ML20235G977
| ML20235G977 | |
| Person / Time | |
|---|---|
| Site: | North Anna  |
| Issue date: | 09/22/1987 |
| From: | Stewart W VIRGINIA POWER (VIRGINIA ELECTRIC & POWER CO.) |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| References | |
| 87-474B, NUDOCS 8709300317 | |
| Download: ML20235G977 (6) | |
Text
_ _ - - _ _ _ _ _ _ _ _ _ _ _ _ _ _ _
VIItOINIA ELECTitIC AND POWE11 COMI%NY t
l HICitMOND,VI HOINI A 2 0 2 61
)
W, 1,, ST H WA NT Vns l'usutumwr l
Nt etsAm orsmATions l
September 22, 1987 1
United States Nuclear Regulatory Commission c; rial No.
87-474B Attention: Document Control Desk N0/DJV:jmj Washington, D.C.
20555 Docket Nos.
50-338 License Nos.
NPF-4 Gentlemen:
it VIRGINIA ELECTRIC AND POWER COMPANY w
NORTH f.NNt. POWER STATION UNIT 1 C:
REQUEST FOR APPROVAL TO STARTUP AND y
OPERATE AT 50 PERCENT POWER ao 30 O
3>
tb Following the North Anna Unit 1 steam generator tube rupture event of Jgly I3, 1987, Virginia Electric and Power Company agreed to obtain concurrence $ ppm the NRC prior to Unit I startup (Mode 2).
This agreement was confirmedGFy your letter dated July 22, 1987. We have since completed our evaluation of the tube rupture event.
The results of our evaluation and our request for approval to return Unit 1 to service was submitted to the NRC by our letter dated September 15, 1987.
As the NRC review of this matter may extend beyond early October when Unit I will be ready for startup, we are requesting approval for startup reduced power of 50% of the licensed limit. Approval of and operation at a this request will permit unit startup and operation at 50% while NRC review is completed.
Operation at 50% does not present a safety concern for the reasons stated below.
The results of our evaluation of the steam generator tube failure are presented in our " North Anna Unit 1 July 15, 1987 Steam Generator Tube Rupture Event Report," revision 1,
dated September 15, 1987.
Further details of the evaluation are presented in Westinghouse technical report STD-7.2.4-7126,
" North Anna Unit 1 Steam Generator Tube Rupture and Remedial Actions Technical Evaluation," dated September 12, 1987.
In summary, we have conc 3uded that the tube failure occurred due to high cycle fatigue.
The fatigue mechanism has been determined to be due to a combination of stresses imposed by tube denting at the support plate and vibration due to fluid clastic instability.
Modifications are being made to the steam generators to address the failure cause. These modifications and other corrective actionc are discussed in detail in the above reports and form the basis for our conclusion that the unit can be restarted and operated safely.
8709300317 870922 DR ADOCK 050 8
1 Our basis for restart of the unit is summarized here.
1.
The steam generators are being modified to include a downcomer flow l
resistance plate (DFRP).
The DFRP will reduce the steam generator I
recirculation flow and is expected to result in the improvement in tube l
" stability ratio".
needed to preclude further tube failures of active tubes by the fluid elastic instability mechanism. " Stability ratio" is a relative measure of the potential for tube vibration due to fluid l
elastic instability. Evaluations by Westinghouse have concluded that a 10% improvement in stability ratio should provide the necessary reduction in fatigue usage (reduced amplitude of vibration) to preclude a further tube failure by this mechanism over the remaining life of the steam generators.
(Refer to Section 4.1.1 of the Westinghouse report).
A safety evaluation of the DFRP modification has been prepared and the need to update the FSAR analysis of the steam generator tube rupture event for operation above approximately 59% power with the DFRP installed has been identified.
The UFSAR revision and safety analysis required for operation above 50% power will be submitted by a separate letter.
Additionally, some tubes are conservatively being preventative 1y plugged to provide additional margin with respect to this mechanism.
These tubes are being preventatively plugged with a sentinel plug on the cold leg side and a standard mechanical plug on the hot leg side.
l In the event that one of these tubes contains a pluggable eddy current indication, the tube will be plugged with a standard mechanical plug and its neighbor tubes will be plugged with the sentinel plug. As a result of this change, certain Row 8 tubes will become the most susceptible active tubes for fluid elastie instability. The most l
susceptible Row 8 tube has a stability ratio which is at least 14% less than the most susceptible tube in Row 9.
Therefore the cocbined effect of the DFRP and preventative plugging 1s a reduction in stability ratio of at least 24%, well in excess of the 10% criteria referred to previously.
3.
The failed tube (Row 9, Column 51), or possibly its neighbor tubes, will be stabilized and plugged to prevent further damage to adjacent tubes.
If the failed tube is stabilized, its neighbor tubas will be preventatively plugged with a sentinel plug.
4.
An extensive eddy current testing (ECT) program has been completed and tubes having ECT indications meeting established plugging criteria will be plugged prior to restart.
