ML20235J269
| ML20235J269 | |
| Person / Time | |
|---|---|
| Site: | LaSalle  |
| Issue date: | 07/10/1987 |
| From: | Allen C COMMONWEALTH EDISON CO. |
| To: | NRC OFFICE OF ADMINISTRATION & RESOURCES MANAGEMENT (ARM) |
| Shared Package | |
| ML20235J272 | List: |
| References | |
| 3317K, NUDOCS 8707150472 | |
| Download: ML20235J269 (7) | |
Text
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One First National Plaza, Chicago, Illinois Address Reply to: Post Office Box 767 Chicago, Illinois 60690 0767 July 10, 1987 U.S. Nuclear Regulatory Commission Attn: Document Control Desk Washington, DC 20555
Subject:
LaSalle County Station Units 1 and 2 Proposed Amendment to Technical Specifications for Facility Operating l
NRC Docket No. 50-373 and 50-374 References (a): Risk Based Evaluation of Technical Specification Problems at the LaSalle County Station Nuclear Station, EPRI Research Project 2142-2.
(b): Federal Register Vol. 51 No. 44 dated March 6, 1986
Dear Sir:
Pursuant to 10 CPR 50.90 Commonwealth Edison proposes to amend Facility Operating License NPF-ll.
This amendment is being submitted for your staff's review and approval and is in accordance with ref6rence (b).
The proposed amendment will allow removal of the Main Steam Line Isolation from Main Steam Tunnel (MST) temperature and differential temperature sensors. The alarm function from the s'ensors will be retained to provide early indication of potential steam leaks.
Attachment A provides an introduction and discussion. Attachment B provides copies of the changes to be made to the Facility Licenses.
Commonwealth Edisor. has reviewed this document and finds that no significant hazards exists. This review is documented in Attachment C.
A risk analysis evaluation is enclosed as Attachment D of which Sections 1,2,4,6,7 and appendix B provide support for this proposed amendment. Attachment E contains copies of applicable pages of the LaSalle Updated Final Safety Analysis Report (UFSAR). Attachment F contains a portion of a Sargent and Lundy (S & L) report in which the basis for the setpoints was investigated.
Commonwealth Edison is notifying the State of Illinois of our request for this amendment by transmitting a copy of this letter and attachments to the designated state official.
In accordance with 10 CFR 170, a fee remittance in the amount of
$150.00 is also enclosed.
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1 US NRC July 10, 1987 i
The effective date of this amendment should be 45 days after the date of issuance.
If you have any additional questions regarding this matter, please contact this office.
Very truly yours, C. M. Allen Nuclear Licensing Administrator Attachments 4
cc: Paul Shemanski - NRR I
Regional Administrator - RIII NRC Resident Inspector - LSCS M. C. Parker - IDNS 3317K
l-1 ATTACHMENT A i
I TECHNICAL SPECIFICATION CHANGE REOUEST LASALLE COUNTY STATION UNI'fs 1 AND 2 BACKGROUND AND DISCUSSION I
3317K i
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BACKGROUND As part of the nuclear. boiler leak detection system, the Main Steam
' Tunnel (MST) ambient. temperature and ventilation system differential temperature sensors are used for the detection of small leaks in the MST.
The temperature sensors for these parameters are currently set to initiate a control room annunciator whenever an air temperature increase corresponding to a 5 gpm leak is detected and to cause a Group I Isolation when a 25 gpm leak is detected.
Since approximately 90 percent of the reactor building air flow
. passes through the main steam tunnel, detection of small leak rates requires l
the trip-setpoints to be set very close to normal operating temperatures.
Any. disruption of reactor building ventilation flow or rapid changes in reactor building air temperature can cause a spurious Group I Isolation.
(Main Steam Isolation Valve (MSVI) closure). Disruptions in reactor building ventilation flow can be occur from random ventilation system l
failures or by clogged ventilation filters caused by environmental factors, such as agricultural dust during the spring plowing season and by snow in the winter.
During the' spring and fall seasons rapid decreases in outside air temperature at night cause the reactor building ambient temperature to' drop, which in turn causes a rapid increase in the differential temperature between the MST ventilation intake and exhaust'to a point very close to the isolation setpoint. The plant may be required to operate for extended periods of time under these conditions which further increases the risk of a 1
group I isolation.
since spurious group one isolations are a significant challenge to
_ the plant and require the use of safety systems to remove stored energy, commonwealth Edison (CECO) believes that an improvement is safety is gained by removal of the Main Steam Tunnel (MST) Temperature and Differential Temperature isolations.. This belief is based on the close proximity of the setpoint.to the operating values as well as consideration to the stress under which the control room operators must function when the plant is operating close to the isolation setpoint.
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DISCUSSION The fellowing variables are used to detect steam leakage outside of the primary containment and to cause a Group I (MSIV) Isolation:
- MST high ambient temperature and high differential temperature.
Low main steamline pressure (operating condition 1 only).
- High main steamline flow rate.
- Low reactor water level.
