ML19345G108
| ML19345G108 | |
| Person / Time | |
|---|---|
| Site: | Humboldt Bay |
| Issue date: | 01/10/1975 |
| From: | Crane P PACIFIC GAS & ELECTRIC CO. |
| To: | Goller K Office of Nuclear Reactor Regulation |
| References | |
| 332, AO-DPR-7-74-4, NUDOCS 8103020430 | |
| Download: ML19345G108 (6) | |
Text
I,.
M".I,p, Mi f.%.z M,d W tut PACIFIC G-A S A N D ELECTR,IC C O M PANY FOWE l
77 B E ALE S T R E ET. S A N F R A N CISCo, C ALIFoR NI A 94106. (415) 781 4211 g
page_e
.3 O H N C. MORRISSEY N.
..-.. s = = o
.a vics cessierar ano ar= cent counset ia h &m:(" N
= =,.,=,33.j.....
u ALcoLu M. FuReusH
/,j N
v s i;::r.Lt.:"/...
k.'
a*5cce a< <at cou $<6 weouau e. muoa=
- 3 _'h f; i
,..,',',,'[,""'
c na a o r s v. v4 = oa u o e u
.,,,,f
,-j
- y*f,',?,1y,J,.g****
p, :
of,,;;pt;;p;;.,
uatcot u a. u..xitto.
,f_,
ensur 4. c na a r. s =.
lq g",:g*.g.j **-
I ~'
, S/ January 10, 1975 e., s. t.. L a.o.
3.,,.EE.f.~.
n",9
=<enano4.etanne jy
,g y,4;a.,,*agge;,--..
sona m.aie,ao i'
g y
..u,.
..:L /
- r.'::':.~
- .=~~ ~
tn, *o ~S, t :n.w ~:::...
s Mr. Karl R. Goller
- %*C."f *.
s Assistant Director for Operating Reactors p
',[' l '
Directorate of Licensing
)I Office of Regulation D
k4 h3 U. S. Atomic Energy Commission d-
'Q,
8
' g Q.Q,h
&gg Washington, D.C. 20545 ce E
.e..
\\
Re: Docket No. 50-133
~
Dear Mr. Goller-On November 27, 1974 the Company submitted Abnormal Occurrence Report No. DPR-7/74-4 covaring the failure of some of the reactor feedwater sparger hold-/.own U-bolts. At the time of the report, two broken U-bolts were undergoing laboratory exami-nation and the feedwater sparger repairs were in progress. The purpose of this report is tg des <. ribe the UIbolt failure mechanism,,_
i determined from this examination and the corrective feedwater N
sparger repairs.
)
g
'Y
<I, a
BACKGROUND c
The reactor feedwater sparger consists of a continuous Q,. v v y
cection of oval cross section pipe located around the inside of~,
y the vessel at an elevation below the top of the core chimney. De?~ -r,/
sparger is supplied with water from one inlet nozzle. The sparger C _1._ /
has holes \\ inch in diameter near the bottom or. both sides to dis-tribute the feedwater uniformly in the vessel.
rhe r,parger is sup-ported by eight gusset type brackets around the inside diameter of the reactor vessel wall and was originally designed to be held in place with U-bolts mounted on each bracket.
The U-bolts, which were fabricated from inch diameter SA-276 Type 304 austenetic stainless steel, fit around the sparger and were held in place with nuts on both sides of the support bracket. The nuts were tack welded to the bracket. The general arrangement of the spar-ger is shown in Figure 20 of,the Final Safety Analysis Report.
P00R ORIGINAL
"?-
r ow6930
-.~ -
('
i Mr. Karl R. Goller Jznucry 10, 1975 On November 17, 1974, while the reactor was shut down for a regularly scheduled refueling outage, it was noted that the sparger had moved 1 to 2 inchcs away from the nozzle and that some of the U-bolts were broken. A close examination showed that six of the U-bolts had broken through on one or both legs (refer to Figure No. 1).
The bolts from the 220 and 1570 positions, consid-ering the feedwater nozzle as the O pos'. tion, were sent to the Vallecitos Nuclear Center for metallographic examination to iden-tify the failure mechanism. These bolts were broken through on both legs just above the uut on the tcp of the support bracket.
RESULTS OF THE METALLOGRAPHIC EXAMINATION Examination procedures consisted of visual, visible dye penetrant, scanning electron microscopy, metallography and hard-ness. The examination showed that the U-bolt failure was caused by high cycle fatigue. The basis for this conclusion is as follows:
1.
Electron microscope examination of the broken bolts showed that the fracture mode was transgranular cleavage. Transgran-ular cleavage is the type of fracture associated with high cycle fatigue.
2.
Metallographic examination of the fracture surfaces showed a completely transgranular mode of failure. The transgranular mode of propagation and the absence of plastic deformation associated with the fracture indicated that the most likely failure mechanism was high cycle fatigue. The material was verified to be solution annealed with a Rockwell Hardness in the range of 93 to 95.
This is normal for commercial bar stock of this material.
3.
A longitudinal section through the slightly worn area where the 1570 bolt contacted the side of the sparger revealed essen-tially no cold working. This indicates that the source of wear was gentle and probably occurred over a long period of ttse.
POSTULATED FAILURE MECHANISM lt is postulated that the U-bolts vibrated in resonance with input from a source such as the feedwater pump or recircula-tion flow.
It is further postulated that the sparger itself was l
not vibrating or that the vibration amplitude was very low. This is based on the following findings:
l
[
l
(
1 Mr. Kcrl _ R. Goller Jinu ry 10, 1975 1.
Visual examination of the underside of the sparger showed no unusual wear marks. If the aparger were vibrating it would be expected that grooves or chatter marks would be worn into the bottom of the sparger from contact with the sparger support brackets.
