ML20198B479

Forwards Evaluation of Seismic & Other External Events Related Portions of Info Submitted by Licensee of South Texas Project in Response to GL 88-20,Suppl 5,individual Plant Exam for External Events

ML20198B479
Person / Time
Site:	South Texas
Issue date:	03/30/1995
From:	Shao L NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
To:	Hodges W NRC OFFICE OF NUCLEAR REGULATORY RESEARCH (RES)
Shared Package
ML20198B394	List: ML20198B387 ML20198B411 ML20198B479
References
GL-88-20, NUDOCS 9812180175
Download: ML20198B479 (11)

Text

- - - - - - - - - - - -

March 30, 1995 i ~ HEM 0'lANDUM To: Wayne M. Hodges, Director l

l Division of Systems Technology, RES .

FROM:  !

Lawrence C. Shao, Director &isinal signed by Imrence C. Shao Division of Engineering Technology, RES l

SUBJECT:

STAFF EVALUATION REPORT OF SOUTH TEXAS l IPEEE SUBMITTAL Attached is our evaluation of seismic and other external events related i portions of information submitted by the licensee of South Texas Project (STP) l in response to Generic Letter 82-20, Supplement 5, Individual Plant Examination for External Events (IPEEE). We have not reviewed fire related portions.

I The licensee's IPEEE submittal has relied on an early probabilistic safety l analysis (PSA) conducted for STP units. Since the staff reviewed the PSA and l issued a Safety Evaluation Report, the current staff evaluation supplements l l the earlier evaluation to draw conclusions with respect to objectives of an IPEEE.

In summary, we conclude that the licensee's studies conducted in support of IPEEE meet the intent of the generic letter, and no vulnerabilities to the i

seismic and other external events have been identified. l The attached staff evaluation was prepared by T.Y. Chang of the Division of l l( Engineering Technology. Please contact him or N. Chokshi should you have any questions.

Attachment:

As stated cc w/att:

E. Beckjord D. Morrison T. Speis

)ISTRIBUTION:

Iranch r/f C. Serpan P. Kadambi RES 2C-5 M. Drouin LCS (Sig File)

J. Murphy J. Chen M. Cunningham G. Bagchi l DOCUMENT NAME: g:\stp.ser n . n., - ~ s. r c.n m a m ,.c.n e n n u-u.-

0FFICE EIB:RES SGEB:RES l (._ SGES:RES C DD:DEj W M l D:Dff:MS- lE

[ NAME Tchang prg NChokshi 4 4 AMurphy 4/4f JCr/pq4/ P LSNhru l DATE 3/M/95 ~ 3/10/95 3/30/95 ' 3/ 6 /95 ' 3/n/95 l

l 9812180175 981215 PDR ADOCK 05000498 P PDR 4

~

d. .

, SOUTE TEXAS PROJECT I

outline of Staff Evaluation Report on IPERE Review ]

1.0 EACKGROUND (;

2.0 SEISMIC EVEETS 2.1 STP PSA Results 2.2 Methodology 2.3 GL 88-20. sunnlement 4 Enhancements  : ,

l 2.3.1 Plant Walkdowns 2.3.2 Relay Chatter .

2.3.3 Liquefaction 2.4 fontainment Performance g i

s 2.5 Staff conclusions !I 3.0 EXTERNAL FLOODS

_ 3.1 STP PSA Results i

3.2 Staff conclusions '

j (-

4.0 OTHER EITERNAL EVENTS 1

. 4.1 STP PSA Results '

l

4.1.1 High Winds 1

1 4.1.2 Aircraft Crash-4.1.3 Transportation and Other External Hazard 4.2 staff conclusions . 1 1

5.0 OVERALL STAFF CONCLUSIONS

6.0 REFERENCES

Y t

v-. 1 - , , -

l ,

. SOUTE TEXAS PROJECT

( l Staff Evaluation Report on IPEEE Review  ;

1.0 EACKGROUND l Generic Letter 88-20, Supplement 4 (with enclosed NUREG-1407, l Procedural and Submittal Guidance, Ref. 1) was issued by the NRC

on Jun6 28, 1991, which requested that all licensees perform I

) individual plant examination of external events (IPEEE) to l l

' identify plant-specific vulnerabilities, if any, to severe I accidents and report the results together with any licensee-

{ determined improvements and corrective actions to the commission.

