05000245/LER-1997-009-01, :on 970212,reactor low-low Level ECCS & Primary Containment Initiation Setpoints Were Not Conservative. Caused by Deficient Setpoint Methodology.Calculations Will Be Revised & TS Change Initiated

:on 970212,reactor low-low Level ECCS & Primary Containment Initiation Setpoints Were Not Conservative. Caused by Deficient Setpoint Methodology.Calculations Will Be Revised & TS Change Initiated

ML20136H274
Person / Time
Site:	Millstone
Issue date:	03/14/1997
From:	Robert Walpole NORTHEAST NUCLEAR ENERGY CO.
To:
Shared Package
ML20136H257	List: 05000245/LER-1997-009, Forwards LER 97-009-00,documenting Condition That Was Discovered at Millstone Nuclear Power Station,Unit 1 on 970212.Util Commitments Made within Ltr,Listed 05000245/LER-1997-009-01, :on 970212,reactor low-low Level ECCS & Primary Containment Initiation Setpoints Were Not Conservative. Caused by Deficient Setpoint Methodology.Calculations Will Be Revised & TS Change Initiated
References
LER-97-009-01, LER-97-9-1, NUDOCS 9703190104
Download: ML20136H274 (4)

LER-1997-009, on 970212,reactor low-low Level ECCS & Primary Containment Initiation Setpoints Were Not Conservative. Caused by Deficient Setpoint Methodology.Calculations Will Be Revised & TS Change Initiated

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(iii) 10 CFR 50.73(a)(2)(v), Loss of Safety Function 10 CFR 50.73(a)(2)(vii), Common Cause Inoperability
2451997009R01 - NRC Website

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NRC FORM 366 U.S. NUCLEAR REGULATORY COMMISSION APPROVED BY oMB NO.3160-0104 l

(4 95)

EXPIRES 04/30/98

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FAclLrTY NAME m DOCKET NUMBER (2)

PAGE (3)

Millstone Nuclear Power Station Unit 1 05000245 1 of 4 TITLE I4)

Reactor Low-Low Level ECCS and Primary Containment isolation Initiation Setpoints Not Conservative EVENT DATE (5)

LER NUMBER (6)

REPORT DATE (7)

OTHER FACILITIES INVOLVED (8)

MONTH DAY YEAR YEAR SEQUENTIAL REVISION MONTH DAY YEAR FACILITY NAME DOCKET NUMBER NUMBER 02 12 97 97 009 00 03 14 97 J

OPERATING THIS REPORT IS SUBMITTED PURSUANT TO THE REQUIREMENTS OF lo CFR 5: (Check one or more) (11) i MODE m)

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20.2201(b) 20.2203(a)(2Hv) 50.73(aH2)(i) 50.73(aH2)(viii)

LEVEL (10) 000 20.2203(a)(1) 20.2203(a)(3)(i)

X 50.73(aH2)(ii) 50.73(a>(2 Hx>

POWER 20.2203(a)(2)(i) 20.2203(a)(3)(ii) 50.73(a)(2)(iii) 73.71

..............m 20.2203(aH2)(ii) 20.2203(a)(4) 50.73(aH2)(iv)

OTHER 20_2203(a)(2)(iii) 50.36(cH1) 50.73(a)(2)(v) specify in Abstract below l

> G 20.2203(a)(2Hiv) 50.36(cH2) 50.73(a)(2)(vii) or in NRC Forrn 366A i

LICENSEE CONTACT FOR THIS LER (12)

NAME TELEPHONE NUMBER l Include Area Code)

Robert W. Walpole, MP1 Nuclear Licensing Manager (860)440-2191 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DESCRIBED IN THIS REPORT (13)

CAUSE

SYSTEM COMPONENT MANUFACTURER REPORTABLE

CAUSE

SYSTEM COMPONENT MANUFACTURER REPORTABLE TO NPROS To NPROS SUPPLEMENTAL REPORT EXPECTED (14)

EXPECTED MONTH DAY YEAR f

(if yes. complete EXPECTED SUBMISSION DATE).

j SUBMISSION f

YES NO ABSTRACT (Limit to 1400 spaces, i.e., approximately 15 single-spaced typewritten lines) (16)

