05000336/LER-1997-015-02, :on 970415,deficiency of Containment Air Recirculation Coolers During Design Basis Accidents, Occurred.Caused by Inadequate Consideration of Potential Failure Modes.Finalize Calculations

:on 970415,deficiency of Containment Air Recirculation Coolers During Design Basis Accidents, Occurred.Caused by Inadequate Consideration of Potential Failure Modes.Finalize Calculations

ML20141G086
Person / Time
Site:	Millstone
Issue date:	05/15/1997
From:	Joshi R NORTHEAST NUCLEAR ENERGY CO.
To:
Shared Package
ML20141G026	List: 05000336/LER-1997-015, Forwards LER 97-015-00 Per 10CFR50.73(a)(2)(v).Also Includes Commitments Made in Ltr 05000336/LER-1997-015-02, :on 970415,deficiency of Containment Air Recirculation Coolers During Design Basis Accidents, Occurred.Caused by Inadequate Consideration of Potential Failure Modes.Finalize Calculations
References
GL-96-06, GL-96-6, LER-97-015-02, LER-97-15-2, NUDOCS 9705220156
Download: ML20141G086 (5)

LER-1997-015, on 970415,deficiency of Containment Air Recirculation Coolers During Design Basis Accidents, Occurred.Caused by Inadequate Consideration of Potential Failure Modes.Finalize Calculations

Event date:
Report date:
Reporting criterion:	10 CFR 50.73(a)(2)(v), Loss of Safety Function
3361997015R02 - NRC Website

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NRC FORM 366 u.S. NUCLE AR REGULATORY COMMISSION APPROVED BV OMB NO. 3150 0104 (4-951 l

i EXPiRfs04/30/98 LICENSEE EVENT REPORT (LER)

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PCCILITV NAME (13 DOCKET NUMBER (2)

PAGE136 Millstone Nuclear Power Station Unit 2 05000336 1 OF 5 TITLE (4)

Performance Deficiency of Containment Air Recirculation Coolers During Design Basis Accidents (Response to Generic Letter 96-06)

[

EVENT DATE (5)

LER NUMBER 16)

REPORT DATE (7)

OTHER FACILITIES INVOLVED (8)

SE U N

FACiUTV NAMF~

DOCKET NUM8ER MONTH DAY YEAR YEAR MONTH DAY YEAR E

NU R

04 15 97 97

-- 015 --

00 05 15 9/

OPERATING THis REPORT is SUBMITTED PURSUANT TO THE REQUIPIcNTs OF 10 CFR 1: (Check one Or more) (11)

MODE (9)

N 20.2201(b) 20.2203(aH2)(v) 50.73(aH2Hi>

50.73(aH2)(vm)

POWER 20.2203(aH1) 20.2203(aH3HO 50.73(aH2Hn) 50.73(aH2)(x)

LEVEL (10) 000 20.220w2HO 20.2203(aH3Han 50.73(a)(2Hm>

73.71 20.2203(aH2Hn) 20.2203(aH4) 50.73(aH2Hiv)

OTHER 20.2203(aH2Hu0 50.36(cH1)

X 50.73(aH2Hv) specify In Abstract below 20.2203(aH2Hiv) 50.36(cH2) 50.73(aH2Hvn) in NRC Form 366A LICENSEE CONTACT FOR THis LER O2)

NNE TELEPHONE NUMBER linClude Aree Codel R. G. Joshi, MP2 Nuclear Licensing (860)440-2080 COMPLETE ONE LINE FOR EACH COMPONENT FAILURE DEsCRIBEDIN THis REPORT (13)

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CAUSE

SYSTEM COMPONENT MANUFACTURER

CAUSE

SYSTEM COMPONENT M ANUF ACTURE R PRDS PRDS SUPPLEMENTAL REPORT EXPECTED (14)

EXPECTED MONTH DAY Y?AR submission [f

X No DATE M S) 12 16 M

es, complete E XPECTED sUBMISSloN DATE).