f In a letter dated December 23, 1991 to the NRC (Ref. 2), Houston

Lighting & Power Company (HL&P) stated that the South Texas i

Project Probabilistic Safety Assessment (STP PSA) report for the

! South Texas Project Electric Generating Station (STPEGS) submitted to the NRC on June 15, 1989 (Ref. 3) constituted the completion of the STP IPEEE and the information therein satisfied the information requested in Supplement 4 of GL 88-20. On August 28, 1992, HLEP further submitted " Level 2 PSA and IPE" (Ref. 4) i to the NRC, which addressed expected plant performance and <

l vulnerabilities for both internal and external events. HL&P

{ provided additional information subsequently on October 21, 1992 i (Ref. 5) and January 14, 1993 (Ref. 6) to supplement previous STP PSA and in response to NRC staff questions.

k The original STP PSA was reviewed by the staff and a Safety Evaluation Report (SER) related to external events (without fire) was issued on August 31, 1993 (Ref. 7). The present Staff Evaluation Report supplements the earlier SER by discussing issues associated with the use of existing PSA to conduct IPEEE studies.

Among the five external event.s requested to be assessed by supplement 4 of GL 88-20, review of seismic events, external floods, and other external events (high winds and tornadoes, aircraft crash, transportation and other external hazards) are covered in this input to the Staff Evaluation Report.

2.0 pBI8MIC EVENTS 2.1 STP PSA Results Most of the evaluations made in the previous staff SER on STPEGS external events (Ref. 7) still apply for the IPEEE. However, because of the new results of STPEGS level 2 PSA (Ref. 4), the total core melt frequency (including both internal and external events) was reduced from 1.7E-4/yr (from level 1 PSA result, Ref.

3) to 4.4E-5/yr. Therefore, some of the findings regarding numerical values in the " Summary and conclusions" section of the

( _

1

I l .

j( afore-mentioned here to reflect this staffchange. SER on STPEGS external events are modified I

' The STP level 1 PSA (Ref. 3) compared the frequencies of the seismic-initiated plant damage states and core damage with the

frequencies resulting from all initiatcra to determine if any  ;

3 i seismic and to core scenarios damage.were major contributors to a plant damage state '

i By such comparison HP&L had detarmined that i i the seismic contribution to core damage is less than 0.1% of the total based on the EPRI seismic hazard curve. This number would i

j change to less than 0.3% if the reduced total core melt frequency ,

from STP level 2 PSA (Ref. 4) is used. In either case, two plant 1 damage states (PDSs), designated as HXXXS and MAXXS, dominated the seismic-initiated core damage and account for about 45% and

! 37% respectively of the seismic CDF. The major contributors to j

PDS HXXXS are seismically induced loss of offsite power

{ coincident with seismic failures of onsite electrical components, i

j and loss of AFW storage tank, leading to the loss of DC control power and feed water to the turbine-driven AFW pump. The major j contributors to PDS MAXXS are loss of the essential chilled water system, leading to the loss of 4.16KV switchgear and AC power, j followed by the loss of CCW.

4

{ 2.2 Methodoloav l' For the seismic review of the IPEEE (hereafter referred to as

" seismic IPEEE"), the two units of the STPEGS were classified by l{ Supplement 4 of GL 88-20 as " Reduced Scope" plants east of the

! Rocky mountains.

j The licensee used an existing seismic probabilistic safety assessment (PSA) contained in the STP PSA (Ref.3) together with j

enhancements suggested in Supplement 4 of GL 88-20 for the i seismic IPEEE. This approach was one of the two methodologies j listed in Supplemsnt 4 of GL 83-20 for the seismic IPEEE and was found by the staff to be acceptable. These enhancements are

.I described below for STP seismic IPEEE.

l 4

2.3 GL 88-20. Sunclement 4 Enhancements l 2.3.1 Plant Walkdowns a

! The two units of STPEGS are relatively new (C0 dates are 1988 and 4

1989 respectively for Unit 1 and 2), and there is in general a i higher requirements on the seismic design and construction on these never plants as compared to the older vintage ones.

j In spite of the new vintage of the plants, HL&P conducted several j walkdowns (Ref. 4), each of which were focused to address i specific severe accident issues. In particular, walkdowns were i performed to address PSA level 1 seismic, human factors, internal

!' fires, and PSA level 2 containment performance issues. For 4

2 4

4 l

1

t' seismic walkdown, the scope included the review of structures, .

( systems, and components for fragility analysis, spatial interactions, and seismic margins analysis. In addition, the NRC staff participated in a walkdown (May 30-31, 1990) to familiarize  !

i itself with the plant and make judgements regarding the assumptions made in the PSA. Therefore, it is concluded that '

walkdowns conducted during the STP PSA development satisfy the intent of GL 88-20, Supplement 4.

