NSD-NRC-96-4843, Forwards Copy of Revised Section 3, Initiating Event Frequency Evaluation of WCAP-13856, AP600 Implementation of Regulatory Treatment of Nonsafety-Related Sys Process
| ML20128N935 | |
| Person / Time | |
|---|---|
| Site: | 05200003 |
| Issue date: | 10/14/1996 |
| From: | Mcintyre B WESTINGHOUSE ELECTRIC COMPANY, DIV OF CBS CORP. |
| To: | Quay T NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NSD-NRC-96-4843, NUDOCS 9610170077 | |
| Download: ML20128N935 (17) | |
Text
~
x
- p m
Westinghouse Energy Systems hx 355 Electric Corporation Pittsburgh Pennsylvania 15230-0355 NSD-NRC-96-4843 DCP/NRC0624 4
Docket No.: STN-52-003 l
October 14, 1996 Document Control Desk U.S. Nuclear Regulatory Conunission Washington, D.C. 20555 ATTENTION:
T.R. QUAY j
SUBJECT:
ADVANCE COPY OF SECTION 3 OF WCAP-13856
Dear Mr. Quay:
Enclosed is an advanced copy of the revised Section 3, Initiating Event Frequency Evaluation, of WCAP-13856, "AP600 Implementation of the Regulatory Treatment of Nonsafety-Related Systems Process" (RTNSS). This section (stamped " draft") summaries the initiating event frequency evaluation performed to determine the significant nonsafety-related systems, structures, and components (SSCs) for the AP600.
The results and conclusions of this updated initiating event evaluation remains the same as provided in revision 0 of WCAP-13856.
This advanced copy of a revision to Section 3 of WCAP-13856 is sufficient to allow the NRC to review the initiating event evaluation area of RTNSS.
Please contact Cynthia L. Haag on (412) 374-4277 if you have any questions concerning this transmittal.
~
h ")
Brian A. McIntyre, Manager Advanced Plant Safety and Licensing
/nja Enclosures cc:
D. Jackson, NRC (I copy enclosure)
N. J. Liparuto, Westinghouse (w/o enclosure) i
\\
I 9610170077 961014 PDR ADOCK 05200003 PDR A
l Enclosure to Westinghouse Letter NSD NRC 96-4843 October 14, 1996 l
- e, i
l l
2954A
31
'3
' INITIATING EVENT FREQUENCY EVALUATION
[
An evaluation was performed to study the importance of the nonsafety related systems to the initiating event frequencies using the calculation of shutdown and at-power initiating event l
frequencies in the AP600 PRA.
1 The initiating events identified in the PRA were reviewed for the impact of nonsafety related j
system unavailability. The assessment of the importance of nonsafety-related systems, j
structures, and components (SSCs) on initiating event frequency is based on the specific PRA methodologies used to calculate the initiating event frequencies.
For the purpose of evaluating the importance of the nonsafety related SSC unavailability to the calculation of initjating event frequency, the specific baseline PRA and focused PRA sensitivity study initiating events were categorized by the PRA methodology used to calculate the initiating event frequencies. Eleven categories of initiating events were identified for l
at-power and shutdown conditions. A brief. discussion of the initiating event frequency calculational methodology for each category is provided to assist in understanding the process used to determine the importance of nonsafety-related SSC reliability on the initiating event frequency. The specific categories are listed below along with the section of this report that
(
evaluates the importance of the nonsafety-related SSCs:
I AT-POWER INITIATING EVENTS Section 3.1 - Main Steam Line Stuck Open Safety Valve Section 3.2 - RCS Leak Section 3.3 - LOCAs Section 3.4 - Secondary Side Breaks Section 3.5 - Transients Section 3.6 - Anticipated Transient Without Scram Section 3.7 - Miscellaneous Special Initiators S.HUTDOWN INITIATING EVENTS I
Section 3.8 - Shutdown LOCA Section 3.9.- Shutdown Loss of Offsite Power
=
Section 3.10 - Shutdown Loss of Decay Heat Removal Section 3.11 - Shutdown RCS Overdrain l
Draft for Revisior) 1. October 1996 mA3284w 3.wpf.1b 101496 I
. 3-2 The initiating events and the associated initiating event frequencies calculated in the baseline PRA and focused PRA sensitivity study are provided in Chapter 2 of the AP600 PRA. As discussed in Chapter 2 of toe AP600 PRA, the initiating event frequencies were determined using several different methodolog!es.
