ML20043A420

LER 90-006-00:on 900412,P-6 Permissive Signal Received & Source Range Flux Doubling (Srfd) Actuation Occurred.Caused by Inadvertent Reset of Srfd Block.Integrated Plant Operations Procedures changed.W/900514 Ltr

ML20043A420
Person / Time
Site:	Comanche Peak
Issue date:	05/14/1990
From:	William Cahill, Hood D TEXAS UTILITIES ELECTRIC CO. (TU ELECTRIC)
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-90-006, LER-90-6, TXX-90158, NUDOCS 9005220058
Download: ML20043A420 (9)

Text

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=M ' Log # TXX-90158- ,

M l File #'10200 1 Ci - 907 .'

C- C' Ref. # 10CFR50.73 .

7UELECTRIC : 10CFR50.73(a)(2)(iv)

-May 14, 1990-Eurcutive Yke President '

U. ; S.: Nuclear Regulatory Commission .

Attn: Document Control Desk Washington, D. C. .20555'

SUBJECT:

n-COMANCHE' PEAK STEAM ELECTRIC STATION ,

DOCKET N0. 50-445' REACTOR PROTECTION SYSTEM ACTUATION:

' LICENSEE EVENT REPORT 90-006 Gentlemen: 4

)

Enclosed.is-Licensee Event-Report 90-006-00 for-Comanche Peak Steam. Electric Station Unit 1, " Source' Range: Flux Doubling. Actuation Due to Inadvertent = Reset ,

of Source Range. Flux Doubling Block." ,

Sincerely,.

William J. C hill, Jr. <

DEN /daj Enclosure c - Mr. R. D. Mart'in, Region IV Resident Inspectors, CPSES (3)

I l i

9005220058 900514 PDR fiDOCK 05000445 ,

S PDC 400 North Olive Street LB 81 Dallas, Texas 7D01

Enclosure to TXX 90158 -

NRC FORM 306 U.S. NUCLE AR REOUL.A TORY COM M ISSsON j APPROVED OMB NO.31640104 EXPIRES:4/3@92 l ESTIMATED BURDEN PER RESPONSE TO COMPLY WITH TH:S INFORVATON i COLLECTON REQUEST:' 50.0 HRS. FORWARD COMMENTS REGARDING 1 BURDEN ESTIMATE TO THE RECORDS AND REPORTS MANAGEMENT LICENSEE EVENT REPORT (LER) 8 RANCH (P 630). U.S. NUCLEAR REOULATORY COMMISSON. WASHINGTON.

DC. 20655. AND TO THE PAPERWORK REDUCTON PROJECT (31540104).

OFFCE OF MANAGEMENT AND BUDGET, WASHINGTON, DC. 20603.

F.cady Narn. p) .

Dock.t Nonts.r (2) Pag.(3)

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Nam. T.a.ston. Nunti.e D. NORMAN HOOD SUPERVISOR. COMPLIANCE 8Y1Y7 8 I 9I 7 l- 15l 8I 8 I 9 Corma.t. On. Un. For Each Cormoneet Fauwe. D.senb.d in This Report (13)

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$ M h _f I I I I I I I $$3M i Il i ll I E '3 M I II I I I I MM I II l l l l Day MNNE har Sucol.m.ntal R. cort Esp.ct.d O 4) Ew.ct.d Month Subnisaien Y., (tf y.s. cormiet. Emp.ct.d Subtressen Dat.) No l l l e cuun no uoo. .t . - yio n.,n,+.,ac.,y, n..ni )06) l l At 1726 CST on April 12,1990, Comanche Peak Steam Electric Station Unit 1 was in Mode 2,

Startup.- Concurrent with the startup, the Source Range / Intermediate Range Nuclear Instrumentation overlap verification was being performed. At 1729 CST, the P-6 permissive signal was received and at approximately 1730 CST, a Source Range Flux Doubling (SRFD)

Actuation occurred. The most likely root cause was a momentary neutron flux peak above the P-6 permissive setpoint and then below the setpoint, unblocking the previously blocked SRFD signal, resulting in an SRFD Actuation. At 1745 CST, the Reactor Operator discovered that an SRFD Actuation had occurred. At 1747 CST, the SRFD signal was re-blocked and affected components restored to their original position, Corrective actions include: 1) A change to Integrated Plant Operating procedures, instructing the operator to hold the SRFD Block switch in the block position while neutron flux level is passing through the P-6 permissive setpoint, and 2) Changing the SRFD Actuation alarm window color from yellow to red to aid in identification. Further corrective action includes the distribution of a Lessons Learned package to alllicensed operators discussing operator performance concerns and operator alarm response concerns.