2.3.2 Relay Chatter Relays, in this context, include components such as electric relays, contactors, and switches that are prone to chatter.

NUREG-1407 stated that for plants in the reduced scope, relay chatter reviews are not necessary. In addition, because of the newer vintage, the two units of the STPEGS are not USI A-46 plants. Therefore, no action is needed for STPEGS regarding relay chatter review.

2.3.3 Liquefaction i According to NUREG-1407, no evaluation is necessary on the potential for soil liquefaction and associated effects on reduced scope plants such as the STPEGS.

2.4 Containment Performance

( HL&P, in its August 28, 1992 (Ref. 4) letter to the NRC, submitted results of a level 2 PSA, which has a full-scope, up-to-data containment analysis. Containment vulnerability was addressed for both internal and external events.

In general, HL&P found from its level 2 PSA that STPEGS has a very low potential for loss of containment integrity and no apparent containment vulnerabilities exist that have not already been identified and eliminated. ~In the early stages of STP PSA program, enhancements were made to selected containment isolation valves to reduce the' likelihood that a sequence involving loss of AC power would result in an inability to isolate any open

• containment isolation valves. The level 2 PSA has not uncovered additional opportunities to enhance containment performance-via hardware modifications. In conclusion, no vulnerability to the performance and integrity of containment was identified by HLEP due to earthquakes.

2.5 Staff conclusions With some mi or modifications, most of the conclusions below are taken from the previous staff SER on the STPEGS external events (Ref. 7).

( -

3 l

1 .

1 i .

(1) When EPRI seismic hazard curve and total core damage

' ( frequency from STP level 2 PSA are used, the seismic J

contribution to core damage is less than 0.3% of the total.

(2) The fragility approach used in this PSA is an acceptable i approach used in the past PSAs. Limited review of the i calculated fragility values indicate that values are similar j

to those used in other studies. The use of puddle weld for i

anchoring 4.16KV switchgear would be a concern at sites with '

i higher seismicity as this represents a non-ductile i !

construction, therefore, as a prudent engineering practice, I i the licensee may want to investigate an alternate anchoring l j system in the future at an opportune time. -

4 l (3) The seismic PSA methodology used by the licensee is a state-s of-the-art approach used in many seismic PSAs.

l (4) In accordance with NUREG-1407, the IPEEE submittal should contain information to address other seismic issues such as USI A-45, GI-131 and USI A-46.

{;

HL&P, after an evaluation of l NUREG-0933, Rev. 12, "A Prioritization of Generic Safety i Issues" and the level 2 PSA, concluded (Ref. 4) that none of

• i USI A-45 and GI-131, nor any of the current GSIs or USIs are vulnerabilities or risk outliers, and each of these issues i were resolved by the level 2 PSA, which included external j events. USI A-46 was determined to be not applicable to i

)

STPEGS, since the two units of STPEGS are not A-46 plants. l

!(

1 (5) HL&P, in conclusion, determined that no seismic

}

vulnerabilities was found at STPEGS in its review. I i

3.0 EITERNAL FLOODS

{

i i 3.1 STP PSA Results t

l. HL&P, based on an analysis of three fundamental sources of

! flooding at the site (namely, storms, off-site dam breaks, and

on-site das breaks), constructed (Ref. 3) a set of events that l identified those failure causes and mechanisms that could result -

in site flooding, and determined that the source event of the greatest importance with respect to risk is the failure of the

. main cooling reservoir (MCR). NL&P then examined a set of I

failure scenarios specific to the MCR, and q'uantified the I likelihood of MCR breach. Then a set of scenarios impacting ~

! plant equipment due to the MCR flooding event was identified, and

! the annual core damage frequencies of the scenarios that follow a i flood-induced loss of off-site power and flooding of risk-related i equipment leading to core damage were determined. The total l

! frequency of the external flood-initiated scenarios is 2.1E-8,  !

] which has negligible contribution to the total annual core melt

frequency of all initiating events.

l(-

4 l

i .

e l .*

• s.

HL&P, however, in responding to staff questions, stated that j (.

(Ref. 4):

j (1) The failures of the proposed Columbus Band Dan has not been i included in the STP PSA since it has yet to be constructed.