ABSTRACT (Limit to 1400 spaces, t.e., approximately 15 single-spacedtypewrittentines) (16)

Cr April 15,1997, it was determined that the potential existed for waterhammer and two-phase flow to occur in the Containment Air Recirculation (CAR) Cooler piping and therefore, could cause the piping to fail such that the CAR Coolers may not be able to perform their safety function during accident conditions. In addition, it was also determined that certain containment penetrations were susceptible to thermally induced overpressurization On September 30,1996, the Nuclear Regulatory Commission informed licensees in Gene'ric Letter (GL) 96-06 that the potential existed for the containment air cooler water systems to be susceptible to either waterhammer or two-phase j

flow conditions % ring postulated accident conditions. GL 96-06 requested that licensees evaluate this condition and j

additionally determine if piping systems that penetrate containment are susceptible to thermally induced overpressurization when isolated. On January 28,1997, Northeast Nuclear Energy Company (NNECO) responded to the requested actions identified in Generic Letter 96-06 and stated that any issues discovered would be reported in accordance with the provisions of 10 CFR 50.73. It was determined that the Reactor Building Closed Cooling Water j

System (RBCCW) and certain containment penetrations were susceptible to the conditions identified in GL 96-06.

j The cau"

.he event was inadequate consideration of potential failure modes during the system initial design and design basis venfication Corrective actions include finalizing calculations to determine the effects of conditions identified in GL 96-06 and completing evaluations of containment piping penetrations susceptible to thermally induced overpressurization dunng accident conditions. Based on the results of these evaluations, actions will be taken as necessary.

9705UO156 970515 PDR ADOCK 05000336 S

PDR

NHC FOHIVI 366 A u.S. NUCLE AR REGULATORY COMMISSION i4 95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (U ooCKET LER NUMBER (6)

PAGE 13)

SEQUENTIAL REVisloN YEAR NUMBER NUMBER 2OF5 Millstone Nuclear Power Station Unit 2 05000336 97 015 00 TEXT (If more spaceis required use additionalropres of NRC Form 366Al (11) 1.

Description of Event

On April 15,1997, it was determined that the potential existed for waterhammer and two-phase flow to occur in the Containment Air Recirculation (CAR) Cooler [BK) piping and therefore, could cause the piping to fail such that the CAR Coolers may not be able to perform their safety function during accident conditions. In addition, it was also determined that certain containment penetrations were susceptible to thermally induced overpressurization. On September 30,1996, the Nuclear Regulatory Commission informed licensees in Generic Letter (GL) 96-06 that the potential existed for the containment air cooler water systems to be susceptible to either waterhammer or two-phase flow conditions during postulated accident conditions. GL 96-06 requested that licensees evaluate this condition and additionally determine if piping systems that penetrate containment are susceptible to thermally I

induced overpressunzation when isolated. On January 28,1997, Northeast Nuclear Energy Company (NNECO) responded to the requested actions identified in Generic LeNr 96-06 and stated that any issue-tiscovered would be reported in accordance with the provisions of 10 CFR 50.73. It was determined that the Reactor Building Closed Cooling Water System (RBCCW)[CC), which supplies cooling water to the CAR Coolers, and certain containment penetrations w :....,... ale to the conditions ic'ent:'ied in GL 96-06. At the time of discovery of this event, the unit was defueled.

The potential for two-phase flow was postulated to occur during a Loss of Offsite Power (LOOP) coincident with a design basis Loss of Coolant Accident (LOCA). The high heat content of containment atmosphere under accident conditions being drawn over the CAR Fan heat exchanger coils prior to initiation of pumped liquid flow has been calculated to result in reaching saturation conditions for the liquid in the coils. When the RBCCW pumps are ie-energized, the pumped liquid flow acts to collapse the steam void in the CAR Cooler piping. The resulting waterhammer could affect the integnty of either the heat exchanger coil or its associated piping, resulting in a loss of integnty of one or both trains of the RBCCW system and a loss of containment integnty.

This event is being reported in accordance with 10 CFR 50.73(a)(2)(v)(D), any condition that alone could have prevented the fulfillment of the safety function of structures or systems that are needed to mitigate the consequences of an accident. This criterion required a prompt report which was made on April 15,1997 in accordance with the requirements of 50.72(b)(2)(iii).

In NNECO's response to Generic Letter 96-06, actions that were taken to determine the susceptibility for waterhammer and two phase flow in the RBCCW system included:

1. Development of a thermal-hydraulic model of the RBCCW system.

2. EE. ablishment of containmen; pressure and temperature conditions for the evaluation of the CAR Cooler performance during LOCA and Main Steam Line Break (MSLB) events.

