ML20118A834
| ML20118A834 | |
| Person / Time | |
|---|---|
| Site: | Waterford  |
| Issue date: | 09/18/1992 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML20118A830 | List: |
| References | |
| NUDOCS 9209280235 | |
| Download: ML20118A834 (5) | |
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t SAFETY EVALVATION BY THE OFFICE OF NUCLEAR REACTOR REGULATION-RELATED TO AMENDMENT NO. 76 TO FAtlllTY OPERATING LICENSE NO. NPF-38 ENTERGY OPERATIONS. INC.
FATERFORD STEAM ELECTRIC STATION. UNIT 3 DOCKET NO. 50-382 1.0 -INTRODUCTION By application dated April 24, 1992, as supplemented by letter _ dated August 27, 1992, Entergy Operations, Inc. (the licensee) submitted a proposal-to reodify Waterford 3 Technical Specification (TS) Surveillance Requirement:
4.7.1.5,." Main Steam Isolation Valves," and Table 3.3-5, " Engineered Safety -
Features Response-Time."
This modification proposes to increase the allowed Main Steam Isolation Valve (MSIV) closure time from 3 seconds to 4 seconds during surveillance testing.
In response to questions to the licensee, Entergy Operations, Inc., submitted a-letter dated August 27, 1992, to clarify the reasons-for the increase andL further state the-basis for the codes /meth'ods used-in-the analysis. -This information was for clarification and does'not_. change or affect-the staff's no:
significant-hazards consideration published on June 24, 1992-(57 FR 28200).
2.0 EVALUATION-MSIV closure time is significant for several events analyzed in the Final-Safety Analysis Report (FSAR)' Chapter 6.2 and' Chapter 15. These analyses can be categorized into two groups' based on the assumed MSIV closure time-being
-greater:than.or less than 4 seconds.- The events: analyzed with a MSIV_ closure t
time greater than 4. seconds are not affected by this proposed change. Those i
events are:
Full. Power Double-Ended Steam Line. Break (SLB) Inside Containment L
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with Concurrent Loss of Offsite Power-
~ Double-Ended Steam Line Break Inside. and Outside Containment During a
Mode 3 Operation with Concurrent Loss of Offsite Power
-Steam System Piping Failure:. Pre-Trip Power ' Excursion Analysis Outside-Contair 9nt with-Loss of Offsite Power.
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- The analyses that use less than a 4-second MSIV closure time were evaluated in detail by the licensee and are discussed in the sections that follow.
2.1 Main Steam Line Break (FSAR Section 6.2)
The existing main steam line break (MSLB) analyses, which are the limiting analyses for the MSIV closure time, are located in Section 6.2 of the FSAR.
They assume that the MSIV remains fully open and then closes instantaneously 3.0 seconds after receiving a signal to close.
The resulting mass and energy release cause an increase in containment temperature and pressure.
The licensee reanalyzed the mass and energy released to containment for the three most limiting, in terms of peak containment temperature and pressure, MSLB cases using a 4.0 second, instead of a 3.0 second, MSIV closure time.
These reanalyses use a more detailed representation of the steam line piping and pressure drop from the steam gercrator to the MSIV while using the same code / method as used in the original analysis, in addition, the licensee took credit for the decrease in the MSIV flow area during the 4.0 seconds the valve is closing instead of assuming instantaneous closure at the end of 3.0 seconds.
Consequently, less mass and energy was calculated to be released to the containment for all three cases when a 4.0 second MSIV closure time was used.
An anclysis of containment peak temperature and pressure using the revised mass and energy releases would result in lower peak values. Therofore, the current MSLB analyses in the FSAR bounds the peak containment temperature and
- rnsure that would be calculated with the revised 4.0 second MSIV closure time.
The three most limiting MSLB cases used by the licensee were 75% power with a containment cooling train failure, 102% power with a containment cooling train failure, and 75% power with the failure of one MSIV to close.
NRC Information Notice 91-069, " Errors In Main Steam Line Break Analyses For Determining Containment Parameters," addressed the consequences that may result from not considering the possibility that feedwater could continue to flow following a MSLB inside containment.
This could lead to a licensee choosing a MSLB case that does not maximize the post-accident containment temperature and pressure.
Section 6.2.1.4 of the licensee's Final Safety Analysis Report states in part that "A single failure of one main feedwater isolation valve during a postulated main steam line break accident is accommodated by closure of the main feedwater regulating valve upon receipt of a main steam isolation signal-(MSIS). Thus, assuming a single failure, feedwater flow to the affected steam generator is terminated by the closure of the main feedwater regulating valves within 5 seconds after receipt of signal." Therefore, Waterford 3 is not susceptible to the condition described in Information Notice 91-069.
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- 2.2 Increned Main Steam Flow Due to OnQrbine Bypass Valve Failina in the Opfn Position (FSAR Section 15.1.1.3)
During this transient the steam flow increases, causing excess heat removal from the reactor coolant system (RCS). The excess heat removal reduces RCS temperature and pressure and increases core power due to a negative moderator temperature coefficient (MTC).
The increase in core power decreases the departure from nucleate boiling ratio (DNBR) such that at 18.2 seconds after
..ie initiating eved, a low DNBR reactor trip signal is generated. At 255.2 seconds the low steam generator pressure generates a MSIS which closes the MSIVs 2 seconds later.
The times of interest in this event are the time of minimum DNBR and the time of reactor trip.