} However HL&P believed failure of both the existing upstream dans

(Mansfield, Buchanan and S. W. Freese dams) and the proposed upstream dam will not raise the flood level above the design

! basis flood resulting from breach of the MCR embankment. HL&P i further stated that it will include the effect of the failure of

the upstream Columbus dam in addition to Mansfield, Buchanan and S. W. Freese dans in an update of the STP PSA at the time j Columbus Band Dam is constructed.

i

(2) No impact on the essential cooling pond (ECP) by external

flooding has been included in the PSA since HL&P expects such

! impact to be negligibly small, because of its design assumptions '

) concerning the ECP. Furthermore, the MCR water does not provide l a safety-related function. The south embankment of the ECP is '

designed to withstand the effects of an MCR embankment breach.

, Therefore, the loss of the MCR as the result of an MCR embankment

]

! breach does not impact the availability of safety-grade cooling j i water (through ECP) for the plant's essential safety features.  ;

) HL&P has, therefore, not included the impact on the MCR by  ;

i external flooding in the PSA. l subsequently, the licensee informed the NRC (Ref. 6) that it has

( also evaluated potential of flooding caused by the water from the i ECP or MCR flowing freely by gravity in the event of valve j failures in the connecting lines, and concluded that the l possibility of such an event is negligible.

The staff, in its SER on STPEGS external events (Ref. 7),

{ generally agreed with HL&P's above statsments.

d 3.2 Staff conclusions ,

{ The following conclusions are taken from the previous staff SER j on the STPEGS external events (Ref. 7). .

i The core damage frequency due to external flooding has been j estimated at 2.1E-8 in the PSA. The failure of the proposed i

Columbus Bend dan upstream of the site has not been included in the PSA since it has not been constructed yet. However, NL&P stated that the effect of the failure of both the existing and i the proposed upstream dans will not raise the flood level above

the design basis flood resulting from breach of the MCR embankment. ML&P further stated that it will include the effect of the failure of Columbus Band dam in addition to the existent

dans upstream in the STP PSA update after Columbus Bend dan has I been constructed. Therefore, as long as the design basis at the

site envelopes the potential failures of upstream dams, the .

! l 4

}( 5 i

I l

j .* *

• 6.

(, external flooding is not likely to be a significant contributor to core damage.

i 4.0 OTPER EXTERMAL EVENTS i 4.1 STP PSA Resulta 4

i During the NRC review of the STP PSA, the staff requested HL&P i (1) to compare the FSAR design criteria (flood level, wind speed, Rgs, etc.) with the 1975 Standard Review Plan guidance and 1

initiating event frequencies and corresponding criteria used in the PRA analysis in a tabular form; and (2) to report if any

'. changes have occurred at or near the site to alter the design information described in the FSAR.  :-

l -

ML&P provided the requested information in a March 15, 1991 i submittal (Ref. 8). The submittal stated that Table 1 therein (IPEEE Survey For STPEGS - Design Basis / Regulatory l l

Guides / Standard Review Plan) and its referenced sections of i STPEGS UFSAR showed that STPEGS in general conforms with the 1975 '

j SPP guidance. Ref. 8 is briefly discussed below.

l l 4.1.1 High Winds l The high wind design criteria, in terms of wind speeds, pressure j drops, and missiles, used at the STPEGS are generally in

conformance with guidance in the SRP. The STP PSA shows a j

( calculated mean annual frequency of a tornado at the site to be j 1.7E-5, and the annual frequency of excessive tornado wind speeds (in excess of 360 mph) to be 8.3E-9.

i 4.1.2 Aircraft Crash

! AspertheUpdatedFinalSafetyAnalysisReportfUFSAR)atotal l frequency of aircraft crash from both general aviation and US air i carriers is 2.9E-6. The STP PSA shows the corresponding i initiating frequency to be 7.0E-7. Therefore the aircraft crash

! scenario has no significant impact on the CDF.

J I In response to a staff question regarding the aircraft crash data i used in the PSA versus the data used in the UFSAR, HL&P has

} stated (Ref. 9) that the frequency of general aviation accidents i per mile in the PSA is 2.3E-7 while it is 1.8E-7 in the UFSAR.

l The reason for the difference is the use of slightly different j historical data bases. ELEP did not see any need to revise the j PSA in view of the insignificant quantitative difference in the i results.

4.1.3 Transportation and other External Hazurd

]

i

] These events were screened out based on a scoping study.

1 i

i {

4

\

6 i

a i._ .__ _ , _ _ _ - __

4.

4.2 staff conclusions

(

In the staff SER on the STPEGS external events (Ref. 7), the staff generally agreed with the above assessments made by HL&P.

5.0 OYERALL CONCLUSIONS The overall conclusions of the staff SER on the STPEGS external events (Ref. 7) are modified below to take into consideration (1) the updated information contained in the STPEGS level 2 PSA (Ref.

