05000423/LER-1991-002-01, :on 910202,setpoint Drift Identified During Testing of Main Steam Safety Valves.Caused by Inadequate Design.Nine out-of-tolerance Valves Reset within Specified +1% Tolerance

:on 910202,setpoint Drift Identified During Testing of Main Steam Safety Valves.Caused by Inadequate Design.Nine out-of-tolerance Valves Reset within Specified +1% Tolerance

ML20029B603
Person / Time
Site:	Millstone
Issue date:	03/04/1991
From:	Hulme N, Scace S NORTHEAST NUCLEAR ENERGY CO.
To:	NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM)
References
LER-91-002-01, LER-91-2-1, MP-91-194, NUDOCS 9103130100
Download: ML20029B603 (4)

LER-1991-002, on 910202,setpoint Drift Identified During Testing of Main Steam Safety Valves.Caused by Inadequate Design.Nine out-of-tolerance Valves Reset within Specified +1% Tolerance

Event date:
Report date:
4231991002R01 - NRC Website

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March 4. 1991 MP-91-194 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555

Reference:

Facility Operating License No. NPF-49 Docket No. 50-423 Licensee Event Report 91-002-00 Gentlemen:

This letter forwards Licensee Event Report 91-002-00 required to be subrnitted within thirty (30) days pursuant to 10CFR50.75(a)(2)(i), any operation or condition prohibited by the plant's Technical Specifications.

Very truly yours, NORTHEAST NUCLEAR ENEROY COMPANY kejktA e eae.x

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cace Director, Millene Station SES/NDH:ljs Attachment: LER 91-002-00 cc:

T. T. Martin, Region 1 Administrator W. J. Raymonci, Senior Resident ins 3ector, Millstone Unit Nos.1, 2 and 3 D. H. Jaffe, NRC Project Manager, siillstone Unit No. 3

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l Between 0400 and 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> on February 2,1991, with the plant at b~r power in Mode 3 (Hot Standb 00$0 psia and f 4f deprees Fahrenhen, setpoint drift was identified wh'.se testing the Main Steam Sa ahes.

1 Nm out of twent,s vahes failed the a te tolerance band set br b Lnical Specification Table 3.7-3.

r No immediate cecrective acuan was necessary unce the plant wa, m Hot Standby.

l The cause of the safety valve setpoint dnft is inadequate dtsign. The safety vahes do not maintain their i

setpomt within the i 1Ce tolerance specified m Techmcal Specificauons. The highest / lowest deviation observed was +2.lc i-24 The setpomF on all of the failed safety valves were reset to Technical Specification r

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Desermtin, of Event Between 0400 and 1600 hours0.0185 days <br />0.444 hours <br />0.00265 weeks <br />6.088e-4 months <br /> on February 2,1091, with the plant at O'i power in Mode 3 (Hot s

Standbyh at 2250 psia and $46 degrees Fahrenhe t, setpoint drift was identihed while testing Mam Steam Safety Yalves. Nine of twenty vahes failed the A l'i tolerance band set b) Technical Specibcation Table 3.7-3. No immediate operator action was required since the plant was in Hot Standby at the time the valves were being tested.

The valves were being tested per an anproved Mamtenance Surseillance proced,d on the valves that fa ure. On iniual tests of the twent) valves, eight failed high and one failed low. The worst deviation obserse high was 2.19. The vahe that failed low had a deviation of 2Er. All of the failed vahes were reset to within the A l'i tolerance.

Cause nf Event The cause of the safety valves drifting from their setpoints is inadequate design. Setpoint drift is an acknowledged occurrence that is documented industry wide. and has previously occurred at Millstone Unit 3. Investigations have been performeci, and root causer, have been postulated. However, the manufacturer has been unable to identify any specific cause for the drift.

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Annhan of Event This event is reportable in accordance with the requirements of 10CFR50.73(al(2)(i). plant Technical Specihcation 3.7.1.1. Table 3.7-3 requires all Steam Lme Safety Yalves to lift at a l'i of their hit setting during plant operations. The lift setting pressure corresponds to ambient condiuons of the vahe at nominal operating temperature and pressure. It is n'sumed that the setpoint drift occurred on these valves sometime during plant operations smce it was discovered soon after gomg into Hot Standby. The valves had been satisf actorily tested previous to this incident.

