ML19289C601

LER 78-020/01T-1 on 780701:growing Crack Was Observed in Torus Shell Adjacent to Weld.Caused by Failure of a Certain Welding Process to Produce the Desired Results in Metallurgical Properties

ML19289C601
Person / Time
Site:	Vermont Yankee
Issue date:	01/15/1979
From:	Conway W VERMONT YANKEE NUCLEAR POWER CORP.
To:	NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION I)
Shared Package
ML19289C598	List: ML19289C597 ML19289C601
References
LER-78-020-01T, LER-78-20-1T, NUDOCS 7901190109
Download: ML19289C601 (6)

Text

N#C f OitM 374 U.S. NUCLEAR REGULATORY COMMISSION (7 m UPDATE REPORT - PREV 10UE REPORT DATE 7/19/78

. . LICENSEE EVENT REPORT LER 78-20/1T (PLEASE PRINT OR TYPE ALL REQUIRED INFORMA1IONI CONTROL BLOC (: l i

l l l l l lh 6

[ojt]

8 9 VlTlVlYlS}1l@l0l0l-10 14 0 l 0 l 0 l 0 l- l 0 l 0 25l@l 264 l 1UCENSE LICENSE NUYbER l 1 l 1TYPE l 1JQl@lSt l LATad l@

7 LICENSEE CODE lb CON'T lOl.l ','g,",' l L l@[ 0 ] 5 l 0 l 0 l 0 l 2 7 168@l690 l 7EVENT l 0 lDATE 1 l 7 l 874@]O l 1REPORT l 1 l DATE 5 l 7 l 980l@

6 15 7 64 61 DOCK ET NUMBER EVENT DESCRIPTION AND PROBABLE CONSEQUENCES h o I 2 l o 3 l See attachment entitled " Vermont Yankee Torus Crack Causes and Corrective Actions"l o 4 I I

10151 I I

101611 o i I 1

o a l 80 i 8 9 C DE CODE SU8C E COMPONENT CODE SUSC DE S ODE 7

o 9 d 9 l Sl Al@ l Xl@ l Z @ lV lE lS lS lE lL @ l El@ ]@

to 11 12 13 18 19 20 SEOUENTI AL OCCUR AENCE REPORT REVISION

_EVENTVEAR REPOHTNO. CODE TYPE NO.

@ LER AO l7 l8 l g l0 l2 0l l/ l 0l 1l lTl 30 g31 l 1[

32

_ 23 22 2J 24 16 27 28 29 TAKE ACTC ON PLANT iT4 HOURS 22 SB IT FC hi b U. SUPP LI E MAN FACTURER lDl@lXl@

JJ J4 lAl@

35 lAl@

36 l0 l7 l4 l4 40l 3/

lY @

41 l Ylg 42

]@

43 l44 Cl 3l 1l 0l@47 CAUSE DESCRIPTION AND CORRECTIVE ACTIONS i o I

[i[il See attachment entitled " Vermont Yankee Torus Crack Causes and Corrective Actions"1 l

li 171 l l

i 3 i l

li l4 I I 80 7 8 9 ST S  % POWE R oTHER STATUS ISCO R DISCOVERY DESCRIPTION Welder Observation li l5 "l l El@

1 0l0l@ln iz NA 44 l l45C l@l4s 80 l

A2TiviTY CO'O r E NT RELEASE AMOUNT OF ACTIVITY LOCATION OF RELEASE NA l

[L{c_j W @D OF RELEASEW@l NA l l 80 7 8 9 10 11 44 45 PERSONNEL f XPOSUHES NUMBER TYPE DESCRIPTION i 7 l 0l 0 Oj@lZl@l NA l

,E nSONNE L ,N;UL S NUMBE R DESCRIPTION @

l @ l0l0l0l@l 7 8 9 11 12 NA 80 l

' " ^

1 iSE sc"^?E To 's

'^c'""

@ 790119o[o3 M y@l 8 9 10 NA 80 l

7 DESCRIPTFOP4 2 o ISSUE @l Press Relcase - July 1, 1978 I lllllllllll12 7 8 9 10 68 69 80 &

W. F. Conway pggyg; W-N-M {

NME OF PREPARER

VERMONT YANKEE TORUS CRACK CAUSES AND CORRECTIVE ACTIONS YANKEE ATOMIC ELECTRIC COMPANY DECDiBER, 1978

. l INTRODUCTION / BACKGROUND In anticipation of the plant modifications to be required as a result of the Mark I Containment Program and in order to ensure that Vermont Yankee could meet the December 1980 goal of completing all modifications, Vermont Yankee began installation of torus column gussets in May 1978.

