ML19332F169
| ML19332F169 | |
| Person / Time | |
|---|---|
| Site: | Sequoyah  |
| Issue date: | 12/08/1989 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19332F168 | List: |
| References | |
| NUDOCS 8912130489 | |
| Download: ML19332F169 (3) | |
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UNITED STATES i
L l8 NUCLEAR REGULATORY COMMISSION o
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WASWNGTON, D. C. 20556 ENCLOSURE l
SAFETY EVALUATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION CHICKAMAUGA RESERVOIR SEDIMENT DEPOSITION AND EROSION TENNESSEE VALLEY AUTHORITY j
SEQUOYAH NUCLEAR PLANT, UNITS 1 & 2 DOCKET NOS. 50-327 AND 50-328
1.0 INTRODUCTION
By letter dated June 20, 1989, the Tennessee Valley Authority (the licensee) l requested a change to the Sequoyah Technical Specifications regarding the maximum allowable river water temperature'at the intake of the essential raw-cooling water-(ERCW) supply headers.
Specifically, the licensee requested raising the maximum allowable temperature from 83 degrees to 84.5 degrees Fahrenheit.
Justification for. this change was based on a re-analysis of the failure.of the Chickamauga Dam. The staff approved this technical specifica-tion change in its letter dated August 15, 1988 and noted that the sedimenta-tion and erosion calculations for the ERCW intake structure following the failure of the Chickamauga Dam had never been evaluated by the NRC.
The staff requested that the licensee provide the reservoir cross sections used for the unsteady flow analysis, the geologic cross sections through.the reservoir, and the sediment scour and deposition calculations.
The licensee provided the requested information in its letter of May 18, 1989 with theLexception of the geologic cross sections through the reservoir.
The Sequoyah site is located on the Tennessee River upstream of the Chickamauga Dam. The Chickamauga Reservoir is the river water upstream of the Chickamauga Dam and includes the water at the Sequoyah site. The ERCW at the Sequoyah site takes water from the Tennessee River to provide coolirig water to the Sequoyah plant.- It is the ultimate heat sink for the plant during an accident.
2.0 EVALUATION The licensee used an unsteady flow model, coupled with an erosion and depo-
- sition model to analyze silt movement in the river channel around the ERCW forebay. The unsteady flow model used was the TVA Simulated Open Channel Hydraulics model which has been widely used throughout the industry for dam failure analysis and other unsteady flow open channel hydraulics computations.
The input to this model consisted of 29 channel cross sections to define the hydraulic geometry extending from river miles 471.00 to 529.90.
Chickamauga Dam is located at river mile 471 and the ERCW intake structure is at river mile 484. The dam was assumed to fail totally and instantaneously at normal maximum pool elevation.
The unsteady flow model was used to develop a time history of water surface elevation and velocities at the ERCW intake structure cross section.
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k The time history of velocities at the ERCW intake cross section was used to I-calculate sediment transport using the Laursen equation which relates sediment concentration to the ratio of actual tractive force to critical tractive force. Actual tractive force is a function of flow and channel geometry, l
Critical tractive force is a function of rediment size and geometry.
Using these relationships the licensee determined a maximum sediment concentration of 2,000 parts per million (ppm), and a maximum average channel velocity of 4.5 feet per second.
Sediment deposition was-calculated using the settling velocity of the sediment particles and the residence time of the particles in the pump forebay channel. The residence time was such that essentially all the sand being transported settles out before reaching the pumps. However, ninety percent of the silt reaches the pump intakes without settling. The l
-licensee concluded that sediment deposition at the ERCW pumps wou d not be significant.
In addition. the licensee concluded that the potential for silt blockage of the pumps is low because the operating pumps will force flow channels through the depositing silt. The staff is in agreement with the licensee's conclusions.
Both of the redundant trains of the ERCW system are required to be operable when either of the Sequoyah units are above 200'F (i.e. reactor Modes 1, 2, 3 or 4). With one train inoperable for any reason including the ERCW pumps, the operating units must begin shutting down within 72 hours8.333333e-4 days <br />0.02 hours <br />1.190476e-4 weeks <br />2.7396e-5 months <br /> and be at or below 200 F (i.e., Mode 5) within 38 hours4.398148e-4 days <br />0.0106 hours <br />6.283069e-5 weeks <br />1.4459e-5 months <br />.
With both ERCW trains inoperable, the-operating units must begin shutting down in one hour and be at or below 200*F within 36 hours4.166667e-4 days <br />0.01 hours <br />5.952381e-5 weeks <br />1.3698e-5 months <br />.
These requirements will assure that the plant has at least one ERCW train operating during and after a dam failure to prevent silt blockage of the ERCW intake structure when the units are above 200*F.
Because the velocities at the ERCW intake are in the deposition regime, no erosion of the river bed is expected at the intake structure.
Therefore, the geologic cross sections through the reservoir are not needed.
3.0 CONCLUSION
S The staff, however, has reviewed the licensee's methodology and conclusions.
The staff concludes that the licensee's assumptions and method of approach are reasonable and. adequate for the analyses. However, the licensee did not address the uncertainty (or possible error) inherent in the type of calculation performed and what this uncertainty could do to the conclusions regarding sediment concentrations. The staff, however, considers the assumption of complete and instantaneous failure of Chickamauga Dam to be conservative. Any realistic failure mode would be considerably less severe and result in lower rates of sediment transport within the stream channel.
The staff agrees that the licensee's conclusion that sand will not reach the intake forebay is a valid conclusion particularly when less severe failure modes of the dam are considered.
In regard to silt deposition, probably the most uncertain parameter, the staff agrees that any pumps started imediately after the dam failure, can be expected to maintain unblocked flow channels.
Therefore, the staff, concludes that, even though the licensees analysis was not a rigorous bounding analysis, the conservatism of the licensee's initial assumptions is sufficient to accept the licensee's analysis.
The staff agrees with the licensee's conclusions that the sediment deposition will not prevent
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3 the ERCW intake structure from performing its intended safety function. The staff also concluded that the requested data on the geologic cross sections through the Chickamauga Reservoir is not needed to conclude thet the ERCW intake structure will perform its intended safety function.
Although the staff concludes that there are no safety concerns about silt blockage of the ERCW intake structure, it recommends that the licensee review the Sequoyah emergency operating instruction for the Chickamauga Dam failure about the potential of silt blockege of the intake structure when no ERCW pumps may be operating.
4.0 REFERENCES
1.
June 20, 1988, letter from M. J. Ray (licensee) to NRC, providing Technical Specifications Change Request 88-21.
2.
August 15, 1988, letter from S. Black (NRC) to S. A. White.(licensee)..
approving Technical Specification Change Request 88-21.
l 3.
May 18, 1989, letter from M.J. Ray (licensee) to NRC, transmitting Chickamauga Reservoir Sediment Deposition and Erosion Information.
Principal Contributor:
R. Wescott Dated: December 8, 1989 l
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