2.
Visual examination of the thermal sleeve-sparJer junction showed this area to be in excellent condition. If the sparger were vibrating significantly it would be expected that cracks would be created at this junction.
CORRECTIVE REPAIRS Corrective repairs were performed under water using specially designed tools. All of the U-bolts and associated nuts were removed from the sparger support brackets. There are no lost or unaccounted for parts in the reactor vessel. The basic tools used in this phase of the repair were a commercial hydraulic bolt cutter and nut cracker modified by the plant staff for this parti-cular application.
The sparger was then recentered using a commercial hydraulic jack with special fixtures built at the plant so that the sparger could be jacked against two of the sparger support brackets. The force required to move the aparger back in place was calculated to be less than 2000 pounds.
After the sparger was recentered, the feedwater line inside of the drywell was examined by the visible dye penetrant method. No relevant indications were found. The feedwater nozzle was also examined by radiography. This examination showed that the thermal sleeve which attaches to the sparger was in its proper position. The end of the thermal sleeve appeared to be normal in all respects.
Redesigned sparger restraints were installed at each l
sparger support bracket. These restraints are substantially stron-ger in design and clamp tightly around the sparger. They are not directly attached to the sparger support brackets as were the orig-inal U-bolts. The nuts which bolt the restraints to the sparger were tightened to 40 foot pounds and secured by staking to prevent j
them from working loose. Legs from the restraints are designed to engage the wall bracket to limit any sparger movement away from the feedwater nozzle due to hydraulic forces, while allowing the sparger to move as a result of thermal growth (see Figure 2).
l I
-..... ~.
(
1 Mr. Karl R. Gollcr Jxntiary 10, 1975 The new restraint design should not be susceptible to the same types of vibration problems that were postulated for the original U-bolt design. The reason for this is that the new restraints are held in firm contact with the sparger. The new restraints were fabricated in the plant from solution annealed ASTM A-276 Type 304 stainless steel bar and flat stock.
Twenty-three days elapsed from the time the sparger prob-lem was first discovered (November 17, 1974) until the corrective repairs were completed (December 9,1974). All of the repair work to the sparger was performed by plant personnel working from the refueling platform on the reactor extension tank to minimize radi-ation exposure. The total accumulated exposure to perform the repair work is estimated to be only 6.5 man-rem. All special tools and the new restraints were built by plant personnel. Engineering support was provided by General Electric Company's Nuclear Engin-eering Division (San -Jose) and Vallecitos Nuclear Center and the
(;ompany's Department of Engineering Research.
FUTURE INSPECTION PROGRA'4 During the 1975 and 1976 regularly scheduled refueling and maintenance outages a thorough visual inspection of the feed-water sparger, all support brackets and all sparger restraints will be made. In addition, each restraint will be checked to assure that it is still clamped tightly to the sparger. During the 1976 outage one of the restraints will be removed and replaced by a new restraint of similar design. The removed restraint will be subjected to a thorough examination by the Company's Department of Engineering Research. Future inspection programs will be based upon the results of these examinations.
Ver t uly yours, 1
l l
/
Philip A. Cr e, Jr.
y cc:
Mr. R. H. Engelken, Director Directorate of Regulatory Operations Region V 1990 N. California Boulevard Walnut Creek Plaza, Suite 202 Walnut Creek, California 94596
t8e9%64 5 5 >** %l8 S S
=-
y v
1 i
l Q*J v
y l
This' fitting installed A3 4,, Nuts only on restraints.
(Torqued to 40
)
Installed at 22* and 337 ft-2bs and staked) l locations Y
~
/ ~
T p_
g
/
~
i gSparger Restraint J
y f
r I
i l
\\
g
- 3 1
.4, L';;>.
s..
~
1s s
. As; _lC
.. N_. :.:~. :q.fk.s. P00R ORIGINE-
's pport..
N N
e
'(,
s.m -
s..
'N.,'..,
(
Bracket
-.,..}'.(i.,d '. '
g' g
~
'i -
,.,,. r g
- 1:-
. ; s..
q
, y, K
+
y g
'" Q y
~;
........ = ~
- ~
[
s
~
/
1" Figure No. 2 - Basic Redesigned Feedwater Sparger Restraint (Illustrative Only) i 4
PACir C gas AND ELECTRIC COMPANY
- 1-3f5
- one SAN FRANCISCO. CALIFORNI A
^
l
-=a.=___
l P18MitO O*4 DatPO Dec. 9000 4 CLE A RPas8tf SofS l
u.inu ::.co e/en r
(
-)
%*g % % % ],y\\ g i
Q
't.
N-a 4
'N 4
\\
N 4
%a [
Sparger r
?J
\\
180 f
y A;
2' 0
(
g]
90 7
'T U-Bolt
~
l3,
%\\
k\\
O N
Restraint
\\
l
\\
%/
'N D
4 4
s Sparger Support Bracket X
- 'N q
k l
-Feedwater Nozzle l
(')j Thermal Sleeve i
)
Arrow (4) indicates broken -Bolt P00R ORIG NAL Figure No. 1 - Feedwater Sparger Plan View Showing Location of Failed U-Bolts (Illustrative Only)
PACIFIC GAS AND ELECTRIC COMPANY om g,u SAN FRANCISCO. CAUFORNIA 1-3-75 Nrne m.m o o. a m o.....oc ca u m.a
..n
.. _ _..