ML20087N523
| ML20087N523 | |
| Person / Time | |
|---|---|
| Site: | Vermont Yankee File:NorthStar Vermont Yankee icon.png |
| Issue date: | 07/31/1982 |
| From: | GENERAL ELECTRIC CO. |
| To: | |
| Shared Package | |
| ML20087N517 | List: |
| References | |
| NUDOCS 8404030538 | |
| Download: ML20087N523 (12) | |
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4 SAFETY EVALUATION OF THE PROPOSED 140% MAIN STEAM LINE FLOW TRIP SETTING FOR THE VERMONT YANKEE NUCLEAR POWER STATION JULY 1982
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Approve E.&d. Romesberg, Mihager Services Licensing Fuel & Services Licerising Safety & Licensing Operation 8404030530 840326 PDR ADOCK 05000271 P
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IMPORTANT NOTICE REGARDING CONTENTS OF THIS REPORT (Please Read Carefully)
This report was prepared by General Electric solely for Vermont Yankee
'i Nuclear Power Corporation for Vermont Yankee's use with the U.S. Nuclear Regulatory Commission (USNRC) for amending Vermont Yankee's operating license of the Vermont Yankee Nuclear Power Station. The information contained in this report is believed by General Electric to be an accurate and true representation of the facts known, obtained, or provided to General Electric at tne time this report was prepared.
The only undertakings of the General Electric Company respecting inforca-tion in this document are contained in the " Contract between Vermont Yankee Power Corporation and General Electric Company for Spare and Renewal Parts and Nuclear Service," March 25, 1974, as amended, and nothing contained in this document shall be construed as changing said contract. The use of this information except as defined by said contract, or for any purpose other than that for which it is intended, is not 4
authorized; and with respect to such unauthorized use, neither General Electric Company nor any of the contributors to this document makes any representation or warranty (express or implied) as to the completeness, accuracy, or usefulness of the information contained in this document or that such use of such information may not infringe privately owned rights; nor.do they assume any responsibility for liability for damage of any kind which may result from such us of such information.
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r-TABLE OF CONTENTS U
e 7
1.
Introduction and Summary 2-8 es 2.
Safety Evaluation e
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1.
INTRODUCTION AND SU$ MARY It is proposed that the main steam line high flow isolation signal at the Vermont Yankee Nuclear Power Station be changed from 120% to,140% rated steam line flow. The basis for the selection of 140% as a requirement I
qg for the automatic isolation is that this setting permits the plant to continue to operate at full power with one of the four main steam lines isolated. This will improve plant operating characteristics and reduce unnecessary challenges to the system.
This document presents the safety analysis for the proposed technical specification change.
It is shown that the setpoint change from 120% to 140% flow will not pose an unreviewed safety question.
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r; 2.
SAFETY EVALUATION I.
Basis for Main Steam Line Flow Trip Setting The 120% high steam flow isolation signal was originally' established g
o'n the two steam line plants, Oyster Creek and Nine Mile Poin't. The basis for this setting was not due specifically to a safety limit, but was sep high enough to avoid spurious trips during normal operation and yet low enough to minimize the consequences of main steam line'(MSL) breaks of any size for a plant of that steam line configuration.
Vermont Yankee is a four steam line plant. The basis for the selectionof{he140%ofratedsteamflowasarequirementforthe automatic isolation is that this retting permits the plant to continue to operate at full power with one of the four main steam lines isolated (i.e., during required MSIV testing). This results in an average of 133% of rated steam flow in each of the remaining steam lines. To improve their operating characteristics and reduce unnecessary challenges to the system, most operating BWR/3 and 4's, and all requisition plants, have already incorporated the 140%
setpoint into their technical specifications.
II.
Radiological Consequences A.
Large Steam Line Leaks:
Large steam leaks (greater than 140% of rated flow) outside containment are detected by the main steam line flow restrictor differential pressure sensors.
If flow exceeds the trip setting in any line, an isolation of all main steam lines will be initiated. The MSIVs will close in 3 to 5 seconds after receiving the isolation signal.
One of the design basis accidents analyzed in the Vermont Yankee FSAR Sect. ion 14.6.5 is a guillotine break of one of the main steam lines. Choked flow._
of the two phase blowdown is assumed to exist until MSIV closure, which is conservatively assumed to be 10.5 seconds following the break, _ resulting in the loss of 60,000 lbs of coolant. This break results in the bounding dose rete due to the two phase flow out of the break. The calculatyd dose at the site boundary is well below the limits of 10CFR100.
For g
this event, the maximum flow rate in the broken line would be l
the maximum permitted by the flow limiting venturi, that is, about 200% of rated steam line flow. This is considerably higher than the proposed isolation valve trip setting of 140%
rated flow. ' Therefore, changing the trip setting from 120% to 140% rated flow will not change the results of this design basis accident (DBA).
B.
Small and Medium Steam Line Leaks:
Small and medium steam leaks in the main steam line (less than 140% of rated flow) are detected by the following:
1.
