05000301/LER-2011-004
| Point Beach Nuclear Plant Unit 2 | |
| Event date: | 06-13-2011 |
|---|---|
| Report date: | 10-13-2011 |
| Reporting criterion: | 10 CFR 50.73(a)(2)(iv)(A), System Actuation |
| Initial Reporting | |
| ENS 46957 | 10 CFR 50.72(b)(2)(iv)(B), RPS System Actuation |
| 3012011004R01 - NRC Website | |
Description of the Event
On 06/13/11 at 1924 CDT, during beginning-of-life (BOL) physics testing with the reactor critical in MODE 2, an automatic reactor trip occurred while operators were inserting a shutdown bank during dynamic rod worth testing of Shutdown Bank A control rods. At approximately 1915 CDT, while inserting the Shutdown Bank A control rods from 228 steps to 2 steps, the Group 2 portion of the Shutdown Bank A step counter [STC] stopped at 152 steps, while the Group 1 portion of the step counter continued to insert.
Based upon the potential misalignment of the control rods, rod motion was stopped to evaluate the step counter condition. As a result of the previous rod insertion, there was a negative startup rate and reactor power was decreasing through the intermediate range.
The evaluation resulted in the decision to manually trip the Unit 2 reactor and to suspend BOL physics testing in order to determine the cause of the apparent step counter discrepancy. Approximately 30 seconds after the decision was made to manually trip the reactor and during preparations to manually open the reactor trip breakers, an automatic reactor trip occurred as a result of the power level dropping below the P-6 interlock. At the P-6 interlock, both source range nuclear instruments automatically energize, and the source range high flux reactor trip (greater than 1.5 x 10-E5 cps on one of two instruments) is unblocked.
Both source range nuclear instruments [DET] energized; however, one of the instruments immediately pegged high on counts and voltage. When this occurred, the source range high flux reactor trip logic (one-out-of-two) [JD] was satisfied, and a reactor protection signal actuated that resulted in the automatic reactor trip. Actions were taken by Operations in accordance with emergency operating procedures to stabilize the reactor in MODE 3 and a post-trip event investigation was initiated.
On June 14, 2011 at 2055 CDT, EN 46957 was amended to state the reactor was subcritical at the time of the trip. Subsequent review of this situation by NextEra resulted in a determination that at the time the test sequence was initiated, the reactor was critical and in MODE 2. Accordingly, the non-emergency report was amended on October 12, 2011, to reflect EN 46957 as originally submitted on June 13, 2011, at 2229 CDT.
This license event report is being reported in accordance with 10 CFR 50.73(a)(2)(iv)(A) and (B), "... an event or condition that resulted in manual or automatic actuation of the Reactor Protection System including reactor scram or reactor trip.
This event was not a safety system functional failure. The failure of the 2N31 source range detector high voltage power supply resulted in the automatic trip of the Unit 2 reactor on the required one-out-of-two logic.
Cause of the Event
The cause of the event was the failure of the nuclear source range detector high voltage power supply.
Analysis of the Event
Plant systems functioned as required following the automatic reactor trip. Control rods fully inserted into the core. Neither the emergency core cooling system nor the auxiliary feedwater system actuated; the emergency diesel generators did not start; and power continued to be supplied from off-site sources. The reactor coolant system had forced circulation following the trip and the atmospheric steam dump valves were used for decay heat removal from the steam generators.
Analysis of Safety Significance The source range neutron flux trip protects against reactivity excursions during reactor startup from subcritical conditions proceeding into the power range. The reactor is tripped when one-out-of-two source range channels are above the trip setpoint. This trip, which provides protection during reactor startup, can be manually blocked when one-out-of two intermediate range channels are above the P-6 permissive setpoint.
There are also two intermediate range channels that provide protection to the reactor. When both of these intermediate range channels are below the P-6 permissive setpoint, the reactor trip is automatically reinstated. The intermediate range trip is automatically blocked when two-out-of four high power range signals are above the P-10 permissive setpoint of approximately 10% power. The source range trip setpoint is between the P-6 permissive setpoint (P-6 allows the manual de-energization of the source range high voltage power supply) and the maximum source range power level detection limit.
When the source range detector failed, the one-out-of two logic to trip was established, resulting in the automatic reactor trip. Since the reactor was subcritical, the reactivity transient was small and the potential for core damage was minimal. Thus, the safety significance of the event was low. There was no impact on the health and safety of the public or plant personnel as a result of this event. There was no radiological release nor any release to the environment. Accordingly, the nuclear safety significance of this event was low.
Corrective Actions
The high voltage power supply for the source range detector was replaced. The root cause evaluation for this event determined a life cycle management plan had not been established for the periodic replacement of nuclear instrumentation high voltage power supplies. The preventive maintenance program has been revised to perform periodic replacement of the power supplies. Completion of the corrective actions to develop the life cycle management program and replace the high and low voltage power supplies is being tracked in the site corrective action program.
Similar Events There were no similar reportable events.
Component Failure Data
Manufacturer Description Model Number Power Designs High Voltage Power Supply UPMD-X54W-M1