3. Establishment of conditions.1nd assumptions for the evaluation of potential two-phase conditions in the RBCCW system.

4. Using the established containment and RBCCW system conditions, the existing thermal-hydraulic model was used to evaluate the RBCCW system for two-phase conditions and water hammer phenomena during LOCA and MSLB events.

5 Various modes of operation of the RBCCW system were analyzed for two-phase flow conditions following a LOCA.

M; FORM 366A 44 951

RC roRM 366A u.S. NUCLEAR REGULATORY COMMISSION LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET LER NUMBER (6)

PAGE (3)

SEQUENTIAL REVISION AR NUMBER NUMBER 3OF5 Millstone Nuclear Power Station Unit 2 05000336 97 015 -

00 TEXT lit more space is required, L se additronalc 3poes of NRC Form 366A) (17)

6. The heat loads imposed on the CAR coolers during both LOCA and MSLB events were evaluated. Only the injection phase modes of operation of the RBCCW system were considered following a MSLB because a containment sump recirculation is not expected to occur dunng this event. Injection and recirculation mode of operation were considered for the LOCA event.

Additionally, determinations were made if the piping systems that penetrate containment are susceptible to thermally induced overpressurization during either normal or post-accident conditions. An initial screening process determined that eighty-nine (89) penetrations to the unit's containment were potentially susceptible to overpressurization conditions due to fluid thermal expansion during either normal or post-accident conditions. Of the 89 penetrations that were evaluated, it was determined that nine (9) had the potential for thermally induced overpressurization.

II.

Cause of Event

The cause of the event was inadequate consideration of po%ntial failure modes during the system initial design and design basis venfication. For postulated accident cond:tions, the potential for steam formation within the RBCCW side of the CAR Cooler and subsequent waterhammer, and the potential for thermally induced overpressurization of piping which penetrates containment was not addressed by the system design.

4 111. Analysis of Event The function of the Containment Air Recirculation and Cooling System is to remove heat from the containment atmosphere during normal operation. The system has two redundant, independent and separate subsystems, each consisting of two CAR cooling units. In the event of a LOCA or MSLB accident, the system, in conjunction with the containment spray system, provides a means of cooling the containment atmosphere to reduce the containment building pressure and thus reduce the leakage of airborne and gaseous radioactivity. The RBCCW system provides cooling for various primary plant components credited for normal plant operation and for performance during and following design basis events. The !BCCW functions to transfer heat from the primary r

components to the service water system and then to the ultimate heat sink. There are two independent trains of RBCCW, each capable of cooling all plant equipment that is required to be operable during and following a design basis event.

Two-phase flow (i e., both steam and liquid) in cooling water systems associated with the CAR Coolers can significantly affect the ability of the containment air coolers to remove heat under design basis accident (DBA) conditions, and can interfere with the cooling of other safety, elated components. These cooling water systems were designed assuming single-phase flow conditions (i.e. liquid only) and containment heat transfer analyses are based on this assumption.

The potential for two-phase flow was postulated to occur during a LOOP coincident with a design basis LOCA.

The high heat content of containment atmosphere under accident conditions being drawn over the CAR Fan heat exchanger coils prior to initiation of pumped liquid flow has been calculated to result in reaching saturation conditions for the liquid in the coils. When the RBCCW pumps are re-energized, the pumped liquid flow acts to collapse the steam void in the CAR Cooler piping The resulting waterhammer could affect the integrity of either the heat exchanger coil or its associated piping, resulting in a loss of integrity of one or both trains of the RBCCW system and a loss of containment integnty.

