ML19345G581
| ML19345G581 | |
| Person / Time | |
|---|---|
| Site: | Midland |
| Issue date: | 02/11/1981 |
| From: | TELEDYNE ENGINEERING SERVICES |
| To: | |
| Shared Package | |
| ML19345G578 | List: |
| References | |
| TR-4599-2, NUDOCS 8104070551 | |
| Download: ML19345G581 (27) | |
Text
I l
O l CONSUMERS POWER COMPANY l O a^CxSON. "ICsIGAN 49201 O.
O O
TR-4599-2 O
CONTINUED INVESTIGATION OF THE FAILURE OF MIDLAND UNIT 1 RPV ANCHOR STUDS ANALYSIS REPORT O-FEBRUARY 11,1981 O
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MTERWNE ENQNEERNGSSMCES 303 BEAR HILL ROAD "
WALTHAM, M ASSACHUSETTS 02154 617-890-3350
.O 4,99040S5 I
- O WTELEDGE Technical Report TR-4599-2 mM O
TABLE OF CONTENTS
- O' PAGE
1.0 INTRODUCTION
1
'O
2.0 CONCLUSION
S 1 3.0 DISCUSSION 2 l
l 3.1 Determination of Actual Stud Tempering Temperature 2 O 3.2 Quenching Response of Heat 000 Material 4 3.3 Tempering Response of Heat 000 Material 4 3.4 Distribution of Hardness 5
'O Figures 7-19 Appendix A
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1.0 INTRODUCTION
l Teledyne Engineering Services (TES) under contract to Bechtel Asso-O-
ciates Professional Corporation studied the preservice failure of three Reactor Vessel Anchor Studs in Consumers Power Company's (CPC0) Midland l Unit 1. The results of this failure analysis were presented in TES reports l TR-3887-1, TR-3887-1 Addendum 1, and TR-3887-2. While the investigation g summarized in those reports clearly shows the cause of failure was stress corresion in studs that were harder than expected for specific material, what was lacking was an explanation of how the studs came to be so hard.
O Subsequently, TES, under contract with CPC0 has conducted further testing, the primary goal of which was to explain the cause of .the high hardness. Secondary goals were to measure hardness distributions along the length of the' studs and determine why some studs exhibited hardness g gradients while others did not. The results of this further effort were presented in TR-4599-1 "Further Investigations of High Hardness in Midland Unit 1 RPV Anchor Studs."
.g This report provides a more detailed discussion and interpretation of the results presented in TR-4599-1 to the extent that these results relate to the cause for high hardness.
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2.0 CONCLUSION
S
- 1) Based on the results described in TR-4599-1, it is TES's opinion that the high hardness is a result of the RPV anchor studs never reaching g the intended tempering temperature of 850UF. The exact tempering tempera-ture achieved cannot be specifically stated, and may have varied from one 3
stud to another or along the length of a _ stud. but appears to have been equivalent to 750 F for stud 3.
O Technical Report WM TR-4599-2 MM g 2) For a given average hardness for a stud produced with material j characteristics of the heat 000 material, a variation in hardness of 4 hRC about the average hardness could be expected within a given stud with higher variations from stud to stud, and heat to heat.
O 3.0 DISCUSSION 3.1 Determination of Actual Stud Tempering Temperature
'O Since the probable explanation of the high hardness in the Midland studs is that they were not adequately tempered, it is desirable to estimate the original tempering temperature. For material which has al-O ready been tempered at one temperature, retempering at a lower temperature should not significantly lower its hardness, whereas tempering at a higher temperature will lower hardness. By retempering at various temperatures material which has 'aiready been tempered, the original tempering tempera-g tures can be estimated from changes in hardness. Figures 1 and 2 show hardness as a function of retempering temperature for samples of stud 3.
The difference between Figures 1 and 2 is that each figure is O based on hardness measurements of material from within different locations in stud 3. Since the original hardness was different, the retempering results have been separated. However, they both show essentially the same 0
result; a marked change in hardness between 750 and 850 F, confirming that g the original tempering temperature of stud 3 was at or below 750 F. This is more evident in Figure 3 where change in hardness has been plotted against retempering temperature for all the data in Figures 1 and 2.