The MSIVs close long after (255.2 seconds) those events, therefore, increasing the closure time from 3 seconds tn 4 seconds will not affect plant response.
2.3 A Steam Line Break at Hot Zero Power Outside Containment with Concurrent loss of Offsite Power (FSAR Section 15.1.3)
In this event, the main steam line is assumed to rupture upstream of the MSIV shortly after a shutdown from full-power operation. This transient causes a large steam generator mass release and radiological consequences that are bounded by the full-power steam piping failure event outside containment:
Pre-Trip Power Excursion with Loss of Offsite Power (FSAR Section 15.1.3.3).
lhe latter event is analyzed with an MSIV closure time of almost 11 seconds, bounding the 4 second closure time.
2.4 feedwater System Pioe Break with loss of Offsite Power (FSAR 15.2.3.1)
During this transient, saturated liquid flows from the affected steam generator through the break causing an instantaneous loss of feedwater to the steam generator.
This causes a gradual heat-up of the primary and secondary systems. The ruptured steem generator empties causing a rapid increase in RCS temperature and pressure. A reactor trip on high pressurizer pressure occurs at 15.4 seconds.
With the concurrent loss of normal AC power, the plant will undergo a simultaneous loss of forced reactor coolant flow and a turbine trip.
Losing the heat sink and reactor coolant pump flow generates a concern regarding the RCS peak pressure.
An increase in MSIV closure time enhances cooling, causing a lower peak pressure. Consequently, the increase in the HSIV cicsure time to 4 seconds will not adversely affect the consequences of this transient.
2.5 Loss of Normal Feedwater Flow with an Active Failure in the Steam Bypan System (FSAR Section 15.2.3.2)
The loss of feedwater flow increases steam generator pressure and temperature causing the turbine bypass valves to open, increasing
<1 flow and RCS cooldown rate. When the bypass control valves fail t
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of a closure signal, the cooldewn of the primary system causes core power to increase due to a negative moderator tenperature coefficient. The maximum heat flux will cause the reactor to trip at 42.6 seconds on low steam generator water level. At 77.3 seconds a main steam isolation signal (MSIS) is penerated due to low steam generator pressure causing the MSIVs to close 3 seconds later.
Changing the HSly closure time from 3 seconds to 4 seconds will not affect the transient results significantly with respect to DNB or radiological consequences since (1) the MSIV closure occurs after the maximum heat flux and reactor trip and (2) the secondary mass release through the MSIV in the one additional second is not significant.
However, the increase in MSIV closure time can cause a slight increase in RCS temperature which will have a negligible effect on the course of the transient.
2.6 Sinale Reactor Coolant Pumo Shaft Seizure with a Stuck Open Second.gy Safety Valve (FSAR Section 15.3.3.2)
Approximately I second into this transient, a reactor trip signal is generated due to a low DNBR. The main turbine trip and reduction in feedwater flow causes steam generator pressure to increase, which opens the steam generator safety valves.
Assuming one safety valve remains open throughout the transient, the steam flow through the valve reduces the pressure in the steam generator.
The low steam generator pressure generates a MSIS - at about t
700 seconds into the transient - causing the HSIV to close 3 seconds later.
Changing the closure time will not have an effect on thermal margin since the minimum DNBR occurs before the MSIV closure.
3.0 SAFETY CONCLUSION Based on the review of the FSAR Chapter 6.2 MSLB event, the staff concludes that the proposal to increase the time for MSIV full closure by 1.0 second, as discussed in TS 4.7.1.5 and Table 3.3-5 of the TSs, is acceptable because the mass and energy calculated to be released inside containment with the new closure time is bounded by the original FSAR analysis when credit is taken for flow choking during MSIV closure. This more realistic portrayal of MSIV valve closure is also acceptable.
Based on the review of the Chapter 15 events that have an MSIV closure time of less than four seconds, the staff concludes that the increase of the MSIV closure time from 3 seconds to 4 seconds either (1) has no effect, (2) has a negligible effect, or (3) is bounded by a previously analyzed event. The v
staff, therefore, finds the licensee's prop-41 to modify TS Surveillance 4.7.1.5 and Table 3.3-5 acceptable.
't o 3.0 STATE CONSULTATI08 In accordance with the Comission's regulations, the Louisiana State official was notified of the proposed issn= e-af the amendment.
The State official had no coments.
4.0 ENVIRONMENTAL CONSIDERATION
The amendment changes a requirement with respect to installation or use of a facility component located within the restricted area as defined in 10 CFR Part 20 and changes in surveillance requirements. The NRC staff has determined that the amendment involves no significant increase in the amounts, and no significant change in the types, of any effluents that may be released offsite, and that there is no significant increasa in individual or cumulative occupational radiation exposure. The Comission has previously issued a pro-posed finding that the amendment involves no significant hazards consideration and there has been no public comment on such finding (57 FR 28200).
Accordingly, the amendment meets the eligibility criteria for categorical exclusion set forth in 10 CFR 51.22(c)(9).
Pursuant to 10 CFR 51.22(b) no environmental impact statement or environmental assessment need be prepared in onnection with the issuance of the amendment.
5.0 00NCLUJ1QH The Comission has concluded, based on the considerations discussed above, that: (1) there is reasonable assurance that the health and safety of the public will not be endangered by operation in the-proposed manner, (2) such activities will be conducted in compliance with the Comission's regulations, and (3) the issuance of the anendment will not be inimical to the common defense and security or to the health and safety of the public.
Principal Contributors:
M. Snodderly S. Brewer Date:
September 18, 1992