A review of the Updated Final Safety Analysis Report (UFSAR) requirements was performed to determine the origin of the 5 gpm and 25 gpm limits for the MST ambient and differential temperature setpoints. The UFSAR requires in Table 5.2-8 that the MST be monitored for high ambient temperature and differential temperature and that these variables provide alarms and isolation signals. UFSAR Table 7.3-2 provides the temperature trip setpoints and allowable values for these instruments (140 degrees F for ambient temperature and 36 degrees F for differential temperature) are set to detect a 25 gpm leak, which is inconsistent with UFSAR Section 7.3.2.2.3.3, which requires that the MST ambient temperature and differential temperature sensors detect leaks in the range of from 1% to 10%
of rated steam flow (1% of rated steam flow is approximately 282 gpm).
Section 7.3.2.2.3.3. also states that:
The main steamline space high temperature trip is set far enough above the temperature expected during operation at rated power to avoid spurious isolations, yet low enough to provide early indication of a steamline break.
It is clear from a review of plant operating experience that the setpoints do not provide an adequate safety margin to prevent spurious trips.
That review shows that three spurious trips have occurred to date.
The design bases for the Primary Containment Isolation System (PCIS) are provided in section 7.3.2.1 of the UFSAR. Paragraphs a and d of this section state that:
a.
To limit the release of radioactive ruoterials to the environs, the primary containment and reactor vessel isolation control system shall, with precision and reliability, initiate timely isolation of penetrations through the primary containment whenever the values of monitored variables exceed preselected operational limits.
d.
To provide assurance that conditions indicative of a failure of the reactor coolant pressure boundary are detected to fulfill safety design basis a, primary containment and reactor vessel isolation control system inputs shall be derived from variables that are true, direct measures of operational conditions.
. Sargent & Lundy's (S & L) analysis (Attachment F) of the leak detection system has shown that the ambient and differential temperature setpoints cannot be accurately established with respect to quantifying leakage because the temperatures being sensed are directly affected by outside environmental conditions. Therefore the MST ambient and differential temperatures cannot be considered a precise or reliable indication of MST conditions. Nor can these variables be considered to be a true and direct measure of plant operational conditions.
A review of the following references, was performed by S & L (Attachment F) to determine the design basis and logic for the leak 6etection system:
Regulatory Guide 1.45 " Reactor Coolant Pressure Boundary Leak Detection Systems."
Standard Review Plant 5.2.5 "Reactnr Coolant Pressure Boundary Laakagc."
General Electric (GE) Company's Design Specification for Leak Detection (GE Document Number 22A2870, Revision 7).
General Electric Company's Design Specification Data Sheet (GE e
Number 22A2870AA, Revision 3 " Leak Detection System.")
Tne regulatory documents do not specify quantitative requirements for the temperature monitoring systems used for detecting leakage outside of the primary containment.
Instead, it appears that they specify only requirements for the capabilities of system designs required to detect leakage within the primary containment.
In fact, the regulatory guide indicated that humidity, temperature or pressure monitoring of containment atmosphere should be considered as alarms or indirect indication of leakage to the containment.
The specific requirements for the ability to detect a 5 gpm leak and cause an alarm as well as the ability to detect a 25 gpm leak and cause an isolation to occur, are contained only within the GE design j
specification.
There are no references to regulatory documents in the reference section of the GE design specification, and it appears that the 5 and 25 gpm values originated from the GE System Design Engineer. Within the GE specification, the words describing the use of temperature sensors to detect steam leaks appear as follows:
" Alarm should be actuated by a temperature rise corresponding to the steam leakage of 5 gpm and automatically isolate the system on 25 gpm."
. ~A risk-based evaluation has been completed to determine the effect on plant safety of removing the ambient and differential temperature trips from the Primary containment Isolation System (PCIS) Group I isolation logic (reference a).
This evaluation was sponsored by the Electric Power Research Institute (EPRI) to determine the usefulncss of risk-based methodt..ogy in achieving technical specification improvements. The MST ambient and differential temperature trips were included in the report as one of the areas evaluated. Applicable portions of this report have been included in Attachment D.
The analysis concluded in section 4.9, that the reliability of the MSIVs to close in response to a steam line break outside the containment is not significantly compromised by removal of the MST temperature trip sensors as part of the trip logic. The small increase in risk to plant safety due to small steam line breaks which quickly propagate to large breaks is more than offset by the reduction in risk to plant safety posed by the challenges to plant safety systems caused by spurious MSIV closures. The report recommends retention of the MST high temperature alarm function to provide detection of small steam leaks.
Based on tne above discussion it is proposed that the Units 1 and 2 j
1 Technical Specifications be amended to eliminate the Main Steam Tunnel high ambient temperature and high ventilation differential temperature trips from the PCIS technical specifications (see Attachment B).
Upon approval of this proposed amendment the PCIS logic will be modified to eliminate the MST temperature trips from the Group I isolation logic and the UFSAR will be amended as necessary in the next revision of the UFSAR. The MST high ambient and differential temperature alarms will be retained and reset to temperature setpoints corresponding to 25 gpm in order to provide control room operators with early indication of potential steam leaks in the MST outside of the primary containment.
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