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g It might be argued that the lowered hardness of the specimens retempered at 850 F might result solely from additional time at the temper-ing temperature. However,. Figure 4 shows that tempering for 'up to 8 hours9.259259e-5 days <br />0.00222 hours <br />1.322751e-5 weeks <br />3.044e-6 months <br /> at 850 F only lowers hardness about 1 HRC below the hardness for retem-g pering for-1 hour at 850 F. Since normal experimental error in hardness testing is at least 21 HRC, this change is not significant.
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O Technical Report TR-4599-2 3 W@
M O Figures 5, 6, and 7 show the original tempering furnace tem-perature records for three out of five temper heats of studs during temper-ing (the other two records are consistent with the three shown but did not reproduce well for this report), showing the studs spent several hours in O the furnaces. However, these are plots of furnace temperature and not stud temperature. According to these records the studs spent a total time of about 4 to 41/2 hours in the tempering furnace, of which for 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> the U
furnace was at 850 F.
O.
Figure 8 is the furnace tempering record for one of the heat 00 batches where the goal was a 925 F tempering temperature. In this case the thermocouple was attached to a stud and thus this temperature record is for
'O the stud surf ace temperature. Here the stud was in the furnace for at least 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br />, of which 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> were required to reach 925 F. The studs required at least four hours to reach 850 F. Based on this observation it is apparent that the heat 0 and 000 studs which were to be tempered at O 850 F would have required at least four hours to reach temperature. In fact the studs probably never did reach 850 F, since in order to reach U
850 F in four nours the furnace had to be set higher, at 925 F, which results in a faster heating rate (higher driving force) when compared to O the heating rate when the furnace is set at 850 F. As the stud temperature approaches the furnace temperature, the heating rate slows few studs in heat 00.
~O By making assumptions about the heat-transfer characteristics and using the furnace char t for the heat 00 studs it is p'ossible to show that at least 51/2 hours are required for the studs to reach 850 UF, and in 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> the studs will only reach 'about 810UF. These calculations are
- O ' shown in Appendix A. This analysis gave the studs the benefit of the doubt since: a) it assumed they were already at 250 F, when the starting temper-ature may really have been lower, b)' assumed the furnace temperature was
.25 F above the target temperature, and c) assumed no time was required for O .the furnace to warm up, when Figures 5, 6, and 7 show the furnace itself needed 1-1 1/2 hours to reach 850 F.
a
Technical R: port TM TR-4599-2 N SBt\/ ICES q
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3.2 Quenching Response of Heat 000 Material One possible alternate explanation of the high hardness for the Midland Unit 1 anchor studs is that they consist of material character-ized by high hardenability. Thus the as-quenched hardness, being high, would result in higher hardnesses after tempering.
The Jominy . test results shown in Figures 4 through 9 of
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TR-4599-1 and sumarized here as Figure 9, clearly show this material can be characterized as having hardnesses on the low side of hardenability limits for this material. Therefore, it is not possible to explain the high as tempered hardness on the basis o' high as-quenched hardness in this n'"
material.
3.3 Tempering Response of Heat 000 Material Having the Jominy specimen results allows a calculation of the as-quenched surface hardness for the studs. If the Jominy bars are then tempered, an estimation can be made of the as-tempered surf ace hardness for different tempering temperatures. These calculations assume that accurate
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' estimates can be made of the initial quenching heat transfer rates.
Charts are available to relate bar diameter, quenching medium, and location across the bar diameter, to equivalent Jominy location. These charts exist in several different forms. The one presented. here in Figure 10' is f rom the Metals Handbook Volume 1, Page 492, 9th Edition.
From information supplied by CPC0, the quenching medium was oil moving at 2.4 fpm. This is nearly equivalent to still oil (H = 0.20),
which is a slower quench than used for Figure 10. Further we assume a non-scaling atmosphere was used in the furnace, since no scale or decarbur-
_ization was seen on these studs in the previous investigation (scaling
O Technical Report WM TR-4599-2 5
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tends to slow the quenching rate) and the heat treatment procedures call for an endothermic atmosphere. Based on this, in Figure 10, the stud surf ace is equivalent to a Jominy location of 6/16ths. From Figure 9, the O hardness at this location is 49.3.
