ML20207E973
| ML20207E973 | |
| Person / Time | |
|---|---|
| Site: | Davis Besse  |
| Issue date: | 12/22/1986 |
| From: | Williams J TOLEDO EDISON CO. |
| To: | James Keppler NRC OFFICE OF INSPECTION & ENFORCEMENT (IE REGION III) |
| References | |
| 1-698, NUDOCS 8701050262 | |
| Download: ML20207E973 (6) | |
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MB TOLEDO h EDISON Docket No. 50-346 JOE VVILLIAMS. JR seme w. w (4191249 2300 License No. NPF-3 (419N41%223 Serial No. 1-698 Paiuala MUTIt:G December 22, 1986
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@ M-h Mr. James G. Keppler, Regional Administrator United States Nuclear Regulatory Commission FILE dD' Region III 799 Roosevelt Road Glen Ellyn, IL 60137
Dear Mr. Keppler:
During the System Review and Test Program (SRTP) at the Davis-Besse Nuclear Power Station several surveillances and tests were run on the Main Steam Isolation Valves (MSIVs). These tests initially utilized an acceptance criteria for the valve response time of five seconds. Mode 4 testing required by the Technical Specifications was successfully completed within this time restraint on both MSIVs on December 19, 1986. During additional testing in Mode 3, one main steam isolation valve (MS-101) exhibited greater than a five second response time.
In review, it became apparent that MSIV response times and identified functions were inconsis-tent between the Technical Specification and the Bases. A detailed review of the analyses and bases was undertaken. This letter discusses the details of the review and supports the conclusion of MSIV operability.
The results of one review shows that the appropriate function described in the Technical Specification Bases for MSIV response time is mitigation of a steam line break. Its analyzed response time from the low steam line pressure condition of the accident to MSIV closure is six seconds. This response time was met by MS-101 on December 21, 1986, and as such the valve is operable.
The MSIVs are installed in the two main steam lines between the steam generators (SGs) and the turbine to provide isolation of the uneffected SG in the event of a steam line break (see attached Figure 1).
Automatic closure of the MSIVs is initiated by either the Steam and Feedwater Rupture Control System (SFRCS) upon low pressure in a steam line or upon a high differential feedwater line or pressure between the steam generator and the main Safety Features Actuation System (SFAS) upon a high-high containment pressure. Closure of this valve isolates the steam line as part of the containmenc integrity and also isolates the SG to maintain it as a heat sink.
In the former case, the main steam lines along with the feedwater lines are considered Type III containment penetrations since i
they are not a part of the reactor coolant pressure boundary (Type I) and I (
are not connected directly 'to the containment vessel atmosphere (Type II).
The MSIVs can also be closed by manual push button.
g THE TOLEDO EDtSON COMPANY EDISON PLAZA 300 MADISON AVENUE TOLEDO, OHIO 43652 8701050262 861222 0gg 2 9 1986 PDR ADOCK 05000346 D
D:ckst No. 50-346 License No. NPF-3 Serial No. 1-698 December 22, 1986 Page 2 Operability and closure time requirements for the MSIVs are identified in four different sections of the Davis-Besse Technical Specifications as noted below:
Section 3.3.2.1 (Safety Features Actuation System Instrumentation) Table 3.3-5 (page 3/4 3-18) identifies that the response time for the MSIVs be less than or equal to ten seconds, including the diesel generator starting sequence loading delays. The basis for this Technical Specification states that this response time is commensurate with the time limit assumed in the Safety Analysis. SFAS would be loss of coolant accident related.
Scction 3.3.2.2 (Steam and Feedwater Rupture Control System Instrumenta-tion) Table 3.3-13 (page 3/4 3-29) identifies an isolation time of six seconds for the MSIVs. The bases for this section also state that this response time is commensurate with the time limits assumed in the Safety Analysis. SFRCS would be steam and feedwater line break related.
