ML20065K227

Proposed Tech Specs Deleting Requirement to Perform Weekly Stroke Test of High Pressure TCVs

ML20065K227
Person / Time
Site:	Seabrook
Issue date:	11/13/1990
From:	PUBLIC SERVICE CO. OF NEW HAMPSHIRE
To:
Shared Package
ML20065K219	List: ML20065K214 ML20065K227
References
NUDOCS 9011160141
Download: ML20065K227 (5)

Text

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8 INSTRUMENTAT ON ,

., 3/4.3.4 TORBINE OVERSPEED PROTECTION l.!MITING' CONDITION FOR' OPERATION ~

.3.3.4. At least one Turbine Overspeed Protection System shall be OPERABLE.

, APPICABILITY: MODES 1,'2, and 3. ,

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4 zy a. With one stop valve or one control valve per high pressure turbine ,.

• Steam lineLinoperable and/or with one intermediate stop valve or one M eheat intercept valve per low pressure turbine steam line inoperable, e restore the inoperable valve (s) to OPERABLE status within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br />, i 1 or close at least one' valve in the affected' steam line(s) or' isolate  ;

'the turbine from the steam supply within . cia next 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.

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b. Withithe above required Turbine Overspeed Protection System otherwise '

, , inoperable, within 6-hours isolate the turbine from the steam supply.

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St'fvEILLANCE RE0U'IREMENTS l

l ^4.3.4.1 :The provision $ ofISpecification 4.0.4 are not applicable. '

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• 4.3.4.2 The above required Turbine Overspeed Protection System shall be I

^ demonstrated OPERABLE: -

a. Athleast once per'7 days by cycling each of the following valves 4

through at least one complete: cycle from the running position:

1) Four high pressure / turbine-stop valves,and-

2) %ur H gt pr ::ur:' turbine ge rner 1 9 ::, = d ,

G) Six low pressure combined intermediate. valves.

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b. At least once per 31 days by direct observation of.the movement of

.-each of the above valves %th" rough one complete cycle from the~ running

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c. At 'least'.once per 18 months by performance of a CHANNEL CALIBRATION' c o. the Turbine Cverspeed Protection Systems, and Atcleast once per 40 months by disassembling at.least one of each of

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, -G ,; , the above v' alves and performing a visual and' surf ace inspection' of valve seats, {isks, and stems and verifying no unacceptable flaws or

'q excessive' corrosion. If unac'ceptable flaws'or excessive corrosion are-found, all.-other valves of that type shall be inspected.

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New Hamp Yankee Novemt 13, 1990 DESCRIPTION OF CHANGE i

The proposed change deletes the requirement to perform a stroke test of the righ pressure

. turbine control valves on a weekly basis. This test frequency was originally based upon the turbine manufacturer's (General Electric) recommendation, which was based upon test frequencies established for fossil fuel turbines which typically operate at higher temperatures and pressures than nuclear plants. Operating experience at nuclear plants over the past ,

three decades has indicated significantly lower failure rates than those from which these recommendations were derived. Based upon these findings, GE issued Technical Information Letter (TIL) No. 969, based on an overnced reliability analysis which revises the recommended testing frequency for these valves. The new test frequency specified by this

.TIL is monthly testing for the control valves. This testing is specified in Surveillance Requirement 4.3.4.2b.

o  : Additionally, editorial changes with respect to valve nomenclature have been indicated to

' provide consistency throughout the Technical Specification.

SAFETY EVALUATION OF PROPOSED CHANGES New Hampshire Yankee has reviewed the proposed change in accordance with the criteria specified in 10 CFR 50.92 and has determined that the proposed change would not L Involve a significant increase in the probability or consequences of any accident previously evaluated. The Seabrook Station Probabilistic Safety Assessment (SSPSA) estimates the likelihood of generating turbine missiles and analyzes the most probable

l. consequences. The result, discussed below, indicates that the contribution from L turbine missiles to public risk is negligible, i

The total mean annual frequency of turbine missile generation was estimated by the SSPS A to be 8.3 ' x - 10'*. The conditional probability of damage to structures and systems as calculated in FSAR Section 3.5.1.3 was used.

. The resulting turbine missile damage frequencies for structures are listed in SSPSA Table . 9.9-4. From this list, six common cause initiating events were chosen and i included in the plant model for- quantification. It should be noted that these are Initiating event frequencies only, .and not core damage /offsite release frequencies,

? The six scenarios are discussed below:

a& b. Steam Line Break (TMSLB) and Loss of Condenser Vacuum (TMLCV) were both conservatively assumed to occur with a conditional probability of one,

'given a turbine missile had been generated. These were included as initiating events in the SSPSA. However, the mean annual frequency of steam line breaks outside containment (SLBO - 6.04 x 10'*) and loss of condenser vacuum

'(LCV 0.42) from other causes totally dominate any contribution from turbine missiles (8.3 x 10 for TMSLB and TMLCV). Given this event, a loss of-l 1

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New Hampshire Yankee November 13, 1990 m

