05000285/LER-1986-001, :on 860702,reactor & Turbine Generators Tripped on Low Steam Generator Level After safety-related Inverter Failure.Caused by Loss of 120-volt Ac Instrument Power to Electrohydraulic Control Unit.Power Restored
| ML20212G451 | |
| Person / Time | |
|---|---|
| Site: | Fort Calhoun  |
| Issue date: | 08/02/1986 |
| From: | Adams J, Andrews R OMAHA PUBLIC POWER DISTRICT |
| To: | NRC OFFICE OF ADMINISTRATION (ADM) |
| References | |
| LER-86-001, LER-86-1, LIC-86-404, NUDOCS 8608130523 | |
| Download: ML20212G451 (4) | |
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U.S. NUCLE 4.4 kEGULATORY COMMISBN)88 (9431 AM110VED 0e40 NO. 3190-4104 LICENSEE EVENT REPORT (LER)
DOCKET NutsetR (2i FAGE G PACILITY NAMG (1)
Fort Calhoun Station, Unit No.1 o 1s10101o1218 15 1 loFl 013 ftTLE le Reactor Trip Caused by Instrument Inverter Failure EVENT DATE ISI LER NuesetR to)
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, n.1 On July 2,1986, at 0534, while the plant was operating at 100% power, the reactor and turbine generator were automatically tripped on low steam generator level after the failure of a safety related instrument inverter. The trip was initiated by a closure of the turbine control valves which was caused by the loss of 120 VAC instrument power to the electrohydraulic control unit. Other consequences of the loss of power included inoperability of the steam dump and bypass valves and failure of the feedwater valve rampdown circuitry.
These failures combined to cause an abnormal post-trip pressure transient in the reactor coolant system. Alternate power was quickly restored to the failed I
instrument bus and, within a few minutes, the plant was placed in a normal hot I
shutdown condition.
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we,., ass. nm During normal operation at 100% power, an instrument inverter trouble alarm was received in the control room at 0534 on July 2,1986. Control room operators quickly diagnosed a failed instrument inverter feeding bus AI-40A and dispatched an equipment operator to the switchgear room to manually reenergize the bus by closing the breaker on a bypass transformer also feeding bus AI-40A. The inverter failure placed the Reactor Protective System (RPS) in a half-trip condition since the RPS operates on a two-out-of-four logic and the failed inverter was one of four feeding the independent channels of the RPS. About ten seconds after the inverter failure, a reactor trip occurred when a second channel trip was received on the steam generator B low level trip unit.
Several unusual transients were noted in the moments following the trip:
1.
Reactor Coolant System pressure increased to approximately 2400 psia for a short period of time.
This caused PORV's to be actuated.
2.
Steam generator pressure increased to the setpoint of the secondary safety valves causing them to be actuated.
3.
Overfeeding the steam generators resulted in abnormally high level and subsequent overcooling of the primary system. As a result, RCS pressure decreased to a low of about 1725 psia 4.
Steam dump and bypass valves could not be opened.
5.
The operating charging pump stopped and the two backup pumps could not be started.
Within a minute of the reactor trip, the equipment operator had reenergized the lost instrument bus and control room operators were soon able to restore the plant to normal shutdown condition.
A diagnosis of computer and recorder information revealed the following explanation for transients seen after the inverter failure. The deenergized instrument bus AI-40A supplies power to EHC panel AI-50 with no alternate power.
A turbine first stage pressure transmitter which sends a signal to the EHC load control circuitry is powered from Al-50.
Loss of power caused a loss of signal to the load control unit.
This ultimately resulted in the turbine control valves going closed without a reactor trip.
The load rejection before reactor trip explains the high pressure seen in the primary system and the low steam generator level earlier in the transient.
The high steam generator pressure was due to the loss of a heat removal path that occurred when inverter power was lost to the steam dump and bypass controllers. Also, a relay which energizes to open the steam dump valves on a turbine trip receives its power from AI-40A.
These valves again became operable when power was restored to the bus.
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LICENSEE EVENT REPORT (LER) TEXT CONTINUATION A> aoveo ous No uso-oio.
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_nc n.,, mar,m n The overfeeding of the steam generators after the trip occurred because the main feedwater regulating valves did not rampdown as they normally would. A relay which energizes after a trip to initiate the rampdown is powered from AI-40A and therefore did not perform its function when the reactor tripped.
Operator action
was required to restore steam generator level to normal once the overfeeding was discovered.
The loss of the inverter also caused deenerization of relays which control the operation of backup charging pumps.
The loss of these relays would have shutdown the backup charging pumps if they had been operating (they were not).
The operating pump should have kept running unless the charging pump selector switch was not in its normal position. When AI-40A was reenergized, all chargir.g pumps regained operability.
Investigation of this problem is continuing.
The loss of a safety related inverter does not put the plant in an unsafe condition. There are four such inverters which supply power to the four channels of the Reactor Protective System and to the Engineered Safety Features logic.
Loss of a single inverter cannot prevent these systems from performing their functions.
It is apparent, however, that the failure of inverter A led to several undesirable consequences.
Several steps were taken to reduce the probability of this incident being repeated.
These steps are summarized below.
An emergency modification was performed to transfer the power supply for AI-50 from bus AI-40A to bus AI-42B which is supplied by an inverter which has automatic transfer to alternate power. This reduces the probability that the initiating scenario will be repeated on the loss of an inverter.
This modification was completed before the plant was returned to power. An Engineering Evaluation Assistance Request has been initiated to study the desirability of redistributing the loads on AI-40A so that the loss of a single inverter would not have all the consequences as described above.
The failure of an inverter due to a problem on the DC-DC converter has been seen before in other instrument inverters at Fort Calhoun Station, but this is the first time that such a failure has caused a reactor trip or any other kir. af adverse consequence.
Consultation with the vendor has been taking place to determine how to improve the reliability of the inverters.
The loss of load aspect of this event was compared to the loss of load analysis in the Updated Safety Analysis Report (USAR).
The values of critical parameters during the transient were, in all cases, more conservative than those predicted in the USAR.
A similar loss of load trip occurred on May 28, 1976, in which the turbine control valves partially closed while the plant was operating at 100% power.
That event was not caused by a loss of inverter power.
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Omaha Public Power District 1623 Hamey Omaha. Nebraska 68102-2247 402/536 4000 August 5, 1985 LIC-86-404 U. S. Nuclear Regulatory Commission Document Control Desk Washington, D. C. 20555 Reference: Docket No. 50-285 Gentlemen:
Licensee Event Report for the Fort Calhoun Station Please find attached Licensee Event Report 86-01 dated July 2,1986.
This report is being submitted per requirements of 10 CFR 50.73.
Sincerely, R. L. Andrews Division Manager Nuclear Production RLA/DJM/me Attachment cc:
J. E. Gagliardo Reactor Projects Branch U. S. Nuclear Regulatory Commission Region IV 611 Ryan Plaza Drive, Suite 1000 Arlington, Tx. 76011 Mr. D. Sells, NRC Project Manager Mr. P. H. Harrell, NRC Senior Resident inspector INP0 Records Center American Nuclear Insurers SARC Chairman PRC Chairman Fort Calhoun File (2)
Licensee Contact b
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