ML023240262

Proposed License Amendment to Eliminate the Turbine Missile Design Basis

ML023240262
Person / Time
Site:	South Texas
Issue date:	11/14/2002
From:	Sheppard J South Texas
To:	Document Control Desk, Office of Nuclear Reactor Regulation
References
NOC-AE-02001335, STI: 31452163
Download: ML023240262 (14)

Text

Nuclear Operating Company South TCas Pro/cd LE-C GenrItM$ Stabon P.

Box 289 Wadworth Tcxas 77483 3 November 14, 2002 NOC-AE-02001335 10CFR50.90 STI: 31452163 U. S. Nuclear Regulatory Commission Attention: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852 South Texas Project Units 1 & 2 Docket Nos. 50-498, 50-499 Proposed License Amendment to Eliminate the Turbine Missile Design Basis STP Nuclear Operating Company (STPNOC) proposes to amend the operating licenses for South Texas Project Units 1 and 2 to delete the UFSAR turbine missile design basis.

STPNOC has determined that the turbine missile contribution to risk is so small that no special measures are required beyond normal commercial operating practices.

STPNOC determined the proposed change requires prior NRC approval as a departure from a methodology as described in 10CFR50.59(c)(1)(viii). The safety evaluation, environmental evaluation, and determination of no significant hazards are attached.

The STP Plant Operations Review Committee has reviewed the proposed amendment and recommended it for approval. The STP Nuclear Safety Review Board has approved the proposed amendment.

STP Nuclear Operating Company requests 30 days for implementation of the amendment after approval.

NOC-AE-02001335 Page 2 If there are any questions, please contact Mr. A. W. Harrison at 361-972-7298 or me at 361-972-8757.

I declare under penalty of perjury that the foregoing is true and correct.

Executedon: I[,

-o.

J. J. Sheppard Vice President and Assistant to the President and CEO awh

Attachment:

Description of Changes and Safety Evaluation

NOC-AE-02001335 Page 3 cc:

(paper copy)

(electronic copy)

Ellis W. Merschoff Regional Administrator, Region IV U.S. Nuclear Regulatory Commission 611 Ryan Plaza Drive, Suite 400 Arlington, Texas 76011-8064 U. S. Nuclear Regulatory Commission Attention: Document Control Desk One White Flint North 11555 Rockville Pike Rockville, MD 20852 Richard A. Ratliff Bureau of Radiation Control Texas Department of Health 1100 West 49th Street Austin, TX 78756-3189 Cornelius F. O'Keefe U. S. Nuclear Regulatory Commission P. 0. Box 289, Mail Code: MNI 16 Wadsworth, TX 77483 A. H. Gutterman, Esquire Morgan, Lewis & Bockius LLP M. T. Hardt/W. C. Gunst City Public Service Mohan C. Thadani U. S. Nuclear Regulatory Commission R. L. Balcom Reliant Energy, Inc.

A. Ramirez City of Austin C. A. Johnson AEP - Central Power and Light Company Jon C. Wood Matthews & Branscomb C. M. Canady City of Austin Electric Utility Department 721 Barton Springs Road Austin, TX 78704

Attachment NOC-AE-02001335 Page 1 Description of Changes and Safety Evaluation

1.

Description STPNOC proposes to eliminate the turbine missile design basis from the STP Unit 1 and Unit 2 UFSAR. STPNOC's analyses have determined that turbine missiles pose no credible threat to the nuclear safety design of the STP units and warrant no special regulatory consideration at STP. As described in Section 2, the change involves eliminating the design basis description from the STP UFSAR and removing the Turbine Overspeed operating specification from the STP Technical Requirements Manual. No Technical Specifications are affected by the proposed change.

2.

Proposed Change Section 3.5.1.3 of the STP UFSAR describes the station design basis for turbine missiles.

Technical Requirements Manual (TRM) Specification 3/4.3.3.4 contains the operational requirements for turbine overspeed components credited for the prevention/mitigation of turbine overspeed events that could contribute to the generation of a turbine missile.

