05000458/LER-2016-003, Regarding Operations Prohibited by Technical Specifications Due to Reactor Control Rod Drift During Core Alterations
| ML16098A312 | |
| Person / Time | |
|---|---|
| Site: | River Bend  |
| Issue date: | 03/17/2016 |
| From: | Maguire W Entergy Operations |
| To: | Document Control Desk, Office of Nuclear Reactor Regulation |
| References | |
| RBG-47667 LER 16-003-00 | |
| Download: ML16098A312 (4) | |
| Event date: | |
|---|---|
| Report date: | |
| 4582016003R00 - NRC Website | |
text
- ===-Entergy RBG-47667 March 17, 2016 U.S. Nuclear Regulatory Commission ATTN: Document Control Desk Washington, DC 20555
Subject:
Licensee Event Report 50-458 I 2016-003-00 River Bend Station - Unit 1 Docket No. 50-458 License No. NPF-47 RBF1-16-0033
Dear Sir or Madam:
Entergy Operations, Inc.
River Bend Station 5485 U.S. Highway 61 N St. Francisville, LA 70775 In accordance with 10 CFR 50. 73, enclosed is the subject Licensee Event Report.
This document contains no commitments. If you have any questions, please contact Mr. Joseph Clark at 225-381-4177.
Sincerely, tv~~
William F.~e Site Vice President Enclosure cc:
U. S. Nuclear Regulatory Commission Region IV 1600 East Lamar Blvd.
Arlington, TX 76011-4511
~j, Licensee Event Report 50-458 I 2016-003-00 March 17, 2016 RBG-47667 Page 2of2 NRC Sr. Resident Inspector P. 0. Box 1050 St. Francisville, LA 70775 INPO (via ICES reporting)
Central Records Clerk Public Utility Commission of Texas 1701 N. Congress Ave.
Austin, TX 78711-3326 Department of Environmental Quality Office of Environmental Compliance Radiological Emergency Planning and Response Section Ji Young Wiley P.O. Box 4312 Baton Rouge, LA 70821-4312
NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY OMB: NO. 3150-0104 EXPIRES: 01/31/2017 (02-2014)
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
YEAR
- 6. LER NUMBER I
SEQUENTIAL I REV NUMBER NO.
2016 003 00
- 3. PAGE 2
OF 2
On January 19, 2016, at 5:28 am. CST, while conducting core alterations, an alarm was actuated in the main control room alarm indicating that a reactor control rod had drifted out ofthe 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 intenriediate 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 applicabilify 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 positi~n 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.
REPORTED CONDITION
, the NRC may not conduct or sponsor, and a person is not required to respond to, the information collection.
YEAR
- 6. LER NUMBER I
SEQUENTIAL I REV NUMBER NO.
2016 003 00
- 3. PAGE 2
OF 2
On January 19, 2016, at 5:28 am. CST, while conducting core alterations, an alarm was actuated in the main control room alarm indicating that a reactor control rod had drifted out ofthe 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 intenriediate 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 applicabilify 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 positi~n 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.