Enclosure to TXX 90158 -

NRC FOfW 306A U.S. NUCLEAR REQULATORY COMMIS$10N AppROYED OMB NO. 315CkO104 ESTNATED BURDEN PER RES COMPLY wtTH THIS INFORMATION LICENSEE EVENT REPORT (LER) g',$%,",'My; 4"fjc'g,^",M/ffg^

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j F ar,iley Name (1) Docket Nureer (2) e LER Nureer (6) Page (3)

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l. DESCRIPTION OF WHAT OCCURRED L

A. PLANT OPEFlATING CONDITIONS BEFORE THE EVENT On April 12,1990, Comanche Peak Steam Electric Station (CPSES) Unit 1 was in Mode 2, Startup, with the reactor (Ells:(RCT)(AB)) critical at approximately 1 E-10 amps, as indicated on the Intermediate Range Nuclear Instrumentation (Ells:(RI)(JC)). Reactor Coolant System (RCS)(Ells:(AB)) temperature and pressure -

were 556 degrees Fahrenheit (F) and 2250 pounds per square inch gage (PSIG),

respectively. The RCS was borated to approximately 1069 parts per million (ppm).

Centrifugal Charging Pump-01 (CCP 01)(Ells:(P)(CB)) was running, taking suction:

from_the Volume Control Tank (VCT)(Ells:(TK)(CB)).

l B. REPORTABLE EVENT DESCRIPTION UNCLUDING DATES AND APPROXIMATE TIMES OF MAJOR OCCURRENCES) i Event Classification: An event or condition that resulted in an automatic - .

i actuation of any Engineered Safety Feature (ESF), including the Reactor Protection

. Syctem (RPS).

At 1416 CST on April 12,1990, the Source Range Flux Doubling (SRFD) signal was manually blocked prior to Mode 2 entry In accordance with the approved Integrated 4

Plant Operating (IPO) procedure for plant startup from hot standby to minimum load.

At 1726 CST, Unit 1 reached initial criticality. Concurrent with the IPO, the initial Startup (ISU) procedure was being utilized to operationally align nuclear instrumentation to verify proper overlap between the Source Range (Ells:(RI)(JC))

arid the Intermediate Range Nuclear Instrumentation, during power increase, in 4

accordance with the ISU, the Source Range High Flux (SRHF) Trip would not be -

blocked until the Source Range indicated approximately 3E+04 counts per second (CPS). The SRHF Trip setpoint is 1E+05 CPS.

At approximately 1727 CST on April 12,1990, the Unit Supervisor (utility-licensed) and the Reactor Operator (RO) (utility-licensed) made the decision to proceed through the Source Range / Intermediate Range overlap region at a slower than normal startup rate to allow time to perform the overlap verification. The startup ,

procedure recommends a startup rate of 0.5 decades per minute (DPM). The startup rate was initia.Ily established at 0.3 DPM and control rods (Ells:(ROD)(AA)) were inserted to further reduce the startup rate to approximately 0.2 DPM.

._ _ _ __ _ _ . - _ ~ _ _ . _

o Enclosura to'TXX 90158 unc ronu . - u. .uucuan acouwoar couwam ,,,mo a . uo_

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010 013 OF 018 a.i % . - . % - unc o,.1 w .m n At 1729 CST on April 12,1990, the P-6 permissive signal was received (1E-10 amps)'and at appro_ximately 1730 CST, an SRFD Actuation occurred. As a result of  :

the SRFD signal, valves 1-LCV-112D & -112E [ isolation valves (Ells

(ISV)(CB)) from J

the Refueling Water Storage Tank (RWST)(Ells

(TK)(BE))) automatically opened and ;

L valves 1-LCV-112B'& -112C [ isolation valves from the VCT] automatically closed which shifted the CCP-D ' suction from the VCT to the RWST. Both pairs of isolation valves are considered part of the Chemical and Volume Control System ~

e (CVCS)(Ells:(CB)).

The P 6 permissive signal provides a permissive to block the SRHF Trip when the neutron flux level has entered the intermediate range. - At 1731 CST on April 12, -

t 1990, nuclear instrumentation overlap data was taken and the SRHF Trip blocked.

Neutron flux was then raised to approximately 1E-08 amps on the Intermediate

Range Nuclear instrumentation and stabilized.

- At 1745 CST on April 12,1990,'the RO observed a VCT High Level alarm

, (Ells:(ALM)(IB)) coincident with slowly decreasing neutron flux. Upon investigating.

l these conditions, the RO discovered that an SRFD Actuation had occurred. At 1747 ,

CST, the RO re-blocked the SRFD signal and realigned CCP-01 suction to the VCT.

I An event or condition that results in an automatic actuation of any ESF, including the RPS, is reportable within 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> under 10CFR50.72(b)(2)(li). At 2015 CST on April 12,1990, the Nuclear Regulatory Commission Operations Center was notified via  :

the Emergency Notification System of tne ovent as required.