STPNOC proposes in this change to eliminate the UFSAR Section 3.5.1.3 design basis description and to delete the TRM requirements. Markups of the proposed changes are attached for the staff's information.

There are no changes required for the Technical Specifications.

3.

Background

Section 3.5.1.3.5 of the UFSAR states:

The results of the turbine missile analysis have demonstrated that the probability of damage to safety-related components is less than 10-7 per year, which satisfies regulatory requirements. This probability is maintained below this value by maintaining the probability of turbine missile generation below 10-4 per year, which is accomplished by the South Texas Turbine System Maintenance Program described in section 3.5.1.3.4.

STPNOC analyses have determined that there is no need to credit the Turbine System Maintenance Program to maintain an acceptably low probability of damage to safety related components.

Attachment NOC-AE-02001335 Page 2 Although no change to the STP Technical Specifications is required, STPNOC determined the proposed change requires prior NRC approval as a departure from a methodology as described in 10CFR50.59(c)(1)(viii). Section 3.2 of NEI 96-07, "Guidelines for 10CFR50.59 Implementation", specifically names turbine missiles as "an accident previously evaluated in the FSAR (as updated)". The evaluation methodology is specifically described in the UFSAR; consequently, changes to the described methodology (e.g., deletion) requires prior NRC approval.

4.

Technical Analysis The orientation of the turbine relative to Safety Related Systems, Components, and Structures is shown in STP UFSAR Fig. 3.5-1 (attached). The only safety-related systems, structures or components located within the +/- 50 strike zone are the ECW buried piping and a small portion of the Unit 1 Diesel Generator Building (DGB). The ECW piping is buried to a sufficient depth to prevent damage from a direct strike. The probability of striking the Unit 1 DGB with a missile ejected from the Unit 2 turbine is very small. This very favorable orientation of the STP turbines justifies elimination of the turbine missile generation from the safety analysis. The probabilistic assessment described below shows that the likelihood of core damage, given a turbine missile is generated, is insignificant.

A probabilistic risk assessment study was performed to evaluate the probability of core damage given that the main turbine of either Unit generates a missile. It does not involve a significant increase in the probability of occurrence or the consequences of an accident or malfunction of equipment important to safety. The probability of core damage, given that a turbine missile is generated, is 3.74E-08 for shear failure and 3.09E-08 for shear and rotational failure.

Assuming the likelihood of a turbine missile is less than 1.OE-04/yr, the core damage frequency (CDF) is less than 1.OE-1 l/yr. The result is significantly below the acceptable levels for core damage frequency change justified by Regulatory Guide 1.174. As shown above, the acceptability of the result does not depend on maintaining the turbine missile generation frequency below 1.OE-04/yr.

From this evaluation, it can be concluded that testing of the turbine governor and intercept valves generates unnecessary exposure to a turbine trip without commensurate benefit to nuclear safety.

The proposed change meets the other criteria of RG 1.174:

1. The change is consistent with current regulations. Approval of the proposed change does not involve an exemption to any regulation.

2. The change is consistent with the defense-in-depth philosophy. STP is not proposing to change the design basis of the plant or how the plant responds to transients or accidents.

Attachment NOC-AE-02001335 Page 3

3. The change retains sufficient safety margins. The PRA was performed with conservative assumptions with regard to the consequences of turbine missile impact on safety related structures. STP does not expect the reliability of the turbine systems to be significantly affected by the proposed change. Because of the critical nature of the turbine and associated systems to overall plant performance and reliability, STP will have an effective maintenance and monitoring program for the turbine for commercial reasons. Because of the very low safety significance associated with turbine missile generation, no additional monitoring is warranted.

4. As discussed above, the changes in risk are not significant and are offset by the reduction in the potential for plant trip from reduced testing of the turbine governor and intercept valves.

Attachment NOC-AE-02001335 Page 4

5.

Regulatory Safety Analysis 5.1.