On February 12,1997, with the plant in COLD SHUTDOWN, a review of a previously completed setpoint calculation l

from October 25,1994, identified the reactor vessel low-low level Emergency Core Cooling System (ECCS) and j

Primary Containment isolation System (PCIS) initiation setpoints were potentially non-conservatively set. A review of j

tha instrumentation uncertainty associated with the low-low level ECCS and PCIS initiation functions revealed that the instrument uncertainty error is greater than the instrument trip setting band required by Technical Specitications (TS). Therefore, ECCS and PCIS initiation could occur beyond the reauired TS limits. This event was promptly reported on February 12,1997, pursuant to 10CFR50.72(b)(2)(i) as an unanalyzed condition. The ECCS and PCIS low-low level initiation instrumentation was determined to N it' operable as a result of this discovery.

t Tha cause for exceeding TS limits is the use of an original setpoint methodology which did not explicitly evaluate individual contributors to the overall instrument uncertainty. There were no safety consequences as a result of this evsnt. The TS will be changed or a new analysis will be performed to ensure that ECCS and PCIS actuation will occur consistent with the plant's design basis including consideration of instrument uncertainties. All calculations that have bsen completed as part of the Setpoint Verification Program will be reviewed to determine if other non-conservative astpoints exist.

9703190104 970314 I

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l NRC FORM 3::6A U.S. NUCLEAR REGULATORY COMMISSION R95)

UCENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 1 05000245 NUMBER NUMBER 2 of 4 97 009 00 TEXT Uf more space is required use additionalcopies of NRC Form 366A) (17}

l.

Description of Event

l On February 12,1997, with the plant in COLD SHUTDOWN, a review of a previously completed calculation from October 25,1994, identified the reactor vessel low-low level ECCS and PCIS initiation setpoints were potentially non conservatively set. This calculation determined that the low-low Level Indicating Switches (LISs) could have approximately 7 inches of error (-4.3/+3.0 inches); however, the current TS require the ECCS and PCIS low-low water level instrument trip settings be set at 79 inches above the Top of Active Fuel (TAF) -0/+4 inches. In order to maintain the maximum margin to both TS limits, the low-low level setpoint was set at 81(12) inches above TAF. Applying the new calculated error, the actual instrument trip could occur beyond either the lower or the upper TS limit. The upper limit is specified to prevent premature Main Steam Isolation Valve (MSIV) isolation of tha turbine main condenser, and the lower TS limit is credited in the accident analysis.

This event was promptly reported on February 12, 1997, pursuant to 10CFR50.72(b)(2)(i) as an unanalyzed condition. The ECCS and PCIS low-low level initiation instrumentation was determined to be inoperable as a result of this discovery.

This condition was not reported on October 25,1994, when the calculation was completed due to a lack of clear guidance for addressing potential adverse conditions that could arise from the implementation of the new j

setpoint.

11.

Cause of Event

f i

The cause for exceeding TS limits is the use of an original setpoint methodology which did not explicitly evaluate individual contributors to the overall instrument uncertainty.

1 ll1. Analysis of Event The ECCS system is designed to initiate and carry out remedial actions to mitigate plant abnormal and accident evsnts, e.g., cool and isolate the reactor vessel. The ECCS instrumentation detects a need for core cooling systems operation, and the trip systems initiate the appropriate response. A low-low water level (79 inches above TAF) condition in the reactor vessel could indicate that reactor coolant is being lost through a breach in the reactor coolant pressure boundary and that the core is in danger of becoming overheated as the reactor coolant inventory is lost.

i The design basis of the PCIS system is to acceptably mitigate the consequences of accidents and to protect egainst the gross release of radioactive materials from the primary containment. Protection is achieved through i

ths automatic isolation of appropriate pipelines that penetrate the primary containment whenever monitored

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variables exceed their pre-selected operational limits. The low-low reactor water level trip setting completes the isolation of the primary containment and reactor vessel by initiating closure of the Group I and Group V isolation valves and any other Group 11 or Group ll1 valves that must be shut to isolate minor process lines.