The design of the installation is shown in Figure 1. The purpose of the modification is to increase the load carrying capacity of the torus shell to column attachment.

The gussets are welded to the torus shell and torus column.

The torus shell material is a medium carbon steel plate

( ASME specification SA-516 Cr 70) . Because of the level of carbon and other elements in the steel the materic' can be hardened and embrittled by rapid cooling from weldin; mperatures to room temperature.

The ASME Code requirements for welding d 9t normally require any special welding precautions for welding c SA 516 plate in the thickness range of the torus (0.584 inches nominal thickness). However, since the installation would be performed with the torus containing water it was concluded by Yankee Atomic Electric Company (engineering agents for Vermont Yankee) that special welding procedures should be developed to avoid a weld hardened-area on the torus shell. The technique chosen was the " butter-temper process".

In this process a layer of low carbon non-hardenable weld metal is deposited on the torus shell. This is called the butter layer.

Because of the rapid cooling of the weld metal and shell, caused by the massive heat sink of the torus water, a hardened area is established in the weld heat af fected zone (HAZ). In order to reduce the hardness of the HAZ a second layer of non-hardenable veld metal, called the temper layer, is deposited over the butter layer. The heat from the tenper layer provides a heat treating ef fect to reduce the HAZ hardness.

This technique was developed for the Mark I Torus program and utilized at several other reactor sites prior to the Vermont Yankee installation.

On June 27, 1978, a crack was observed visually in the torus shell material adjacent to one of the butter-temper pads. Visual examination revealed cracks at several other pad locations.

An ultrasonic examination of the first cracked area was conducted and it was determined that the crack was less than 0.250 inches deep.

Since visually that was the largect crack, and since that crack did not infringe on the minimum required wall thickness of the torus, it was decided to grind out the crack while plant operation continued.

Grinding was initiated using approved procedures and with required hold points to ensure that the minimum wall thickness requirements was not violated.

On July 1, 1978, the crack had been ground down to 0.250 inches and liquid penetrant examination of the ground area showed that the crack was still present. The area was examined again using ultrasonic techniques and it was observed that the crack had grown in depth to the point that the minimum wall thickness of the torus may have been penetrated; thus an im=ediate orderly shutdown was initiated.

FAILURE ANALYSES Several metal samples were removed from the torus in order to dete rmine the morphology of the cracks. The failures exhibited classical hydrogen induced cracking characteristics.

In addition, hardness measurements taken in the HAZ showed values equivalent to hardened steel, indicating that little or no tempering had been achieved.

No material abnormalities were observed. A review of actual material certifications for the torus plate material was conducted; the material composition and mechanical properties were within the range of values specified for SA 516.

CAUSE OF CRACKING The cracking of the HAZ was caused by the failure of the particular butter-temper weld process utilized to produce a HAZ that was tempered from the hardness produced by the butter layer. The rapid cooling produced a hardened HAZ and increased the likelihood of hydrogen being trapped in the weld and surrounding base metal. These factors, coupled with welding stresses from the gusset welds, resulted in a total stress environment in the HAZ that exceeded the yield strength of the material. Because of the reduced ductility, resulting from the high hardness, the ability of the HAZ to plastically deform was reduced and the material cracked.

CORRECTIVE ACTION The corrective action consisted of physically removing, by grinding, all of the hardened base metal and restoring the necessary wall thickness

utilizing welding procedures that properly addressed the heat input and slow cooling considerations of the butter-temper process.

The repaired areas were radiographed, ultrasonically examined and magnetic particle examined. The magnetic particle examination was conducted at least 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> af ter welding on a particular pad was completed, as required by I6E Bulletin 78-11.

Since the torus minimum wall was violated, a Class A containment test was conducted.

1.

TORUS SilELL BUTTER-TEMPER PAD AND WELD TO TORUS

\ s LARGE GUSSET __

ASSEMBLY ->

1

),

SMALL GUSSET ASSEMBLY

(

EXISTING I-BEAM

\#

COLUMN SUPPORT

/

/

FIGURE I