05000220/LER-2003-001
| Event date: | 04-22-2003 |
|---|---|
| Report date: | 06-23-2003 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(i)(a) 10 CFR 50.73(a)(2)(i)(A), Completion of TS Shutdown 10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications |
| 2202003001R00 - NRC Website | |
I. Description of Event
On April 21, 2003, Nine Mile Point Unit 1 (NMP1), having recently ended a refueling outage, was at low power preparing for technical specification (TS) required testing of six solenoid-actuated pressure relief valves (also referred to as Electromatic Relief Valves or ERVs). At 2056 with power approximately 23 percent, TS required testing of the six ERVs began. At 2117 solenoid-actuated pressure relief valve, ERV-111, failed to open when the manual switch was taken to the open position. TS 3.1.5.a requires that Turing power operating condition whenever the reactor coolant pressure is greater than 110 psig and the reactor coolant temperature is greater than saturation temperature, all six solenoid-actuated pressure relief valves shall be operable" If TS 3.1.5.a is not 3.1.5.b requires that lithe itaator coolarT prifiure—aila the reactor coolant temperature shall be reduced to 110 psig or less and saturation temperature or less, respectively, within ten hours' TS 3.1.5.b was entered when ERV-111 failed to open. The testing of the remaining ERVs continued and was satisfactorily completed at 2212. At 2230, NMP1 began the TS required shutdown. At 0055 the reactor was brought sub-critical and at 0250 on April 22, 2003 NMP1 exited TS 3.1.5.b. During the cooldown and depressurization to less than 110 psig, the cooldown rate, as determined by reactor coolant temperature, in two of four recirculation loops, marginally exceeded 100 degrees Fahrenheit (F) per hour.
NMP1 has 6 ERVs, 3 on each main steam line. The ERVs are pail of the automatic depressurization system (ADS). Each ERV discharges to the suppression chamber. In the event of a small line break, the ERVs provide a means for depressurizing the reactor coolant system, allowing coolant injection by the core spray system.
The ERVs are pilot operated valves (Dresser Industries model 1525-VX). Energizing a solenoid opens the pilot valve. For ERV-111 the solenoid operated pilot valve Is SOV-01-102A (model CR9503-213C manufactured by General Electric). A red indicating light, when Illuminated, shows that the solenoid has stroked to open the pilot valve.
The ERV test (N1-ST-C2) consists of manually actuating the ERV with the reactor at pressure and then monitoring resultant system conditions. When the switch for ERV-111 was taken to the open position, the red Indicating light did not illuminate, which indicated that the SOV did not stroke. Additionally, temperature and acoustic monitoring data collected downstream of ERV-111 did not indicate that ERV-111 had opened.
The solenoid contains two operating coils, a low resistance coil and a high resistance coil. A built-in cut-out switch bypasses the high resistance coil when the solenoid Is not energized. Movement of the solenoid armature —opens the cut-out-switch -and-places the high resistance doll in series with the low resistance coil.
Troubleshooting Identified that high resistance in the cut-out switch contacts had prevented the solenoid from actuating.
During cooldown and depressurization, coolant temperature in two of four operating loops marginally exceeded the allowed maximum cooldown rate of 100 degrees F in one hour. The largest cooldown in either of these two loops was 101 degrees F in one hour and this cooldown rate lasted for approximately three minutes. In the other two operating loops the cooldown rates were 99 degrees F in one hour and 100 degrees F in one hour. The cooldown rate was reduced to less than 100 degrees F in one hour by securing auxiliary steam loads.
II. Cause of Event
The cause of the ERV failure to open was high resistance on the cut-out switch for solenoid valve SOV-01-102A which limited coil current and prevented the SOV from operating. The cause of the high resistance was an inadequate preventive maintenance procedure. Although the preventive maintenance procedure required cleaning the contacts, a measurement of contact resistance was not required.
NRC FORM WA /1-70r111 DOCKET (2) FACILITY NAME (1) LER NUMBER (6) PAGE (3)
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II. Cause of Event
The cause of exceeding the maximum allowed cooldown of 100 degrees in one hour, in two of four recirculation loops was due to procedural inadequacy. The shutdown procedure, N1-OP-43C, did not provide sufficient guidance to promptly secure steam loads to prevent exceeding a cooldown of 100 degrees F, when cooling down with low decay heat loads. A contributing cause was ineffective corrective action. A similar event occurred in May 1997, immediately following a refueling outage. A planned scram from 18 percent power resulted in a cooldown of 86 degrees F in a one hour period. Since the scram was from low power following a refueling outage, the decay heat load was low. An evaluation concluded that the plant response was to be expected for the operating conditions. The previous corrective action was not adequate to preclude recurrence.
