ML20128L734
| ML20128L734 | |
| Person / Time | |
|---|---|
| Site: | 05200001 |
| Issue date: | 02/16/1993 |
| From: | Fox J GENERAL ELECTRIC CO. |
| To: | Poslusny C Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9302190359 | |
| Download: ML20128L734 (6) | |
Text
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e DdCKET: 52-001 DATE: 1////93 NOTE T0: Document Control Desk FROM: Chester Poslusny, PM, NRR
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SUBJECT:
00CKETING OF ABWR INFORMATION RELATED TO DESIGN CERTIFICATION REVIEW Document Date: 2 /k/'7)
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February 16,1993 Docket No. STN 52-001
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Chet Poslusny, Senior Project Manager Standardization Project Directorate Associate Directorate for Advanced Reactors and License Renewal Office of the Nuclear Reactor Regulation
Subject:
Submittal Supporting Accelerated AllWR Review Schedule - Proposed Severe Accident Input
Dear Chet:
Enclosed is draft GE proposes as severe accident input into Tier 2. This list was developed by consideration of the key issues deemed as important to the ADWR ability to mitigate a severe accident. Reference was made to SECY 90-016 and its progeny to ensure that the full complement ofissues was examined.
Only a few of these items are suitable for inclusion in ITAAC. Based on discussions with both GE and NRC management, it is concluded that the crosstie for the firewater system, the containment overpressure protection system and the lower drywell flooder should be identified in ITAAC. Ilowever, given the state of the art in accident mant.gement, it is not appropriate to specify these systems in detail in ITAAC. The details should only be in Tier 2.
Please provide a copy of this transmittal to John Manninger and Bob Palla.
Sincerely, Y
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- J. k Fox X
. Advanced Reactor Programs
_ (([v' cc: Carol Buchholz (GE)
Jack Duncan (GE) fufr/
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No man Fletcher (DOE)
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5-ABWR Features to Mitigate Severe Accidents The ABWR has been designed to prevent the occurrence of a core damage accident.
In fact, the probability of a core damage accident is less than I ci,ance in 1 million.
This represents an improvement in severe accident prevention when compared to.
current plants. In the extremely unlikely event of a core damage accident, the ABWR containment has been designed with specific mitigating capabilities. These capabilities not only mitigate the consequences of a severe accident but also address uncertat Jes in severe accident phenomena. The capabilities are listed below along-with a discussion of the specific severe accident phenomena that the mitigation desise is addressing. The severe accident issues addressed are consistent with the issues discussed in SECY 90416.
Firewater Addition System This system not only can play an important role in preventing core damage, it is the primary source of water for Hooding the lowe) drywell should the core become damaged and relocate into the containment. The drywell spray mode of this system not only provides fbr debris cooling, but it is capable of directly cooling the upper drywell atmosphere and scrubbing airborne fission products. This system has i
suflicient capacity to cover the core det
+ vessel and provide debris cooling and l
scrub fission products released as a rem af continued core-concrete interactions.
The firewater addition system operating in the drywell spray mode will aho reduce the consequences of a suppression pool bypass or containment isolation failure. This is due to the fission product removal function performed by this mode of operation.'
Fission products will be scrubbed by the sprays prior to leaving the containment.
The firewater addition system has been sized to optimize the containment pressure response. The system is capable of delivering water to the containment up to the setpoint pressure of the COPS system. The flow rate, nominally 0.055 m3/sec at I
runout and 0.044m3/sec at the COPS setpoint, is suflicient to allow cooling of the core debris, while maximizing the time until the water level reaches the bottom of the vessel, at which point it is turned off.
Lower Drvwell Flooder The lower drywell flooder system has been included in the ABWR design to provide alternate cavity flooding in the event of core debris discharge from the reactor vessel and failure of the firewater addition system. This system is actuated from the melting.
of a fusible plug. The temperature set point for the plug is 533 K. The system consists of ten 4 inch diameter lines located about 4 m below the normal suppression pool l
water icvel discharging into the lower drywell about 1 m above the floor. Assuming only 9 of the 10 flooders open, the total Gooder flow would be 97 kg/s. By flooding after the introduction of core material, the potential for energetic core-water interactions during debris discharge is minimized. The Gooder will cover the core I
debris with water providing for debris cooling and scrubbing any fission products released from the debris due to core-concrete interactions.
7 CEB93-09
p Containment Overpressure Protection The COPS is part of the atmospheric control system end consists of two 8-inch diameter overpressure relief rupture disks mounted in series on a 14-inch line which connects the wetwell airspace to the stack. This system will provide for a scrubbed release path in the event that pressure in the containment cannot be maintained-below the structural limit. This controlled release will occur at a containment.
pressure of 0.72 MPa (90 psig). This system is beneficial for several of the severe accident issues. In cases with continued core-concrete attack, or those with no.
containment heat removal operational, the containment will pressurize. The COPS provides a controlled release path which will mitigate the fission product releases.
l This is an example of how uncertainties in severe accident behavior, i.e. debris coolability, are addressed by the ABWR design.
Vessel Depressurization Th. ABWR reactor vessel is designed with a highly reliable depressurization system.
This qstem plays a major role in preventing core damage, however, even in the event ;
of a severe accident, the RPV depressurization system can minimize the affects of 1 -
high pressure melt ejection. If the reactor vessel would fail at an elevated pressure, fragmented core debris could be transported into the upper drywell. The resulting heatup of the upper dowell could pressurize and fail the drywell. Parametric analyses performed in Section 19AE of the ABWR SSAR indicate that even in the event of direct containment heating, the probability of early drywell failure is quite low. The RPV depressurization system further decreases the probability of this failure mechanism.
Lower Drywell Design The details of the lower drywell design are important in the response of the ABWR -
containment to a severe accident. Six key features are described below.