5.
An augmented surveillance program for monitoring steam generator primary-to-secondary leakage is being implemented.
This program is based on the use of several radiation monitors and sampling to detect and quantify primary - to - secondary leakage.
The program is designed to detect leakage during the early stages of fatigue failure se that an orderly shutdown can be accomplished.
Administrative controls addressing leak rate limits, operator actions and monitoring equipment operability are being prepared.
The above measures should provide adequate assurance that a similar tube failure will not recur.
However, pending completion of your review and approval of 100% power operation, power level will be limited to 50 0 Operation at 50% power results in a further, significant improvement in stability ratio of about 30%, which is well in excess of the 10% improvement required to preclude a further tube failure by this mechanism.
This improvement is due to the reduction in steam generator recirculation flow and void fraction which occurs at reduced power.
(See the attached discussion of dependence of stability ratio on load.) The net effect of operation at 50%
power in combination with the DFRP and preventative plugging is to reduce the stability ratio to less than unity, thus indicating that the tube is stable.
[
Therefore it is very unlikely that a fatigue crack would initiate due to fluid einstic instability at 50% power.
It should be noted that the augmented primary-to-secondary leakage monitoring program will be implemented at all times above Mode 3 operation.
If you have any questions on this matter, please contact us.
Very truly yours, W. L. Stewart Attachment cc:
U. S. Nuclear Regulatory Commission 101 Marietta Street, N.W.
Suite 2900 Atlanta,_CA 30323 Mr. J. L. Caldwell NRC Senior Resident Inspector North Anna Power Station l
1
' Attachment DEPENDENCE OF STABILITY RATIO ON LOAD The influence of thermal load on fluid elastic stability ratio in the U-bend is discussed below. A discussion of the general trend is followed by a discussion for a partial load of 50% power.
General Trend The steam flow rate from the steam generator is strongly dependent on thermal load.
As the load increases the steam flow rate increases.
The maximum steam flow rate occurs at full load.
Since most of the vapor generation occurs in the straight leg portion of the tube bundle, when the secondary flow passes through the U-bend the total vapor flow is nearly equal to the steam flow from the steam generator.
The vapor velocity in the U-bend is thus porportional to the steam flow rate.
As steam flow rate increases with load, the circulation ratio decreases. The total mass flow rate (liquid plus vapor) through the tube bundle is nearly constant and is independent of load above 60% load. However, since the specific volume of vapor is 20 to 30 times higher than that of the liquid, the volumetric flow rate increases with increase in load.
Thus the velocity in the U-bend increases and the density of the two phase mixture decreases with increase in load.
The flgid elastic stability ratio is proportional to the square root of (Rho)V.
The net effect on the stability ratio from the flow conditions alone isanincreasewjthload.
The figure shows the effect of load on the square root of (Rho)V component of the stability ratio.
This figure provides post modification reduction factors relative to full load operating conditions prior to the installation of the downcomer flow resistance plate as a function of thermal load.
Note that this figure is based on aggregate flow rates and circulation ratios. Although it does not account for detailed distributions of velocity and density along the tubes, it is a good means of comparing the influence of load since the detailed distributions will have only a secondary effect.
A decrease in load is accompanied by a decrease in void fraction.
Thus a decrease in load would increase tube damping and further reduce stability ratios.
For a tube with low damping, this contribution would be small.
- However, the reductions in stability ratio with reductions in load would be equal to or greater than the factors shown on the attached figure.
Stability Ratio at 50I Power At 50% of full load, the effects on stability ratios from velocity and density alone are to reduce the stability ratio by a factor of two relative to full load prior to the modification.
- Further, this will be accompanied by a
reduction in void fraction of over 10%. The reduction in void fraction, if considered, would further reduce the stability ratio at the power level of interest (i.e. 50% power).
l Tube Fatigua Conclusions The net effect of operation at 50% in combination with installation of the downcomer flow resistance plate (DFRP) and preventative plugging of the most j
susceptible tubes is as follows:
Factor Reduction in Stability Ratio DFRP and Operation 46%
at 50% Power Preventative Plugging 14%
Total 60%
Even assuming worst case tube stability ratios prior to the modifications as discussed in the Westinghouse report, the net effect of operation at 50% power after the modifications is to reduce the stability ratio of the most susceptible tubes to less than the critical value of unity.
Therefore, a
fatigue crack will not initiate due to fluid elastic instability at 50% power.
I l
.~
NORTH ANNA STEAM GENERATOR WITH DFRP MODIFICATION INFLUENCE OF LOAD ON STABILITY RATIO i
1
,- 0
/
0.9
\\
/
1
/
0.8 0.7
/
-/
0.6 0.5 Tvn t 9
M 0.4 NIE 0.3 i
0.2 Z
0.1 0
20 40 60 80 100 Load %
l l
Note 1. Normalized Relative to 100% Load Conditions Prior to the Downcomer Flow Resistance Plate (DFRP) Modification.