No Significant Hazards Consideration STPNOC has evaluated whether or not a significant hazards consideration is involved with the proposed amendment by focusing on the three standards set forth in 10CFR50.92, "Issuance of amendment," as discussed below.

1)

Does the proposed change involve a significant increase in the probability or consequences of an accident previously evaluated?

Response: No.

The turbine missile generation probability will not be significantly increased by elimination of the regulatory commitments in the UFSAR. No plant changes are proposed that would significantly increase the probability of turbine missile generation. Turbine missile generation does not pose a credible threat to safety related components and consequently has no potential to increase radiological consequences.

2)

Does the proposed change create the possibility of a new or different kind of accident from any accident previously evaluated?

Response: No.

The proposed changes involve no physical modification of the plant or different operating configurations.

3)

Does the proposed change involve a significant reduction in a margin of safety?

Response: No.

Turbine missiles do not constitute a credible threat to nuclear safety at STP. They are not a consideration in any plant safety analysis. Changing the regulatory commitment with regard to design for turbine missiles has no effect on any margin of safety.

Conclusion Based upon the analysis provided herein, the proposed amendments do not involve a significant hazards consideration.

Attachment NOC-AE-02001335 Page 5 5.2.

Applicable Regulatory Requirements STP's UFSAR states that STP meets the intent of Regulatory Guide 1.115 Rev. 1 "Protection Against Low-Trajectory Turbine Missiles". The Regulatory Guide supports compliance with General Design Criterion 4, "Environmental and Missile Design Bases,"

of Appendix A to 10 CFR Part 50, which requires, in part, that structures, systems, and components important to safety be appropriately protected against the effects of missiles that might result from equipment failures.

As described in the Technical Analysis, STPNOC has determined that turbine missiles do not pose a credible threat to safety related components at STP. Consequently, there is no adverse effect of the proposed change on the regulatory design basis of the plant.

6.

Environmental Consideration 10 CFR 51.22(b) specifies the criteria for categorical exclusion from the requirements for a specific environmental assessment per 10 CFR 51.21. This amendment request meets the criteria specified in 10 CFR 51.22(c)(9).

The specific criteria contained in this section are discussed below.

(i) the amendment involves no significant hazards consideration As demonstrated in the No Significant Hazards Consideration Determination, the requested license amendment does not involve any significant hazards consideration.

(ii) there is no significant change in the types or significant increase in the amounts of any effluents that may be released offsite The requested license amendment involves no change to the facility and does not involve any change in the manner of operation of any plant systems involving the generation, collection or processing of radioactive materials or other types of effluents. Therefore, no increase in the amounts of effluents or new types of effluents would be created.

(iii) there is no significant increase in individual or cumulative occupational radiation exposure The requested license amendment involves no change to the facility and will not increase the radiation dose resulting from the operation of any plant system.

Furthermore, implementation of this proposed change will not involve work activities that could contribute to occupational radiation exposure. Therefore, there will be no increase in individual or cumulative occupational radiation exposure associated with this proposed change.

Based on the above it is concluded that the change meets the criteria specified in 10 CFR 51.22 for a categorical exclusion from the requirements of 10 CFR 51.21 relative to specific environmental assessment by the Commission.

Attachment NOC-AE-02001335 Page 6

7.

Licensing Document Changes UFSAR Changes:

Table 3.12-1, "Regulatory Guide Matrix": Change listing for RG 1.115 from "B" - meets the intent to "G" - no commitment to this guide.

Section 3.5.13

[Note to reviewer: The figures associated with these UFSAR sections will also be deleted.]

Turbine missiles have been evaluated not to be a credible threat for the STP design basis.

3.5.1.341 Introduetion: The potential for-damage to safety related targets which include safety related structur~es, systems and components due to turbine failure was evaluated prior-to r-eceiving the operating licensing to detem~ine whether additional proetection, beyond that inherently provided by plant building orientation and existing structur-al shielding, need be proevided to further-reduce the probability of damage. Th total probability of damage was calculated for-each seismnic Category 1 tar-get by evaluating the product of the probability for-missile gener-ation, the probability of striking the tar-get, and the probability of the baidFer failing to proetect the tar-get.

