05000458/LER-2016-003

LER-2016-003, Operations Prohibited by Technical Specifications Due to Reactor Control Rod Drift During Core Alterations

River Bend Station - Unit 1
Event date:	1-19-2016
Report date:	03-17-2016
Reporting criterion:	10 CFR 50.73(a)(2)(i)(B), Prohibited by Technical Specifications
4582016003R00 - NRC Website

LER 16-003-01 for River Bend Station, Unit 1, Regarding Operations Prohibited by Technical Specifications Due to Control Blade Drift During Core Alterations

ML17163A208
Person / Time
Site:	River Bend
Issue date:	06/08/2017
From:	Vercelli S P Entergy Operations
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
RBF1-17-0067, RBG-47764 LER 16-003-01
Download: ML17163A208 (5)

Reported lessons learned are incorporated into the licensing process and fed back to industry.

Send comments regarding burden estimate to the FOIA, Privacy and Information Collections Branch (T-5 F53), U.S. Nuclear Regulatory Commission, Washington, DC 20555-0001, or by internet e-mail to Infocollects.Resource@nrc.gov, and to the Desk Officer, Office of Information and Regulatory Affairs, NEOB-10202, (3150-0104), Office of Management and Budget, Washington, DC 20503. If a means used to impose an information collection does not display a currently valid OMB control number, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.

REPORTED CONDITION

On January 19, 2016, at 5:28 a.m. CST, while conducting core alterations, an alarm was actuated in the main control room alarm indicating that a reactor control rod had drifted out of the fully inserted position. At the time, a fuel bundle was being raised out of the core, and the control rod in the same cell drifted out one notch with no "withdraw" command present. This condition actuated a corresponding alarm on the refueling platform, and system interlocks stopped the platform hoist with the fuel bundle partially withdrawn.

One subset of instrumentation in the core neutron monitoring system is the intermediate range monitors (IRM). The function of the IRMs, as required by Technical Specifications, is to actuate a trip of the reactor protection system upon a "high flux" signal, as might occur during a reactor startup. This function required to be operable only in MODE 5 (refueling) if all reactor control rods are not fully inserted, and in MODE 2 (reactor startup). Since it was not anticipated that the plant would enter the applicability for that IRM function until near the scheduled time of reactor startup, the surveillance requirement for that function had not been performed to verify operability at the time of this event. When the control rod drifted out of the fully inserted position, the applicability of the "high flux" trip function was inadvertently entered. As the function was not operable at the time, this event constituted operations prohibited by Technical Specifications.

INVESTIGATION and IMMEDIATE CORRECTIVE ACTIONS After a detailed assessment of the situation, the fuel bundle was manually lowered back to into the core. The system interlock preventing any intentional control rod motion was temporarily bypassed, and the control rod was fully re-inserted with a normal "insert" command at 11:58 a.m. The bypass on the control rod motion interlocks was then removed.

Later on the same day, the surveillance test procedures for six IRM channels were successfully performed, re-establishing operability of the "high flux" function. The drive mechanism for the control rod has been disabled, and the control rod will remain fully inserted for the remainder of the current fuel cycle.

The causal analysis for this event will be completed when the control rod can be removed for inspection during the next refueling outage. The results of that investigation will be provided in a supplement to this report.

SAFETY SIGNIFICANCE

River Bend Technical Specifications require that adequate shutdown margin exists at all times. It is required that the reactor core is subcritical (or at any time can be made so) by a specified margin of a percentage of the total reactivity in the core. One of the base assumptions of the shutdown margin calculation is that the control rod with the highest reactivity is fully withdrawn. In this event, one control rod moved out of the fully inserted position by only one notch. Therefore, the assumption of the shutdown margin calculation was not violated, and the calculation remained fully bounding.

No other control rods were affected. The fuel bundle and the control rod were both returned to their original positions. 'This event was, thus, of minimal safety significance to the health and safety of the public.