ML20085N672
| ML20085N672 | |
| Person / Time | |
|---|---|
| Site: | Dresden, Quad Cities, 05000000 |
| Issue date: | 11/02/1983 |
| From: | Rybak B COMMONWEALTH EDISON CO. |
| To: | Harold Denton Office of Nuclear Reactor Regulation |
| References | |
| RTR-NUREG-0737, RTR-NUREG-737, TASK-2.K.3.18, TASK-TM 7556N, NUDOCS 8311110150 | |
| Download: ML20085N672 (6) | |
Text
Commonwrith Edison O
One First National Ptara, Chicago, Illinois C'
Address R: ply to: Post Office Box 767 Chicago. Illinois 60690 November 2, 1983 Mr. Harold R. Denton, Director Office of Nuclear Reactor Regulation U.S. Nuclear Regulatory Commission Washington, DC 20555
Subject:
Dresden Station Units 2 and 3 Quad Cities Station Units 1 and 2 Status of NUREG 0737 Item II.K.3.18 l
Modification of ADS Logic NRC Docket Nos. 50-237/249 & 50-254/265 References (a):
T.J. Rausch letter to D.G. Eisenhut dated December 28, 1982.
(b):
D.B. Vassallo letter to D.L. Farrar dated June 3, 1983.
(c):
D.M. Crutchfield letter to D.L. Farrar dated June 3, 1983.
(d):
B. Rybak letter to H.R. Denton dated July 18, 1983.
Dear Mr. Dentor.:
As committed to in Reference (d), Commonwealth Edison is submitting, in the form of an attachment to this letter, the proposed ADS bypass timer setting.
Also included are justification for the proposed setting and an analysis of the impact of this modification on current accident analyses, including line breaks inside or outside the drywell, isolation events with or without a stuck open relief valve, and ATWS events.
i One signed original and sixty (60) copies of this transmittal are provided for your use.
Very truly yours, 8311110150 831102 PDR ADOCK 05000237 P
PDR B.
'ba Nuclear Licensing Administrator 1m cc:
NRC Resident Inspector - Dresden NRC Resident Inspector - Quad Cities R. Bevan - NRR
(/
R. Gilbert - NRR i(\\
7556N
e ATTACHMBNT ANALYSIS OF ADS LOGIC CHANGE IN RESPONSE TO NUREG-0737 ITEM II.K.3.18 1.0 Summa ry The BWR Owner's Group Evaluation of NUREG-0737 Item II.K.3.18, Depressurization System Logic (NEDE-30045, February 1983), describes the modification option which is to be implemented at Dresden and Quad Cities.
This option consists of adding a timer that will bypass the high drywell pressure initiation logic during a sustained low RPV level, thus permitting ADS initiation on low level alone.
A manual ADS inhibit switch will also be provided.
The intent of this modification is to f acilitate automatic ADS operation when required during events which do not directly result in a high drywell pressure signal (isolation events or high energy line breaks outside the drywell).
ADS operation is necessary in cases where no high pressure makeup systems are available, so that the reactor is depressurized to the point where low pressure systems can provide core cooling.
For Dresden and Quad Cities, an acceptable timer setting is 8.5 minutes.
Justification for this setting and a discussion of the effects of this modification on the various accident scenarios are given in the following sections.
I t
l
2.0 Timer Setting Justification Analyses presented in NEDO-24708A, " Additional Information Required for NRC Staff Generic Report on Boiling Water Reactors," indicate that adequate core cooling is assured for isolation events when ADS actuation occurs within 10 minutes after RPV level reaches the low-low level setpoint.
Therefore, 10 minutes is an upper bound for the bypass timer setting.
These results are bounding for Dresden and Quad Cities, where the level 1 setpoint is approximately seven feet above the top of the active fuel for these plants, as opposed to the one foot level for the BWR 4-6 assumed in the referenced analysis.
Therefore, ADS will actuate approximately 5 minetes sooner, the resulting core uncovery is shorter, and core heatup is less.
Small break LOCA results presented in Chapter 6 of the Quad Cities FSAR also indicate that complete core uncovery occurs in the small break case where ADS must activate, rather than the partial core uncovery shown in Figure 3-2 of NEDE-30045.
The resulting peak cladding temperatures (PCT) are approximately 200'F lower than those experienced in the design basis LOCA.
More recent analyses for fuel bundles with drilled lower tie plates have resulted in still lower PCT's.
The generic results shown in the Owner's Group Evaluation are therefore conservative for the Dresden and Quad Cities Stations.
Based on the above discussion, a bypass timer setting of 8.5
- 0.5 minutes will result in adequate core cooling for isolation events, with or without a stuck open relief valve, in which high pressure makeup is unavailable.
3.0 Accident Impact 3.1 Line Breaks Inside Drywell The ADS modification will have no ef fect on reactor system response to large or small breaks (liquid or steam) inside the drywell.
ADS operation is not required to mitigate large breaks; small break accidents will pressurize the drywell, and ADS will operate via the normal logic scheme if low reactor level is attained for 120 seconds without recovery.
~
3.2 Isolation Transients and Breaks Outside Drywell The current logic design does not actuate ADS automatically for isolation events due to a system transient or line break outside the drywell, since these events do not result in high drywell pressure.
The single exception is a transient which results in a stuck open safety valve, which discharges directly to the drywell at Dresden and Quad Cities.
This event therefore is similar to the small break case discussed above.
For other isolation events (with or without a stuck open relief valve), the modification will provide automatic ADS actuation if reactor low level is sustained until the new bypass timer times out (8.5 minutes).
Low pressure injection systems will then provide the necessary core cooling without immediate operator action.
The addition of a manual inhibit switch provides the operator additional flexibility in responding to isolation events.
Due to the level 1 setpoint at 7 feet above the core for Dresden and Quad Cities, it is possible to maintain adequate cooling even though a 1
sustained low level signal is present.
Such a situation could occur following a late isolation condenser (Dresden) or RCIC (Quad Cities) initiation; if the operator is controlling the reactor level above the core, he may wish to inhibit ADS although a low level signal is present.
Such a situation can be addressed in the plant emergency procedures.
3.3 ATWS Events ATWS events at Dresden and Quad Cities which result in low reactor level (level 1) will also close the main steam isolation valves, therefore preventing use of the main condenser as a heat sink.
The BWR Emergency Procedure Guidelines instruct the operator to lower the reactor water level to the top of the active fuel in order to reduce reactor power if the MSIV's have closed.
In this case, the addition of an ADS inhibit switch will allow the operator to prevent ADS initiation indefinitely rather than repeatedly reset the existing ADS timer.
Less severe ATWS events which do not result in low level and MSIV closure will be unaf fected by the ADS modification.
It should be noted that ATWS events which result in safety valve discharge to the drywell will cause high drywell pressure; in this case, ADS actuation may occur via the normal logic chain unless the operator utilizes the new inhibit switch.
3.4 Conclusion The ADS logic modification will not detrimentally affect any current accident analysis, and will provide for automatic ADS actuation if required for isolation l
In addition, the new ADS inhibit switch will provide the operator with additional flexibility L
6 in c.sses where ADS actuation may be signalled by the logic although not actually required to provide adequate core cooling.
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