The averaged tempered Jominy results are shown in Figure 11.
For the 6/16ths location the tempered Jominy data gives the following O average tempered hardnesses that would be expected for tempering at 650 F, 7500 F, 850 F, and 950 F.
Hardness (HRC) at
.O Tempering Temperature (UFJ Location 650 750 850 950 Surface 46.4 45.6 41.8 39.3 O
These results are plotted in Figure 12 along with the average surface hardness measured in stud 35.(Reported in Table I of TR-4599-1).
The data indicates a tempering temperature less than 850 F.
O 3.4 Distribution of Hardness Figure 13 is a plot of the distribution of hardnesses along the O- length of stud 35 which were reported in TR-4599-1. Clearly, for an average hardness of 44 HRC it is conceivable that regions with a hardness as high as 48 HRC and as low as 40 HRC would be found.
'O in TR-3887-2, TES attempted to determine the range of varia-tion that might be expected .for a large sampling plan for a given target hardness. TES concluded that material which has a nominal hardness of some value based on a limited sampling in accordance with some specification O
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0 could, in fact, have regions where the hardness was as much as 3 HRC higher if it were more extensively sampled. For example, if the nominal hardness were 3S HRC, a hardness ranging as high as 41 HRC would be expected if a
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O- larger nuncer of samples were measured.
l The results shown in Figure 13 show that a variation as high as
[ 24 HRC could be expected for this stud material. Actually, since the data O; in Figure 13 only represents one stud, a higher variation would be expected
.for multiple studs within a heat of material and an even higher variation would be expected from multiple heats of material.
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J O' . .O: , Oi :O LO. .O O O O .O Technical . Report - TR-4599-2 WTELED YNE
., ENGINEERING SERVICE 48.0
- ~ ;:
.{
i
+
i-
^l
+ 1 Standard Deviation 4 6. 0 ' - . .. f.i ......................,.................................,,,,,,,
t
-l Average liardt..:ss 44.0 -
Surface Stud 35
. . . . . . . . . . . . . . .. . . . . . . . . . . . . . . ....... .. .. .. . 1.S.ta.n.dar.d Deviation 4 2. 0.. -
e - U
- x -
'C 40.0 -
. 1 .
i
'30.0 - 1 Figure 12 36.0 - Comparison of average dominy hardness after tempering (black circles) at the 6/16ths location to the average surface haraness of stud 35 f
- 34. 0 ' - , , , , ' '
600 700 000 900 10'00 TEMPERI'NG TEMPERATURE
+O Technical Report I9 M
TR-4599-2 O O 50 - O 40 - O E 8 E g 30 - O 5 g
-jz 20 -
O l' 10- -O-
- 0-39.0-I40.0 4 41.0 , 42.0 43.0,44.0l45.0l46.0147.0 48.0 9
- -39.9 40.9 41.9 42.9. 43.9 44.9 45.9 46.9 47.9 48.9 Hardness Range (HRC)
'(X -i:
4 Figure 13 h Distribution of Surface Hardness for Stud 35. This figure based on
. results reported in Table I of TR-4599-1
O
" WTA AWNE NE$N5"} "*P ENGNEERING SERVICES O
O O~ iO APPENDIX A O O O O O
O Report Y A-1 N SBR \/)CES Lumped Heat Capacity Method O, When an object of some uniform TemperatureoT is placed in an environment of some different Temperature (T.) and if the size of the object is sufficiently small such that the object does not change measurably b the temperatu"e of the environment into which it is placed, and if the object is assumed to have a uniform temperature as it attains the temperature of the surrounding env ronment, then the Lumped Heat Capacity Method
- may be used to calculate the temperature of the object as a function of time. The temperature O~ at any time, t > o is found from the following relation:
T - T. --(hA/pCV)t~
=e To - T.
O where: T = Temperature of object at time t t = time from placement in environment To = Temperature of object at t=o O T.= Temperature of surroundings h = heat transfer rates A = Surface area of object p = Density of object-O C = Heat capacity of object v = Volume of object-For the heat treatment of the Midland studs all the above terms may be , quantified except h.
