05000336/LER-2008-004
| Millstone Power Station - | |
| Event date: | 05-24-2008 |
|---|---|
| Report date: | 07-14-2008 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(B), System Actuation 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| LER closed by | |
| IR 05000245/2008004 () | |
| 3362008004R00 - NRC Website | |
1. Event Description On May 24, 2008 at 09:38 while at less than 0.01% power in Mode 2, the Millstone Power Station Unit 2 (MPS2) reactor automatically tripped due to a loss of normal power (LNP) event. At the time of the LNP, reactor startup was in progress, the reactor was critical and power was below the point of adding heat. Plant electrical power was supplied from the reserve station service transformer (RSST) [EA, XFMR]. The LNP was caused when the low-side supply breakers [BKR] from the RSST to the 4160 volt (4.16KV) and 6900 volt (6.9KV) buses unexpectedly opened.
Opening the supply breakers to the buses removed power from the reactor coolant pumps (RCP) [P] and control element drive mechanism motor-generator (CEDM MG) sets. As a result, automatic reactor trip signals were initiated on low reactor coolant flow and low RCP speed. Loss of power to the CEDM MG sets resulted in control rods inserting into the reactor core. The emergency diesel generators started and loaded as expected. The Engineered Safety Feature Actuation System responded as expected for a loss of power event.
This reactor trip meets the Performance Indicator criteria for "Reactor Trip with Complications" since after completing standard post trip recovery actions, operators entered into the emergency operating procedure for "Loss of Offsite Power/Forced Circulation".
The operators' response to the event was affected by the repeated cycling of the 120 volt AC non-vital instrument panel (VR11) between the normal and alternate power supplies. This system provides AC power to controllers and instrumentation for non-safety related components. While the system is not vital to the safe shutdown of the unit, it affects plant operation. The repeated cycling led to spurious alarms, repeated starts of the non-running charging pumps, letdown isolation and erroneous indication of main steam safety valves opening.
(MPS3) instrument air was established within 16 minutes. Air system pressure remained within acceptable ranges.
Also, there was an increase in containment sump level due to leakage from the RCP vapor seals. The RCP vapor seal leakage was re-directed to the volume control tank at 10:40. Both trains of containment radiation monitors were rendered inoperable by the loss of power. The "A" train containment radiation monitor was restored at 11:25.
Offsite power was restored to the "C" 4.16KV vital AC bus at 10:35 on May 24, 2008. Offsite power was restored to the "D" 4.16KV vital AC bus at 12:30 on May 25, 2008. Core heat removal via natural circulation was established and maintained throughout the event until approximately 11:30 on May 25, 2008 when the "B" and "D" reactor coolant pumps were restarted.
This event is being reported pursuant to 10 CFR 50.73(a)(2)(iv)(A) as an event that resulted in manual or automatic actuation of systems listed in 10 CFR 50.73(a)(2)(iv)(B).
2. Cause The most probable cause for the reserve station service low-side supply breakers opening is a spurious signal that was not sufficiently filtered by the primary audio tone circuitry due to degradation of the tone generation/filtering circuitry.
The RSST protective circuitry design is that an audio tone trip actuation from a single channel results in a signal to open the reserve station service low-side supply breakers. This spurious signal resulted in a primary audio tone trip actuation, which caused the low-side supply breakers to open.
The purpose of the RSST audio tone circuitry is to isolate power to the RSST should a fault be detected in the transmission lines to or from the RSST. If an electrical fault is detected on the high side (345KV) of the RSST, a signal is sent to an audio tone transmitter in the switchyard, which sends a signal to an audio tone receiver in the main control room (MCR). The receiver then sends a signal to the breakers on the low side (6.9KV and 4.16KV) of the RSST to trip open. Similarly, if an electrical fault is detected on the low side of the RSST, a signal is sent to an audio tone transmitter in the MCR, which sends a signal to an audio tone receiver in the switchyard. The receiver then sends a signal to the breakers on the high side of the RSST to trip open.
3. Assessment of Safety Consequences This event is considered to be of low safety significance. The loss of normal power to the 6.9 KV buses caused a coast down of the reactor coolant pumps. As expected, a reactor trip signal was generated on low reactor coolant pump speed and low reactor coolant system flow. Since the reactor was operating at less than 0.01% power prior to the trip, there was no approach to the specified acceptable fuel design limits, including DNBR (Departure from Nucleate Boiling Ratio). Both emergency diesel generators started and loaded as expected. Offsite power was restored to the "C" 4.16KV vital AC bus at 10:35 on May 24, 2008. Offsite power was restored to the "D" 4.16KV vital AC bus at 12:30 on May 25, 2008. Charging and letdown were utilized to maintain RCS inventory control and auxiliary spray was utilized to control RCS pressure. Core heat removal via natural circulation was established and maintained throughout the event until approximately 11:30 on May 25, 2008 when the "B" and "D" reactor coolant pumps were restarted. RCS heat removal was maintained using auxiliary feed water and the atmospheric dump valves. Containment heat removal was maintained by the containment air recirculation cooling units. There was no challenge to any of the fission product barriers. Based on the above, there was no adverse effect on the health and safety of the public.
4. Corrective Action Since the most probable cause was a spurious signal within the primary audio tone circuitry, the primary audio tone circuit has been disabled. A modification is being developed to improve the reliability of the audio tone circuit.
Note: The backup audio tone circuit is in service to isolate power to the RSST should a fault be detected in the transmission lines to or from the RSST.
Longer term corrective actions are being addressed in accordance with the Millstone Corrective Action Program.
5. Previous Occurrences No previous similar events or conditions were identified.
Energy Industry Identification System (EIIS) codes are identified in the text as [XX].