ML20079R805
| ML20079R805 | |
| Person / Time | |
|---|---|
| Site: | 05200001 |
| Issue date: | 02/07/1994 |
| From: | Fox J GENERAL ELECTRIC CO. |
| To: | Poslusny C Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9402140316 | |
| Download: ML20079R805 (7) | |
Text
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' GE Nuclear Enargy.
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ll5 n;.l% A, Wha 5to sa, CA 95125 i
February 7,1994 Docket No.52-001 j.
1 Chet Poslusny, Senior Proj,ect Manager Standardization Project Directorate Associate Directorate for Advanced Reactors and License Renewal Office of the Nuclear Reactor Regulation
Subject:
Subm.ittal Supporting Accelerated ABWR Schedule -
Response to Open Item F19.2.3.2.1-1
Dear Chet:
Enclosed is a SSAR markup addressing the subject open item pertaining to ACRS
.I concern with equipment tunnel protection.
Please provide a copy of this transmittal to John Monninger.
EJ Sincerely,
( _
h4 Ji -Fox Advanced Reactor Programs cc:
Joe Quirk GE Alan Beard GE Carol Buchholz GE Jack Duncan GE Norman Fletcher DOE) l
> 10000s JNfD4413 9402140316 940207
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23A6100 Rev. 3 ABWR standard Safety Analyais Report 00.0.1 Behavior of Access Tunnels if core debris is entrained out of the lower drywell and into the access tunnels, it is possible that the integrity of the tunnels could be compromised. This depends on several key factors, such as the amount of debris entrained into the tunnels, whether the debris remains in the tunnels, the heat transfer characteristics between the debris and the tunnel walls, and the strength and loading of the tunnel material.
00,0.1.1 Potential for Debris to Enter Tunnel Based on the configuration of the lower drywell and the equipment contained therein, it is highly unlikely that debris will be carried into the tunnels unless there is significant debris entrainment. Based on recent work at INEl, local failure of the lower head is expected. In fact, the drain plug located at center of the bottom head appears to be the dominant failure location. A localized failure should result in a concentrated discharge from the center of the lower head. Immediately below the reactor vessel are the CRD mechanisms. Splashing off of the CRDs is notjudged to result in a significant amount of debris transport to the tunnels. Since the debris is likely to be discharged from the center of the CRD array, radial movement through a foren of venical structures is not expected and transport of the debris outside of the CRD array is notjudged to be likely.
In fact, the CRDs will tend to columnate the flow, since they are long, vertically oriented and have little change in cross section along their length.
Approximately 6 meters below the bottom of the vessel is the equipment platform constructed from thin steel grating material. This grating is located at about the elevation of the tunnel bottom. No other structures exist at or above this elevation to divert the discharging debris into the tunnel. The grating surface area is small compared to the overall cross sectional area of the lower drywell and the thermal properties of the debris would result in immediate melting of the grate. Further, the center of the equipment platform, where the debris is likely to flow, does not have any grating to allow movement of the CRDs during refueling. Thus, the presence of the equipment platform is not expected to result in significant sphtshing of the debris into the tunnels.
Earlier work reported to the ACRS (Letter from Carol E. Buchholz to Dean llouston,
" Draft Response to issures Raised at the September 22-24,1993 Severe Accident Subcommittee hlecting licld in Portland, Oregon", CEB93-33, dated December 13, 1993) [ note this will be incorportated into the SSAR] concluded that, using Ishii's methodology, debris entrainment thresholds were only reached for high pressure melt ejection events with very large vessel failure areas. Based on work done at INEL and comained in the expert clicitations in NUREG 1150, the ABWR SSAR assigns a probability of.1 to a large (> 2 m2) vessel failure area. Combining this with the probability of a high pressure core melt with mel ejection, this scenario cor.sititutes t
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23A6100 Rev. 3
'ABWR Standard Safety Analysis Report only 2% of all core damage events. Thus, the potential for debris entrainment and the transport of debris to the access tunnels isjudged to be quite low for the ABWR.
00.0.1.2 Bounding Calculation Assuming Debris Enters Tunnel Bounding calculations are performed to address those very low probability scenarios in which debris is transported into the tunnels.
Each access tunnelis a circular steel tube, approxineately 11 meters long and 4.6 meters in diameter. The thickness of the steelis 2 cm. The bottom of the tunnelis located 7.4 meters below the bottom of the RPV. Since the low water level in the suppression pool is 6.2 meters below the RPV, the portion of the tunnel that will be in contact with core debris will be submerged. The opening from the tunnel to the lower drywell is a rectangle centered on the tunnel axis. The height of the personnel access is 3.55 meters and the height of the equipment access is 3.8 meters; both are 2.2 meters wide. Due to the reduction in area at the lower drywell wall intersection (i.e. the curb that is formed at the entrance to the tunnel), it will be assumed that any debris transported into the tunnel will remain there.
00.0.1.2.1 Amount of Debris Entrained into Access Tunnels As noted above, debris can only be entrained during a high pressure core melt scenario.