~).
However, using the tempering furnace records where the thermocouple was attached to the stud, (Figure 8 of this report), we can determine the quantity -(hA/pCV) et any time since we know T, T o
, T. and t.
~
~Since charging _the furnace lowers its temperature, the temperature in the furnace will not be constant, and hence the numerical value of -(hA/pCV) will change also, increasing as the furnace temperature increases. It will most nearly approach the value for which the Lumped Heat Capacity Method can be used_ as T approaches T. and T. has stabilized. If we plot (hA/pCV) as a function of T and t, we can extrapolate to the value of (hA/pCV) for (T.-T)->0 Values for hA/pCV are tabulated in the following table and plotted in Figure 1.
- Reference : Heat Transfer by J.P. Holman,1972 McGraw-Hill, pages 88-91
TN Technical Report M TR-4599-2 i A-2 O n T T. To t (hA/o CV) 500 950 250 1.38 0.319
.j 600 950 250 2.125 0.327 l 700 950 250 2.75 0.376 800 950 250 3.5 0.440 V,l '
900 950 250 5.0 0.530 0 T. is assumed' to be 950 F to provide some AT as T+T. The extrapolated value of (hA/pCV) is 0.57 Given .this extrapolated constant three questions can be asked:
.O
- 1) H w well does it predict the time required for a stud to reach 925 F when.T= = 950 F and To = 250 F q T - Ts ,go.57t To .T.
925 - 950 , .057t 250 - 950 !O~ t = 5.8 hours, compared to 6 hours actually required from figure 8 which .is excellent agreement. i i ! 's ~
- 2) How long would. it take for a stud to reach 850 F if To= 250 F
-and T.= 875 F?
- g-
- 850 - 875 , g0.57t 250 -'875-5.58 hrs = t
'O 6
O. Technical Report TN TR-4599-2 A-3 MM o 3) How hot will the studs get in 4 hours assuming To = 250 F and T. = 875 F? T - 875 = e(0.57)(4) g 250 - 875 T = 811 F l l This analysis has made assumptions which introduce errors. All gj such errors tend to reduce the predicted time required for studs to reach temperature. The assumptions were:
- 1) Assumed the furnace was already at temperature, when in fact, time was required for the furnace to reach
'g temperature.
- 2) Assumed furnace temperature was higher than it actually
-O
- 3) Assumed studs were already at 250 F when they probably were nearer room temperature.
O Thus, the predicted times presented here should be considered lower limits only, and the predicted temperatures upper limits.
- O For comparison, the time required for a single stud placed in a
, ;- furnace to reach 850 F is calculated
- to be about 15-30 minutes.
..i If ~1 O
- Private correspondence (1/29/81) with Prof. William Leslie, University 3 of Michigan, Ann Arbor n
O WTEl.ED(fE Technical Report TR-4599-2 0 0,60 - O
.-0.55 -
.O i
- 0.50 -
o-l 0.45 - O t p I Y
- 2 l- 5 0.40 - --
lO l l
- l. . 0. 35 _ __
! O e 0 30 ' i i
- g b
i i i. i 750 i 650- 600 i 550 500 i 450 i 400 950 900 850 800 Temperature I Figure Al LO I. t l-(0. .
Encicsure 3 Serial ll52h Bechtel Associates ProfessionalCorporatioI$~ 777 East Eisenhower Parkway Ann Amor, yen $n assuseees P.O. Boa 1000, Ann Arbor, Mchgan 48106 023809 -
SUBJECT:
HCAR 37 (issued 12/28/79) Broken Reactor Vessel Anchor Stud in Unit 1 INTERIM REPORT 5 DATE: March 6, 1981 PROJECT: Consumers Power Company . Midland Plant Units 1 and 2 Bechtel Job 7220 Introduction The discrepancies discussed in this report concern the failed reactor pressure vessel (RPV) anchor studs in Unit I and the proposed method of repair.