Temperature sensors inside the steam tunnel which provide an alarm at 160*F and an isolation trip at 200*F.
2.
Steam line flow sensors which provide steam line flow readings to the operators and isolation at the prescribed high-flow setpoint.
3.
Area radiation monitors in the turbine building which provide alarms in the control room when exposed to high radiation.
An estimate was performed to represent the most bounding case for maximum amount of steam release from the MSL. It assumes that steam line flow increases to a value just under the high flow isolation setpoint, takes no credit for high temperature or high steam line flow trip, and assumes 10 minute operator..
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action time before isolation. For 120% and 140% steam 'line flow trip satpoints, the maximum mass losses would be 243,135 and 486,270 lbs, respectively. These mass losses were calcu-lated assuming 120% and 140% flow, respectively, in all four steam lines simultaneously. This results in a very conserva-
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tive, bounding value. Based on LOCA analysis with approved evaluation models for these small and intermediate breaks, steam is the only effluent released. Thus, the radiological dose is still much less than the design basis accident in the FSAR and well below the 10CFR100 limit.
The bounding estimate described above does not take credit for the high temperature sensors in the steam tunnel.
In reality, these sensors would further limit the mass loss due to a leak in the steam tunnel. At the request of Vermont Yankee Power Corporation, an analysis was performed to determine a best l
estimate for mass loss under these conditions.
An analysis was performed to develop a set of break curves, showing tunnel temperature as a function of time after break, for several break flow rates. This set is shown in Figure 1.
Using Figure 1, the total mass losses prior to isolation were obtained for the following cases:
(a) 160*F alarm sensing time plus 10 minute operator action, and (b) 200'F sensing time plus five second tech spec MSIV closure time. These mass losses are presented in Tables 1 and 2.
Combined the results of cases (a) and (b) above, a bounding mass loss curve was developed for the MSL break inside the steam tunnel and shown in Figure 2.
Comparison of Tables 1 and 2 indicates that for break flows greater than 1.8% of rated steam line flow the 200*F automatic trip occurs well before the
'160*F alarr plus 10 minute operator action.
For conservatism, the bounding mass loss curve accounts for 10 minute operator i
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action after the 160'F alarm for break flows less than 20 lb/sec (S% rated steam line flow).
Based on this bounding curve, the maximum mass loss prior to isolation on high steam tunnel temperature would bp,12,360 lbs which is far less than the mass losses determined in the g
previously discussed analyses.
In conclusion, for small and intermediate breaks in the steam tunnel, the temperature sensors will effectively limit the mass loss, independent of the high flow isolation setpoint.
III. Conclusions Changing the Technical Specifications high flow setting from 120% to 140% rated steam line flow will not affect the radiation dose reported in the FSAR nor will it pose undue risk to public health.
It will, however, improve the plant operating characteristics and reduce unnecessary challenges to the system.
e._
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TABLE 1 MASS LOSS (1b) IN STEAM TUNNEL AS DETERMINED BY 160% TEMPERATURE SENSOR LIMIT
- BREAK SENSOR TRIGGER ACTION TIME FLOW TIME (sec)
(sec)
MASS LOSS s
(1b/sec)
(%),
(REACH 160*F)
(add 10 min)
'(1b) 6 (1.3) 280 880 5280 8
(1.8) 181 781 6248 16 (3.6) 32 632 10112 20 (4.4) 18 618 12360 30 (6.7) 8.6 608.6 18258 45 (10.0) 4.9 604.9 27221 60 (13.3) 3.4 603.4 36204 80 (17.8) 2.3 602.3 48184 160 (35.6) 1.0 601.0 96160
- Time to reach 160'F in tunnel plus 10 minutes operation action time
+ Percent of each steam line rated flow j
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j TABLE 2 MASS LOSS (1b) IN TUNNEL AS DETERMINE 0 BY 200*F TEMPERATURE SENSOR LIMIT
- i BREAK SENSOR TRIGGER ACTION TIME r
FLOW TIME (sec)
(sec)
MASS LOSS (1b/sec)
(%),
(REACH 200*F)
(add 5 sec)
(1b) 8 (1.8) 501 506 4048.0 16 (3.6) 172.5 177.5 2840.0 20 (4.4) 106.5 111.5 2230.0 30 (6.7) 44 49 1470.0 45 (10.0) 20.3 25.3 1138.5 60 (13.3) 12.9 17.9 1074.0 80 (17.8) 8.7 13.7 1096.0 160 (35.6) 3.8 8.8 1408.0 300 (67.0) 1.9 6.9 2070.0 440 (98.0) 1.3 6.3 2772.0
- Time to reach 200*F in tunnel for automatic trip plus 5 seconds
+ Percent of each steam line rated flow 6
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FIGURE 2.
SOUNDING MASS LOSS CURVE (St:am Tunnel Tamperature Sensors)
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Mass loss assuming 160*F-alarm + 10 min. operator action
-- Mass loss assuming 200*F isola-tion + 5 sec. closure time.
G Proposed bounding mass loss curve.
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