and a rapid, but brief, discharge pressuie increase on the pump before the pump rotation came to a complete stop. A discharge pressure of approximately 50 pounds per square inch absolute (psia) was recorded on the system's monitoring computer. This value exceeds the 45 psia Safety Limit (SL) defined for the Normetex pump discharge bellows pressure, per Technical Safety Requirement (TSR) 2.3.2.1. The Plant Shift Superintendent (PSS) declared the No.1 Normetex Pump High Discharge Pressure System inoperable at 2105 hours0.0244 days <br />0.585 hours <br />0.00348 weeks <br />8.009525e-4 months <br /> on October 17,1998. The NRC Senior Resident Inspector was notified of the event. On October 18,1998, at 0415 hours0.0048 days <br />0.115 hours <br />6.861772e-4 weeks <br />1.579075e-4 months <br />, a voluntary notification was made to the Nuclear Regulatory Commission (NRC) informing them of the SL exceedance. NRC assigned Worksheet No. 34926 to this event notification.
A pre rious SL exceedance occurred on August 26,1998, when the C-315 No. 2 Normetex pump tripped due to an unplanned closure of the discharge block valve. A discharge pressure of approximately 46 psia was recorded. A voluntary 24-hour notification was made to NRC, followed by a voluntary 30-day written report of the event investigation (ER-98-23). The investigation determined that the design of the Normetex Pump High Discharge Pressure System was inadequate to preclude transient pressures from exceeding the 45 psia SL. As a result of that event, coupensatory actions were put in place to minimize the risk of exceeding the 45 psia SL. The compensatory measures included stationing an operator at the Normetex pump control panel to monitor and control pump suction and discharge pressures below procedurally implemented limits.
The operator was not expected to take actions to prevent the transient in the event of a discharge valve closure, but to ensure that the steady-state operating pressures would meet the initial conditions necessary to avoid exceeding the SL. Engineering Evaluation EV-C-821-98-018, "Normetex Pump Pressure Limitations," documented the assumptions and calculations for the determination of these pressures limits. Limits for the C-315 No. I Normetex pump were established at 3.7 psia maximum suction pressure and 29.5 psia maximum discharge pressure.
The operator monitoring the C-315 No.1 Normetex pressures on October 17,1998, reported a suction pressure of 3.5 psia and a discharge pressure of 27.5 psia were being maintained, which were below the procedurally defined limits. He heard the general building alarm and determined from the control panel annunciators that the No.1 Normetex pump had shut down and its air-operated l
suction and discharge valves had closed. The operator then opened the discharge vent valve to evacuate the remaining UF in the pump. The operator did not observe a pressure increase prior to 6
the Normetex pump shutdown alarm, t
l Docket No. 70-7001 l
GDP 98-1074 Page 2 of 6 l
In response to the Normetex pump shutdown alarm, a second operator went to the pump to check l
for local control. panel alarms, which might indicate the cause of the pump shutdown. The pump l
main drive motors were tripped, but no local control panel alarms were actuated. There was no observable damage to the pump or expansion joint bellows or out-leakage of UF. Operations in the 6
vicinity of the pump were restricted to preserve evidence, with the exception that manual suction l
and discharge block valves were closed to permit long term isolation. When isolated the pumps do not contain significant quantities of UF (less than five pounds). Recognizing that the SL may have 6
been exceeded, at 1917 hours0.0222 days <br />0.533 hours <br />0.00317 weeks <br />7.294185e-4 months <br /> on October 17,1998, all operating Normetex pumps were placed in standby and the plant was placed on recycle.
On October 18,1998, the United States Enrichment Corporation (USEC) submitted to NRC a Request for Enforcement Discretion related to TSR 2.3.2.1. The enforcement discretion was needed to allow cascade operations to retum to normal operations, even though the plant could not demonstrate that the Normetex Pump High Pressure Discharge System would always function to i
meet the TSR 2.3.2.1 operability requirements for the Normetex pump discharge bellows pressure.
The justification for this request was based on the low safety significance associated with a failure of the discharge expansionjoint. Similarjustification was provided in the Certificate Amendment Request (CAR) which was submitted on September 11,1998, and is currently in the review / approval process. The Certificate Amendment will delete TSR 2.3.2.1, "Normetex Pump Discharge Pressure," and TSR 2.3.3.1," Normetex Pump Discharge Pressure System."