Background
The anchor studs in. question are 2-1/2 inches in diameter and 7 feet, 4 inches long, embedded in the reinforced concrete RPV pedestal. The anchor studs were purchased from Mississippi Valley Structural Steel of St. Louis, Missouri; fabricated by Southern Boli. and Fastener of Shreve-port, Louisiana; and heat-treated by J.W. Rex of Lansdale, Pennsylvania. These studs were received on site by Bechtel in early 1976, embedded in concrete by Bechtel in April 1977, and tensioned by Babcock & Wilcox Construction Company in late July 1979. The first stud failure was discovered on September 14, 1979. The second and third stud failures were reported on December 20, 1979, and February 5, 1980, respectively. Investigative Action
- Teledyne Engineering Services (TES) performed a metallurgical failure analysis for Bechtel. The resulting reports discuss the stud failure
! investigation and the use of the present studs for service. The root. I cause of the studs' failure has been established by TES acting for j Consumers Power Company. The cause of the failures is the excessive hardness of the studs which is due to improper heat treatment. Consumers Power Company will submit the final report for this investigation to the NRC. Because of concern over the installed anchor studs, a design modification has been proposed as outlined in Reports 1 and 2. The studs have been detensioned to a nominal stress of 6 ksi.
Bechtel Associates ProfessionalCorporation MCAR 37 Interim Report 5 March 6, 1981 023809 Page 2 The brackets supporting the shield plug at the top of the cavity will be strengthened and modified to provide upper lateral supports to the RPV. Bechtel has calculated the stresses in the studs and upper lateral support brackets based on preliminary loads provided by Babcock & Wilcox for the accident condition of a combined seismic and loss-of-coolant accident event. Bechtel has found those stresses to be within the allow.*ble range. Where the stresses are more than half the lift-off load, the studs have been or will be proof tested to twice the design stress. Based on the preliminary loads, Bechtel has proceeded with the design of the upper lateral support (ULS). This design has been issued for procurement; however, construction is held pending NRC approval. New elastic spring constants for this design have been calculated and transmitted to B&W for use in confirming the preliminary loads. B&W has started the analysis of the primary loop based on the new support system. Because -the proper operation of this repair depends on the determination of the initial setting of the bumper at the end of the brackets so that
- - it approaches but does not contact the RPV at operating temperature, several meetings have been held between B&W, Consumers Power Company, and Bechtel to determine the criteria to be used to set the initial gap from the bumper to the RPV. B&W is presently. conducting a heat transfer i ' analysis to determine the effects of the bumper on the vessel and bumper I
heat-up on the concrete. In addition, modifications of the RPV insula-tion are being considered. Contact has been made by Consumers Power Company and Bechtel with Ray-mond Bolting Services regarding bolt tensioning. A procedure for reten-sioning Unit 1 to less than 6 ksi (5.0 + 0.5 ksi) has been developed and sent to Raymond for review. A procedure to detension the studs in Unit 2 to enable Raymond to use the lif t-off values from Unit 2 to i evaluate the scatter in bolt tension, which was found during the deten-sioning of the_ Unit I studs, has been developed and is being reviewed by Raymond. Corrective Action , I l Reactor Pressure Vessel Support-Modification for Midland Nuclear Power l: Plant, Midland, Michigan, Report 1, July 1980, was transmitted to Region III by Serial 9330 on July 24, 1980. Report 2, which'provides ! the analytical techniques for design, was transmitted by. Consumers Power j Company Serial 9787 on December 10, 1980. In addition, responses to questions from NRC have been transmitted by the following Consumers Power , Company letters: j i l . 1. Serial 11936 on January 20, 1981. - I I
-2. Serial 11334 on February 18, 1981 }
- 3. Serial 11345 on February 24, 1981 l
[ L !
Bechtel Associates Professional Corporation MCAR 37 Interim Report 5 March 6, 1981 02380S Page 3 Safety Implications - If uncorrected, this deficiency could adversely affect the safety of operation of the Midland plant at any time. throughout the plant's expected life. l Reportability This condition was reported to the NRC by Consumers Power Company under 10 CFR 50.55(e) on September 14, 1979. Submitted by:L EOI. T Approved by:M. _ g.c j p fyg 7f, Concurrence.by: /) ' , g/ #4 SS/ LED /sg < - -
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