ML112270139

Request for Additional Information Response Clarification from Davis-Besse Nuclear Power Station in Support of License Renewal Application Review

ML112270139
Person / Time
Site:	Davis Besse
Issue date:	08/15/2011
From:	John Parillo NRC/NRR/DRA/AADB
To:	Travis Tate NRC/NRR/DRA/AADB
Parillo J, NRR/DRA, 415-1344
References
TAC ME4613
Download: ML112270139 (7)

Text

August 15, 2011 MEMORANDUM TO: Travis L. Tate, Branch Chief Accident Dose Branch Division of Risk Assessment Office of Nuclear Reactor Regulation FROM: John G. Parillo /RA/

Accident Dose Branch Division of Risk Assessment Office of Nuclear Reactor Regulation

SUBJECT:

REQUEST FOR ADDITIONAL INFORMATION RESPONSE CLARIFICATIONS FROM DAVIS-BESSE NUCLEAR POWER STATION IN SUPPORT OF LICENSE RENEWAL APPLICATION REVIEW (TAC NO. ME4613)

Based on a review of the severe accident mitigation alternatives (SAMA) assessment, the U.S. Nuclear Regulatory Commission (NRC) issued a request for additional information (RAI) to FirstEnergy Nuclear Operating Company (FENOC) by letter dated April 20, 2011. The enclosed documents several clarifications to the RAI responses supplied by FENOC to support the NRCs review of the SAMA assessment for the Davis-Besse Nuclear Power Station (Davis-Besse).

Docket No.: 50-346

Enclosure:

RAI Response Clarifications CONTACT: John G. Parillo, NRR/DRA (301) 415-1344

ML112270139 OFFICE NRR/DRA/AADB NAME JParillo DATE 08/15/2011

RAI Response Clarifications from Davis-Besse Nuclear Power Station in Support of License Renewal Application Review Response to RAI 1.d Clarify whether the scope of the 2008 gap self assessment included Level 2 as well as Level 1 internal events, and whether a review of internal flooding and the high winds hazard was performed.

FENOC Response to RAI 1.d The 2008 gap self assessment:

• included Level 2 as well as Level 1

• did not include internal flooding

• did not include high winds Response RAI 4.b Clarify whether the escalation factor for the updated analysis was the same one as used in the original analysis. If not provide the new factor and its basis. Clarify whether transient population was considered in the updated analysis.

FENOC Response The population escalation factor used for the updated analysis (accounting for the Canadian population) is the same as was used for the original analysis.

Transient population (between 0-30 miles) was considered in the original analysis. The same transient population was considered in the updated analysis.

Response to RAI 5.b Clarify which and to what extent applicable severe accident mitigation alternatives (SAMAs) meet the intent of improving seismic capacity for the boron water storage tank (BWST).

Address in your response the cited SAMAs (i.e., AC/DC-01, CC-10, and CW-09) and SAMA CC-19.

FENOC Response SAMA candidate CC-10 considers providing an in-containment reactor water storage tank. This SAMA candidate would meet the intent by providing a tank independent of the BWST.

Response to RAI 5.d Clarify whether the automatic actions that were identified and evaluated in the response to RAI 5.d.ii are the only candidates or meant to be representative of other possibilities. Clarify that further unevaluated automating options do not remain.

Enclosure

Describe the probabilistic risk assessment (PRA) modeling assumptions used to calculate the SAMA benefits for AC/DC-26R and OT-08R similar that shown in Table E.7-1 of the ER.

FENOC Response The following SAMA candidates evaluate* automating operator actions:

• AC/DC-14 (Table E.7-1) - makes the station blackout diesel generator (SBODG) and corresponding human failure event (HFE) perfectly reliable.

• AC/DC-25 (Table E.7-1) - provides dedicated direct current (DC) power for auxiliary feedwater pump (AFP) control and eliminates the need for local manual control.

• AC/DC-26 (Table E.7-1) - provides an alternator/generator driven by the AFPs to provide DC power for the AFPs and eliminates the need for local manual control.

• AC/DC-27 (Table E.7-1) - makes the HFE to refuel the SBODG fuel tank perfectly reliable.

• AC/DC-28R (RAI 5.d) - automatically starts and loads the SBODG on Bus D2 upon loss of power to the bus.

• CC-19 (Table E.7-1) - makes the HFEs for switchover of high pressure injection (HPI) and low pressure injection (LPI) suction from the borated water storage tank (BWST) to the containment sump for loss of coolant accidents (LOCAs) perfectly reliable.

• CC-22R (RAI 7.d) - automates refill of the BWST.

• CW-26R (RAI 7.a) - automates reactor coolant pump (RCP) trip on high motor bearing cooling temperature.

• FW-17R (RAI 7.e) - automates start of the motor-driven feedwater pump (MDFP) in the event the automated EFW is unavailable.

• OT-08R (RAI 5.d) - automatically starts and loads the SBODG on Bus D2 upon loss of power to the bus in combination with automatically starting the MDFP.

As described in the response to RAI 5.c, internal events and LERF basic events (including HFEs) with a risk reduction worth (RRW) equal to or greater than the cost of a procedure change were identified and evaluated. Hardware modifications were also considered based on RRW values. This method was judged to identify all potentially cost beneficial automating options.

SAMA candidate AC/DC-28R evaluates automatically starting and loading the SBODG on Bus D2 upon loss of power to the bus for Davis-Besse.

