ML20215K644
| ML20215K644 | |
| Person / Time | |
|---|---|
| Site: | 07109044 |
| Issue date: | 04/30/1987 |
| From: | CHEM-NUCLEAR SYSTEMS, INC. |
| To: | |
| Shared Package | |
| ML20215K627 | List: |
| References | |
| NUDOCS 8705110281 | |
| Download: ML20215K644 (28) | |
Text
..
b REPORT ON THE EVALUATION OF THE l-13G BOTTON PLATE /SHELL WELD
.l -
APRIL,1987 i
Chem-Nuclear Systems, Inc.
t 220 Stoneridge Drive Columbia, South Carolina 29210 4
870511Ct281 070415 ADOCK 07109044 PDR PDR C
l
TABLE OF CONTENTS 1.0 HISTORY AND TECHNICAL BACKGROUND 2.0 CASK EXAMINATION AND TESTING
3.0 TECHNICAL EVALUATION
3.1 GENERAL DISCUSSION 3.2 MECHANICAL LOADING 3.3 THERMAL LOADING 3.4 " ICE PLUG" EXPANSION 3.5 METALLURGICAL / WELDING
4.0 CONCLUSION
S 5.0 REPAIR PROCEDURE 6.0 CORRECTIVE ACTION REFERENCES APPENDICES APPENDIX A -- DERIVATION OF WELD STRESS INTENSITY VERSUS TRAPPED WATER VOLUME (0169W)
1.0 HISTORY & TECHNICAL BACKGROUND In late January 1987, Chem-Nuclear Systems, Inc. (CNSI) received notification from operating personnel at the Consumers Power, Big Rock Point (BRP) Plant of apparent problems with our 1-13G cask (C of C 9044). The plant personnel noted that there was leakage of water-l emanating from a crack (s) in the weld joining the outer cylindrical i
barrel to the bottom plate (see Figure 1 - Weld A). CNSI recomended l
that the cask be placed in the fuel pool and the radioactive waste material be removed. Following this operation, the cask weld was re-examined and several " linear" type cracks were visually observed in the affected weld area.
The cask was then closed and shipped empty to the CNSI facility in Barnwell, SC for further examination and repair.
I The NRC was notified of the occurrence and per Reference 1, the affected cask and comparable casks were removed from service.
)
j Subsequent examination of the other casks owned by both the General Electric Co. and CNSI revealed no prior history of this type of problem nor any indication of failure.
Following this, the other casks were again placed into service. Following decontamination of the 1-13G, a detailed physical examination of this cask was commenced.
The 1-13G cask configuration is shown in Figure 1.
The cask's shell and-baseplate is fabricated from 304 stainless steel.
There are a total of six GE-1600s (1-13G/1-13C) in use, of which two were purchased by CNSI in l
1978. These casks have been certificated and in continuous operation for approximately 17 years. There is no other reported incidence of a comparable weld cracking with these casks.
Two other circumstantial aspects of the cask weld cracking incident are noteworthy:
(1 ) The empty cask was exposed to very cold (below 0*F) weather for i
several days prior to insertion in the pool.
(2) Chemical analysis of the water leaking from the failed weld i
indicated a considerable difference in isotopic composition when compared to BRP pool water.
(0169W) l 4
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2.0 CASK EXAMINATION AND TESTING 4
1 Both the 1-13G and 1-13C casks were subjected to the following tests:
4 (1) Visual examination of all exposed surfaces.
(2) Dye penetrant (PT) testing of all exposed welds on the cask surface and inner cavity.
(3) Leak testing examinations of the waste cavity and all possible regions that could result in water inleakage to the lead annulus.
This included the waste cavity, the lifting ear bolt holes, and the cask lid bolt holes. The leak tests performed included a soap bubble test of all accessible joints and a pressure decay test of the waste cavity for more than 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />.
The testing of the 1-13C cask was successful. The leak testing verified that the cask cavity and lifting ear reinforcement ring welds were leak tight. Only a few very small rounded PT indications were observed -- with no distinctive pattern. These were repair welded and the cask returned to service in accordance with Reference 2.
