ML20148N756
| ML20148N756 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 03/31/1988 |
| From: | Mroczka E NORTHEAST NUCLEAR ENERGY CO., NORTHEAST UTILITIES |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| GL-87-05, GL-87-5, NUDOCS 8804070319 | |
| Download: ML20148N756 (12) | |
Text
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NORTHEAST UTILITIES a.n.r.i omc... s.m.n sir..i. 8.mn. conn.ci,cui l
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ae mss *0 March 31, 1988 Docket No. 50-245 B12803 Re: Generic Letter 87 05 U.S. Nuclear Regulatory Commission Attn:
Document Control Desk Washington, D.C. 20555 Gentlemen:
Millstone Nuclear Power Station, Unit No. 1 Additional Information Concernina Generic Letter 87-05 Generic Letter 87-05(I) requested information from BWR owners regarding their intended actions to determine if drywells at their facilities have degraded by a corrosion mechanism similar to that which occurred at the Oyster Creek Nuclear Generating Station from 1980 to 1986.
Northeast Nuclear Energy Company (NNECO) providg)therequestedinformationconcerning W ilstone Unit No. I on May 12, 1987 In this letter, NNECO committed to perform visual inspections of the eight 1-inch drain lines and ultrasonic thickness measurements of drywell shell plates.
The purpose of this letter is to provide further information subsequent to the completion of inspections performed during the 1987 refueling outage.
During the 1987 refueling outage the following actions were taken concerning the drywell and the drywell shell:
o A detailed visual inspection on the eight 1-inch drain lines, o
Ultrasonic thickness measurements on the drywell shell plates adjacent to the sand cushion.
(1)
R. M. Bernero letter to All Licensees of Operating Reactors, Applicants for Operating Licenses, and Holders of Construction Permits for Boiling Water Reactors (BWRs) with Mark I Drywells (Generic Letter 87-05),
dated March 12, 1987.
(2)
E. J. Mroczka letter to U.S. Nuclear Regulatory Commission, "Millstone Nuclear Power Station, Unit No.1, Response to Request for Additional Information Concerning Assessment of Licensee Measures to Mitigate and/or Identi fy Potential Degradation of Mark I Drywells (Generic Letter 87-05)," dated May 12, 1987.
9804070319 890331 PDR ADOCK 05000245 A
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i U.S. Nuclear Regulatory Commission B12803/Page 2 March 31, 1988 The results of the inspections show that the sand pocket drain lines are L
functioning properly as originally designed.
The ultrasonic thickness measurements performed confirm that the shell liner thickness is within ASTM thickness tolerance criteria and the shell liner shows no signs of plate degradation.
Attachments 1 and 2 present detailed engineering evaluations of the inspection results for the sand pocket drain lines and the drywell v
liner, respectively, i
E Should you have any questions concerning this matter, please contact us.
j Very truly yours, c
NORTHEAST NUCLEAR ENERGY COMPANY h/
11/
E F K Mroczkaf Senior Vice President cc:
W. T. Russell, Region I Administrator M. L. Boyle, NRC Project Manager, Millstone Unit No. 1 i
W. J. Raymond, Senior Resident Inspector, Millstone Unit Nos. 1, 2
)
and 3 i
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i 4
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Docket No. 50-245 B12803 Millstone Nuclear Power Station, Unit No. 1 Results of Sand Pocket Drain Line Inspection March 1988
ENGINEERING SUMARY SAND POCKET DRAIN LINE I
INTRODUCTION in accordance with NNEC0's response to NRC Generic Letter 87 05 titled "Request for Additional Information -Assessment of Licensee Measures to Mitigate and/or Identify potential Degradation of Mark I.Drywells," the drain lines that are connected to the sand cushion located between the drywell wall and the surrounding concrete were inspected.
The Millstone Unit No. I sand cushion design is open to the gap between the drywell shell and surrounding concrete.
The only path for the water to drain from the sand cushion is through the 1-inch plastic drain lines located at the bottom of the sand cushion.
In order to properly inspect the sand cushion drain lines, a visual inspection of each line was performed, as well as a closed circuit television (CCTV) camera inspection during the past 1987 Millstone Unit No. 1 Refueling Outage.
l i
SAND CUSHION DRAIN LINE DESIGN DETAIL The sand cushion drain line detail is shown in Figure 1.