C. STATUS OF STRUCTURES. SYSTEMS. OR COMPONENTS l

THAT WERE INOPERABLE AT THE START OF THE EYENI AND THAT CONTRIBUTED TO THE EVENT Not applicable - no structures, systems, or components were inoperable at the start

~

i of the event that have been determined to have contributed to the event.

D. CAUSE OF EACH COMPONENT OR SYSTEM FAILURE. IF KNOWN Not applicable - no component or system failures have been identified.

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. . . ~l Enclosure to TXX-90158 -

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COMANCHE PEAK - UNIT 1 0151010 l'Ol 4 I 4 l 5 910 -

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0IO OI4 OF 0l8 ioi o n- . - ... ..onanc F- 3 A.3 (in E. FAILURE MODE. MECHANISM. AND EFFECT OF EACH _;

FAILED COMPONENT Not applicable - no failed components h' ave been identified.

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F. FOR FAILURES OF COMPONENTS WITH MULTIPLE FUNCTIONS. LIST OF SYSTEMS OR SECONDARY FUNCTIONS THAT WERE ALSO AFFECTED il Not applicable - no component failures have been identified.

4 G. FOR FAILURES THAT RENDERED A TRAIN OF A SAFETY SYSTEM L INOPERABLE. AN ESTIMATE OF THE ELAPSED TIME FROM THE ,

DISCOVERY OF INOPER ABILITY UNTIL THE TR AIN WAS RETURNED TO SERVICE Not applicable - no failures have been identified.

H. THE METHOD OF DISCOVERY OF EACH COMPONENT OR SYSTEM FAILURE OR PROCEDURAL ERROR Not applicable - no component or system failures have been identified.

l

l. CAUSEOEllilLEVERT The cause of the SRFD Actuation was due to the inadvertent roset of the SWD Block. There are two ways to reset (unblock) the SRFD Block: manually a id automatically.

1. A manual reset switch (Ells:(HS)(JG)) is provided to unblock the SRFD Block t

below the P-6 permissive setpoint.

, 2. When the neutron flux level drops below the P-6 permissive setpoint, the Source I

Range Nuclear Instrumentation re energizes to monitor the neutron flux level as it leaves the intermediate range, the P-6 permissive signa! resets and automatically resets (unblocks) the SRFD Block.

l

Enclosure to TXX 90158 NRC FORM 1864 U.S. NUCLE AR REGLuTORY COMM'. SON APPROVED OWS NO. 31840104 ESTIMATED RURDIN PER RF8 h COMPLYWITHTHISINFORMATON ,

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010 015 OF 018 w (~. - . ..o. = NRC % =A.m n Root Cause:

The root cause of the inadvertent reset of the SRFD Block cannot be conclusively.-

. determined, however, the potential causes have been identified and evaluated a bolow'

1. The manual reset switch may have been inadvertently operated.' However, the SRFD Actuation occurred after the P-6 permissive signal was received. The P-6 permissive signal renders the SRFD manual reset switch inoperable as long as the P-6 permissive signalis present. Since the SRFD Actuation occurred after the P-6 permissive signal was received, the inadvertent operation of the SRFD manual reset switch is not supported.

2. A logic failure may have allowed the P-6 permissive signal to inappropriately <

reset the SRFD Block. However, the protection logic is such that the P-6  :

permissive signal must first be received (toggled on) and then reset (toggled off) to allow the SRFD Block to be reset. A protection logic failure of this type is very L unlikely since a logic failure would either fall on or off. Following this event, a subsequent reactor startup was performed. The correct operation of the P-6 permissive was verified, thereby confirming the proper operation of the protection logic. Therefore, a logic failure is not supported.

3. The intermediate range neutron flux level fluctuated abos e and below the P-6 permissive setpoint, causing a momentary P-6 permissive signal.. A slower than normal startup rate had been established, which may have allowed the neutron.

flux level to momentarily peak above the P-6 permissive setpoint and then return below the setpoint. This is consistent with the logic, as discussed above, and would allow the SRFD Block to be reset. The sequence of events indicates 1 that the SRFD Block was reset (SRFD Actuation occurred) after the P 6 permissive signal was received, and therefore supports this as the most likely cause of the event.

During this event the SRFD Actuation alarm had been received but not responded to for a period of approximately tourteen minutes due to the similaritics between two 1 alarms:1) SRFD Actuation, and 2) SRFD. The SRFD Actuation alarm window (Ells:(ALM)(IB)) is yellow in color and is lccated immediately above the reactor controls (Ells:(JC)(JD)). This alarm is not expected to alarm during startup since the  ;

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SRFD Actuation i.s manually blocked prior to the initiation of the startup. Immediately

~above this alarm is the second alarm, SRFD alarm. The SRFD alarm window is'also l

yellow in color. The SRFD alarm is expected to alarm during startup and had alarmed several times during the course of the startup.