Evaluation of the initial setpoint results have determined that the as-found setpoints did not result in any adverse safety implications. The safety vahes lifted within the lower pressure bounds analyzed for a Steam Generator tube rupture coincident with loss of offsite power, and the high pressure bounds of the Main Steam System design.

Setpoint drift on Main Steam Safeties has occurred in the past. This has been recognized by other utihties and, therefore, is not a problem unique to Millstone. Historical informauon provided by Nuuear Network and NPRDS entries has indicated a general concern that a l'1 safety valve tolerance is too restrictive. Millstone 3 is pursuing a relaxation of the i l'1 criteria in favor of a 139 tolerance, lY.

Correethectier, The nine out-of-tolerance valves were reset to within ihe specihed 21% tolerance.

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A&hoonM informntion Similar esents concerrung setpomt dnft have occurred to the Prewunrer Safets Yahes as well as the Ninin Steam Safety Vaives. The fouowing n a summary of the previously reponed setpomt drif t problems:

L_E P. Number luk 87-009 Earh Libiag of Prenun7er Salenes for Undetermined Reasons67-036 Setpomt Dnft on Ntam Steam Safety Yahes W-010 Setpomt Dnit on N1ain Steam Safety Yahes Due to Unknown Causes P-02f Lifting of Prenun7er Safeties Above the Allowed Tolerance The folloutng factors base been mvesugated as powible reasons for vahe dnft:

Problems associated with the h - oc assist devicertei,t method, Disc / seat bondmg which could cause high hft values.

Relaxation of the spnng matenal over a penod of time at elevated temperatures that would result in the low hit values.

A direct contnbutor to the high safet) vahe hit pressures was the use of an erroneous spnng constant for setpomt calculations. After a 10CFR21 nouficauon from the manufacturer. Niilistone Umt 3 personnel evaluated the effect of the error on the settmps for the mam steam safety vahes. The resuh was a : ic 7 psig merease m rehef prnsure when apphed to the "as-left" values recorded dunng the previous salety vah'e suncillance. None of the safety vahe settmg> exceeded the +1ci tolerance allowed by the Techmcal Spe:ihcotions. Howeser, the increase in the actual "as-left" rehef pressure settmgs resuhed in the values being closer to the +1ri tolerance and reduced the margin available for valve dnft. This i

reduced margin indicates why b of the 9 vahes exceeded the +1ri tolerance dunng the most recent tests.

The vahes were reset using the correct spnng constant. Also, more accurate digital test equipment, vice analog pressure pages, is currently being used, which shodd provide an increase m accurac). The change m test method measurement is expected tc favorabl3 t.ffect future *as-found" results.

Disc / seat bondmg was evaluated previousl> by Niillstone Unn 3 personnel, and found not to be contributing to setpoint drift, Yalves sent out to a repatt facihty for refurbishment have not displayed material defects m the seatmg surfaces which are mdicathe of disc bondi.4, Sho: term relaxauon of spring matenah aho has been evaluated by Niillstone Unit 3 penonnel. The sprmg matenal utihred f

would start to relax due to maienal creep if temperatures were to exceed 700 degrees Fahrenhen. Test resuhs have shown that 160 degrees Fahrenheit is the highest temperature seen by the spnngs.

Furthermore, the manuf acturer mdicates spnng relaxation would only occur over a ume penod of 10 to 20 3 cars. Therefore, spnng relaxation is not a factor in valve setpomt dnft.

Based on the recurrence of setpoint drift on N1ain Steam Safet) Valves, the need for a review of N1ain Steam Safeties setpomt tolerance has been identified. A recommended 2 3ri tolerance is still be:ng reviewed for Techmcal Specificauon implementauon.

This event has been diuemmated on Nuclear Network.

Ells Codes Snirm Comnenent Vendor N1ain Steam System Rehef Yahe - RV Drewer (D243) go.m m

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