HITACHI GE Hitachi Nuclear Energy James C. Kinsey Vice President, ESBWR Licensing PO Box 780 M/C A-55 Wilmington, NC 28402-0780 USA T 910 675 5057 F 910 362 5057 [email protected]Docket No. 52-010 MFN 06-301 Supplement 2 January 29, 2008 U.S. Nuclear Regulatory Commission Document Control Desk Washington, D.C. 20555-0001 Subject: Response to Portion of NRC Request for Additional Information Letter No. 31 RAI Number 21.6-4 Supplement I The purpose of this letter is to submit the GE Hitachi Nuclear Energy (GEH) response to the U.S. Nuclear Regulatory Commission (NRC) Request for Additional Information (RAI) sent by the Reference 1 NRC letter. GEH response to RAI Number 21.6-4 Supplement 1 is addressed in Enclosure 1. If you have any questions or require additional information, please contact me. Sincerely, as C. Kinsey - President, ESBWR Licensing
U.S. Nuclear Regulatory CommissionDocument Control DeskWashington, D.C. 20555-0001
Subject: Response to Portion of NRC Request for AdditionalInformation Letter No. 31 RAI Number 21.6-4 Supplement I
The purpose of this letter is to submit the GE Hitachi Nuclear Energy (GEH)response to the U.S. Nuclear Regulatory Commission (NRC) Request forAdditional Information (RAI) sent by the Reference 1 NRC letter. GEH responseto RAI Number 21.6-4 Supplement 1 is addressed in Enclosure 1.
If you have any questions or require additional information, please contact me.
Sincerely,
as C. Kinsey -President, ESBWR Licensing
MFN 06-301 Supp 2Page 2 of 2
Reference:
1. MFN 06-203, Letter from U.S. Nuclear Regulatory Commission to DavidHinds, Request for Additional Information Letter No. 31 Related to theESBWR Design Certification Application, dated June 23, 2006.
Enclosure:
1. MFN 06-301 Supp 2 - Response to Portion of NRC Request for AdditionalInformation Letter No. 31 - Related to ESBWR Design CertificationApplication - RAI Number 21.6-4 Supp 1
cc: AE CubbageGB StrambackRE BrownDH HindseDRF
USNRC (with enclosure)GEH/San Jose (with enclosure)GEH/VVilmington (with enclosure)GEH/Wilmington (with enclosure)0000-0071-8515
Enclosure 1
MFN 06-301 Supplement 2
Response to Portion of NRC Request for
Additional Information Letter No. 31
Related to ESBWR Design Certification Application
RAI Number 21.6-4 SO0
MFN 06-301 Supp 2 Page I of 7Enclosure I
NRC RAI 21.6-4 S01
This request asked General Electric (GE) to provide additional information on thedepressurization operations during an Anticipated Transient Without Scram (ATWS). The stafffinds the information that GE submitted in relation to Phenomena Identification and RankingTable (PIRT) ranking and models contained within TRACG for simulating depressurizationduring an ATWS complete for review. However, GE has not submitted any demonstrationcalculations of this event. Before the staff approves TRACG's capability of performing thiscalculation, it would need for GE to submit some demonstration calculations. GE indicated thatEmergency Operating Procedures (EOPs) have not been established at this time to instruct anoperator to depressurize during an ATWS event. Therefore the staff does not find it necessary toapprove this function of TRACG to support the ESBWR design certification. Should EOPs beestablished that instruct the operators to depressurize during an A TWS event, the staff would liketo evaluate TRACG demonstration calculations at that time to ensure TRACG's capability ofsimulating the event. If GE requests approval of this capability of TRACG at this time, GE willneed to submit demonstration calculations of this event.
GEH Response
Operator initiated depressurization is not expected during ATWS scenarios for the ESBWR.This is because the calculated suppression pool temperature is well below the Heat CapacityTemperature Limit (HCTL) curve for all limiting ATWS events, as reported in DCD Rev 4.
In order to study a postulated operator initiated depressurization behavior during ATWS,TRACG depressurization analysis results for the most limiting ATWS event, Main SteamIsolation Valve Closure (MSIVC) event, is provided in Appendix A. The MSIVC event wasreanalyzed with one significant exception. The Safety Relief Valves (SRVs) were held open sothat the vessel dome pressure vs. suppression pool temperature response has a similar slope tothe HCTL curve coincident with the initiation of the standby liquid control system (SLCS)injection at about 189 seconds into the transient. Holding the SRVs open simulated an operatorinitiated depressurization event.
Following the initiation of depressurization the reactor vessel pressure decreases. Additionally,the suppression pool temperature increases due the blowdown steam flow from the vessel. Therelative rates of vessel pressure decrease vs. suppression pool temperature increase are expectedto be consistent with the HCTL curve reported in Section 15.5.4 of DCD Tier 2. In operatingplants this curve is the design limitation of a plant's ability to depressurize. If the suppressionpool temperature exceeds the HCTL value for any given dome pressure the RPV could not besafely depressurized. As reported in DCD Tier 2, calculated suppression pool temperatures arewell below the HCTL curve for all limiting ATWS events.
MFN 06-301 Supp 2 Page 2 of 7Enclosure 1
Appendix A
TRACG depressurization analysis results for the most limiting ATWS event, Main SteamIsolation Valve Closure (MSIVC) event, are provided below.
The results shown in Figure 21.6-4 SOI-1 indicate that the slope of the suppression pooltemperature vs. the vessel pressure is very similar to the Heat Capacity Temperature Limit(HCTL) curve, and as depressurization proceeds, the rate of suppression pool temperatureincrease is less than the HCTL curve. This indicates that the as-designed ESBWR suppressionpool can adequately handle an operator initiated depressurization during ATWS.