- Due to the similarities in these two alarms, the operating staff acknowledged the

SRFD Actuation alarm as another SRFD alarm and did not immediately realize the difference. When the RO discovered that an SRFD Actuation had occurred, the SRFD signal was re-blocked and CCP-01 realigned to the VCT.

J. SAFETY SYSTEM RESPONSES THAT OCCURRED -

The following safety system actuated automatically as a result of the event. The -

appropriate components within this system operated as designed upon receipt of the SRFD signal.

Chemical Volume and Control System Train 'A' and 'B'.

[

K. FAILED COMPONENT INFORMATION l

Not appticable - no failed components were involved.

II. ASSESSMENT OF THE SAFETY CONSEQUENCES AND IMPLICATIONS OF THIS EVENT An SRFD Actuation signalis provided to protect against inadvertent boron dilution accidents during reactor shutdown by automatically transferring CCP suction from the VCT to the RWST. This actuation occurs if one of two Source Range Nuclear Instruments l detects an increase in neutron flux by a factor of two within ten minutes. SRFD actuation j signals are required by Technical Specifications in Mode 5, Cold Shutdown, through Mode 3, Hot Standby. The SRFD signalis manually blocked prior to entry into Mode 2, Startup, per procedure. Based on the following factors, this event did not adversely affect the safe operation of CPSES Unit 1 or the health and safety of the public.

1. The SRFD Actuation occurred as designed but was not required to mitigate an actual boron dilution transient. The SRFD Actuation had been manually blocked prior to entry into Mode 2, Startup, per procedure. However, due to a momentary neutron

. . /--

1- ,

Enclosure thTXX 90158 -

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l flux level peak above the P 6 permissive setpoint and then below the setpoint, the .

SRFD Block was reset and an SRFD Actuation occurred. Chemistry samples of the >

reactor coolant were taken and analyzed prior to the event at 1455 CST on April 12, . -

l 1990, and after the event at 1814 CST on April 12,1990.' The boron concentration 6 levels were 1069 ppm and 1113 ppm, respectively. This confirmed that the SRFD

' Actuation was not required to mitigate an actual boron dilution transient. .

2. The appropriate safety system actuated automatically upon initiation of the SRFD ,

signal and would have performed its design function if an actual boron dilution  :

transient had occurred. The possible dilution accident situations have been ,

analyzed in FSAR Chapter 15 and it has been concluded that the successful s automatic actuation of the CVCS isolation valves will terminate the dilution transient, initiate boration, and the reactor will be left in a stable condition.

Ill, CORRECTIVE ACTIONS A. ACTIONS TO PREVENT RECURRENCE Root Cause:

A momentary neutron flux peak above the P-6 permissive setpoint and then below the setpoint, reset (unblocked) the SRFD Block resulting in an SRFD Actuation.

Corrective Action: J Integrated Plant Operations procedures have been changed to instruct the RO to

hold the SRFD Block switch in the block position while the neutron flux level is passing through the P-6 permissive setpoint. This will prevent a momentary P-6 permissive signal from resetting the SRFD Block.

The following corrective action addresses the contributing factor identified relative to the delayed operator response to the SRFD Actuation alarm during the event.

Contributina Factor:

The SRFD Actuation alarm had been received but not responded to for

approximately fourteen minutes due to the physical similarities in the SRFD Actuation ,

and the SRFD alarms. l

i Enclos.ure td TXX 90158 )

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010 018 OF 018 r..i - . . . n.,umr .nin Corrective Action:

The SRFD Actuation alarm window color has been changed from yellow to red. This '

will aid the RO in identifying that an SRFD Actuation has occurred.

B. ACTION TAKEN ON GENERIC CONCERNS IDENTIFIED AS A DIRECT -

RESULT OF THE EVENT:  :

Generic Imolications :

Senior Reactor Operator (SRO) (utility licensed) and RO alarm response to SRFD events has been less than adequate. '

Corrective Action:

A Lessons Learned package dealing with the concerns of operator performance and alarm response was distributed by the Manager, Operations to all SRO and ROs.

t T he package addrmes the Operator alarm response concerns identified in ,

previous Licensee Event Reports (LER), including the event described in this LER.

The Manager, Operations has met with each shift crew to discuss the concerns  !

Identified in this Lessons Learned package.

I IV. PREVIOUS SIMILAR EVENTS Although there have been several previous events (LER 90 001, LER 90 002) that .

t resulted in SRFD Actuations, the root causes of those events were unrelated to the root I cause of this event. The corrective actions taken to resolve the root causes of the previous I

events would not have prevented this event. Therefore, no previous similar events have been reported pursuant to 10CFR50.73.

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