The results of the testing of the 1-13G were as follows:
o Visible cracking indications were found in the cask shell to bottom plate weld area, o
Additional linear indications were found in this region following PT testing (see Figure 2).
o Indications (of a minor nature) were found in other areas of the cask. These did not compromise the containment (as indicated by leak tightness) and are repairable.
Additionally metallurgical testing of the affected weld was performed.
Three metal samples were carefully removed from the weld area (two samples from regions with cracks and one from an area with no indications). The metallurgical test results are reported in Section 3.5.
These were performed by an outside testing laboratory (Westinghouse-Pittsburgh).
(0169W)
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3.0 TECHNICAL EVALUATION
3.1 General Discussion A general evaluation was made of all possible causes of weld failure.
Due to the previous long and failure-free operating experience with this cask, it was felt that the failure scenario was cask specific and not generic.
It was recognized that the failure could be a result of a combination of several factors. The specific areas identified were:
1 o
Unusual mechanical loading o
Thermal loadings o
Welding metallurgy The mechanical and thermal loadings were evaluated analytically and the results presented in Sections 3.2, 3.3 and 3.4.
Testing of the weld metallurgy required removal of weld material and spectrometric testing. Those results are presented in Section 3.5.
3.2 Mechanical Loading Mechanical loading on these casks occur during truck transport, pool loading operations at the reactor, and other handling operations at Barnwell.
CNSI has operated the 1-13G cask for over 8 years with no reporting of or other indication of poor handling, short distance drops, etc. The mechanical condition of the cask body was good (i.e., no evidence of bottom plate, side wall, or cavity plate bowing or other visible deformation).
Our experience with plant operations indicates reasonable caution is taken in cask lifting / lowering operation to prevent excessive jarring of the cask and contents.
However, this cask lifting operation leads to the highest mechanical loading of this cask.
Hence our analysis focused on cask lifting..
The case examined considers the hydraulic pressure of the lead on the bottom plate (lead is not bonded to cask walls) during a vertical lift (see (0169W)
Figure 3). An acceleration factor of 1.3 was assumed. The resultant stress intensity is 9,515 psi.
Since the normal material yield stress is 30,000 psi, this is acceptable. Additionally, the effect of multiple lifts (alternating stress cycles) was examined and shown to be small. The-conclusions are:
o The stresses due to handling are much smaller than design-allowable
. stresses.
o Alternating stresses are much smaller than the endurance limit (for infinite cycles) of the shell material.
Hence, we considered that weld failure, solely as a result of mechanical handling loads, is very unlikely.
3.3 Thermal Loading 3.3.1 Thermal Shock The next failure mechanism examined was the thermal shock of placing a cask exposed to a cold ambient temperature into warm pool water. The differential thermal expansion between the shell wall and baseplate would result in a stress concentration at the material junction (weld region).
Conservative values were used to estimate this effect:
o Cask temperature
-20*F o
Pool water temperature
+80*F o
Weld stress concentration factor 1.30 The calculation results are shown in Figure 4.
This calculation is conservative, since it assumes an instantaneous temperature gradient.
In reality, the temperature differential moderates with time.
The calculated results are for stresses in the 20,000-40,000 psi range.
However, this type of loading is typically a secondary stress, and material damage is not expected (0169W)
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l STRESSES DUE TO MECHANICAL LOADING I
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unless a stress value exceeding two Sy (approx. 60,000 psi) is exceeded. We concluded that excessive weld strain due to thermal shock is not a probable cause of weld indication.
The discontinuity stresses are within the elastic " shake-down" limit for even the severe thermal shock scenario postulated.
3.4 " Ice Plug" Expansion There are clear indications that water was trapped in the lead annulus.
Even a small existing hole in the weld region could permit water inleakage under the hydrostatic head of 40-45 feet of pool water.
Typically, the cask would be submerged in a reactor fuel pool for several hours.
Any trapped water would freeze and expand (typically 9-10 percent) where subjected to the temperatures observed at BRP.
The expanded, incompressible ice would load the cask side wall in the manner sketched in Figure 5.
(This loading pattern is greatly exaggerated for illustration purposes.) The resulting weld shear stress was estimated parametrically as a function of entrapped water height.
The derivation of the weld stress intensity as a function of water expansion volume is included as Appendix A.