The evaluation of the importance of the nonsafety-related SSC unavailability to the initiating i
event frequency requires identifying ap,s.opriate criteria for use in determining the importance of the nonsafety-related SSCs. The btving three criteria were developed for use in the
)
evaluation:
Criterion.1, Are nonsafety-related SSCs considered in the cciculation of the initiating event frequency?
Criterion 2 Does the unavailability of the nonsafety-related SSCa sigruficantly affect the calculation of the initiating event frequency?
Criterion 3 Does the initiating event significantly affect core damage frequency and large
)
release frequency for the focused PRA?
The criteria are applied to the individual initiating events in each of the initiating event categories. For each initiating event, if the response to any one of the three criteria is "No,"
then the unavailability of the nonsafety-related SSCs is not important to the calculation of initiating event frequency for the focused PRA. The discussions in Sections 3.1 through,3.11 include the results from applying the screening criteria to the initiating events discussed in i
each section. Section 3.12 provides a summary of the results of the evaluation and Table 3-1 shows the results of the criteria application. Figure 3-1 shows a diagram of the evaluation process.
]
The third screening criterion was developed for initiating events where nonsafety-related SSCs affect the. calculation of the initiating event frequency, but the initiating event itself is not significant to the focused PRA from the perspective of its contribution to the core damage frequency and the large release frequency. The rationale for this criterion is that a change in the nonsafety-related SSC unavailability can impact the calculated initiating event frequency, but the change does not have a significant effect on the core damage frequency and large release frequency., For the purposes of this screening criteria, individual initiating events with Draft for Revision 1, october 1996 m:\\3284w-3.wpf;1b-101496
3-3 e contribution,to_ core damage frequency and large release frequency of less than approximately 10 percent are not considered to be significant.'
AT-POWER INITIATING EVENTS 3.1
- MAIN STEAM LINE STUCK OPEN SAFETY VALVE A plant specific calculation was performed to determine the initiating event frequency for spurious opening of the steam generator safety valves and power-operated relief valves. The steam generator system, including the steam generator safoty valves and the power-operated relief valveblock valves are safety-related. The design of the steam generator power-operated relief valve is such that the valve itself and the closing function of the actuator are safety related, while the valve actuator opening capability (required for this initiating event) and the required support functions (compressed Cr plant control system, de power) are nonsafety-related. The initiating event frequency calculation implicitly includes the nonsafety-related component initiators that contribute to spurious actuation of the steam generator power-operated relief valve. Since this initiating event considers nonsafety-related SSCs in the calculation of the initiating event frequency, the response to Criterion 1 is "Yes."
The initiating event frequency calculation considers the historical data for spurious opening of '
both the steam generator safety valves and the power-operated relief valve. The historical data reflects the difference in the number of safety valves per plant compared to the number of power-operated re!ief valves per plant. Based on a comparison of the contributions from each type of valve, the contribution from the relatively small population of power-operated relief valves is not significant when compared to the contribution from the larger population of safety valves in the historical data. Since nonsafety-related SSCs do not significantly affect the calculation of initiating event frequency, the response to Criterion 2 is "No," An additional consideration for this event is that increased unavailability of the associated nonsafety-related SSCs will reduce the reliability of the power operated relief valves to open on demand and therefore, reduce the initiating event frequency.
The response to Criterion 2 is "No." Therefore, the nonsafety-related SSCs associated with this event are not considered to be important with respect to their effect on this initiating event.
