PNO-V-86-022, on 860413,reactor Tripped After Achieving Criticality. Caused by Minor Control Rod Misalignments. Incorrect Estimated Critical Position Caused by Incorrect Xenon Tables in Operations Physics Data Summary Book

PNO-V-86-022:on 860413,reactor Tripped After Achieving Criticality. Caused by Minor Control Rod Misalignments. Incorrect Estimated Critical Position Caused by Incorrect Xenon Tables in Operations Physics Data Summary Book

ML20210L048
Person / Time
Site:	San Onofre
Issue date:	04/24/1986
From:	Huey R, Narbut P, Tatum J NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION V)
To:
References
PNO-V-86-022, PNO-V-86-22, NUDOCS 8604290159
Download: ML20210L048 (2)

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PRELIMINARY NOTIFICATION OF EVENT OR UNUSUAL OCCURRENCE--PNO-V-86-22 Date:04/24/86 Thio preliminary notification constitutes EARLY notice of events of POSSIBLE safety or public int r:st significance. The inforraation presented is as initially received without verification cr cvaluation and is basically all that is known by Region V staff on this date.

FACILITY: Southern California Edison Company Licensee Emergency Classification San Onofr< Unit 3 Notification of Unusual Event DOCKET NO. 50-362- ilert Site Area Emergency General Emergency XX Not Applicable SUhJECT: PREMATURE CRITICALITY AND REACTOR TRIP At 11:16 a.m. on April 13, 1986, while the licensee was performing a recovery from a spurious rret r trip which had occurred the previous day, the reactor tripped af ter achieving criticality at a withdrawal position lover than predicted by the Estimated Critical Position (ECP). The licensee made a 4-hour report of the reactor trip to the NRC Duty Officer at

-3:00 p.m. PST. This ENS report indicated that the reactor had tripped on departure from nucleate briling ratio (DNBR) and local power density (LPD) trip signals generated by the core pr:tiction calculator (CPC), because of penalty factors resulring from minor control rod misalignments. The resident inspectors have been conducting follow-up inspection of this event, cnd have determined that the following sequence of events occurred:

s O Operations personnal calculated an ECP of 60 inches on group 6, based on an estimated criticality at 11:00 a.m. Although two previous startups had been conducted since the first refueling outage which ended in early 1986, this was the first startup with cignificant xenon present.

O Regulating groups 1, 2, and 3 were withdrawn to their upper group limits. However, they were not individually aligned to uniform height at the fully withdrawn position, as cpecified in the procedure, because of preoccupation with achieving criticality near the 11:00 a.m. time on which the ECP was based. As a result, some group 1 rods were inserted cpproximately one inch more than group 2 rods, which generated an out-of-sequence penalty.

factor signal to the CPC. The operators' intent was to dress the rods after criticality

-was achieved before the penalty factors became effective (at 10E-4% power).

O When group 4 rods reacned approximately 80 inches withdrawn, the reactor achieved criticality (at approximately 10E-5% power). This was not immediately recognized by the operator and rod withdrawal continued to approximately 114 inches before the critical condition was recognized. The reactor controls were being manipulated by a reactor cperator trainee under the direct supervision of a licensed reactor operator. Oth'e r licensed personnel were present in the control room.

O When criticality was recognized, the operator began inserting control rods. However, power had by this time reached 10E-4% power, at which point the CPC reactor trips were enabled.

Increasing penelty factors as the rods were inserted initiated DNBR and LPD reactor trips ct a rod position of 98 inches on group 4. Actual unsafe DNBR or LPD conditions did not cxist. Peak reactor power during the transient was approximately 10E-2%, with startup rates in the range of 1.5 to 1.7 decades per minute.

8604290159 860424 PDR I&E PON-V-86-022 PDR m ,_

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.D Tht licensee determined on April 13. following the trip, that the incorrect ECP was caused by incorrect xenon tables (used to predict the reactivity worth of xenon) in the Operations Physics Data Summary Book. Due to improper administrative control of the ECP data, the xenon tables for cycle 1 operation were being used instead of the tables required for cycle 2. Indications are th t the licensed operator who was performing the startup (i.e., supervising the trainee) did n:t d; vote appropriate attention to available indications of approaching criticality and was not cnticipating criticality "at any time" as directed by the approved procedure. Since the reactor w:nt critical with rods inserted below the zero-power insertion limit (60 inches on group 5), it io algo possible that adequate shutdown margin was not maintained. Inspections into this and oth2r aspects of the event are continuing.

Following a post-trip review by the licensee and an appropriate addition of boron. Unit 3 started up leter on April 13 and has been operating at full power since April 14. The reactor trip of April 13 was reported in the Morning Report on April 14, 1986, consistent with information cvailable at that time.

This information is current as of COB on April 23, 1986.

CONTACT: P. Narbut R. Huey/J. Tatum FIS 463-3730 714-492-2641 DISTRIBUTION H St. MNBB Phillips E/W Willste AirRights2'3I5Mail:

Chairman Palladino ECO NRR IE NMSS ADM:DMB Comm. Zech PA OIA RES DOT:Trans Only Comm. Bernthal MPA AEOD Comm. Roberts ELD Comm. Asselstine Regions:

SECY INPQ2M7 NSAC G.*g 7 ACRS Licensee: I',';r 57'

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CA (Reactor Llcensees) REGION V: FORM 211 PDR Resident Inspector __ (Revised 3/14/83)