The results are plotted in Figure 6.
This figure illustrates that a trapped water height of only 0.5 inch could result in a weld stress intensity in excess of 173,000 psi.
This phenomenon has the potential for very high weld stresses for small quantities of water.
3.5 Metallurgical / Welding Three metal samples were removed from the affected cask weld.
Two samples (No.1 & 2) were removed from the cracked region, and No. 3 was removed from a "non-failed" region.
A spectrometric analysis was performed on all three samples to check for the use of improper weld filler material or poor welding. The test results are enclosed.
The following key results are noted.
o A normal stainless weld uses 308 stainless filler metal. The weld sample met 308 specifications with the exception of some noticeable lead contamination.
(0169W)
STRESSES DUE TO THERMAL SHOCK REMOTE FROM
+ 4T DISCONTINUITIES
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STRESSES DUE TO THE FREEZE-UP
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X FLOOR FIG.5
I 200.000 180,000 173,220
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10 INCREASE IN CAVITY VOLUME (INCH 3)
Volume of 1/2" in height of Standing Water in the Cavity = 552 in3
' Volume Increase following Freeze-Up (Unrestricted) = 55 in3 3
Effect of Approximately 20% Restricted Growth (10-11 in ) = 173,000 psi.
EFFECT OF ICE EXPANSION ON BOTTOM PLATE WELD STRESS INTENSITY FIGURE 6
o The lead contamination was only found in the failed weld and was at least 10-25 times higher than the unfailed region, and well in excess of permissible levels.
o Lead contamination in weld material would clearly lead to reduced joint strength.
The exact reduction in strength cannot be accurately quantified, o
Lead contamination was undoubtedly the result of poor welding practice and was a factor in the failure of the affected weld.
I (0169W)
WESTINGHOUSE ELECTRIC CORPORATION ADVANCED ENERGY SYSTEMS DIVISION TEGN10GY DEPARTHENT ANALYTICAL LABORATORIES Analytical Report 12919 GEMICAL ANALYSIS of STAINLESS STEEL ALLOY Submitted to HISHAM SHANKHANI CHEM-NUGEAR SYSTEMS INC.
OSBORN ROAD B ARNWELL, S. C.
29812
SUMMARY
Three samples identified as Sample #1, #2 and #3, were received fra Chem-Nuclear Systems Inc.
The samples were reported to represent several weld locations from a radioactive shipping cask.
The casing material was identified as 308 stainless steel.
A metals specification analysis was requested including the determination of lead.
April 9, 1987 Approved by -
WC Lawrence Kardos, S'enior Scientist Page 1 of 2
Analytical Laboratories y Advanced Energy Systems Division y AESD ALf 12919 AL Service #
87 617 87-618 87-619 MATERIAL ID.
STAINLESS STEEL WELD SAMPLES Specimen No.
I 2
3 308 Specs.
Metals Concentration in Weight Percent -
Fe MATRIX ELEMENT: Remainder by Difference Cr 19 0 18.7 18.4
- 18. to 23 Ni 90 8.8 87
- 8. to 12.
Mn 1 30 1 39 1 32 2.0 max Mo 0.27 0.22 0.28 0 50 max Co 0.099 0.091 0.0 97 no spec Cu 0.205 0.198 0.206 no spec P
0.020 0.025 0.0238 0.04 max Pb (1) 0.14 2 03 0.29 2 03
<0.01 no spec 0
0 Carbon not requested for analysis 0.20 to 0.40 Sulfur not requested for analysis 0.04 max Silicon not requested for analysis 2.0 max Remarks:
The above material meetn the metals criteria for a 308 steel.
The carbon specification identifies 308 from 302, 303, and 304 steels.
(1)
A 3 sigma value is included for the lead measurement.
i METHODS OF ANALYSIS Metals ICPS, Inductively Coupled Plasma Spectrometry.
Operator File RMcK 12919 Page 2 of 2
. - ~,.
4.0 CONCLUSION
S The key factors in our evaluation are:
After 17 years of operation, with five virtually identical casks, this o
weld cracking phenomenon had not been previously observed, o
The baseplate-to-shell weld is the final containment weld for the structure following lead pour.
