ML19318C050

LER 80-010/01T-0:on 800516,during Cold Pressure Test,Leakage Discovered at Gasket Joint Between Pump Casing & Pump Cover on Reactor Coolant Pumps RC-3A,3B & 3C.Closure Studs Corroded by Boric Acid.Leaks Caused by Deteriorated Gaskets

ML19318C050
Person / Time
Site:	Fort Calhoun
Issue date:	06/17/1980
From:	Fleuhr J OMAHA PUBLIC POWER DISTRICT
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION IV)
Shared Package
ML19318C040	List: ML19318C039 ML19318C050
References
LER-80-010-01T, NUDOCS 8006300475
Download: ML19318C050 (6)

Text

l UCENSEE EVENT REPORT o- (PLEASE PRINT OR TYPE ALL "sOUIRED INFORMATION)

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EVENT DESCRIPTION AND PROBABLE CONSEQUENCES h lo !2I l A cold pressure test was being performed on the RCS prior to unit startup. Test l 4 I o 131 I pressure was approximately 180 psig. Leakage was discovered at the gasketed joint l lo i4 l l between the pump casing and the pump cover on reactor coolant pumps RC-3A, 38, and I!

IoIsl I 3C. S ibsequent inspection revealed that significant corrosion damage had occurred l 1o le i I to a number of the closure studs on all three pumps. No Technical Specification i l o 17 I I LC0's were violated as a result of this condition. l I

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44 47 CAUSE DESCRIPTION AND CORRECTIVE ACTICNS 11 101 L ! aakage fecm tho onmos was found to be caused bv detarioration of the Flexitallic I li ll i l oaskets. Damaca to the closure studs is believed to have been caused by boric acid 1 It i2II attack. The affected cumos were disassenbled. the defective caskets reolaced, and I l i 131 I new studs installed as recuired. l I i 14 I l l 7 8 9 80 SA  % POWER OTHER STATUS 015 0 Y DISCOVERY DESCRIPTION I1 45I[3]@ l0l0l0l@l NA l lBl@lRoutineStartuoInspection l ACTIVITY CO TENT RELEASED CF RELEASE AMOUNT OF ACTIVITY LOCATION OF RELEASE li is8l9W @ W@l 7 10 11 NA 44 l l 45 NA 80 I

PE3SONNEL EXPOSURES NUMBER TYPE DESCRIPTION l1 l 7 I l 1 l 1 l 8 l@l E l@l 27 Man-Rem rec'd. by Maintenance. Encineering & Contractnr l PERSONN E L INJLRt ES NUMBER DIESCRIPTION l

li la l8 l9O l O l 2 l@l12 Minor ini:' ries occurred durino reoair of oumos 7 11 80 LOSS CF OR OA '. *ME TO FACILITY

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LER 80-010 Omaha Public Fow::r District Fort Calhoun Station Unit No.1 Docket No. 05000285 ATTACHMENT NO. 1 Safety Analysis It is probable that the corrosion damage to the reactor coolant pump closure studs did not occur over a short period of time and must have existed prior to the refueling shutdown. However, even with the damaged studs discovered during this inspection, the 1 gpm reactor coolant system leak rate limit discussed in the Technical Specifica-tions was not exceeded during power operation.

The pump cover is secured to the pump casing by means of 16 closure studs. The createst stud damage was observed on pump RC-3B; 3 of the studs were substantially corroded with a minimum diameter of 1 to 1-1/2" (original diameter of approximately 3-1/2") and a fourth stud exhibited significant corrdsion, although the stress area was not markedly decreased. Durin'g disassembly of the pump, all but one of these studs were subjected to a stress of 23,500 psi without failure. It is evident that, since normal operation of the reactor coolant pumps did not cause failure of any studs, the corroded studs were not overstressed even with the reduced stress area.

It is concluded that the gasket leakage and corrosion damage which occurred on the reactor coolant pumps did not create a con-dition which jeopardized the health and safety of the public or District personnel.

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LER 80-010 Omaha Public Power District Fort Calhoun Station Unit No. 1 Docket No. 05000285 ATTACHMENT NO. 2 Cause Description / Corrective Actions On the evening of May 15, 1980, plant operating personnel were performing a cold pressure test of the reactor coolant system prior to plant startup. The primary system was pressurized to approxi-mately 180 psig using one low pressure safety injection pump. A routine primary system leakage inspection by operations personnel revealed leakage coming from reactor coolant pump RC-3C. At this time the low pressure safety injection pump was secured. On the morning of May 16, 1980, the system was again pressurized using the low pressure safety injection pump. An inspection by mainte-nance personnel revealed leakage from the shaft seal on pump RC-3C.