On October 18,1998, at 0700 hours0.0081 days <br />0.194 hours <br />0.00116 weeks <br />2.6635e-4 months <br />, NRC orally issued USEC a Notice of Enforcement Discretion (NOED) which allowed the plant to return to normal operations but required the compensatory actions defined in the earlier NOED to remain in effect to minimize any pressure transients in the Normetex pump discharge lines. At 0720 hours0.00833 days <br />0.2 hours <br />0.00119 weeks <br />2.7396e-4 months <br /> on October 18, 1998, the PSS declared the Normetex Pump High Discharge Pressure Systems operable, in accordance with the NOED. The No.1 Normetex pump remained inoperable, pending determination of cause of the pump shutdown.
NRC issued a written NOED on October 20,1998. The NOED stated the intention to continue to exercise discretion not to enforce compliance with TSR 2.3.2.1 until issuance of the certificate amendment.
Engineering Evaluation EV-C-821-98-026 was prepared, with concurrence by the Plant Operations Review Committee, to ensure all actions required by TSR 1.6.1.2 were met prior to restart of the No.
1 Normetex pump. TSR 1.6.1.2.a and b did not require action because of the nature of the event.
l TSR 1.6.1.2.c requires a technical evaluation be conducted to determine if any damage has occurred l
and to evaluate the ability of the system to be restarted. This Engineering Evaluation concluded that there was no damage to the C-315 No.1 Normetex pump discharge expansion joint as a result of the momentary SL exceedance.
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l Docket No. 70-7001 i
GDP 98-1074 Page 3 of 6 CAUSES OF EVENT A.
Direct Cause The direct cause of the pump discharge pressure increase was the closure of the discharge block valve on the running pump. The Normetex pump is basically positive-displacement in nature, and compression of gas into a blocked discharge line results in a rapid pressure rise. The discharge block valve design uses air pressure to actuate a diaphragm that opens the valve. Loss of air pressure closes the valve. Air is supplied to the valve via three solenoid valves arranged in series such that each must be energized to enable air to be supplied to the discharge block valve. If any of the three solenoids de-energizes, or fails closed, air is vented and the discharge block valve closes. In this case, solenoid valve PY-212Cl, the first valve in series from the plant air source, failed.
This failure mode was confirmed during testing associated with the event investigation.
When PY-212Cl was energized, it chattered loudly and vented significant amounts of air out ofits vent port. If the solenoid valve had been operating properly, this port would have been closed off when the valve was energized. The discharge block valve opened, but one to two minutes later it closed, indicating that PY-212Cl was not maintaining sufficient air pressure to the valve to keep it open. A short time later the test was repeated, but the solenoid valve had degraded to the point that it seized, keeping the block valve closed.
Solenoid valve PY-212Cl is a non-safety-related component of standard commercial quality.
j It is estimated that this solenoid had been in service 5-8 years. The safety function of the Normetex pump protective systems involves shutdown and isolation of the pump, thus loss of air to the Discharge Block Valve does not compromise this function. The solenoid valve was removed and disassembled for inspection and evaluation of the failure mode. The interior of the valve was covered with a soot-like deposit. The soot-like material would have interfered with the seating of the valve and is considered the cause of the solenoid valve failure.
A laboratory analysis of the solenoid valve was performed and is documented in detail in KY/L-2039," Failure of ASCO Solenoid Valve in C-315." This analysis determined that the soot-like deposit was mostly silicone and iron. The iron came from wear on the solenoid core, an expected result of chattering. The silicone does not appear to have come from any part of the solenoid valve or valve lubricant. The air supply port of the solenoid valve was free ofsignificant dust, whereas the downstream ports on the discharge block valve side and j
the vent port were coated with the soot-like material. It is thus likely that silicone-based foreign material entered the valve each time it experienced reversed flow (i.e., a venting or valve closure operation). The most likely source is the discharge block valve air-operator 1
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Docket No. 70-7001 GDP 98-1074 Page 4 of 6 diaphragm, which is fiberglass-reinforced silicone rubber. Further inspections are planned to confirm this hypothesis or else identify another source of the foreign material.