Note that in Revision 0. the screening cnteria definition for an initiator to not be a signrficant contributor to core damage frequency i
I or la's 'elease frequency was conservatively chosen as "less than about one percent,* in the absence of any conventionalty estao.ished criteria. since then, this criteria was reviewed agemst risk significance deArutions suggested by EPRI and those that
]
are e id in mamtenance rule applications. Moreover, it was observed hat the nonsafety r9lated system failure probabilrties are j
ulready modeled conservatively in the AP600 PRA so that any trutisting event frequencies calculated by these models already include pessimism. It was concluded that maintaining the one percent criteria would be unduly conservanve. From a risk point of view, in order to avoid unduly flaggang initiating events for potenbal regulatory oversight treatment the screening limit for Cntenon 3 of 10 percent is chosen.
Draft for Revision 1. october 1996 mA3284w-3.wpf;1b-101496 I
. 3-4 3.2 RCS L.EAK For an RCS loak event, the initiating event frequency is derived from a review of historical data that loentifies leaks that result in a RCS leak with an equivalent pipe diameter of less than 3/8-inch and the unavailability of the chemical and volume control system (CVS) to provide reactor coolant system makeup. The components that are considered in the portion of the RCS leak event frequency calculation related to reactor coolant system leakage are safety related. The chemical and volume control system and the associated support systems are nonsafety-related. Since this initiating event considers nonsafety-related SSCs in the calculation of the initiating event frequency, the response to Criterion 1 is "Yes."
)
The effect of overall nonsafety-related system unavailability on this initiator results in a proportional change in the initiating event frequency. It is also possible to evaluate sensitivity to changes in unavailability of specific equipment and components modeled in the chemical and volume control system fault tree. The initiating event frequency changes in proportion to a
the importance of the specific component to the overall nonsafety related system unavailability. Based on a comparison of the probability of a leak initiator with the probability for unavailability of the chemical and volume control system, the nonsafety related SSC impact on the initiating event frequency is significant. Since the nonsafety-related SSCs significantly affect the calculation of initiating event frequency, the response to Criterion 2 is "Yes."
As shown in Tables 52-5 and 52-12 of the PRA, this initiating event is not an important contributor to the core damage frequency and large release frequency for the focused PRA.
Since this initiating event does not significantly contribute to the core damage frequency and large release frequency, the response to Criterion 3 is "No."
The response to Criterion 3 is "No." Therefore, the nonsafety-related chemical and volume control system SSCs and other nonsafety-related SSCs (that are required for normal plant power operation) associated with this event are not considered to be important with respect to their effect on this initiating event.
3.3 LOCAs For LOCA events, the initiators are caused by piping and valve leaks and breaks and by spurious opening of certain safety related valves such as reactor coolant system safety valves or automatic depressurization system valves. The PRA methodology identifies the piping j
segments (or tube segments for tube rupture events) within the appropriate areas of the various systems and calculates the initiating event frequency based on a basic failure rate fer these piping sections.
Draft for Revision 1; October 19%
mA3284w-3.wpf;1b-101496
3-5 There are two, nonsafety-related systems (normal residual heat removal system and chemical and volume control system), identified in the LOCA events, however, all of the sections of the system that could contain reactor coolant and potentially initiate a LOCA event are designed using safety related piping and components.
In addition, these piping sections have redundant safety-related isolation valves that are either normally closed during plant operation or automatically closed following LOCA events. For example, the chemical and volume control system purification loop and discharge piping includes redundant letdown isolation valves and redundant containment isolation valves to isolate leaks that initiate in this piping and to prevent leakage from lines that exit containment.
The piping ' sections of the nonsafety-related systems included in the initiating event frequency calculation are the same sections that are designed using safety related piping as discussed previously.
S nce these initiating events do not consider nonsafety-related SSCs in the calculation of the initiating event frequencies, the response to Criterion 1 is "No." Therefore, nonsafety-related SSCs are not considered to be important with respect to their effect on these initiating events.
