ML20012D480

LER 90-003-00:on 900215,identified Potential Single Failure Scenario That Could Result in Unavailability of Both Core Spray Sys for Long Term post-LOCA Core Cooling.Detailed Heatup Evaluation Performed for Power bundle.W/900316 Ltr

ML20012D480
Person / Time
Site:	Grand Gulf
Issue date:	03/16/1990
From:	Byrd R, Cottle W SYSTEM ENERGY RESOURCES, INC.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
AECM-90-0052, AECM-90-52, LER-90-003, LER-90-3, NUDOCS 9003270394
Download: ML20012D480 (6)

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Vce Presdent I ; Noelcar Operahons i

March 12, 1990 U.S. Nuclear Regulatory Commission Mail Station P1-137 Washington, D.C. 20555 Attention: Document Control Desk Gentlemen:

SUBJECT:

Grand Gulf Nuclear Station Unit 1 Docket No. 50-416 License No. NPF-29 Loss of ESF Division 1 Power Could Effect Long-Term Post-LOCA Core Spray LER 90-003-00 '

AECH-90/0052 Attached is Licenseo Event Report (LER)90-003 which in an Interim report.

Yours truly, wrC%-

WTC:cg

-Attachment cc: Mr. D. C. Illutz (w/a)

Mr. T.11. Cloninger (w/a)

Mr. R. B. McGehee (w/a) gg"g Mr. N. S. Reynolds (w/a)

Mr.11. L. Thomas (w/o) lir. II. O. Christensen (w/a) w Mr. Stewart D. Ebneter (w/a)

F>O@$o Regional Administrator 88 U.S. Nuclear Regulatory Commission

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.g Mr. L. L. Kintner, Project Manager (w/a) orr Of fice of Nuclear Reactor Regulation

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.On February 15, 1990, System Energy Resources, Inc. (SEkI) identified a

! potential single failure scenario that could result in the l- unaval'1 ability of both core spray systems for long term post-LOCA core i cooling.- UFSAR Section 6.3.3.2 states that criterion 5 of 10CFR50.46 is

l. demonstrated generically for General Electric (GE) BWRs in that the core remains covered to at least the jet pump suction elevation and the uncovered region is cooled by core spray. In a postulated LOCA scenario-where the single failure is the ESF Electrical Division 1, the Low Pressure Core Spray (LPCS) system and the Standby Service Water (SSW)

I "A" system would not be available. This would leave the High Pressure L Core Spray (HPCS) system (ESF Electrical Division 3) the only available l core spray. In the GGNS design, the HPCS service water and the Division 1 service water both return via the SSW "A" spray header. The relatively small return flow of the HPCS service water, without added SSW return flow from Division 1 components, would be insufficient to provide effective spray over the SSW "A" cooling tower fill. After approximately 50 to 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br />, HPCS service water temperature could exceed the 90 degrees F design temperature, and the availability of the HPCS system may not be assured. GE has performed an evaluation which demonstrated ECCS criteria are met assuming no credit for spray cooling after the initial 50 hours5.787037e-4 days <br />0.0139 hours <br />8.267196e-5 weeks <br />1.9025e-5 months <br /> of HPCS operation. SERI considers this evaluation adequate for interim operation and is evaluating the ultimate solution to the condition.

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A. Reportable Occurrence:

On February 15, 1990, System Energy Resources, Inc. (SERI)

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identified a condition that could affect the availability of the High Pressure Core Spray'(llPCS) system (EIIS code: BG).for long term post-LOCA core cooling. This condition is-reported pursuant to 10CFR50.73(a)(2)(v).

B.- Initial Condition:

=The plant was operating at 100 percent power at the time of.

discovery.

C. Description of Occurrence:

Criterion 5 of the acceptance criteria for_ECCS Systems in 10CFR50.46 states, "After any calculated successfully initial operation of the ECCS, the calculated core thermal temperature shall be maintained at an acceptably low value and decay heat .

shall be removed for the extended period of time required by the L long-lived radioactivity remaining in the core." UFSAR Section 6.3.3.2 states that conformance to this criterion is demonstrated

[ generically for General Electric (GE) BWRs in that the core I remains covered to at least the jet pump suction elevation and i the uncovered region is cooled by spray cooling.

1.

During a special review conducted as part of the Technical Specification Improvement Program, SERI design engineers identified a situation in.which the HPCS system could become unavailable after about 50 to 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br /> of: post-LOCA operation.

In a postulated LOCA scenario where a failure of ESF Electrical Division 1 occurs, the Division 1 Emergency Core Cooling (ECC)-

systems and support systems (Low Pressure Core Spray, Low Pressure Coolant Injection "A", and Standby Service Water "A")

would not be available for long term core cooling, llPCS, which 1- is powered from ESF Electrical Division 3, would be the only available core spray.

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D. Apparent'Cause: l i

Conformance to Criterion 5 of 10CFR50.46 was demonstrated generically for GE BWRs in section III. A of NEDO-20566, GE's LOCA Topical Report, submitted August 1974. The GGNS FSAR referenced this Topical Report and stated that the uncovered region of the core could be cooled by core spray and/or steam generated in the covered part of the core. GE identified concerns in 1984 that LPCI injection, inside the shroud, could subcool the water within

, some of the fuel bundles and thereby result in a reduction of long-term steam. cooling capability. The FSAR was then revised to_ '

clarify that core spray was the_only method utilized at GGNS for long-term cooling of the uncovered region of the core.