Sacrificial Concrete The floor and walls of the ABWR lower drywell inchide a 1.5 m'eter layer of concrete-above the containment liner. This is to insure that debris will not come in direct contact with the containment boundary upon discharge from the reactor vessel. This.
added layer of concrete will protect the containment from possible early failure.
Basaltic Concrete The sacrificial concrete in the lower drywell of the ABWR has been constructed of low gas content concrete. The selection of concrete type is yet another example of how the ABWR design has striven not only to provide severe accident mitigation, but to also address potential uncertainties in severe accident phenomenon. Here, the uncertainty is whether or not the core can oc cooled by flooding the lower drywell.-
For scenarios in which the lower drywell flooder is unable to cool the core debris,- the concrete type selected will result in a very low gas generation rate. This translates into a long time to pressurize the containment. This is important because time is one of the key factors in aerosol removal.
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- Sump Protection
-The lower drywell sumps are protected such that'a substantial ~ amount of core debris will not enter. This maximizes the upper surface area between the debris and the water and maximizes the potential to quench the core debris.
Increased Floor Area q
The floor area of the lower d well has been maximized to improve the potential for 9
2 debris cooling. The lower drywell floor area of 88 m exceeds the ALWR Utility '
Requirements Document criteria of 0.02 M2/MWth.
Wetwell-Drywell Connecting Vents The flow area between the lower and upper drywell~has been designed in a way toi allow adequate venting of gases generated in the' lower drywell. The connecting vents -
2 flow area is 11.25 m. This is important when considering the steam generation rates associated wi:.h fuel-coolant-interactions in the lower drywell.~
The path from the lower to the upper drywell includes several 90 degree turns. This-tortuous path enables core debris to be stripped prior to transport into the upper-drywell minimizing the consequences from high pressure melt ejection.- Also important when considering high pressure core melt scenarios, the configuration of.
the connecting vents will result in the transport of some core debris directly into the suppression pool. This is preferable to transport into the upper drywell and would result in the debris being quenched with only a slight increase in the suppression -
pool temperature.
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Solid Vessel Skirt The vessel skirt in the ABWR does not have any penetrations which would allow the -
liow of water from the upper drywell directly to the lower drywell. This ensures a very low probability that water is in the lower drywell before the time of vessel failure.
Thus, large sc.de fuel-coolant interactions are precluded.
b Inerted Containment One of the important severe accident consequences is the generation of combustible-gasses. Combustion of these gasses could increase the containment temperature and pressure. The ABWR containment will be operated inerted 'o minimize the impact from the generation of these gasses.
Cc,ntainment Isolation The ABWR containment design has striven to minimize the number of penetrations.
This impacts the severe accident response due to a smaller probability of containment isolation failure. All lines which originate in the reactor vessel or the -
containment have dual barrier protection which is generally obtained by redundant isolation valves.- Lines which are considered non-essential in mitigating an accident:
isolate automatically in response to diverse isolation signals. Lines which may be e
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usefulin mitigating an accident have means to detect leakage _or breaks and may be isolated should this occur.
. Upgraded Low Pressure Piping
- The low pressure piping in the ABWR has been upgraded to withstand higher pressure, This reduces the probability of an interfacing system LOCA and the severe' accident consequences associated with such an event.-
Drywell-Wetwell Vacuum Breakers-The ABWR contains eight 20-inch diameter vacuum breakers which provide positive position indication in the control room. They have also been located high in the wetwell _to reduce potential loads occurring during pool swell. The result of the vacuum breaker design in the ABWR is to reduce the potential for suppression pool' bypass.
Overall Containment Performance The design of the ABWR containment provides for holdup and delay for fiu5 ion product release should the containment integrity be challenged.- Long term containment pressurization is governed by the generatioa of decay heat and non -
condensabic gases. The primary source of non-condensable gas generation is metal-water reaction of the zirconium in the core. This is accommodated by a relatively-large containment volume. 'ihe mitigating systems' discussed above ensure that the decay energy results in steam production. The suppression pool and substantial containment heat sinks absorb this energy, resulting in very slow containment j
response which ensure ample time for fission product removal.
Key Severe Accident Modelling Parameters Table 1 provides a list of key severe accideu modelling parameters. This list has been l
derived from the discussions presented above and from a variety of ABWR severe l
accident evaluations.
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CEB9&O9
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,m Table 1 -
Key Severe Accident Parameters -
Parameter
. Relates to Description Value What Feature?
Core Power 3926 MW Containment Performance El. of Top of Fuel -
9.05 m Containment Performance Normal Water Level 13.26 m Containment Performance-ADS Area
.07m2
- Vessel Depressurization-Total Zr in Core 72,550 kg Containment Performance Concrete Type Basaltic
' Basaltic' Concrete Compartment Volume Lower Drywell 1860 m3 Containment Performance Upper Drywell 5490 m3 Containment Performance Wetwell 9585 m3 Containment Performance Floor Area Lower Drywell 88 m2 Lower Drywell Flooder Upper Drywell 610 m2 Containment Performance Wetwell 507 m2 -
Lower Drywell Flooder -
Overflow Elevation LDW to Wetwell
-4.55 m
- Lower Drywell Flooder UDW to Wetwell 7.35 m Firewater Addition System Heat Sink Surface Area
. Lower Drywell -
589 m2 Containment Performance Upper Drywell 1720 m2 Containment Performance Wetwell 2348 m2 Containment Performance LDW to UDW vent area 11.3 m2 Connecting Vents Lower Drywell Flooder Elevation
-10.5 m Lower Drywell Flooder Area
.073 m2 Lower Drywell Flooder Plug Temperature 533 K-Lower Drywell Flooder L
Suppression Pool Mass 3.6 x 106 kg Containment Performance CEB934)9