III. Analysis of Event
The TS required shutdown of NAAP1 resulting from the failure of ERV-111 Is reportable in accordance with 10 CFR 50.73(a)(2)(i)(A) as a shutdown required by technical specifications. Additionally, the cooldown rate in excess of the TS allowed maximum of 100 degrees F per hour in two of the four operating recirculation loops, is reportable in accordance with 10 CFR 50.73(a)(2)(i)(B) as operation prohibited by Technical Specifications. TS 3.2.2, Minimum Reactor Vessel Temperature For Pressurization, specifies that during reactor vessel heatup and cooldown when the reactor is critical, the reactor vessel temperature and pressure shall satisfy the requirements of Figures 3.2.2.c and 3.2.2.d. Figure 3.2.2.d, Cooldown — Core Critical, is a plot of maximum reactor pressure versus reactor vessel belffine downcomer water temperature and is based upon cooling rates of less than or equal to 100 degrees F in one hour.
Figure 3.2.2.d specifies that temperature Is measured at the recirculation loop suction. Since the cooling rate measured at the recirculation suction for two of the recirculation loops marginally exceeded 100 degrees in one hour, the basis of Figure 3.2.2.d was not met.
Operation of three ERVs is sufficient to depressurize the primary system to 110 psig, which will permit full flow of the core spray system within required time limits. Five of the six ERVs satisfactorily passed their surveillance test.
Therefore five ERVs were operable, providing sufficient depressurization capability.
A qualitative risk evaluation concluded that, based on the risk achievement worth, ERV-111 failing to open was of low risk significance.
Engineering evaluated the impact of the cooldown with the following considerations:
1. Reactor coolant temperature Is used to define vessel Inner diameter (ID) temperature.
2._ The thermal.analysis_assumes adiabatic conditions on the vessel outer diameter (OD) and very high heat transfer coefficient on the vessel ID.
3. Realistic vessel heat transfer coefficients will create a lag time between vessel coolant and vessel ID surface conditions.
A review of the heat transfer coefficients assumed in the analysis compared to realistic values indicated that sufficient lag time exists such that the vessel inner surface would not exceed the cooldown limit of 100 degrees F in one hour, given that the coolant cooldown rate reached 101 degrees F in a one hour period for a maximum of 3 minutes.
Additionally, vessel OD surface thermal couple data confirmed that the vessel OD surface temperature change was approximately 50 degrees F coincident with the recirculation suction temperature change of 100 degrees F in one hour. The OD temperature data confirmed that significant margin relative to the assumed 100-degree through-wall thermal gradient remained. The Engineering evaluation concluded that the cooldown of the vessel inner surface did not exceed 100 degrees F In one hour, 10 CFR 50 Appendix G requirements were not violated, and the overall structural integrity of the reactor vessel was not compromised.
Based on the above, the failure of ERV-111 and subsequent cooldown did not pose a threat to the health and safety of plant personnel or the public.
IV. Corrective Actions
1. Replaced the SOV for ERV-111.
2. Measured cut-out switch contact resistance and Inspected contact coating for the remaining five ERVs and cleaned contacts as necessary.
3. The ERV preventive maintenance procedure was revised to include contact resistance measurement.
4. Revised procedure N1-OP-43C, Plant Shutdown, to provide additional guidance for securing steam loads to control cooldown rate.
5. Operator training will be provided on this event, including actions to address excessive cooldown 6. Initiatives are underway to Improve the effectiveness of the corrective action program, as a result of previously identified weaknesses In the corrective action program
V. Additional Information
1. Failed Components:
SOV-01-102A Model CR9503-213C Manufacturer General Electric 2. Previous similar events:
Licensee Event Report (LER)00-005 discusses a loss of secondary containment due to an inadequate procedure for checking track bay doors closed. The corrective actions are specific to the event. LER 00- 002 discusses an Instance In which a service water check valve failed a surveillance test due to inadequate preventive maintenance. The corrective actions were specific to check valves. The corrective actions for the events discussed above would not have prevented the ERV-111 failure or exceeding the TS allowed maximum cooldown of 100 degrees F in one hour.
3. Identification of components referred to In this Licensee Event Report:
Components IEEE 805 System ID � IEEE 803A Function Core Spray � BM � N/A Automatic Depressurization System � SB � N/A Reactor Coolant System � AD � N/A Main Steam System � SB � N/A Vessel � AD � RPV Valve � SB � V, RV Solenoid � SB � SOL Coil � SB � CL Switch � SB � N/A Contacts � SB � N/A