Section 3.6.3.1 (Containment Isolation Valves) Table 3.6-2 (page 3/4 6-17) lists all containment isolation valves at Davis-Besse and identifies a response time for each. The general basis attached to this specification is that containment isolation within the time limits specified ensures that the release of radioactive material to the environment will be consistent with the assumptions used in the analysis for a LOCA. The basis for the main steam isolation valves on this list, however, is specifically clarified by License Amendment No. 72 dated July 20, 1984 l
(Log No. 1559).
In the NRC Safety Evaluation Report supporting this License Amendment, the only identified function of the MSIVs is to miti-gate the consequences of a steam line break.
Finally, Section 3/4.7.1.5 (Main Steam Line Isolation Valves) identifies full closure of the MSIVs in five seconds. The basis for this section is also steam line break.
It further states that this isolation ensures that no more than one steam generator will blow down in the event of a steam line break. This is required to 1) limit the pressure rise within the contain-ment if the rupture occurs within the containment, and 2) minimize the positive reactivity effects of the Reactor Coolant System cooldown associ-ated with the blowdown. The 'ases further state that the closure time requirement is consistent with the assumptions used in the safety analysis.
From the different specifications above, it is inferred that credit has been taken for the MSIV closure for prevention of two steam generator blowdown following a main steam line break as well as to provide contain-ment isolation. The above examples also served to highlight the incipient inconsistencies relating to the MSIV response time in various sections of the Davis-Besse Technical Specifications.
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Dock 0t No. 50-346 License No. NPF-3 Serial No. 1-698 December 22, 1986 Page 3 Following is an evaluation of MSIV operability in relation to assumptions of its availability and closure time for the steam line break accident as well as for the Loss of Coolant Accident (containment isolation) functions.
Steam Line Break Isolation By a letter dated July 20, 1984 (Log'No. 1559), the NRC issued a license amendmenc to Toledo Edison relating to testing requirements for the MSIVs. The NRC Safety Evaluation Report for this amendment specifically assumed and credited the safety function of this valve as to provide isolation of the steam generator in the event of a steam line break, it is further noted that the steam line break analysis contained in Section 15.4 of the Davis-Besse USAR assumes'that following detection of a steam line break (steam line pressure less 'than 600 psig) steam flow from the unaffected steam generator will be isolated within six seconds. Note that this is also commensurate with the steam line break analysis for Davis-Besse submitted to the NRC (see Serial No. 1208 dated November 16, 1985) to address the single failure vulnerability of the Davis-Besse Auxiliary Feedwater System and SFRCS. This six second time period includes the response time of the actuation system circuitry which consists of the Steam and Feedwater Rupture Control System. It should be noted that this SFRCS response time has been measured at Davis-Besse during past surveillance testing and has been found not to exceed 100 milliseconds. This allows 5.9 seconds for the MSIVs to go from full open to full closed. Further, the required (5.9 second) response time of the valve would be with full steam flow through the valve.
This is because during an actual steam line break condition steam will flow past this valve, in the direction to assist closure, from the unaffected steam generator in an attempt to discharge through the break. Testing done at Davis-Besse on December 21, 1986 (TP 851.55) demonstrates that steam flow improves the response time of this valve. Per TP 851.55, the response time of the MSIV (MS 101) has been measured to be 5.7 seconds with two turbine bypass valves open during the valve stroke. This corresponds to less than 5% of the total steam flow that would be expected during a design basis main steam line break accident. Since the actual steam flow past this valve will be significantly greater during an actual accident condition, the valve is expected to close faster than the measured 5.7 seconds.
In any event, the closure of the MSIV within 5.7 seconds is considered acceptable with the attendant SFRCS system response time, since the overall closure time is within the assumptions of the Davis-Besse Safety Analysis Report.