This seal was subsequently tightened which reduced but did not eliminate the leakage. Further investigation revealed leakage emanating from the gasketed surface between the pump casing and the pump cover. At this time, technical assistance was requested from the pump manufacturer, Bryan Jackson. On the afternoon of May 16, the other three reactor coolant pumps RC-3A, RC-3B, and RC-3D were inspected. Leakage was found to be coming from RC-3A and RC-38. In addition, it was noted that corrosion damage had occurred to a number of the closure studs on pumps RC-3A and RC-3B.

No stud damage was evident on RC-3D. The corrosion had occurred on the exposed shank of the studs.

The pump cover and the pump casing for each reactor coolant pump are constructed of ASTM A-351, Grade CFSM stainless steel.

Sealing between the pump cover and the pump casing is accomplished by means of two concentric 304 stainless steel Flexitallic gaskets.

The cover is secured to the pump casing by 16 closure studs con-structed of ASTM A-193, Grade B7 carbon steel, chrome plated in the thread area, and phosphate coated in the shank area. The studs are approximately 3-1/2" in diameter and approximately 29" in length. During assembly the studs are hydraulically tensioned to approximately 23,500 psi. The residual stress in the studs, when they are held by the nuts, is approximately 15,500 psi.

A leakoff line is included between the two Flexitallic gaskets; this line was originally blanked off with a pipe plug. To aid in determining the source of the leakage, the leakoff line for each pump was unplugged ard the system was repressurized to approxi-mately 180 psig. Pump RC-3A showed. slight leakage through the leakoff line. Pumps RC-3B and RC-3C showed considerably more leakage. No leakage was noted for RC-3D.

l l The covers were removed from pumps RC-3A, RC-38, and RC-3C.

The Flexitallic geskets were inspected and it was determined that deterioration of the gaskets was apparent in some cases. Where deterioration was apparent, it was in the form of loss of asbestos filler, not catastrophic failure. The gasketed surfaces for all three pumps were in good condition with no evidence of steam erosion. \

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LER 80-010 l Omaha Public Power District i Fort Calhoun Station Unit No. 1 l Docket No. 05000285 l

ATTACHMENT NO. 2 (Continued)

Following removal, the 48 used studs were subjected to ultra-sonic, magnetic particle, and visual examinations. A stud was considered to be acceptable for further service if the ASME Section XI acceptance criteria were met and if corrosion had not reduced the minimum diameter to less than 3.320" (the minimum acceptable diameter for new studs). The following is the final status on the studs removed from service.

Number of Studs Reason for Rejection RC-3A RC-3B RC-3C l Excessive corrcsion 4 8 6 Other unacceptable indications 2 2 0 E III T It is emphasized that, of the studs rejected for excessive cor- ,

rosion, only 3 studs on RC-3A and 4 studs on RC-3B were severely l corroded (minimum diameter in the 1 to 1-1/2" range). Full hydraulic  !

tensioner pressure of 4500 psi (stud stress of 23,500 psi) was re- i quired for removal of 47 of the 48 studs; one stud on RC-3A fractured at a tensioner pressure of 3000 psi (stud stress of 15,500 psi).

The affected pumps were reassembled using new studs as required and new Flexitallic gaskets. The nuts were visually inspected prior to reinstallation and one was replaced owing to excessive thread damage.

The closure studs for RC-3A and RC-3B were ultrasonically inspected during the 1980 refueling outage. The studs were examined from the top surface using a straight beam longitudinal wave search unit at a nominal search frequency of 2.25 MHz. No recordable indications were revealed. A visual examination of the shank of the studs was not performed since it was not known that the shank portions of the studs were accessible. Following discovery of the corroded studs, the ultrasonic examination was repeated; once again, no recordable indications were revealed.

In order to preclude future stud damage and unidentified gasket failures, a number of preventive measures have been adopted. First, the leakoff lines between the primary and secondary gaskets have been connected to pressure transmitters so that leakage past the primary gaskets will be detected. Second, the accessible portions of the pump closure studs will be visually inspected at least once per fuel cycle. (The accessible portions were, in all cases, the portions susceptible to corrosion damage.) The original insulation for the pumps has been replaced by insulation which will allow better inspection of the studs. Third, further investigation will be con-ducted in an attempt to ascertain the precise mechanisms responsible for the stud damage.

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9 LER 80-010 Omaha Public Power District Fort Calhoun Station Unit No.1 Docket No. 05000385 ATTACHMENT NO. 2 (Continued)

In order to ensure that wetting of the old insulation did not create the possibility of chloride attack to the pump casing, samples of old insulation were analyzed for Cl , Na+, and SiO2 concentration.

In all instances, the analyses verified that the insuTat:en met the acceptance criteria in Regulatory Guide 1.36. In addition, a sample area of pump casing was liquid penetrant inspected to confirm that no evidence of stress corrosion cracking had occurred.

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LER 80-010 Omaha Public Power District Fort Calhoun Station Unit No.1 Docket No. 05000285 ATTACHMENT N0. 3

Failure Data No problems had occurred previously with the reactor coolant pump gaskets or studs.

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