05000323/LER-2016-001

LER-2016-001, Reactor Trip Breakers Manually Opened During Shutdown Due to a Control Rod Movable Gripper Fuse Failure

Diablo Canyon Power Plant, Unit 2
Event date:	05-30-2016
Report date:	07-28-2016
Reporting criterion:	10 CFR 50.73(a)(2)(iv)(A), System Actuation
3232016001R00 - NRC Website

LER 16-001-00 for Diablo Canyon, Unit 2, Regarding Reactor Trip Breakers Manually Opened During Shutdown Due to a Control Rod Movable Gripper Fuse Failure

ML16210A524
Person / Time
Site:	Diablo Canyon
Issue date:	07/28/2016
From:	Welsch J M Pacific Gas & Electric Co
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
DCL-16-075 LER 16-001-00
Download: ML16210A524 (6)

L Reportable Event Classification This event is reportable pursuant to 10 CFR 50.73(a)(2)(iv)(A), System Actuation.

II. Plant Conditions

Unit 2 was in Mode 4, Hot Shutdown, preparing for restart following the nineteenth refueling outage (2R19).

Ill. Description of Problem

A. Background

The rod control system [AA] is a Design Class II system for positioning the reactor control rods for reactor power modulation by manual or automatic control of control rod banks. The rod control system is provided motive power through the redundant Design Class I Reactor Trip breakers that open to perform the safety-related function of rod insertion by removing all motive power to the control rod drive mechanism. The rod group demand position indication is located in the control room on the control board and indicates the individual rod position demands made for rod motion.

The digital rod position indication (DRPI) system measures the position of the control rod drive mechanism shafts within the control rod drive housings so that the positions of the control rods within the core are verified.

Equipment Control Guideline (ECG) 41.2, "Special Test Exceptions - Position Indication System — Shutdown", requires that position indication systems be operable when the reactor trip breakers are closed. With less than the above required position indication systems operable, the reactor trip breakers are required to be immediately opened. Surveillance Requirement (SR) 41.2.1 requires verification "that the demand position indication system and the DRPI system agree within 12 steps when rods are stationary.

Surveillance Test Procedure (STP) R-1C, "Digital Rod Position Indicator Functional Test," verifies the OPERABILITY of the DRPI System. The rod banks are individually withdrawn in 24 step increments while a comparison is made between the DRPI indication and the demand step counter for each rod. When the bank is fully withdrawn, it is then inserted to the rod bottom light emitting diode (LED) point and the position at which each rod bottom LED was actuated is recorded.

B. Event Description

During plant restart following 2R19, plant operators initiated DRPI operability testing of Shutdown Bank A in accordance with STP R-1C.

On May 30, 2016, at 0930 PDT with Unit 2 in Mode 4, licensed operators responded to an indicated difference between the Rod B4 demand position indication and DRPI indication greater than 12 steps and initiated manual opening of the reactor trip breakers in accordance with plant procedures.

On May 30, 2016, at 1611 PDT, operations made an 8-hour, nonemergency notification in accordance with 10 CFR 50.72(b)(3)(iv)(A).

C. Status of Inoperable Structures, Systems, or Components that Contributed to the Event Control rod moveable gripper fuse (FU49) for Rod B4 was open.

D. Other Systems or Secondary Functions Affected

None.

E. Method of Discovery

The condition was discovered during the performance of STP R-1C as required by plant ECG 41.2.

F. Operator Actions

Licensed operators initiated manual opening of the reactor trip breakers removing motive power from the rod control system in accordance with plant procedures.

G. Safety System Responses

Unit 2 rod control system was deactivated by opening the reactor trip breakers, which deenergized the gripper circuits and control rods that were withdrawn then reinserted into the core as designed.

III. Cause of the Problem

A. Immediate Cause

The rod control demand position indication and DRPI had an observed difference of 14 steps, requiring the immediate opening of the reactor trip breakers.

B. Cause

A failure of the Rod B4 moveable gripper fuse (FU49) prevented Rod B4 from movement upon demand. This caused the Rod B4 to remain at the core bottom as indicated by the DRPI system while the demand counter counted up to 14 steps.

Gripper coil fuses are replaced every refueling outage to prevent failure due to cyclic fatigue. Handling and installation stress can weaken fuses and cause them to fail when first energized. These issues are detected when post maintenance testing is performed.

STP R-1C, Revision 21, did not explicitly specify actions to identify improper DRPI indications prior to exceeding a 12 step difference between rod demand and rod position indication. STP R-1C, Revision 17, dated March 19, 2009, had steps to withdraw the rod bank to 10 steps and then check rod control fuse integrity. These steps were added to STP R-1C, as a corrective action to a previous Licensee Event Report (LER) 2-2003-004-00.

Subsequently, this step was removed in Revision 18 of the procedure dated July 14, 2009. If this step had been in STP R-1C at the time of the event, the faulted fuse would have been detected prior to exceeding the 12-step limit and a manual opening of the reactor trip breakers would not have been required.

IV. Assessment of Safety Consequences

Procedure controlled pre-operation testing discovered the inoperable system prior to pulling more than one shutdown bank. A control rod moveable gripper fuse (FU49) was identified as failed open preventing Rod B4 from being moved, thus, no unanticipated positive reactivity change was possible. At the time of this event, the boron concentration in the reactor coolant system was approximately 2100 ppm; therefore, compliance with shutdown margin requirements was maintained. This event was not safety significant and it did not adversely affect the health and safety of the public.

V. Corrective Actions

A. Immediate Corrective Actions 1. Unit 2 rod control system was deactivated by opening the reactor trip breakers, which deenergized the gripper circuits and control rods that were withdrawn then reinserted into the core as designed.

2. Plant staff identified a failed moveable gripper coil fuse (FU49) for Rod B4 and replaced the failed fuse.

B. Corrective Actions to Prevent Recurrence 1. STP R-1C has been revised to provide adequate guidance regarding confirmation of rod motion prior to exceeding 12 steps demand indication.

2. Plant operators will receive training on the changes to STP R-1C.

A. Failed Components:

Component: Fuse, 10A 250V ceramic cartridge Manufacturer: BUSSMANN # 2432659

B. Previous Similar Events

of a Control Rod F2 moveable coil power supply fuse which caused Rod F2 to remain on the core bottom, as indicated by the DRPI system. Corrective actions included revising STP R-1C to provide adequate guidance regarding confirmation of rod motion prior to exceeding 12 steps demand indication.

However, as noted above, these steps were inadvertently deleted in a subsequent revision to the procedure.

Diablo Canyon Power Plant, Unit 2 05000323