A bounding assessment of the potential benefit of automatically starting the SBODG and loading it on bus D2 upon loss of power to the bus will be performed by removing the human action to start the SBODG from the cutsets (PRA-DB1-11-010-R00). Core damage frequency (CDF) = 8.17E-6/yr.

SAMA candidate OT-08R evaluates automatically starting and loading the SBODG on Bus D2 upon loss of power to the bus in combination with automatically starting the MDFP for Davis-Besse.

A bounding assessment of the potential benefit of automatically starting the SBODG and loading it on bus D2 upon loss of power to the bus with automatically starting the MDFP will be performed by removing the human actions to start the SBODG and MDFP from the cutsets (PRA-DB1-11-011-R00). CDF = 5.43E-6/yr.

• Note that only those SAMA candidates that were evaluated in detail are listed here; SAMA candidates that were screened (or subsumed or already implemented) are not listed even if they considered automating operator actions.

Response to RAI 6.j Provide the increased evacuation speed used in the Case E1 sensitivity analysis.

FENOC Response The increased evacuation speed used in sensitivity case E1 is 1.0 meters/second.

Response to RAI 7.a, 7.b, 7.c, 7.d, 7.e, and 7.f Describe the PRA modeling assumptions used to calculate the SAMA benefits for CW-26R, CC-22R, FW17-17R, and CB-22R similar that shown in Table E.7-1 of the ER.

FENOC Response to RAI 7.a A SAMA candidate (CW-26R) to provide an automatic reactor coolant pump trip on loss of cooling to the RCP seal thermal barrier cooler and loss of seal injection flow was evaluated for Davis-Besse.

A bounding assessment of the potential benefit of automating a reactor coolant pump trip on high motor bearing cooling temperature or on a loss of cooling to the RCP seal thermal barrier cooler and a loss of seal injection flow will be performed by making the operator action to trip the reactor coolant pumps on loss of seal cooling and injection perfectly reliable (PRA-DB1 005-001). CDF = 7.50E-6/yr.

FENOC Response to RAI 7.b As described in the RAI original response: The Davis-Besse design and PRA already includes use of the decay heat removal (DHR) system as a suction source for HPI. For cases in which RCS pressure is too high for adequate flow, the HPI pumps can be aligned to take suction from the discharge of the DHR pumps; this is possible with the BWST as the suction source or with the containment sump as the suction source.

FENOC Response to RAI 7.c As described in the original RAI response: This SAMA candidate considers automating HPI injection on low pressurizer level following a loss of secondary side heat removal where RCS pressure remains high while level drops. This SAMA was a viable consideration for Three Mile Island (TMI) based on plant design and system configuration. At TMI, the HPI system is also the makeup system - there is a single Makeup and Purification system that provides normal makeup as well as standby Engineered Safety Actuation Signal (ESAS)-selected pumps which automatically inject high-pressure water into the RCS from the BWST in mitigation of LOCA

scenarios. In addition, as discussed in Volume 3 of the B&W Emergency Operating Procedure Technical Basis Document (EOP TBD), (Chapter III.C, Lack of Adequate Primary to Secondary Heat Transfer), for all plants except Davis-Besse, HPI cooling must not be intentionally delayed if feedwater is not available. HPI cooling must be established in a timely manner to assure adequate core cooling; it must be started early enough to slow RCS inventory depletion so that HPI cooling will match decay heat before the core is uncovered.

At Davis-Besse, however, the plant design and systems are different from those at TMI. Davis-Besse has a separate HPI safety system in addition to the normally operating makeup system.

The Davis-Besse HPI system is not capable of injecting water into the RCS until pressure reaches ~1600 psig. In addition, because Davis-Besse has two makeup pumps, makeup/HPI cooling can be delayed until the core outlet temperature reaches 600°F provided the RCS PT limit is not exceeded. Although the Davis-Besse PRA considers makeup/HPI cooling in FENOC Response to a loss of feedwater, and the associated operator actions, automating this function was not considered because of the complexity associated with the number of options and systems involved (e.g.,pumps, valves and alignment options, injection line options, bleed options). Consequently, this SAMA candidate was not considered for Davis-Besse.

FENOC Response to RAI 7.d A SAMA candidate (CC-22R) to provide an automatic refill of the borated water storage tank was evaluated for Davis-Besse.

A bounding assessment of the potential benefit of automating refill of the BWST will be performed by making the operator action to refill the BWST perfectly reliable (PRA-DB1-11-006-000). CDF = 9.76E-6/yr.

FENOC Response to RAI 7.e A SAMA candidate (FW-17R) to automatically start the auxiliary feedwater pump when the emergency feedwater system is unavailable was evaluated for Davis-Besse. Based on the Davis-Besse design, this SAMA was interpreted as automatically starting the motor driven feedwater pump in the event both turbine-driven auxiliary feedwater pumps were not available.

A bounding assessment of the potential benefit of automating start of the MDFP will be performed by making the operator actions to start the MDFP perfectly reliable (PRA-DB1 007-000). CDF = 7.03E-6/yr.

FENOC Response to RAI 7.f A SAMA candidate (CB-22R) to use a gagging device that could be used to close a stuck-open steam generator safety valve for a SGTR was evaluated for Davis-Besse.

A bounding assessment of the potential benefit of utilizing a gagging device on stuck open MSSV will be performed by removing MSSV failures to close from the cutsets (PRA-DB1 008-000). CDF = 9.24E-6/yr.