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| Entered date | Event description | |
|---|---|---|
| ENS 51928 | 13 May 2016 20:02:00 | At 1200 (CDT) May 13, 2016, while the plant was operating at 100% power, it was brought to the attention of the River Bend Station Main Control Room staff that an existing design inadequacy could prevent both trains of the Standby Gas Treatment System (GTS) from performing its design function. Under certain specific conditions, the installed Masterpact breakers may not close to allow energization of the filter train exhaust fans. A start signal (reactor level 2, drywell pressure 1.68 psid, annulus high radiation, annulus low flow) combined with a trip signal within a certain time differential, could result in a failure of the breakers to close. As a result of this condition, both Standby Gas Trains were declared inoperable, which required entry into LCO 3.6.4.3 Condition C (requires entering Mode 3 in 12 hours). Declaring both trains of Standby Gas Treatment System inoperable resulted in loss of the safety function since a system that has been declared inoperable is one in which the capability has degraded to the point where it cannot perform with reasonable expectation or reliability. The Standby Gas Treatment System (GTS) limits release to the environment of radioisotopes, which may leak from the primary containment, ECCS systems, and other potential radioactive sources to the secondary containment under accident conditions. At 1240 (CDT) May 13, 2016, one division of GTS, GTS 'A', was manually started from the Main Control Room. This action prevents the breaker failure mode, restored the operability of one train and restored the safety function of the GTS system. LCO 3.6.4.3 Condition A (restore Operability in 7 days) is currently entered for Standby Gas Train 'B'. During the 40 minutes of inoperability, both trains of Standby Gas remained available. At no time was the health or safety of the public impacted. This condition is being reported in accordance with 10CFR50.72(b)(3)(v)(C) as an event that could have caused a loss of safety function to control the release of radioactive material. The Senior NRC Resident was notified.
Further review has determined that the design inadequacy discussed in EN #51928 could adversely effect the ability of the main control building heating, ventilation, and air conditioning (HVAC) system to perform its design safety function, based upon a particular sequence of events occurring within a short window of time (approximately 75 milliseconds). River Bend has implemented compensatory actions to ensure operability of the main control building HVAC system. The Resident Inspector has been notified by the licensee. Notified the R4DO (Miller). |
| ENS 51701 | 29 January 2016 23:00:00 | On January 29, 2016, at 1518 CST, with the plant in cold shutdown, power was lost on reserve station service (RSS) line no. 1. This is one of two sources of offsite power required by Technical Specifications. The power loss de-energized the Division 1 onsite AC safety-related switchgear, causing an automatic start of the Division 1 emergency diesel generator (EDG). The Division 1 reactor protection system (RPS) bus was also de-energized, causing a half-scram signal. Approximately 8 minutes later, a full actuation of the RPS occurred due to a high water level condition in the control rod drive hydraulic system scram discharge volume header. All reactor control rods were already fully inserted. The loss of Division 1 RPS also caused the actuation of the Division 1 primary containment isolation logic. The Division 1 isolation valves in the balance-of-plant systems closed as designed. Both trains of the standby gas treatment system actuated. The loss of RSS no. 1 occurred during post-modification testing on relays at the local 230kV switchyard. The exact cause of the event is under investigation. This event is being reported in accordance with 10 CFR 50.72(b)(3)(iv)(A). The unit remains in cold shutdown with 1 source of offsite power and all 3 (EDG) available. The (NRC) Resident Inspector has been notified. |
| ENS 51568 | 27 November 2015 09:23:00 | At 0431 CST on November 27, 2015, an automatic reactor scram occurred following the trip of the main generator. The generator trip was apparently caused by a partial loss of offsite power, which resulted from a differential ground on the north bus of the local 230 kV switchyard. The ground signal caused the reserve station service line no. 1 to de-energize, which tripped the Division 1 offsite power source to station, as well as the main generator. The plant responded as designed as follows: The Division 1 emergency diesel generator started and tied to the bus restoring Division 1 emergency power. The Division 3 emergency diesel generator started and tied to the bus, restoring power on the Division 3 switchgear. The reactor protection system tripped as designed. Reactor water level was controlled normally with condensate and feed water. A level 3 reactor water level scram signal occurred as expected, and RPV (Reactor Pressure Vessel) water level was restored to normal level band. Reactor pressure was controlled by the bypass valve system, and a normal cool down was initiated. The reactor is being taken to cold shutdown pending an investigation of the event. The loss of power also resulted in a partial loss of normal service water cooling to the plant, and emergency service water cooling automatically initiated per design. At the time of event, the reactor protection system was aligned to the backup power supply, which was momentarily lost. This resulted in multiple system isolations including reactor water clean up, and outboard balance of plant isolations. These isolations were initiated due to loss of offsite power, and all responded as designed. The isolation resulted in a loss of the running decay heat removal pump for the spent fuel pool. The standby pump is available for service and being aligned for service. The plant is currently stable in hot shutdown. Transmission and distribution personnel are currently investigating the ground in the 230 kV switchyard. All control rods inserted. The licensee has notified the NRC Resident Inspector. |