During- the 1984 reviews of the GE Topical Report and concerns raised by'GE over steam cooling, SERI did not recognize that the spray nozzle distribution concern could adversely affect HPCS.

long-term cooling availability. Thus, core spray with either LPCS or HPCS was deemed an acceptable means of long-term cooling for the uncovered area of the core.

E. Supplemental Corrective Actions:

GE completed'its specific evaluation for Grand Gulf Nuclear Station on February 22, 1990. 'The results of the evaluation confirmed that. adequate _long-term post-LOCA core cooling is assured with the two Division 2 LPCI systems. The GE evaluation demonstrated' compliance with all ECCS criteria using established conservative analytical techniques. .SERI'is investigating alternatives for final resolution of this concern. An update report will be' submitted by April 20, 1990 providing our planned resolution of this concern.

F. Safety Assessment-Thermal hydraulic evaluations performed by GE demonstrate that operation of the two LPCI pumps (LPCI "B" and LPCI "C") for long-term cooling will result in substantial spill over of water into the upper plenum which then drains through the fuel bundles.

This flow rate alone may provide adequate cooling to fuel bundles; however, equal distribution of the flow could not be a demonstrated. A detailed heatup evaluation was performed for '

some high power bundles assuming no cooling water flow from the upper plenum. This evaluation showed that long-term peak cladding temperatures (PCT) and local oxidation values were acceptable under the requirements of 10CFR50.46. Additional conservatism included in the GE evaluation are: a 48 hour5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> basin g ..

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heatup time was used when a 60 hour6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br /> heatup time was demonstrated by ,

calculations; no credit was taken for basin cooling capabilities;

• the time limit was set by a 90 degree F llPCS Service Water temperature rather than the ultimate temperature at which equipment is expected to fail; and no credit was taken for lower ,

than the design APLilGR conditions in the upper third of the core.

Long1 term PCT and local oxidation values of approximately 1580 degrees F and 16 percent were obtained. Therefore, compliance with 10CFR50.46 is maintained and thera are no adverse safety consequences that result from the HPCS post-LOCA long-term availability. concerns.

In evaluating long-term post-LOCA core cooling without core

.. spray, GE used the 1979 version of ANSI /ANS 5.1 for calculation of decay heat load. The 1979 ANS 5.1 Decay IIcat Model differs from the previously used 1971 version by a factor of 1.2

-primarily due to differences in the decay heat uncertainty factors. The use of the 1979 standard is conservative and consistent with 10CFR50.46(a)(1)(1) in that sufficient supporting justification exists to show that the analytical technique realistically describes the behavior of the system during a LOCA, 10CFR50.46 was amended in 1988 to allow licensees to use either

. Appendix K or a realistic evaluation model (such as the 1979 ANS 5.1 Decay lleat Model). . In addition, Regulatory Guide 1.157 was issued in May, 1989 to describe alternate methods of implementing 10CFR50.46(a)(1). The Regulatory Guide endorses the 1979 edition of ANS 5.1 as-a generically acceptable method of calculating fission product decay heat. SERI. considers the application of the 1979 edition for determining the interim operability of the llPCS system and long-term post-LOCA core cooling capabilities to be acceptable and recognizes that the use of the standard as a licensing basis would have to be further justified to the NRC staff.

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By design, the HPCS service water system (EIIS code: BS) circulates cooling water from the SSW "A" basin through Division 3 components for heat removal and then discharges back into the SSW "A" basin. In the event that a loss of ESF Electrical Division 1 occurs, the relatively small return flow of the HPCS service water, without the added SSW return flow from Division 1 ,

1 components, would be insufficient to provide effective spray over the SSW cooling tower fill.- After a period of time (approximately 50 to 60 hours6.944444e-4 days <br />0.0167 hours <br />9.920635e-5 weeks <br />2.283e-5 months <br />) the HPCS Service Water temperature could exceed the'90' degrees F design temperature. Thus, long-term cooling of the HPCS service water and consequently the availability of the HPCS system may not be assured.

SERI conferred with General Electric after identifying this situation. GE performed an initial evaluation on February,15, 1990, which concluded that long term cooling could be ensured t after 48 hours5.555556e-4 days <br />0.0133 hours <br />7.936508e-5 weeks <br />1.8264e-5 months <br /> without a core spray system based on the availability of the Division 2 Low Pressure Coolant Injection (LPCI) systems.

As GE continued with more specific evaluations to validate the conclusion, SERI developed contingency plans for the aforementioned scenario which will remain in effect until final resolution of the concern. In the event that Division 1  :

Electrical power supplies were unavailable but the distribution system was functional, the Division 3 Diesel Generator would be aligned'to the Division 1 distrib.ution system to utilize the Low Pressure Core Spray (LPCS)' system for core spray. If the Division 1 Electrical distribution system was not functional, measures would be taken to transfer water between the two SSW basins in an attempt to maintain the Division 1 SSW basin as cool as possible. Three different methods judged to be the most viabic in accomplishing the basin water transfer were established which include utilization of either the Division 2 SSW pump or temporary pumps as needed.

These plans were developed as interim and immediate response to the issue until the GE evaluation could be completed. The GE evaluation has been successfully completed and is described in the Safety Assessment below. To provide additional assurance the interim compensatory plans will remain in effect until the concern is resolved.

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