For spurious ADS actuation events leading to a large LOCA event, the answer to the first criterion question is "Yes" since DAS is one of the means of spurious actuation. However, the answer to the second criteria question is "No" since the contribution of DAS to spurious ADS -
actuation is much less than that of PMS. Thus, spurious ADS actuation leading to a large LOCA is also dismissed due to the "No" response to Criterion 2.
As outlined in Chapter 2 of the PRA, the initiating event frequency for the interfacing system LOCA event is a result of the erroneous opening of normal residual heat removal system isolation valves, due to either hardware failure or operator error, in conjunction with the rupture of safety related normal residual heat removal system components due to overpressurization.
Since these components are safety-related and the operator errors that contribute to this initiating event have no relationship to nonsafety-related systems, the response to Criterion 1
-is "No." Therefore, nonsafety related SSCs are not considered to be important with respect to their effect on the interfacing system LOCA initiating event.
3.4 SECONDARY SIDE BREAKS For the secondary side break events, the initiators are caused by pipe leaks and breaks.
Similar to the calculation for LOCA events, the PRA methodology identifies the piping segments within the appropriate areas of the various secondary systems and calculates the initiating event frequency based on a basic failure rate for these piping sections.
Draft for Rowsion 1. october 1996 mA3284w 3.wpf:1b-101496
5 j
. 3-6 i
- The irritiating event frequency calculation consists of a plant-specific analysis that includes l
pipe segments in several nonsafety related systems that can be leak initiators for specific events.
s
?
a In the AP600 baseline PRA and focused PRA sensitivity study, two main steam line pipe break initiating events were identified. The initiating events listed below include piping j
segments in nonsafety-related systems, as identified below:
Secondary Side Break - Upstream of the Main Steam Isolation Valves or Downstream i
e of the Main Feedwater Isolation Valves Main steam system
]
Main and startup feedwater system Secondary Side Breaks - Downstream of Main Steam Isolation Valves or Upstream of the Main Feedwater Isolation Valves Main steam system
)
Main and startup feedwater system For the two nonsafety related systems identified for the secondary side break events listed above, the initiating event frequency calculation includes both safety-related and nonsafety-related piping sections. Since these initiating events consider nonsafety-related SSCs in the calculation of the initiating event frequencies, the response to Criterion 1 is "Yes."
i The initiating event frequencies for the main steam line br~k transient events are calculated only considering piping integrity for these nonsafety-re:ated systems. Therefore, the operational unavailability of the nonsafety related systems has no impact on the initiating event frequency, assuming that piping integrity is unchanged.
The integrity of this nonsafety-related piping directly affects the calculation of the initiating event frequencies. To provide conservative treatment of the nonsafety-related SSCs (piping) for these nonsafety related systems, piping integrity is assured to impact availability for the purposes of screening against this criterion. Since nonsafety-related SSCs significantly affect the calculation of initiating event frequency, the response to Criterion 2 is "Yes."
As shown in Tables 52 5 and 5212 of the PRA, the main steam line break initiating events are not important c'ontributors to the core damage frequency and large release frequency for the focused PRA sensitivity study. Since these initiating events do not significantly contribute to the core damage frequency or large release frequency, the response to Criterion 3 is "No."
Therefore, the nonsafety-related SSCs (that are required for normal plant power operation) associated with these events are not considered to be important with respect to their effect on these initiating events.
Draft for Revision 1, october 1996 mA3284w 3.wpf.1b-101496
3-7 3.5
- TRANSIENTS i
1 The initiating event frequencies for the transient events are calculated using historical failure data. The historical data for applicable initiating events is sorted into categories for calculating the initiating event frequencies for the specific initiating events, in general, the initiating event frequency is determined based upon the number of initiating events per year from the historical data. Once the historical data used in the calculation of initiating event frequency for a specific event was identified, the historical data that is not applicable to the AP600 design for a specific initiating event is not included in the initiating event frequency calculation.