Results for KEY parameters are listed in Table 21.6-4 SO1-1.
Table 21.6-4 S01-1Parameter Value Time (s)Maximum Neutron Flux, % 229.5 3Maximum Vessel Bottom Pressure, MPaG 9.22 6Maximum Bulk Suppression Pool Temperature, 'C 93.85 720Associated Containment Pressure, kPaG 268 720Peak Cladding Temperature, 'C 787.8 32Peak Dome Pressure, MPaG 9.08 19
Figure 21.6-4 SO1-2 presents both pressure and pool temperature behavior with respect to timefor the depressurization simulation. At beginning of the transient the pressure increases whenthe main steam isolation valve closure occurs and decreases about the time of SLCS injectiondue to the SRVs that are held open.
Figure 21.6-4 SO1-3 shows the fuel, void, boron, and total core reactivity during thedepressurization transient. As expected, the total reactivity values decreased after the SLCSinitiation. The fuel reactivity slightly increased in response to the decreasing power, while allother reactivites remained negative for the duration of the transient. With the injection of boroninto the core with the SLCS initiation, the core average boron concentration increased rapidly.
Figure 21.6-4 SO1-4 presents steam flow, feedwater flow, neutron flux, and average fueltemperature with respect to their rated values. At the beginning of the transient, right after themain steam isolation valves close, there was a pressure increase, as seen in Figure 21.6-4 S01-2.This pressure increase caused voids to collapse at the top of the core. This accounted for thespike in neutron flux at the beginning of the transient. Another notable point on the plot was thesignificant increase in steam flow at 189 seconds. This is attributed to holding the SRVs open.By 700 seconds average fuel temperature had returned toward nominal. Also at the end of thetransient, steam flow and feedwater flow tapered toward 0 %. Figures 21.6-4 SO 1-5, 21.6-4 SOI-6, 21.6-4 SO1-7, and 21.6-4 SO1-8 all show additional characteristics of the depressurizationtransient. Each of these plots followed logically from the events described in Reference [1] forATWS MSIVC cases and was consistent with the observed effects of depressurization describedearlier.
MFN 06-301 Supp 2Enclosure I
Page 3 of 7
Operator initiated depressurization is not expected during limiting ESBWR ATWS events. Itwas shown above that TRACG is capable of simulating the depressurization of the reactor in anATWS MSIVC event. The demonstration calculations are consistent with what was expected fora depressurization case.
Figure 21.6-4 S01-8. Neutron Flux and Core Average Void
Reference:1. TRACG Application for ESBWR Anticipated Transient Without Scram Analyses,
NEDC-33083P Supplement 2.
DCD Impact
DCD will not be changed as a result of this RAI.
LTR NEDC-33083P Supplement 2 will be revised to include a new Subsection 8.1.4 to describethe analysis presented in this RAI response as shown in the attached mark ups.
In order to study a postulated operator initiated depressurization behavior during ATWS, TRACGdepressurization analysis results for the most limiting ATWS event, Main Steam Isolation ValveClosure (MSIVC) event, is provided in this subsection. The MSIVC event was reanalyzed with onesignificant exception. The Safety Relief Valves (SRVs) were held open so that the vessel domepressure vs. suppression pool temperature response has a similar slope to the Heat CapacityTemperature Limit (HCTL) curve coincident with the initiation of the standby liquid control system(SLCS) injection at about 189 seconds into the transient. Holding the SRVs open simulated anoperator activated depressurization event.
Following the initiation of depressurization the reactor vessel pressure decreases. Additionally, thesuppression pool temperature increases due to the blowdown steam flow from the vessel. Therelative rates of vessel pressure decrease vs. suppression pool temperature increase are expected tobe consistent with the HCTL curve. The HCTL curve is shown in Figure 8.1-35. In operating plantsthis curve is the design limitation of a plant's ability to depressurize. If the suppression pooltemperature was in excess of the HCTL value for any given dome pressure the RPV could not besafely depressurized.
Operator initiated depressurization is not expected during ATWS scenarios for the ESBWR. This isbecause the calculated suppression pool temperatures are well below the HCTL curve for all limitingATWS events.
The depressurization case results in reactor shutdown at the beginning of depressurization and theHCTL curve adequately protects the containment from heat up during depressurization. This casedoes not need an uncertainly evaluation.
New Section 8.1.4 of LTR NEDC-33083P Supplement 2
Table 8.1-10 Sequence of Events for MSIVC Depressurization
Time (s) Event
0 MSIV Closure starts
0.3 Feedwater runback initiated
2 IC initiation
4 ATWS trip set at high pressure
5 SRVs open
21 Suppression pool cooling starts
47 Feedwater runback complete
54 Level drops below L2 set point
65 HPCRD flow starts
195 SLCS injection starts
368 Hot shutdown achieved
720 Peak pool temperature
715 High pressure design volume of borated solutioninjected into bypass
The key results from this analysis are presented in Table 8.1-11. and Figures 8.1-29 through 8.1-36.
New Section 8.1.4 of LTR NEDC-33083P Supplement 2
Table 8.1-11 Key Results for MSIVC Depressurization
Parameter Value Time
Maximum Neutron Flux, % 229.5 3s
Maximum Vessel Bottom Pressure, MPaG (psig) 9.22 (1337) 6s
Maximum Bulk Suppression Pool Temperature, 'C (°F) 93.85 (200.93) 720s
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