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D Emergency Feodwater (2.7 x 10 d) and subsequent failure of the Startup Feed

. Pump, and Feed and Bleed Cooling are required to result in a core damage event. The~ resultant core damage frequency is less than 2.2 x 10'*. -j y"' ,

c. Control Room (TMCR) impact was chosen as the most critical location that can' be hit by a turbine missile with relatively high frequency and serious -

consequences. The mean annual frequency of this initiating event (control building impact) is 3.98 x 104 . Most major functions'nceded to mitigate the H effects of the steam line break (which is assumed to occur with a conditional 7

J probability- of one)' are conservatively _ assumed _ to be-' lost without operator J (recovery as a result of the damage to the Control Room. However, this is an 4 '

insignificant contribution to core damage frequency and public risk because it i- 7. :is 'two orders of magnitude less frequent than other scenarios with the same 1

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[J d? ' (A large LOCA;(TMLL). initiating event with a mean frequency of 1_

. 7.44 Lx :10 (containment impact) was included in the~ SSPSA. If the missile (lo '

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, , were to penetrate, damage to multiple' systems is not' expected due to the ,

. spatial ' arrangement of systems. The bounding scenario is assumed to be one

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• or twolsteam generators are damaged leading to a large LOCA, a ioss of .

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containment isolation' and failure of a containment spray train. This scenario '

l is 'also assumed to occur coincident with an independent failure of one high

pressure or . low , pressure -Injection train. Another train of low pressure f injection must- fall' to ' result in core damage, resulting in c mean annual frequency of core damage with containment bypass less than 10.a. Again,- this is an insignificant contribution to core damage frequency and public risk.

e. . Condensate Storage Tank (TMCST) impact in addition to steam line break:and Y loss,of condenser ' vacuum was included as' an initiating event with a mean q

annual frequency of 6.09 x 10. Again,' core damage frequency'is. dominated'

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.by 6ther events with'ioss of emergency feedwater.

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' f. Loss of Primary Component Cooling (TMPCC) water system due to Primary y - A_uxiliary Building impact was included with a mean annual frequency of L27 .

x .10:8 This is an insignificant contribution to core damage frequency and loss .

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-These probabilities. compare with realistic assessments of degraded cores of modern  ;

P.WRs in the. range of 10 to 10 per yeart _Given the conservative analysis in the ,.

. FSAR. and SSPSA the probability of core damage from turbine missiles is judged to U not substantially contribute to public risk.

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1. New Hampshire Yankee .  ;

November 13, 1990

2. Create the poss'bilit? 'of.a new or different kind of accident from any previously 1

.W, . evaluated. The analyses' presented in FSAR Section 15.1 and 15.2 bound the two a possible failure meshanisms which exist for the high pressure turbino control valves a, (ie., the possibil!;y of a control valve not closing lu conjunction with a stop valve not

?' closing,'or' spurious control valve closure). The extension of the testing frequency

$" from weekly. to monthly. does not create a new failure mechanism; therefore, the ,

'L , possibility of a new or different kind of accident is not created '

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3. Involve a significant reduction in.a margin of. safety.; Margin of safety as it relates

~ to.the protaction of safety t' elated structures, systems and components from turbine  ;

missiles is measured in terms of the probability of radiological consequences exceeding: c 10CFR100 limits. FSAR Section 3.5.1.3 specifies the acceptance criteria and analytical no lresultr' for the probability that a turHe missile is generated and strikes a safety.

hj' ' related area 'which may lead do constquences exceeding 10 : CFR - 100 limits. ,

f' g~ Additionally, the Seabrook Stat'.on Probabi'istic Safety Assessment (SSPSA) quantifies i i turbine missile damage frequencies for sevs ral common cause. initiating events. From -(

W the SSPSA,, the. total contribution of arbine missiles to mean' annual core damage, frequency is 4 x 104 (TMCR) or less. Six of the common c:use initiating events were .,

included in the SSPSA plant model for qua.itification of core damage and offsite. l, release frequencies. lThe SSPSAl analysis demonstrates that_.the probability of cor,e.

damage from; turbine missiles provides negligible contribution to public risk. The' SSPSA turbine missile' generation estimates are based on statistical =and analytical data j which show' a' relatively small contribution by overspeed failures versus failures 'at .

ioperating: speed; therefore,Ldamage frequencies would be further' reduced if only ]

turbine missiles generated as La' result of overspeed,were considered, Given that the generation 1of/ turbine missiles is not very : sensitive to changes in control system

. reliability,'the extension of the testing frequency for the high pressure turbine control i valves does n'ot significantly-increase turbine missile damage frequencies and therefore

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doesi not'. result in a significant decrease in the margin of safety.

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b - Additionally, based on engincering Judgement, a slight improvement in safety will be realized by; extending; the high pressure ~ turbine = control valve testing' interval. The decreased frequency of p it power enanges and testing that could cause an inadvertent plant trip'.will  ;

result in less frequent challenges to safety related equipment.

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