The result of this evaluation was that the turbine missile generation pr-obability (P1) is less than 10O4per-year-and the combined prebabilit' of the m~issile strdldng a target and the baRrier-failing to protect the tar-get is less than 10. Therefore, the total1 probability of damnage to safety related components is less than W0-7Te~ef-ye Because plant construcflon will be r-elatively unaffected over-the life of the plant, the combined proebability of striking A-tar~gret -And the bar~rier failing to protect a tar-get wl net change from this initial value of 1 pe-er Ther-efor-e, South Texas has fbocused on maintaining P1 below 10-4 per year to assur-e that the total proebability of damnage to safety related components remains sth.an

.0 per year.!-1 i*s--

maintain below *0' per-year by the South Texas Turbine System Maintenance Progr-am, which is described in.

further-detail in section 3.5.1.3.4.

high pressure turbine rotor-and 3 low pressur-e turbine roetors. The Ifigh Pressure turbine rotor-is of integral forged construction and is noet subject to failure due to stress corrsio crackding or-destructive ever-speed. (Ref. 3.5 2)

The low pressure rotor-s utilize discs installed with a shrink fit to the shaft. The discs r keyed to the shaft to prevent tor-sional slippage under-unusual condition-s Of over-speed and.or-transient temper-atur-e conditions. Each low pressure rotor includes 12 shrun on dises.

Attachment NOC-AE-02001335 Page 7 Three differ-ent styles of disc are used on rotors either-in use or-planned for-use at this rotor-utilizes "medium di'sces" fer 6 of the 12 discs. The latestmonde Of these rotor-s utilizes "heavy discs" for-6 of the 12 discs. Rotor-s utilizing these 3 styles of dises are interc.hangeable in the Lowv Pressure turbines of eaJh unit.

The light discs are keyed to the shaft with 3 round keys installed in the downstream side of the dise bore. Dises of th is design have proven susceptible to stress cefso er-acldng, with cr-acks originating in the keywvay o~r (much less frequently) at the ID ofth disc bor-e. The reeord of inspections of these di

-c. a~s a large data base maintne by Westinghouse, which provides a good statistic-al basiss foor Wo~estfinghouse predictions oe cr-ack initiation and cr-ack przopagation rate probabilities.

The mnedium disc style utilizes a completely different method of keying the disc to th shaft. Mn this style, the keywvay is net located in the highly stressed shrunk on disc boe.

The paft keyed to the shaft is called a "key plate". This key plate is bolted to the disca the lower-stressed point on the disc. The base of this style disc is also slightly beefie~r, hence the terfn m-ediumn disc. These discs are made of steel with imiproeved fractur-e toughness and lower-yield strength for-imnproved r-esistance to disc rupture and crack intatien.

The heavy disc style is similar-to the mnedium disc style, with fu~ther-improevements in metallur-gy, detailed improvements in disc profile, and improevements in blade and steam path design not related to the probability of missile generation-.

moedel. The temperature at the key~way of these discs is below 2002F. No stress eo~oson cacks have been found on discs which operate at this temperature or below. I-n addition, because the rate of pr-opagation of a stres c son r-ack is highly temperature dependent, the rate of proepagation of a cr-ack initiated in one of these discs would be much lower-than in one of the discs 1, 2,oer-3. (Refl. 3.5 35) 3.5.1i.3.3 Turfbine Missile Gener-ation: The probability of missile gener-ation from each disc of each roetor is provided by Westinghouse. The data covers pefiods ot operation of 1, 2, 3, 4, 5, and 10 years. The analysis uses metallur-gical prepel~ies from test eoupons taken during fabdieation of each disc and c alcul ated values of localized eonditions at norfmal operation (stresses and temper-atures at the keyw'ay and/or disc boeFL, etc.), combined with statistical proebabilities of cr-ack initiation, to ealeulate the proebability of disc fupture due to stress coffoesien er-acldng at rated speed and design.