For some e, vents, the available historical data base was used to calculate the initiating event frequency. These events include the following:
l Core power excursion Loss.of RCS flow Loss of offsite power Loss of condenser Loss of main feedwater flow to both steam generators Spurious safeguards actuation l
As described in Chapter 2 of the PRA, the loss of offsite power initiating event frequency is j
based on the frequency provided in Appendix A of the Advanced Light 'Nater Reactor (ALWR)
Utility Requirements Document. The value provided in the ALWR Utility Requirements,
Document is based on historical data, which is also provided.
For other initiating events, some of the historical data was not applicable to the AP600 design.
For these events, the non-applicable data was removed from the calculation of the initiating event frequencies. The events that included data that was inappropriate for the AP600 include the following:
Transient with main feedwater flow i
I Spurious reactor trip Turbine trip Loss of main feedwater flow to one steam generator Loss of main feedwater flow to both steam generators Total loss of main feedwater flow Secondary to primary power mismatch 1
i Draft for Revision 1. october 1996 mA3284w-3.wpf:1b-101496
l l
3-8 t
l For example, the loss of main feedwater event data was removed to exclude events where a plant trip occurred following the loss of a single main feedwater pump. This is due to the AP600 design that allows for continued plant operation following a loss of one main feedwater pump. For loss of main feedwater esents and secondary-to-primary power mismatch events, an adjustment was made in calculating the initiating event frequencies to account for the lower number of steam generators in AP600. Since AP600 has two steam generators and the i
histoncal data includes data from plants that have more than two steam generators, an adjustment to the initiating event frequency is required to prevent calculation of an overly conservative initiating event frequency.
Several of the transient events have been grouped under similar transient event categories.
l These transients have been grouped for quantification purposes. This is possible since the plant response is identical for the group transients. For example the spurious trip and turbine trip events are grouped under the transient with main feedwater category.
l The initiating event frequencies for the seven initiating events listed above are impacted by l
the unavailability of various nonsafety-related SSCs. Chapter 2 of the PRA lists typical
]
nonsafety-related SSCs and associated malfunctions, identified in the historical data, and considered in the calculation of the AP600 initiating event frequencies for these initiators.
Since the seven initiating events consider nonsafety related SSCs in the calculation of the i
a initiating event frequencies, the response to Criterion 1 is "Yes."
l The unavailability of the nonsafety related SSCs impacts the number of events documented in the historical data and therefore, contributes directly to the calculation of the initiating event frequencies. However, some of the associated nonsafety related SSCs are more significant then Mhers in the calculation of the initiating event frequencies. Since some nonsafety related SSCs significantly affect the calculation of the initiating event frequencies, the response to Criterion 2 is "Yes."
l l
As shown in Tables 52-5 and 52-12 of the PRA, six of the seven initiating events discussed in this section do not significantly contribute to the core damage frequency and large release frequency for the focused PRA. These events include the following:
j Core power excursion
=
Loss of main feedwater flow to both steam generators l
Loss of RCS flow Loss of main feedwater flow to one steam generator Loss of condenser l
h Draft for Revision 1. october 1996 mA3284w-3.wpf:1b-101496
3-9 Since these six initiating events do not significantly contribute to the core damage frequency or large release frequency, the response to Criterion 3 is "No." Therefore, the nonsafety-related SSCs associated with these events are not considered to be important with respect to the effect on these six initiating events.
As shown in Table 62 5, the transient with main feedwater flow event does not significantly contribute to the core damage frequency for the focused PRA sensitivity study. However, as shown in Table 52-12, the event is within the cutoff range to the large release frequency.
Since the transient with main feedwater initiating event category contribution to the large release frequency is near the cutoff limit, the response to criterion 3 is "Yes."
The respon'ses to Criteria 1,2, and 3 for the transient with main feedwater initiating event is
~
"Yes.".Therefore, the nonsafety-related SSCs, that are required for normal at-power operation, associated with this event are important with respect to the effect on this initiating event. Section 10 provides a list of important SSCs, the functions they perform and the proposed regulatory oversight recommendations.