Neither the solenoid valves nor the discharge block valve air-operator diaphragm have periodic replacement or internal cleaning / inspection preventative maintenance assigned, since these components were considered to fail safe. This policy will be reevaluated, due to the significance of the undesirable pressure transient experienced when the discharge block valve closes inadvertently. Several degraded solenoid valves, exhibiting minor leaking past the vent port, have been noted in this application, thereby raising suspicion that there may be a common cause. As an interim measure, the solenoid valves in this application at each Normetex pump were examined for vent port leakage. One leaking valve was identified and replaced to minimize risk of another inadvertent discharge block valve closure. The investigation is continuing and will focus on determining the source of the foreign material, defining the extent it impacts other components, and initiating corrective actions to address the issues.
B.
Root Cause The cause of the safety limit exceedance on October 17,1998, was that design of the High Pmssum Discharge System and the safety-related discharge block valve interlock trip signal was inadequate to prevent the transient pressures from exceeding 45 psia, even with the reduced suction and discharge pressure limits defined by the Engineering Evaluation EV-C-821-98-018. The suction pressure limit bounds the pump flow rate. The discharge pressure limit sets the baseline pressure from which a transient begins. The magnitude of the transient itself was empirically determined based on the August 26,1998, event and an earlier pressure excursion on July 7,1998, which did not exceed the SL. Due to the pumps being similar in design, the analysis assumed that observations of discharge pressure increase could be generically applied to any Normetex pump installation. A weakness exists in this assumption. The analysis assumes that the discharge block valve closes rapidly, thereby allowing the steady-state operating pressure to be used as a transient basis (i.e., no pmssure increase during valve closure). If the mode of discharge block valve system failure involves partial or slow closure of the block valve, then an incremental pressure increase occurs before the pump trips. This incremental pressure increase must be added to the steady-state operating pressure to get the transient basis.
1 The post-event tests confirmed that the failure was not complete, since the discharge block valve could be reopened for a time. By nature of the air-operator design (air to open/ spring to close), the application of partial air pressure can mid-position a valve. The interlock trip, assumed to operate in the analysis, requires full closure of the valve to actuate the pump shutdown circuit. Thus, if the solenoid valve vented significant air pressure from the discharge block valve, but not enough to fully close it, the discharge line would be
o Docket No. 70-7001 GDP 98-1074 Page 5 of 6 significantly blocked and cause a pressure increase. Based on a reconstructed peak discharge pressure of approximately 50 psia, it is evident that any pressure increase due to partial valve closure was limited to only 5 to 10 psi, which is the amount above the predicted pressure per the compensatory action calculations. This pressure increase is estimated to have occurred very rapidly, on the order of a few seconds.
At some point, either the High Discharge Pressure System, or, if the valve closed fully, the Discharge Block Valve Interlock, would have tripped the pump. The interlock trip was active and likely initiated the pump trip, based on the pressures observed. However, it is indeterminate whether the interlock or the high pressure trip actually tripped the pump.
A supplemental report on this event is targeted for January 25,1999.
CORRECTIVE ACTIONS A.
Corrective Actions Taken 1.
On September 11,1998, USEC submitted a CAR to NRC to justify the deletion of the TSR 2.3.2.1 SL and the TSR 2.3.3.1 Normetex Pump High Discharge Pressure System as TSR requirements.
2.
On October 17,1998, Systems Engineering examined the solenoid valves in the same application at each Normetex pump in the C-315 Tails Withdrawal Facility for vent port leakage. One leaking valve on the No. 3 Normetex pump was identified. This valve was replaced on November 6,1998.
3.
On October 19,1998, Maintenance replaced solenoid PY-21..-Cl for the discharge valve on the No.1 'Normetex pump in the C-315 Tails Withdraws! Facility.
4.
On November 10,1998, Systems Engineering examined the solenoid valves in the same application at each Normetex pump in the C-310 Product Withdrawal Facility for vent port leakage. No leaking valves were identified.
B.
Corrective Actions Planned To Be Determined 1
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Docket No. 70-7001 GDP 98-1074 Page 6 of 6 EXTENT OF EXPOSURE OF INDIVIDUALS TO RADIATION OR 'IO RADIOACTIVE MAI tiKIALS None -
' LESSONS LEARNED To be determined -
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