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On October 24, 2017 at 1350 EDT, with the plant in Mode 1 at 100 percent power, H. B. Robinson Steam Electric Plant, Unit No. 2 (HBRSEP2), discovered an uncompensated security vulnerability that could have allowed unauthorized or undetected access to a vital area (VA) of the plant. Compensatory measures were promptly established and an immediate investigation was conducted, which did not reveal any evidence of unauthorized entry or tampering.

The cause of this event is attributed to the absence of work instructions to contact security to establish compensatory measures prior to beginning work activities high on a VA barrier wall. The vulnerability was the result of authorized work. All workers involved were validated to have proper permissions to access the area. No unauthorized access to the VA occurred during the time period that the vulnerability existed. This event did not have any nuclear or personal safety implications.

This report is being submitted in accordance with the requirements of 10 CFR 73.71(d), which refers to Appendix G, "Reportable Safeguards Events," Section 1(c).

At 1302 hours Eastern Daylight Time (EDT) on 10 08 2016 with the plant in Mode 1 at 100 percent power, H. B. Robinson Steam Electric Plant, Unit No. 2 (HBRSEP2), experienced a grid perturbation. As a result, HBRSEP2 experienced a reactor trip due to low voltage on the 4kV buses. Plant safety systems responded with the emergency buses separating from offsite power due to emergency bus undervoltage. The emergency diesel generators (EDG) started and powered the 480V emergency buses. 'A' service water pump did not start on the blackout sequencer; however, sufficient service water flow was available from the three operating service water pumps.

This failure did not aggravate this event. The site declared an Unusual Event (UE) at 1317 EDT for loss of power to emergency buses.

At 0011 EDT on 10 09'16, the UE was terminated.

Once the power grid was stable, plant personnel commenced restoration of offsite power to allow shutdown of the the EDGs. During this evolution, at approximately 2323 EDT on 10 . 08 2016, an automatic actuation of the 'B' auxiliary feedwater (AFW) pump occurred due to improper breaker coordination that satisfied the autostart logic for the AFW system.

The apparent cause of the voltage transient in the HBRSEP2 switchyard is a failed fault detection relay, which prevented the grid fault from being immediately isolated. The failed relay has been replaced.

The prevention of flood intrusion on plant grade at H. B. Robinson Steam Electric Plant, Unit No. 2 (RNP), is achieved through lake level control that is performed by two electrically-operated Tainter Gates under normal operation. On 06/062016 and 07 07 2016 with RNP in Mode 1 at 100 percent power, Tainter Gates B and A were respectively identified as degraded due to the failure to meet their functionality test requirements (excessive binding of the the lift gate chains due to corrosion). RNP License Renewal program requires full travel test for the radial arm Tainter Gates to demonstrate functionality of the gates and identify any issues with operation. The Tainter Gates have been repaired, tested and rendered compliant with their original licensing basis function. On August 11, 2016 (ENS #52174), it was determined this event or condition placed RNP in an unanalyzed condition that significantly degrades plant safety.

No actual Probable Maximum Precipitation Design Basis Event (PMP DBE) occurred during the period of time the Tainter Gates were in a degraded condition. Thus, this LER only considers the loss of the Emergency Diesel Generators (EDGs) and the resulting Extended Loss of Alternating Power (ELAP), if a PMP DBE had occurred. This report is submitted in accordance with the requirements of 10 CFR 50.73(a)(2)(v)(A) and 10 CFR 50.73(a)(2)(v)(D), "Prevention of Fulfillment of Safety Function," and 10 CF CFR 50.73(a)(2)(ii) (B), "Degraded or Unanalyzed Condition.

At 1540 hours EST on 01/19/2016 with H. B. Robinson Steam Electric Plant, Unit No. 2, in Mode 1 at 100 percent power, Motor- Driven Auxiliary Feedwater (MDAFW) pump "B" failed its surveillance test due to flow switch FSL-1633B failure to indicate service water flow to the Oil Cooler and Packing Channel Cooler. It was discovered that flow was blocked due to stem and disc separation on the MDAFW pump "A" and "B" cooling flow return isolation valve, SW-115. This blockage rendered both MDAFW trains inoperable for a period longer than allowed by plant Technical Specifications (TS). TS 3.7.4, Auxiliary Feedwater (AFW) System, requires four AFW flow paths and three AFW pumps be operable in Modes 1, 2, and 3, and Mode 4 when steam generator is used for heat removal, allowing 24 hours to restore one of two inoperable MDAFW pumps to operable status. Since two MDAFW pumps were inoperable due to common cause for approximately 52 hours, this condition is reportable as a condition prohibited by TS under 10 CFR 50.73(a)(2)(i) (B), and reportable under 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability of Independent Trains or Channels. The total duration of both trains of MDAFW being unavailable was limited to approximately 3 hours.

An investigation concluded that the failed valve (SW-115) was likely installed in poor condition, without documentation, and outside of design specifications prior to 1978. The valve has been replaced with a valve meeting current design specifications. The health and safety of the public was not jeopardized as a result of this condition.

At 1957 hours EST on 01/28/2015 with H. B. Robinson Steam Electric Plant, Unit No. 2 in Mode 1 at 100 percent power, the plant entered Improved Technical Specifications (ITS) Limiting Condition for Operation (LCO) 3.0.3 due to inoperability of both trains of the Reactor Protection System (RPS). It was discovered that modification work performed during the Fall 2013 refueling outage inadvertently connected in parallel both safety trains of the RPS and both trains of the DC Electrical Distribution System (DC-EDS). This parallel connection rendered both of these systems inoperable because the required independence and redundancy of systems was eliminated.

This condition was present for a time greater than allowed by technical specifications (TS) and is reportable as a condition prohibited by TS under 10 CFR 50.73(a)(2)(i)(B). This condition also rendered inoperable one train of multiple systems designed to mitigate the consequences of an accident and is reportable under 10 CFR 50.73(a)(2)(vii).

Immediate actions taken to restore compliance with regulations included completion of an emergency Work Order that restored the independence of safety trains of both systems and permitted exit of ITS LCO 3.0.3 at 2048 on 01/28/2015.

The condition was entered into the site corrective action program and a root cause investigation is in progress.

At 0041 hours EST on 11/05/2013, with the Unit in Mode 2 and startup low-power physics testing in progress, there was an inadvertent automatic actuation of the Auxiliary Feedwater System due to an 'A' Main Feed Pump trip. While placing the Condensate Polishers in service, a secondary-side perturbation occurred resulting in the loss of the running 'A' Main Feedwater Pump on low suction pressure coincident with low flow. There was no plant damage or personnel injury as a result of this event. An 8-hour, non-emergency notification was made to the NRC per 10 CFR 50.72(b)(3)(iv)(A) due to the valid actuation of the Auxiliary Feedwater System (EN# 49502).

The root cause evaluation has concluded that the cause of this event was an operating error by one individual involving procedure use and adherence.

Corrective actions consist of completion of confidential personnel actions for the Makeup Water Treatment/Condensate Polisher Auxiliary Operator responsible for the event, and enhancement of expectations for supervisory oversight of risk significant evolutions will be employed.