3.6 ANTICIPATED TRANSIENT WITHOUT SCRAM As identified in Chapter 2 of the PRA, there are three A'lWS initiating events considered in the a core damage frequency and large release fregt.ancy calculations for the PRA. The three initiating events are comprised of the following:
ATWS precursor without main feedwater flow ATWS precursor with safeguards actuation ATWS precursor with main feedwater flow available The calculation of the individual ATWS initiating event frequencies is outlined in Chapter 2.
As can be seen from the ATWS events descriptions in Section 2.2.4, the calculation of all three of the ATWS initiating event frequencies consider nonsafety related SSCs. Since the ATWS initiating events considers nonsafety-related SSCs in the calculation of the initiating event frequency, the response to Criterion 1 is "Yes."
The probability of an ATWS event is the combination of the probability of the initiating events as described in Se,ction 2.2.4 of the PRA and the probability of a failure of safety related SSCs to insert control rods. The actual failure probability of the automatic and manual reactor trip is not included in the ATWS frequency calculation; however, automatic and manual reactor trip are modeled as top events in the ATWS event trees, Chapter 4. Therefore, if either the automatic or manual reactor trip are successful, the event does not develop into an ATWS event. Since the failure of safety related SSCs determines if an initiating event develops into Draft for Revision 1; october 1996 mA3284w-3.wpf;1b 101496
i 3 10 1
'an ATWS event, nonsafety-related SSCs do not significantly affect the calculation of the 1
probability of an ATWS, the response to Criterion 2 is 'No."
Since the response to Criterion 2 is "No," the nonsafety-related SSCs associated with this
]
event are not considered to be important with respect to their effect on this initiating event.
3.7 MISCELLANEOUS SPECIAL INITIATORS j
For several initiating events, plant-specific fault trees were developed and evaluated for the specified nonsafety-related systems to determine the initiating event frequencies for these events. Thpse miscellaneous events are typically referred to as special initiators and they
]
include the following:
Loss of component cooling water / service water
=
Loss of compressed and instrument air j
=
Since the plant response to the loss of service water or component cooling water is the same, the initiating event frequencies are combined under the same initiating event category.
Since these initiating events consider nonsafety-related SSCs in the calculation of the mitigating event frequencies, the response to Criterion 1 is "Yes."
The unavailability of these nonsafety-related systems completely determines the initiating event frequency of the associated special initiator. For example, if the overall component cooling water system unavailability ir creases by a specified amount, the initiating event frequency directly increases by this same amount. Since nonsafety-related SSCs significantly affect the calculation of these initiating event frequencies, the response to Criterion 2 is "Yes."
i As shown in Tables 52 5 and 52-12 of the PRA, these initiating events are not important contributors to the core damage frequency and large release frequency for the focused PRA.
Since these initiating events do not significantly contribute to the core damage frequency or large release frequency, the response to Criterion 3 is 'No."
Since the response to Criterion 3 is "No," the nonsafety related SSCs (that are required for normal plarit power operation) associated with these events are not considered to be
- important with respect to their effect on these four initiating events.
Draft for Revision 1. october 1996 m:\\3284w-3.wpf;1b-101496
.. ~ -. - - -. -. - - -. _
3-11
' SHUTDOWN INITIATING EVENTS 3.8 SHUTDOWN LOCA For the shutdown LOCA event, as with the at-power LOCA events, the initiators are caused by piping leaks and breaks, with one exception. As discussed in Chapter 54, the PRA methodology identifies the pipe segments and calculates the initiating event frequency based on a basic failure rate for these pipe segments. The shutdown evaluation considers a number of additional normal residual heat removal system pipe segments in containment that are not included in the at power calculation (almost three times as many pipe segments). For the shutdown L,0CA event, the normal residual heat removal system is assumed to be the source of the LOCA. The normal residual heat removal system piping is safety-related. Therefore, 1
no nonsafety related,SSCs are considered in the calculation of the initiating event frequency from piping breaks or leaks.
