05000237/LER-2001-004
| Dresden Nuclear Power Station Unit 2 | |
| Event date: | 10-21-2001 |
|---|---|
| Report date: | 12-20-2001 |
| 2372001004R00 - NRC Website | |
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Dresden Nuclear Power Station Unit 2 05000237 (If more space is required, use additional copies of NRC Form 366A)(17)
A. Plant Conditions Prior to Event:
Unit: 02 � Event Date: 10-21-2001 Reactor Mode: 1 � Mode Name: Refueling Reactor Coolant System Pressure: 0 psig
B. Description of Event:
Event Time: 0500 Power Level: 0 percent This event is being reported in accordance with 10 CFR 50.73 (a)(2)(i)(B), which requires the reporting of any operation or condition, which was prohibited by the plant's Technical Specifications.
On August 15, 1968, drawings were released which initially established the Torus High Level Switches setpoints for Dresden units 2 and 3. A modification was performed on May 29 1985, which installed new Narrow Range Torus Water Level instrumentation. When this modification was completed, the unit 2 switches' setpoints were both left at an undetermined level. The 1985 modification had no requirement to perform Post Modification Testing (PMT) of the Torus High Water Level Switches.
During the Improved Technical Specification (ITS) development process, on January 12, 2000, site personnel noted that Current Technical Specification (CTS) 3/4.2.B-1.3.d allowed Water Level switches. The TS required a monthly Channel Functional Test (CFT). However, TS states that a channel calibration was "N/A". Channel calibration was "N/A" because the switches were not adjustable. Also, as mechanical devices, these Magnetrol float level switches do not experience drift. Therefore, there has never been a verification that the switch actuates at or below the TS Allowable Value (AV). Operability of the Torus High Level swap of the HPCI Suction to Torus function was supported as all TS requirements were being met. The CFT verified that the instrument performed its safety function.
On July 28, 2000, calculation DRE00-0032, "HPCI Suppression Pool Level Setpoint Error Analysis", was issued to support the ITS conversion. It assumed the Analytical Limit to be 15 feet-8.25 inches and established the TS AV at 15 feet-5.625 inches with the Setpoint at 15 feet-5 inches, the setting tolerance at +/- 0.5 inches, the extended tolerance at +/- 0.5 inches, and the CAL Frequency at 24-months. The assumed AL of 15 feet - 8.25 inches, which was overly conservative based on the original construction design specification calculation, resulting in an overly conservative TS AV. In retrospect, the Standard Improved Technical Specification Basis for this function is to prevent damage to the Suppression Pool (Torus) during Safety/Relief Valve operation. Other calculations, DRE 98-0155 and DRE 98-0135, state that the limit for Safety/Relief Valve operation during accident conditions is 18 feet-6 inches. This Analytical Limit (AL) supported the existing field setpoints, shown in the design drawings.
Therefore, DRE00-0032 was approved with the 15 feet - 8.25 inches AL assumption.
On September 9, 2000, an ITS submittal was made for NRC approval with DRE00-0032's new 24-month CHANNEL CALIBRATION requirement for the Torus High Water Level switches and increased TS 3.3.5.1-1 (formerly 3/4.2.B-1.3.d) function 3.e Allowable Value of approval to implement. NRC approval was obtained for the new surveillance, which was required to be performed within the frequency of 24 months following implementation.
On October 21, 2001 the Instrument Maintenance Department (IMD) performed SR 3.3.5.1.5, 24-month calibration for the first time on unit 2, which was approximately six months following implementation of ITS. HPCI LICENSEE EVENT REPORT (LER) f33), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, And Budget, Washington, DC 20503. If an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
FACILITY NAME (1) DOCKET NUMBER (2) LER NUMBER (6 PAGE (3) Dresden Nuclear Power Station Unit 2 05000237 NUMBER NUMBER (If more space is required, use additional copies of NRC Form 366A)(17) On October 21, 2001 the Instrument Maintenance Department (IMD) performed SR 3.3.5.1.5, 24-month calibration for the first time on unit 2, which was approximately six months following implementation of ITS. HPCI Torus High Level Switches 2-2351-A and 2-2351-B failed. Switch 2-2351-A and B initially and repeatedly tested at 15 feet-6.0 inches, 0.375 inches above the TS AV.
Design Engineering issued a modification package to change the Unit 2 HPCI Torus High Level Switches 2-2351- A and 2-2351-B piping configuration and setpoints. The modification was implemented on November 2, 2001 and successfully tested on November 4, 2001. IMD repeatedly tested the switches at 15 feet — 3.375 inches, 2.25 inches below the TS AV. This successfully completed SR 3.3.5.1.5, 24-month calibration of the Torus High Water Level Switches. Due to the modification performed on the Torus high water level switches and setpoint calculation switches were verified to be within acceptable ranges during there surveillance testing.
C. Cause of Event:
The root cause of the event was determined to be historically poor Post Modification Testing and an overly conservative Technical Specification Allowable Value.
D. Safety Analysis:
The overly conservative analytical limit is assumed to be 15 feet — 8.25 inches resulting in a corresponding overly conservative allowable value of 15 feet — 5.625 inches. These values are to protect the Torus from excessive loading from safety relief valve operation that may exist during accident conditions. The maximum as found setting of 15 feet — 6.0 inches is below the analytical limit of 18 feet — 6 inches as determined by General Electric calculations. Based on the as-found setting of 15 feet — 6.0 inches, the HPCI swap to the Torus would have been accomplished without compromising the safety of the public or the station. Therefore the safety significance of this event is minimal.
E. Corrective Actions:
The calculation was revised to change the existing setpoint.
The existing modification process was evaluated to ensure adequate measures were in place to prevent recurrence of this type of event.
The maintenance department performed a modification to lower the Torus High Water level switches due to failed surveillance test.
F. Previous Occurrences:
A review was conducted via a search of previous condition reports generated. No previous occurrences were identified associated with this type of event.
G. Component Failure Data:
N/A