In certain circumstances, a slight contamination of this weld by lead dilution into the weld metal is possible. Some lead (0.14-0.29%) was observed in the metallurgical sample. This contamination could result in a welded joint with lowered structural strength.
o The observation of water draining from the cracked weld, with a differing chemical composition than BRP pool water, indicates that some water had resided in the lead annulus for an unknown period of
- time, o
The sub-freezing temperatures experienced at BRP were sufficient to j
freeze any residual water trapped in the lead annulus. The expansion
{
of this water, due to freezing, results in a shearing load at the shell/ baseplate joint.
Since the exact water content is unknown, the weld stress can only be estimated.
However, a weld joint with potentially reduced strength is subject to cracking with small j
quantities of " trapped" water / ice, Water flow into a small weld hole is clearly possible for a cask o
submerged in a 40-45 foot deep pool (hydrostatic pressure 15-20 psig). The annulus formed between lead and steel in the cask bottom could " trap" the water, preventing significant water release after cask removal frgm the pool.
In summary, the projected scenario leading to the indicated weld failure is:
(1 )
In previous 1-13 cask pool operations, there was water leakage into the cask lead annulus.
(0169W)
(2) The extremely cold weather at BRP froze the trapped water.
The effect of the expansion on the baseplate / side shell weld, plus some reduced weld strength due to lead contamination, was sufficient to crack the weld.
It is clear that any corrective action must include removal / disposal of the baseplate ard removal of all affected weld material.
It must be assured that all water has been removed from the lead annulus.
The repair procedure must preclude the possibility of any lead contamination of the repair weld.
The repair weld should be full penetration and bonded to
" good" stainless steel material.
A PT inspection of the completed weld must be made.
The affected weld area would be examined on a continuing basis.
Previous experience indicates that this action is sufficient to prevent any further occurrence of this problem.
4 I
(0169W)
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5.0 REPAIR PROCEDURE The proposed repair procedure for the cracked weld is:
(1) Remove the stainless steel baseplate by cutting along the weld seam.
(2) Grind weld groove in cask shell (per Figure 7). Continue to test stainless steel shell material for lead dilution by acid etching.
Ensure that all lead dilution of the stainless steel is removed.
(3) Fonn a groove (nominally 1/8" x 3/4" circumferential) in lead for a backing ring.
(4) Position [1/8" (t) x 3/4" (h)] circumferential weld backing ring / plate between cask shell and lead.
(5) Weld new baseplate to cask shell usin., qualified full penetration weld and ASME qualified welder.
(6) Dye penetrant test the first weld pass.
(7) Dye penetrant test the completed wold.
The acceptance criteria for all welds is PT indication free.
(8) Vacuum dry the lead annulus through connection in baseplate, as follows:
(a)
Evacuate the annulus with vacuum pump until a pressure of 10 millibar or lower is reached.
(b) Vent the annulus until atmospheric pressure is reached.
After one hour, re-evaluate the lead annulus (this precaution eliminates potential of not draining water which forms ice plugs at triple-point of water).
(0169W)
(c) Repeat' step (8.b).
Shut down vacuum pump and close inlet valve.
Monitor inlet pressure gauge for pressure rise for 30 minutes.
Any pressure rise is either due to remaining moisture or gas inleakage to the lead cavity.
(d) Successful vacuum drying and leak testing of the lead annulus is ensured when the pressure gage shows no rise for 30 minutes.
(e) Weigh any water collected in pump cold trap and report.
(9)
Install plug in baseplate connection, seal weld and PT.
(0169W)
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6.0 CORRECTIVE ACTION (1 ) Visually inspect baseplate to shell weld prior to every shipment.
(2) Dye penetrant test the cask baseplate weld annually.
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i REFERENCES i
1.
U.S. NRC letter Mcdonald to House (CNSI) dated 1/30/87.
1 i
2.
U.S. NRC letter Mcdonald to House (CNSI) date'd 2/27/87.
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$4ren b he. 38eIJ IW '
- 175 "
theu Shew b 4e- *Id SI s(Q)'+ t' Chen inf amely e
=2
%.x
=
- s%MW l
+i sq
- 51.768 4 L
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