The detail presented shows a pre formed metal trough 6 inches wide by 3 inches deep. A 1-inch diameter drain line is connected to the bottom of, the metal trough.
The drain line extends vertically downward, turns 90 to a horizontal position, and then slopes downward and extends through the concrete wall.
i This design permits any water to exit the drain line and into the torus room.
j SAND P0CKET DRAIN LINE EXAMINATION RESULTS 1
The following is a chronological list of events which were performed to inspect the sand pocket drain lines.
1 a.
Pre-Outaae Insoection J
NNECO performed a visual inspection on the eight drain lines to l
determine drain line pipe material and inspect for evidence of drain line water leakage. The inspection was performed from the floor of the torus room at elevation -22'.
The drain line pipe was deter:uined to be black flexible polyethylene pipe which extended out from the concrete wall approximately 2 to 3 feet.
No visible water or water stains from where water had exited the drain lines were visible for all eight drain j
lines inspected.
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b.
Outaae Insoections l
1.
Fiberscopic inspections were performed on 5A, SC, SE, and 5G drain lines.
These drain line locations are shown in Figure 1.
Inspection results are as follows:
o Water pockets were present in all four drain lines, o
Drain lines in the concrete wall were not continuously sloped lines as shown in Figure 2.
The lines appeared to be wavy throughout the thickness of the wall.
o Sand from the sand cushion was present in each of the drain lines.
o Water stains and sand were visible on the torus structure at the 11 o' clock position.
The fiberscopic inspection did not reveal the clarity inside the 4
pipe as was originally intended.
In order to achieve better inspection results, a CCTV inspection service w,is obtained.
2.
Drain Line Water Removal
}
Water was removed from inside each of the drain lines by the use of a Randolph pump, and Tygon tubing was placed in the drain line i
from the concrete wall to the sand cushion.
Water was collected l
from each drain line into a sample canister and subsequently j
chemically analyzed.
In order to facilitate future, visual inspections from the floor of the torus room, the drain lines were cut back from the 2 to 3 foot extended length to approximately 2 inches from the wall. This activity permits the water to exit the drain pipe and fall directly onto the torus room floor rather than fall on the torus shell structure.
j 3.
Chemical Water Analysis Water samples were taken from each drain line and were chemically i
analyzed to determine its radioactivity levels and the extent of l
contamination. The water samples indicate that the water was not reactor cavity water.
The existence of the water in the drain l
lines may have been caused from condensation on the drywell shell l
over a period of tinie.
l 4.
CCTV Inspection i
l A CCTV inspection was performed on the eight 1 inch diameter drain i
lines.
The inspection of each drain line was recorded on a VCR l
t
tape and is available for viewing as a permanent record.
The results of the inspection are as follows:
)
o Orain lines in the concrete wall are not continuously sloped i
lines as shown in Figure 1 but have various peaks and i
- valleys, it appears that the lines were not supported during the concrete pour of the wall and that they moved when the concrete was placed.
o Minor amounts of sand from the sand cushion were present in the lower half of the drain line which indicates sand has passed from the cushion into the line.
i o
Minor amounts of water were visible in some of the drain line pockets which indicates that water is filtering through the sand cushion.
(Lines were previously pumped out.)
2 o
There is a wire cloth screen at the base of the metal trough which was not shown in Figure 1.
The screen appears to be passing minor amounts of sand from the sand cushion.
(See Table 1 for details of each screen.)
s o
The sand which is in the drain lines from the sand cushion resembles a very fine beach sand.
The sand is much finer than the mesh on the wire cloth screens, which is why it is passing into the drain line.
CONCLUSION The sand pocket drain lines are functioning properly as intended, although the present as built condition differs from the stated design details.
This l
statement is based on the following engineering facts:
Water is able to filter through the sand cushion, wire cloth o
screen, and 1-inch drain lines as ir.dicated by ws'er stains i
on the torus shell.
After each drain line hr Teen pum;ed dry, minor amounts of water accumulated in some af the drain j
line pockets.
o Although the drain line is not a continuous slope line, the 2
difference in elevation between the lower elevation of the sand cushion and the drain line exit elevation dictates water I
will always drain.
