05000263/LER-2002-001, Re Mechanical Pressure Regulator Failure Causes Reactor Scram

Re Mechanical Pressure Regulator Failure Causes Reactor Scram

ML020910636
Person / Time
Site:	Monticello
Issue date:	03/15/2002
From:	Forbes J Nuclear Management Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
LER-02-001-00
Download: ML020910636 (5)

LER-2002-001, Re Mechanical Pressure Regulator Failure Causes Reactor Scram

Event date:
Report date:
2632002001R00 - NRC Website

text

No Committed to Nuclear Excellen Monticello Nuclear Generating Plant Operated by Nuclear Management Company, LLC March 15, 2002 10 CFR Part 50 Section 50.73 US Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555 MONTICELLO NUCLEAR GENERATING PLANT Docket No. 50-263 License No. DPR-22 LER 2002- 001 Mechanical Pressure Regulatory Failure Causes Reactor Scram A Licensee Event Report for this occurrence is attached. This report contains no new NRC commitments.

Contact Doug Neve, Licensing Project Manager, at (763) 295-1353 if you require further i lmation.

Jeffrey S. Forbes Site Vice President Monticello Nuclear Generating Plant Enclosure c:

Regional Administrator - IlIl NRC NRR Project Manager, NRC Sr. Resident Inspector, NRC Minnesota Department of Commerce 2807 West County Road 75

• Monticello, Minnesota 55362-9637 Telephone: 763.295.5151

• Fax: 763.295.1454 I-

Abstract

While operating at 100% power at 1735 on January 21, 2002, a turbine control valve fast closure (load rejection) signal resulted in a reactor scram. All rods fully inserted and all safety systems functioned as designed. The primary cause of the scram was failure of the main turbine pressure control system. A detailed review of plant computer data revealed that the mechanical pressure regulator (MPR) had been behaving erratically for several days prior to the scram. This erratic behavior eventually caused the MPR to take control from the electric pressure regulator. This initiated rapid cycling of the turbine control and bypass valves which tripped both protection system sub-channels on reduced hydraulic oil pressure at the control valve acceleration relay.

Investigation determined that failure of the MPR was caused by a damaged rate feedback bellows. Following repair of the MPR, and completion of other unrelated maintenance, the unit was returned to service at 1327 on January 27, 2002.

NRC FORM 366 (7-2001)

(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 Forn 366A) (17) investigation after replacement of the bushing assembly revealed an abnormal temporary spiking behavior in the MPR piston.

The erratic behavior of the MPR piston was determined to be a faulty rate feedback bellows. The bellows was found to have a 2-inch crack, another smaller crack, and a pin hole. Discussions with General Electric confirmed that these defects would affect the dampening characteristic of the MPR and cause the erratic behavior which led to the scram. An undocumented modification made to the rate feedback bellows in 1973, in which clamp bars were soldered to the bellows to adjust its spring rate, may have contributed to this failure.

The root cause of this event was determined to be failure to perform adequate preventative maintenance on the MPR.

Corrective Actions

The rate feedback bellows was replaced with a new bellows obtained from another plant. The new bellows meet the original design specifications (without the clamp bars).

Other MPR components were inspected and cleaned. Oil samples were obtained and found to meet specifications. As a precaution, the MPR steam pressure sensing lines were flushed. Linkages and switches were inspected and checked. It is believed that none of these other components contributed to failure of the MPR.

In the future, the MPR piston position will be monitored and trended by the system engineer using the plant process computer. Existing preventive maintenance practices on the MPR will be reviewed and improvements made where indicated.

The affect of the loose primary valve stop adjustment found during the investigation of this event will be investigated for possible impact on the plant transient analyses.

Failed Component Identification General Electric Force-Restored Pressure Regulator, Rate feedback bellows GE Technical Manual GEK-17955, Dwg 945D 604, rev 0 (modified)

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