NRC FoFM 366A (4 9M

Eiid FORM 363A u.S. NUCLE AR REGULAl ORY COMMISSloN (4 95)

LICENSEE EVENT REPORT (LER)

TEXT CONTINUATION FACILITY NAME (1)

DOCKET LER NUMBER (6)

PAGE (3)

SEQUENTIAL REVISloN YEAR NUMBER NUMBER 4 OF 5 Millstone Nuclear Power Station Unit 2 05000336 97

- 015 -

00 iEXT fit more space us required, use additionalcopies of NRC Form 366M i17)

The conclusions that were reached in the response to GL 96-06, as reported on January 28,1997, relative to the susceptibility for waterhammer and two-phase flow in the Containment Air Cooler RBCCW System were:

1. Four locations within the RBCCW system were chosen to be evaluated for two-phase flow conditions. These locations were chosen based on elevations relative to the RBCCW Surge Tank and the expected pressure and temperature conditions. They included, (a) the CAR Cooler Units X-35A and X-35B, (b) Control Element Drive Mechanism (CEDM) Coolers, (c) Reactor Coolant Pump (RCP) Coolers and Lube Oil Coolers and, (d)

Shutdown Cooling Heat Exchangers.

2. In the event of a LOCA or MSLB, with no concurrent LOOP, preliminary results indicated that so long as the RBCCW pumps remained in operation, two-phase flow conditions would not occur in the RBCCW. Therefore, neither degradation of system performance nor waterhammer events would occur under these conditions.

3. In the event of a LOCA or MSLB, concurrent with a LOOP, preliminary results indicated that some void formation would occur in the CAR coolers (X-35A and X-358) and mcy occur in the CEDM coolers. Voiding in the CEDM coolers was not expected to be significant due to the fact that weighted backdraft dampers limited coastdown flow from the CEDM cooler fans and the fans would not restart after a LOOP. No voiding was expected in the RCP Coolers and Lube Oil Coolers, or Shutdown Cooling Heat Exchangers.

In the event of a LOOP, the RBCCW Pumps and the CAR Fans would tnp. Since CAR Fan coastdown is slower than RBCCW pump coastdown, and the CAR Fans restart prior to the RBCCW pumps, the water in the j

upper CAR coolers would reach saturation temperature and voiding would occur in the higher cooling coils and i

at the cooler outlet. Following the restart of the RBCCW pumps, all system voids are expected to condense / collapse. The ongoing analysis has shown that either waterhammer or two-phase flow will occur in the CAR Coolers during post accident conditions.

4. A third condition was post'ulated to occur when an RBCCW pump initially fails to start after a LOOP and flow to the affected header is restored manually some later time during the event. This sequence of events could result in more significant voiding of the affected RBCCW system and could produce a more severe waterhammer condition.

Additionally, a potential for systems to fail to perform their safety function as a result of thermally induced overpressurization of piping systems that penetrate containment was identified as a concern. System and containment integrity could be challenged where piping systems penetrate the containment wall. Of the 89 penetrations that were evaluated, it was determined that nine (9) had the potentialfor thermally induced overpressurization.

Due to the potential of thermally induced overpressurization and the effects of either waterhammer or two-phase flow conditions that could compromise the containment penetration integrity and RBCCW system capability during accident conditions, this condition is considered to be potentially safety significant.,

o RRC FORT 3 366A U.S. NUCLEAR REGULATORY COMMISSION (4 95)

LICENSEE EVENT REPORY (LER)

TEXT CONTINUATION FACILITY NAME (1) oOCKET LER NUMBER (6)

PAGE (3)

SEQUENTIAL REVISION YEAR NUMBER NUMBER 5 OF 5 Millstone Nuclear Power Station Unit 2 05000336 97 -- 015 -

00 1 EXT ll1more space is required. use addstionalcopies of NRC Form 366A) (17)

IV. Corrective Action

As a result of this event, the following actions have been, or will be, performed.

NNECO's response to GL 96-06 (NNECO Commitment No. B16104, January 28,1997) committed to the following prior to entering Mode 4 from the current outage:

1.

Finalize calculations to determine the effect of RBCCW system steam voiding during LOCA and MSLB conditions.

2.

Coniplete evaluation of containment piping penetrations susceptible to thermally induced overpressurization during LOCA and MSLB conditions.

Based on the results of these evaluations, actions will be taken as necessary to preclude or mitigate unacceptable results of the RBCCW DBA steam voiding and thermally induced overpressurization of piping systems that penetrate the containment prior to entering Mode 4 from the current outage.

A supplemental LER will be issued to provide the results of the evaluations and resultant corrective at,tions.

V.

AdditionalInformation

Similar Events

No previous similar events involving performance deficiencies with the CAR Coolers were identified.

Energy Industry Identification System (Ells) codes are identified in the text as (XX].

NRC FOnM 3f 6A (4 95