4) and (2) the IPEEE perspective. l (1) The contribution of the external event's (fire is not i evaluated in this Staff Evaluation Report) to the total core j damage frequency is insignificant. The total point estimate CDF from level 2 PSA (Ref. 4) due to both internal and external events is estimated to be 4.4E-5/yr. The CDF due

to total external events (including all the external events

{ and internal hazards, e.g., fires and floods) is 1.2E-6/yr.

! Thus the contribution to the total CDF frca the external i

events is less than 3% of the total CDP. The previous staff

SER on the STPEGS external events (Ref. 7), based on total l CDF of 1.74E-4/yr of the level 1 PSA (Ref.3), estimated a i

j contribution from the external events to be of less than 1%

of the total CDF. In either case, the staff judges that j external events are not a major contributor to core damage

, at STPEGS.

k i

(2) The reasons for the relatively low CDF due to external events can be contributed mainly to: (a) the site has relatively low hazard of certain external events, (b) the

! two units of STPEGS are relatively new and have modern l design, and (c) the two units of STPEGS each has three

! physically separated redundant safety trains, any one of ,

j which can successfully bring the plant to a safe condition j except in the very unlikaly event of a large-break Loss-of-

Coolant Accident. '.

l j (3) The staff concludes that the licensee has carried out

external event analyses using acceptable state-of-the-art j approaches used in many recent PSAs. External event j initiating event frequencies (e.g., seismic hazard) are also j derived using acceptable methods. The staff also agrees j with the licenses that the external events are not a major j contributor to core damage scenarios at STPEGS and no vulnerability has been found for the external events.

l 1

(4) The general purpose of the IPEEE, as mentioned in GL 88-20, Supplement 4, is for each licensee (a) to develop an i appreciation of severe accident behavior, (b) to understand 4

the most likely severe accident sequences that could occur at its plant under full power operating conditions, (c) to J

7 I

(

z.

.d.

, gain a qualitative understanding of the overall likelihood of core damage and radioactive material release, and (d) if 3

(

necessary, to reduce the overall likelihood of core damage and radioactive material releases by modifying hardware and procedures that would help prevent or mitigate severe accidents. The licensee's PSA is considered detailed enough 4

to meet these objectives.

The review of the level 2 systemic sequences has identified no vulnerabilities or risk-outliers. In addition, HL&P has 1

implamented an accident management program consisting of a i continuing risk-assessment activity and the formation of an i

Accident Management Assessment Committee and a Working l Group. The staff concluded that the STPEGS seismic IPEEE

! has met the intent of the IPEEE delineated in GL 88-20,

Supplement 4.

J

6.0 REFERENCES

l 1. USNRC Generic Letter 88-20, Supplement 4, " Individual Plant l Examination of External Events (IPEEE) for Severe Accident j Vulnerabilities - 10 CFR 50.54(f)," June 28, 1991.

l 1 2. Letter dated December 23, 1991 from S. L. Rosen of HL&P to

! USNRC, subject: South Texas Project Electric Generating i

i Station Units 1 & 2, Completion of the Individual Plant i Examination of External Events for Severe Accident  !

( Vulnerabilities.

?

d l 3. Letter dated June 15, 1989 from M. A. McBurnett, HL&P to j USNRC, subject: South Texas Project Electric Generating Station Units 1 & 2, Probabilistic Safety Assessment.

?

! 4. Letter dated August 28, 1992 from S. L. Rosen of HL&P to j USNRC, subject: South Texas Project Electric Generating

! Station Units 1 & 2, Level 2 Probabilistic Safety Assessment

{ and Individual Plant Examination.

1

! 5. Letter dated October 21, 1992 from W. J. Jump of HL&P to

}

i USNRC, subject: South Texas Project Electric Generating Station Units 1 & 2, Additional Information on the External

! Events Analysis in the PSA.

1

6. Letter dated January 14, 1993 from S. L. Rosen of HL&P to i USNRC, subject

South Texas Project Electric Generating i Station Units 1 & 2, Additional Information on the External

{ Events Analysis in the PSA.

1

7. USNRC SER on External Events PSA for South Texas Project, August 31, 1993.

f,

( 8 i

i 1

pf . *

o. ,

1

, s. Letter dated March 15, 1991 from A. W. Harrison of E&P to

/

\

USNRC, Subjects Additional Information on the External Events Analysis in the PSA.

9. Letter dated November 20, 1990 from M. A. McBurnett of E&P to ilSNRC,

Subject:

Additional Information on the External Events Analysis in the PSA.

t

.