An additional mechanism that contributes to the initiating event frequency is included in the calculation of the initiating event frequency for shutdown LOCAs. The calculation includes the potential for inadvertent operator opening of the normal residual heat removal system discharge valve (s) to the in-containment refueling water storage tank. This mechanism reduces the reactor coolant system inventory by diverting flow from the shutdown cooling a
flowpath. This operator error in this initiating event mechanism has no relationship to nonsafety-related SSCs.
The piping sections for the nonsafety-related normal residual heat removal system included in the initiating event frequency calculation are designed using safety-related piping. The potential for the operator to erroneously initiate a loss of reactor coolant system inventory is not related to the unavailability of nonsafety-related SSCs. Since this initiating event does not consider nonsafety related SSCs in the calculation of the initiating event frequency, the response to Criterion'1 is "No." Therefore, nonsafety related SSCs are not considered to be important with respect to their effect on this initiating event.
3.9 SHUTDOWN LOSS OF OFFSITE POWER A calculation was completed to determine the initiating event frequency for the loss of offsite power during shutdown conditions. The calculation uses the initiating event frequency for this event from the historical data with an adjustment for the length of time spent in shutdown conditions. The calculation considers effects from onsite nonsafety related systems such as the transmission switchyard and the main ac power systems, as well as the offsite power system. Since this initiating event considers nonsafety-related SSCs in the calculation of the initiating event frequency, the response to Criterion 1 is "Yes.'
Draft for Revision 1. october 1996 m:\\3284w 3.wpf 1b 101496
~
3-12 A review of the historical data'used in the calculation of initiating event frequency for this event shows that the contribution from onsite nonsafety-related SSCs such as transformers, high voltage switchyard circuit breakers, or the main ac power circuit breakers are significant j
to the calculation of the initiating event frequency. Since nonsafety-related SSCs significantly affect the calculation of initiating event frequency, the response to Criterion 2 is "Yes."
As seen from Table 52-15 of the PRA, this event during RCS drained conditions contributes to the core damage frequency for the focused PRA. Since this initiating event contributes to the core damage frequency, the response to Criterion 3 is "Yes."
I The responses to Criteria 1,2, and 3 for this initiating event are "Yes." Since the responses to all three criteria are "Yes," the nonsafety-related SSCs (that are required to provide offsite power during shutdown RCS drained conditions) associated with this event are important with respect to their effect on this initiating event. Section 10 provides a list of important SS'Cs, the j
functions they perform, and the proposed regulatory oversight recommendations.
i l
3.10 SHUTDOWN LOSS OF DECAY HEAT REMOVAL A plant-specific fault tree was developed and evaluated to determine the initiating event frequency for two initiating events that represent a loss of decay heat removal during a
shutdown conditions. The two initiating events include the loss of decay heat removal capability due to failure of the normal residual heat removal system and the loss of decay heat removal capability due to failure of the component cooling water or service water system. The evaluation for a loss of decay heat removal during shutdown is provided in Chapter 54 of the i
PRA report. This initiating event results from the loss of a nonsafety related system normally used to provide decay heat removal during shutdown conditions. The nonsafety related SSCs j
considered in this evaluation include the normal residual heat removal system, the component cooling water system, and the service water system. Gince this initiating event considers nonsafety related SSCs in the calculation of the initiating event frequency, the response to Criterion 1 is "Yes."
The unavailability of these nonsafety related systems significantly affects the initiating event frequency. Since nonsafety-related SSCs significantly affect the calculation of these initiating event frequencies, the response to Criterion 2 is "Yes."