over-speed. The design ever-peed analysis also factor-s in the proebability of a design.

over-speed event. The probability of disc rupture is combined with the probability of ds generation for-each disc. The probbilt o ssile generation for-each rotor-is the su of the proebabilities of the installed disc. (Ref-. 3.5 31)

Attachment NOC-AE-02001335 Page 8 The Westinghouse data is used to determine the "probabilistic inter.'al" for eaeh r-otor This interv'al is sel cote d to keep the total for-3 rotor-s below 1 x 10-4 nissiles per-unit pef year-either-by keeping the total for-eaeh rotor-at or-below one third that value, or-by -a moere detailed caleulation using the installed configuration and conser.'ative r~otot operating, intervals. The proebabilistic interval for-individual rotor-. is _et at 60 months it the calculation proevides a longer-intenval. Stai4 up of the turbine following a scheduled outage (refueling outage or-planned outage of sufficient duration to inispect an L.P. rotor) is allowed if and only if the actual operating timfe of the'installed retor-s is less than thei probabilistic interv'al. Some additional analysis of the Westinghouse data is neoessM'toe inter-polate between data points and to obtain a probability density function (missiles per year-) from the overall probabilities _vnin mi sil es over the stated operating period.

If the actual operating time cxceeds the probabilistic inter.'al of one or-more of the installed rotor-s between scheduled outages, the turbine is allowed to operate until the next.

scheduled outage, provided the overall probability of nm-issile generation remains belo!

• 40 -3mi ssiles per year per-unit. Data fromf r-otors cuffent on site show if P! is less tha K le-aot the beinn

-fan oper-ating cycle, P! will be less than 1 x 1 -3 with 19 months more oper-atinig time.

The orientation of the turbine relative to Safety Related Systems, Componients, and Stmcetur-es is show~.n in Fig. 3.5 1. The strike zone of the number-1 through number-5 discs is

• 52 fromn the roetational plane. The only safety related systems, stfuctures or components located within the +/- 52 strike zone are the ECW~ buried piping and a small portion of the Unit 1 Diesel Generator Building. The ECW piping is buried to sufficient depth to prevent damage from a dir-eet strike. The prbblt)f strikdng the Unit 1 DGB with a maissile ejected from the Unit 2 turbine i s Yvey small.

The strike zone of the number 6 discs is 50S to i250 from the r-otational plane. Safety related systems, components, and stmcetures are located in the potential strike zone of some of the number-6 discs on Unit 1 and all of the number 6 discs on Unit 2. Because the number 6 discs have the low.est oper-ating stress and low..est operating temperature, these discs have a mfuch lower probability of disc-fptur-e due to stress eofso cr46l19 and consequential fnissile generation than moest other-discs. The 14 nlumber-6 discs currently on site (six operating roetors plus a spare r-otor-, each with two #6 discs) have individual m-issile generation proebabilities ranging from 6 x 10he4*4W07 '246T6O mionths operation, proebability of failure over entire per-iod).

3.5.1.3.4 Turbine System Mainteniance Progr-am South Texas has established a turbine mntacepormwhich focuses on maintaining the probability of missile generation below 1O per yearfx.nTheprgrm rqurs nsmti maintenance, calibr-ation, and/'or-testing of the functional integrity of turbine component-s at specified intervals. The componenits that are ineluded in this proegramf are the Low Pressur-e Turbine Rotor-s, Turbine Valves, Electrical Over-speed Protection device an Mechanical Over-speed device.

Attachment NOC-AE-02001335 Page 9 predetefned inte-r.als. The interals are unique to ea-h set of rotors and may be differ-ent for-r-eplacement roteor, hence, is detefmined on a rotor specific basis.