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i ECCG related low-Icw water level trips occurs after a finite time following the initiation of a Loss of Coolant Accident (LOCA). The Millstone Unit No.1 analysis assumes this trip occurs at about 600 psia. The change in vsssel pressure results in significant density changes to the reference leg of the level measuring system. In the decreasing direction, this error is approximately

+1 inch, and in the increasing direction, this error is l

.=

.U.S. NUCLEAR REGULATORY COMMISSION (4 95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION Millstone Nuclear Power Station Unit 1 05000245 NUMBER NUMBER 3 of 4 97 009 00 TEXT (11more spare is required, use additional copies of NRC Form 366A) (17) approximately -2.3 inches. When all of the instrument errors are included, the switch cannot be set without violating either the lower or upper limit of the TS.

The current TS values for ECCS low-low water level initiation setpoint are considered analytical limits. Millstone l

Unit No.1 TS do not specify Allowable Values for their instrumentation setpoints. As a result, operation of the low-low level instrumentation is required to occur within the limits specified in the TS.

[

t The plant safety analysis assumes that ECCS is initiated before vessel level reaches 79 inches above TAF. ECCS initiation occurs on either low-low reactor water level or high drywell pressure. LOCA inside the drywell result in receipt of the high drywell pressure condition or occurrence of the low-low level condition. For breaks outside the i

drywell, ECCS initiation may be delayed slightly due to the pressure / density errors in the level measuring instrumentation.

Th3 upper TS limit is not credited in any accident or transient analysis, and premature initiation of the low-low l

lsvel signal will not adversely affect any safety function. Early actuation of the low-low water level switches will j

result in early ECCS initiation and main steam isolation.

in the event the leve! switches trip a few inches lower than the TS minimum, it will not have an adverse impact t

l on LOCA inside the containment, since the high drywell pressure condition is expected to occur prior to the low-i l

low water condition. For small-break LOCAs, Automatic Pressure Relief system initiation could be slightly delayed resulting in a very smallincrease in Peak Clad Temperature for those scenarios where Feedwater Coolant injection is not available, l

For non LOCA events, the potential for later ECCS initiation on low-low water level would delay Isolation i

Condenser and Main Steam isolation Initiation. If the IC fails to initiates, there would be slight reduction in the

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time available for the operator to initiate IC, or other systems, to prevent uncovering of the core.

Other level instrumentation such as Anticipated Transient Without Scram and Reactor Protection System have j

adequately accounted for the effects of density changes on the reference leg. The low-low level ECCS problem i

results from the fact that there is a small band in which the level switches must operate under worst case conditions. Other levelinstrumentation does not specify a band but only a minimum or maximum trip setting.

l l

Thsre were no safety consequences as a result of this event since there was no design basis accidents requiring ECCS or PCIS mitigation.

IV. Corrective Action

Northeast Nuclear Energy Company (NNECO) will review all calculations that have been completed as part of the Sztpoint Verification Program to determine if other non-conservative setpoints exist by May 1,1997.

l NNECO will submit a TS change request or perform a new analysis to ensure that ECCS and PCIS actuation will l

occur consistent with the plant's design basis including consideration of instrument uncertainties by April 15, 1997.

l A review of Setpoint Change Program is ongoing to determine if additional reportablc issues exist.

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k eU.S. NUCLEAR REGULATORY COMMISSION i

L445)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION

' FACILITY NAME (1)

DOCKET NUMBER (2)

LER NUMBER (6)

PAGE (3)

YEAR SEQUENTIAL REVISION

' Millstone Nuclear Power Station Unit 1 05000245 NUMBER NUMBER 4 of 4 97 009 00 TEXT (11more space is required, use additional copies of NRC Form 366A) (17) v V.

Similar Events

LER 94-011-00, This LER involved a non-conservative setpoint for reactor vessel low level scram and low-low level ECCS initiation setpoint due to longer active fuel length making the top of active fuel higher.

LER 96-008-00, This LER involved the discovery that the Anticipated Transient Without Scram Alternate Rod Insert / Recirculation Pump Trip low-low level setpoint was non-conservatively set.

Manufacturer Data Not Applicable, l

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J P3RC FORM 366A (&90 i