4 Although minor amounts of sand are present in the drain line, 1
o no major loss of sand from the sand cushion is evident.
Sand j
is present in the drain line due to its fineness as opposed to other materials caught by the wire cloth screen.
I o
Water samples taken from drain line indicate the water is not reactor cavity water.
4 I
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Millstone Nuclear Power Station, Unit No. 1 Results of Drywell Inspection t
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ENGINEERING
SUMMARY
DRYWELL LINER INSPECTION ULTRASONIC LINER THICKNESS MEASUREMENTS INTRODUCTION In accordance with NNEC0's response to NRC Generic Letter 87-05 titled "Request for additional Information--Assessment of Licensee Measures to mitigate and/or Identify Potential Degradation of Mark I Drywell s," the drywell shell plates adjacent to the sand cushion were inspected.
Ultrasonic thickness measurements were taran in 16 inspection pockets along the shell circumference on the O'-8" level of the Millstone Unit No. I drywell interior.
DRYWELL LINER DESIGN DETAILS The drywell liner shell thickness in the area of the sand cushion at elevation O'-8" is 0.875 inches thick as shown on the original Chicago Bridge and Iron drawings.
The shell is composed of many plate section welded together to form a spherical shell.
See Figure 1 attached.
ULTRASONIC THICKNESS MEASUREMENTS In order to perform the required ultrasonic thickness measurements, sixteen 4" x 4" x 4" inspection pockets of concrete on the O'-8" level were removed by hand chipping.
Freformed sheet metal forms were fabricated to be placed in each of the inspection pockets and were grouted in place. A 4' x 4" grid located within the concrete pocket on the drywell liner shell was established and marked for the current and any future inspections.
Ultrasonic thickness measurements were taken in each of the 16 inspection pockets using the 4" x 4" measurement grid.
Prefabricated sheet metal plugs were then placed in the inspection pocket anti caulked.
The use of prefabricated plugs in each inspection pocket, as well as the marking of the 4" x 4" grid, allows for future inspection utilizing the same inspection points and allows for direct correlation of results.
The drywell liner inspection ultrasonic thickness measurements are presented in Table 1.
As can be seen from the table,16 points were taken in each pocket.
From these readings, the high and low readings were recorded and averaged to obtain the average shell thickness in the area of the concrete pocket.
This average thickness is then compared to the center thickness of the 4" x 4" pocket and finally to the actual design plate thickness.
The average plate thickness for all inspection pockets is 0.865 inches as cocrsred to 0.875 inch design plete thickness.
CONCLUSIONJ i
Based on the recorded ultrasonic thickness measurements performed in the 16 inspection pockets, the measured shell liner thickness is within ASTM l
thickness tolerances and the shell liner shows no signs of plate l
degradation.
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TABLE 1
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-DRYWELL LINER INSPECTION TABULATION OF UT LINER MEASUREMENTS High Readings Low Readings Design No.
No.
No.
Avg.
. Center Plate location Points THK Points THK Points THK THK THK i
(Inches)
(Inches)
(Inches)
(Inches)
(Inches)
SAL 16 0.86 9
0.84 7
0.851 0.84-0.875
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SAR 16 0.88 4
0.86 12 0.865 0.86 0.875 SBL 16 0.88 16 0.88 0.88 0.875 SBR 16 0.88 16 0.88 0.88 0.875 SCL 16 0.88 12 0.86 4
0.875 0.88 0.875 5CR 16 0.86 4
0.84 12 0.845 0.84 0.875 SDL 16 0.86 14 0.84 2
0.858 0.86 0.875 0.86 9
SDR 15 0.88 2
0.84 4
0.857 0.86 0.875 SEL 16 0.86 12 0.84 4
0.855 0.84 0.875 i
SER 16 0.88 16 0.88 0.88 0.875 SFL 16 0.88 16 0.88 0.88 0.875 SGR 16 0.86 6
0.84 10 0.848 0.86 0.875' O.86 7
SHL 16 0.88 1
0.84 8
0.851 0.84 0.875 5HR 16 0.86 6.
0.84 10 0.848 0.84 0.875 Notes:
4" x 4" grid used for UT inspection in 4" x 4" liner pocket.
.