The inspection inter.als have been dete...ned such that the...

ssile generation proebability is less than 10-4 per-year-. In the event the probability falls between W0-and 40 per-year-, the tur.b.ine may be kept iecen the next schcduled outage, at 'hi h

tim.e action can be taken to r.educe the probability to meet the 1.... per-year-limit before r-eturning the tur-bine to srie The tur-bine stop, gover-nor-, r-eheat step, and intercept valves are tested once per: quarter.i valves is disassembled and inspectea at least once per 40u mnontnS. v~alve seats, Eltscs, ana stems receive a visual and surface inspection and are ver-ified as having no unacceptal flaw.s or-excessive coefosion. if unaecceptable flaws or excessive coffosion are found, afll1 other valves of that type shall be inspected.

The electrical over-speed device is calibr-ated at least once every eighteen months.

The mnechanical ever-speed trip is tested during turbine startup under-the turbine startup procedure following each major turbine outage. The mechanical over-speed deviceewl be tested quarterly via oil simulation while the turbine is oper-atingz.

3.541.3.5 Summnar-y and

Conclusions:

The results of the turbine missile analysis have demonstrated that the probability of damage to safety related componnt is-less--than-4le per-year, which satisfies regulator requremnts. This probability is maintained belo'mv this value by maintaining the pr-obability of turbine missile generation belewx-1-0 4 per-year, which is accomplished by th South Texas Turbine System M~aintenance Progr-am descr-ibed in sec-tion35131

Attachment NOC-AE-02001335 Page 10 Technical Requirements Manual (delete specification and associated Bases)

LIMITING CO.NDITION FOR OPERATION 3.3.4 At lIast one Turbine Overspecd Prtection System shall be OPERABLE.

APPLCABII1Y:MODES 1, 2, and 3-.

AGT40N4

a. With one rtop valve or onc governor vgalvo per high pressurc turbine stePa.m IRm9 inoperable and/or with one rohcat Fto aoor oe reheat intercept valve pcr lo9 pressur~e turb-ine steam 1ine inoerbe ostoro the inoperable valve(s) to OPERAB3LE statusic withi 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, or close at least one valve OR the affeeted rsteam line(s) or isolate the turbine from the steam supply within the ne~d 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> bh With the above requ~ired-Tu-rb-ine Overspeed Protection WiHin 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />, isolate the turbine fromn the steam supply-.

,ystem otherwise inoperable SURVE! LLANCE REQUI REMENTS 4.3.4.1 The pro'.isiens of Specification 4.0.4 arc not applicable.

4.3.4.2 The -aboeve required Turbine Ove~rpe-ed-ProtfecinVO System shall11 beA demoInIGstrated Q Pre PAR I-E a. At least once per qua~teF in MODES 1 and 2 when the main turbine is operating by cycling each of the following valves through at least one comnplete cyclIe fromn the

1) Four high pressure turbine stop valves,

2) Four high pre~ssue turbine governor valves,

3) Six low pressure turbine reheat stop valves, and

4) Six low pressr.Ue turbine reheat intercept valvers.

b. At least once per quarter in MO4DES 1 AND 2 w.hen the main turbine is operating-by direct obserwation of the moevement of each of the above valves through one comnplete cYcle fromn the runn posItion, G. At least once per 18 monOths by performaRGED Of a C-HANNEL CALIBRATION on the Turbine Overspeed Protection Systems, and

d. At least once per 10 months by disassembling at least one of each of the above valves and pe~ferming a visual and su~faco inspection of Valve seats, disks, and stems and verifying no unaccGeptable flaws or excessive corrsion. if unacceptable flaws r. exesvecrosion arc found, all other valves. of that type shall b 1nsPeGted.*

• Disassembly and inspecG-tionR Of the low: pressure turbine reheat intercept valves are net required prier to the end of the first 10 moenth interval.

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aa SOUTH TEXAS PROJECT UNITS I & 2 LOW TRAJECTORY TURBINE HISSILE STRIKE ZONE FIGURE 3.5-1 REVISIOlN 4

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