As noted earlier, bases for Technical Specifications Sections 3.3.2.2 and 3.7.1.5 specifically require acceptable valve response time during a main steam line break event, and as concluded in the preceding paragraph, an overall response time of six seconds (which includes 0.1 seconds for SFRCS and 5.9 seconds for the MSIVs) is acceptable based on the assumptions
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. Dockat No.'50-346 License No. NPF-3 Serial No.:1-698 December 22, 1986' Page 4 of the Safety Analysis Report. Consequently, MS-101 which has exhibited a
- response time of 5.7 seconds with less than five percent steam flow is considered to be operable and within the assumptions of the Safety' Analysis Report. With significantly higher steam flows through the valve, as would be expected during an actual steam line break, the closure time is expected to be further decreased.
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Containment Isolation 1
Technical. Specifications 3/4/3.2 Table 3.3-5 " Safety Features Actuation
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System-Response Times" requires that the Main Steam Isolation Valves
[
.(MSIVs) be able to close within ten seconds of an isolation signal being generated. This signal.is generated by high-high containment pressure (set at 38.4 psia). This is the required TOTAL response time of the 4
basis of this requirement, as stated in the Bases section of the Techni-cal Specifications is: "The OPERABILITY of these systems is required to-provide the overall reliability, redundancy, and diversity assumed avail-
- able'in the facility design for the protection and mitigation of accident and transient conditions. -The integrated operation of each of these systems is consistent with the assumptions used in the accident analysis."
~ Technical Specification 3/4.6.1.1' Table ~3.6-2, " Containment Isolation Valves" requires that the MSIVs close in five seconds to accomplish contain-ment isolation. The basis for this in toc Technical Specifications is:
' "The OPERABILITY of the containment isol a valves ensures that tha containment atmosphere will be isolated Itom the outside environment in the event of a release of radioactive material to the containment atmosphere or pressurization of the containment. Containment isolation within the time limits specified ensures that the release of radioactive 4
material to the environment will be consistent with the assumptions used j
in the analyses'for LOCA."
i Therefore both bases refer to the accident analyses in the Final Safety i
Analysis Report (FSAR) for the time requirements. Table 6-8 of the FSAR l
lists the containment isolation function of each valve in the various containment penetrations and the closure time-assumed in accident analyses. For penetrations 39 and 40, the Main Steam Lines, it lists j,
a penetration isolation time of ten seconds. -As long as the entire response l
time of the containment pressure sensing, logic and actuation circuitry (SFAS) and the penetration closure time do not exceed ten seconds, the assumptions of theEFSAR analyses are met.
Based on the above assumptions in the FSAR, the closure of the MSIVs is required within ten seconds.of a high-high containment pressure condition (38.4 psia). Since the measured stroke time (without steam flow) for MS-101 is 6.2 seconds and the SFAS containment high-high pressure string response time has always been measured at less than two seconds, the ten 1
second USAR criteria is met with sufficient margin. Consequently, MS-101
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Dockst No. 50-346 License No. NPF-3 Serial No. 1-698 December 22, 1986 Page 5 is considered to be operable from a containment isolation standpoint.
It should be noted that the main steam lines constitute a Type III penetration for which General Design Criterion 57 of 10CFR50 Appendix A applies. This criterion requires either automatic or remote manual closure of Type III penetrations. Since the automatic closure is available through SFRCS, and manual closure capability is alsd available for this valve, the most fundamental intent of the regulation' requiring isolation is met and exceeded. Consequently, MS-101 is cdnsidered to be operable from a containment isolation perspective.
Based on the above evaluation, it is concluded that in light of analyses summarized and submitted to the NRC and in view of the regulatory requirements identified in 10CFR50, operability of MS-101 is ensured by the recently conducted testing on this valve through TP 851.55, dated December 21, 1986.
Toledo Edison recognizes the inconsistencies in the valve response time requirements as prescribed in varioas sections of the Davis-Besse Technical Specifications. The above evaluation clarifies the technical requirements of the Technical Specifications, however, to minimize the need for complex operability evaluations such as the above, a Technical Specification charge is in preparation to clarify all MSIV response times listed throughout the document and the 1987 Safety Analysis Report Update will clearly address these time requirements in Chapter 15.
Very truly yours, 3C)
LC* -,-
- RFP:plf cc: DB-1 NRC Resident Inspector
po.cket No. 50-346
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