05000530/LER-2004-002, Regarding an Automatic Reactor Trip on Low DNBR Following a Main Turbine Control System Malfunction

Regarding an Automatic Reactor Trip on Low DNBR Following a Main Turbine Control System Malfunction

ML042180167
Person / Time
Site:	Palo Verde
Issue date:	07/30/2004
From:	Danni Smith Arizona Public Service Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
192-01147-DMS/SAB/DJS LER 04-002-00
Download: ML042180167 (6)

LER-2004-002, Regarding an Automatic Reactor Trip on Low DNBR Following a Main Turbine Control System Malfunction

Event date:
Report date:
5302004002R00 - NRC Website

text

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LAFAS A

hsubidisary oqfhniacle 1hfst Capital Corporation Palo Verde Nuclear Generating Station David M. Smith Plant Manager Nuclear Production Tel.

623-393-6116 Fax.

623-393-6077 e-mail: DSMITH10@apsc.com I OCFR50.73 Mail Station 7602 P.O. Box 52034 Phoenix, AZ 85072-2034 192-01147-DMSISABIDJS July 30, 2004 ATTN: Document Control Desk U. S. Nuclear Regulatory Commission Washington, DC 20555-0001

Subject:

Palo Verde Nuclear Generating Station (PVNGS)

Unit 3 Docket No. STN 50-530 License No. NPF-74 Licensee Event Report 2004-002-00 Attached please find Licensee Event Report (LER) 50-530/2004-002 -00 that has been prepared and submitted pursuant to I OCFR50.73. This LER reports an automatic reactor trip on Low DNBR following a main turbine control system malfunction.

In accordance with 1 OCFR50.4, a copy of this LER is being forwarded to the NRC Region IV Office and the Senior Resident Inspector. If you have questions regarding this submittal, please contact Daniel G. Marks, Section Leader, Regulatory Affairs, at (623) 393-6492.

The corrective actions described in this LER are not necessary to maintain compliance with regulations. Arizona Public Service Company makes no commitments in this letter.

Sincerely, DMS/SAB/DJS/kg Attachment cc:

B. S. Malleft M. B. Fields N. L. Salgado NRC Region IV Regional Administrator NRC NRR Project Manager NRC Senior Resident Inspector for PVNGS I5E

APPROVED BY OMB NO. 3150-0104 EXPIRES 7-31.2004

Abstract

On June 7, 2004 at approximately 1458 hours0.0169 days <br />0.405 hours <br />0.00241 weeks <br />5.54769e-4 months <br /> (MST), Unit 3 was operating in Mode 1, Power Operations, at approximately 99 percent power when the main turbine intercept valves fast closed and the control valves ramped closed. The reactor power cutback and steam bypass control systems (RPCB and SBCS) responded to the large decrease in steam flow by inserting group 4 and 5 control rods into the core and opening steam bypass valves to reduce reactor power and maintain reactor coolant temperature. Approximately 10 seconds after the RPCB occurred, a low DNBR reactor trip was initiated by the core protection calculators (CPCs) as a result of rodded radial peaking factor for the control rod configuration.

The cause of the main turbine valves closure is believed to be a control system malfunction in the speed control circuit. The cause of this malfunction has not been determined and the system is currently under a monitoring program/investigation program. The cause of the CPC reactor trip is still under investigation.

Subsequent to this event, a similar event occurred when Unit 2 experienced a main turbine trip, RPCB, and subsequent reactor trip on low DNBR on July 14, 2004.

NRC FORM 366 (7.)

(If more space Is required, use additional copies of (If more space Is required, use additional copies of (if more space Is required, use additional copies of (if more space is required, use additional copies of NRC Form 366A)

The cause of the reactor trip has been attributed to a diminished DNBR margin as a result of the rodded radial peaking factor for the control rod configuration that existed following the RPCB. The cause of this decrease in margin remains under investigation.

If APS determines that the cause(s) are something other than a fault with the speed, control system or a diminished DNBR margin induced CPC reactor trip, then a supplement to this report will be submitted.

7.

CORRECTIVE ACTIONS

Although the root cause of the EHC malfunction has not been identified, the following actions were taken:

Replaced 2 analog load control boards Replaced 4 analog speed control boards Replaced 1 logic speed control board Tightened the backup speed probe amphenol connector Replaced the primary and backup speed probes Currently, a data acquisition recorder is attached to the turbine EHC control cabinet in an attempt to capture data that will help determine the cause of failure.

APS is evaluating whether the Radial Peaking Factors and Rod Shadowing Factors for Control Rod Groups 4 and 5 (inputs to the CPC DNBR calculation) should be reduced to alleviate plant trips post RPCB events.

8.

PREVIOUS SIMILAR EVENTS

Subsequent to this Unit 3 event a similar event occurred when Unit 2 experienced a loss of the main turbine from approximately 100 percent power and a subsequent RPCB on July 14, 2004. In that event a reactor trip also occurred on low DNBR. That event will be submitted in another LER.