1 As seen from Table 52-15 of the PRA, this event is an important contributor to the core damage frequency for the focused PRA. Since this initiating event contributes significantly to the core damage frequency, the response to Criterion is "Yes."
j The responses to Criteria 1,2, and 3 for this initiating event ar.e "Yes." Since the responses to all three criteria are "Yes," the nonsafety related SSCs (that are required for decay heat N'
~
3-13 removal during plant shutdown conditions) associated with this event are considered to be j
important with' re'spect to their effect on this initiating event. Section 10 provides a list of important SSCs, the functions they perform, and the proposed regulatory oversight recommendations.
3.11 REACTOR COOLANT SYSTEM OVERDRAIN For the reactor coolant system overdrain event, two scenarios have been identified for which this accident could occur. The first scenario is a combination of the failure of the hot leg level instruments and operator failure to recognize that the hot leg level instruments have failed.
Scenario two is the combination of a failure of the valves CVS-V045 and V047 to close on the receipt of a' closure single, in conjunction with an operator failure to recognize that the valves have not closed and manually isolate the valves. The failure mechanisms for these two scenarios are detailed in Chapter 2 of the PRA.
i i
The SSCs considered in the reacto.r coolant system overdrain event include the hot leg level instruments, valves CVS-V045 and V047, and protection and safety monitoring system to actuate the components. Since these SSCs are safety-related and the operator errors that contribute to this initiating event have no relationship to nonsafety-related systems, the response to Criterion 1 is "No."
The response to Criterion 1 is "No." Therefore, nonsafety-related SSCs are not considered to be important with respect to their effect on this initiating event.
3.12
SUMMARY
The results of the screening criteria application for each initiating event category are provided in Table 3-1. Section 10 provides a list of important SSCs, the functions they perform, and the proposed regulatory oversight recommendations for nonsafety-related SSCs that impact the calculation of the focused PRA sensitivity study initiating event frequencies, based on the initiating event criteria application.
Draft for Revisiori 1, october 1996 mA3284w.3.wpf;1b-101498
4 3-14 Table 31 Initiating Event Criteria Application ummmmmmmmmmmmmm-Initiating Event Category Criterion 1 Criterion 2 Criterion 3 Are nonsafety related Does the unavailability of Does the initiating event SSCs considered in the nonsafety-related SSCs significantly affect core the calculation of the significantly affect the damage frequency and initiating event calculation of the initiating large release frequency frequency?
event frequency?
for the focused PRA?
3.1 Main Steam Yes No N/A Line Stuck Open Safety Valve 3.2 RCS Leak Yes Yes No 3.3 LOCAs No N/A N/A 3.4 Secondary Yes Yes No Side Breaks 3.5 Transient Yes Yes Yes with MFW Flow Other Yes Yes No Transients 3.6 ATWS Yes No N/A 3.7 Miscellaneo Yes Yes No us Special Initiators 3.8 Shutdown No N/A N/A LOCA 3.9 Shutdown Yes Yes Yes loss of offsite power 3.10 Shutdown Yes Yes Yes loss of decay heat removal 3.11 Shutdown No N/A N/A RCS Overdrain Draft for Revision 1: October 1996 mA3284w-3.wpf:10-101496
. - _. -. _ = _ _. _.
3-15 l
Uneveliability of Critorion1 the sesociated Are nonsafety-related SSCe considered in nonsafety related the calculation of the initiating event S8Ce la not frequency?
Important to the focused PRA Yes UMd Crtterion 2 h6 Does the unavaliability of the nonsafety.
"' N related 88Ce signancently affect the No- -e SSCela not cetoule#en of the initieung event important to the
- - - - ~'
f m PRA m
Yes Unevailability of Crtterion 3 the==ww i
Does the initiating event signincently noneedsty related efloat ooes damage frequency and large S8 cele not frequency for the focused PRA importantto the focused PRA j
Yes Identify rc "2,. ' ^: ' SSCs, proposed missions, and proposed regulatory oweight.
Figure 3-1 Evaluation of Nonsafety-related SSCe impact on initiating Event Frequency Draft for Revision 1 October 1996 m:\\3284w-3.wpf;1b-101496
.