NUREG 1150 indicates an upper bound of 40% for the core debris that exits the vessel at the time of vessel failure, albeit at a low probability (10%). It isjudged more likely that only 10% will exit during the initial blowdown. From previous estimates of debris entrainment discussed above and the fact that the access tunnels are dead end volumes, it is likely that the detris will not be entrained into the tunnels. Ilowever, it is assumed that all of the debris exiting the vessel will be entrained along the lower drywell walls, and since each tunnels occupies 6.5% of the perimeter of the lower drywell,0.65% of the core debris,1675 kg, will enter each access tunnel during the blowdown. This debris is assumed to be instantaneously carried into the tunnel and will quickly spread to a depth of 3.5 cm. In addition,it is assumed to be molten with little superheat. This is consistent with the direct containment heating analysis contained in Attachment 19EA.
00.0.1.2.2 Heat Transfer to the Tunnel Wall As soon as the debris comes into contact with the tunnel shell, the interface will immediately assume a temperature that is between the steel and debris temperatures.
This initial contact temperature can be calculated by assuming that both the steel and delvis are semi-ir. finite slabs and equating the heat flux at the interface.
k, U,, n - T) k Ec.o4) 3 3
c (00-1) q',i =
=
d"cl t
st 2
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~*a 23A6100 Rav. 3 ABWR standard safety Analysis neport For Tc,o - 2500 K, and Tst,o = 373 K, the initial contact temperature is 1087 K. The interface will remain at this temperature until the thermal bounday reaches the outside of the shell; it will then increase (as will be discussed below) becausethe heat transfer to the water can not keep up with the heat supplied
- the debris.The thickness of the thermal tmundary can be expressed as 5,3,,,,, a Et It takes approximately 19 seconds for the thermal bounday to reach the water side of the tunnel shell The thermal boundary in the debris, on the other hand, takes more than 200 seconds to reach the upper surface of the debris.
In order for the steel shell to achieve steady state, the water on the outside of the steel shell must be able to remove heat as fast as it is supplied by the debris. Steady state conduction through steel 2 cm thick with surface temperatures of 373 K and 1087 Kis 6
2 1.07x10 W/m. The critical heat flux for a downward oriented horizontal plate at one 5
2 annosphere is only 4.5x10 W/m (Reference 1). Although the critical heat flux increases with pressure, steady state can not be achieved in this situation.
C0.0.1.2.3 Tunnel Wall Integrity in order to the effect of transient behavior simple, bounding calculations are performed to estimate the times associated with the transient. As the thermal boundary layers penetrate the materials, the magnitude of the heat flux at the interface is falling.
The time at which the debris begins to supply less than CIIF to the interface can be calculated by k AT c c (043) q" cur = bct Approximating the efTect of the heat of fusion by assuming the bulk corium temperature to be 3000 K and assuming the interface temperature remains 1087 K, approximately 194 seconds are required for the heat flux at the interface to drop to C11F. If the steel shell can maintain its integrity for longer than this time, the tunnel could remain intact.
The time for the entire thickness of steel to heat to 1200 K (the temperature at which the steel is assumed to lose all strength) can be compared to the time to supplyless than CIIF. If it is long compared to 194 seconds, the tunnel may remain intact. This time is computed by equating the total heat supplied by the debris to the heat transferred to the steel while the thermal boundary layer is growing plus the heat necessary to raise the steel from its steady state temperature profile to a uniform 1200 K. (Note that the heat transferred during the boundary layer growth takes into account, in a crude way, the
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23 61C0 Rev. 3 IABWR:
saniantsektyAnalysis neport
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i heat that is given up to the water prior to dryout. It is assumed that after 19 seconds, critical heat flux has been reached, and there is essentially no heat transfer to the water.)
t 19 OA s
-f9"c,odt = j 9"si,od t + Ps,Cp 3,j (1200-T,(x))dx
- (004) 3 0
0 0
The debris is assumed to be infinite during this time, and the steel is assumed to tiehave as an infinite slab during the first 19 seconds. It is also assumed that the contact -
temperature is constant, at 1087 K, during the entire time. Solution of thisindicates that the tunnel shell will reach 1200 K at approximately 46 seconds. Thus, this rather crude analysis indicates that the tunnel may fail in the unlikely event that debris is entrained.
00.0.1.3 Impact of Tunnel Failure Failure of the tunnel wall will occur at the lowest point. This will result in a flow path from the lower dnjwell vapor space into the suppression [x>ol. As indicated earlier, there willinitially be at least 1 meter ofwater above the booom of the tunnel.Thus, no fission product bypass of the pool will occur. Since the event Scing considered is a high pressure melt scenario with entrainment of debris, the operator niust initiate the firewater addition system in drywell spray mode to prevent high themperature failure of the drywell. This action will indirectly result in additional water being added to the -
suppression pool as itspills from the upper drywell, through the connecting vent system.
to the wetwell. Thus, several meters of water will be present above the tunnel failure -
elemtion to prmide scrubbing offission products.
00.0.1.4 Conclusion It is unlikely for core debris to be entrained or splashed into the access tunnels.
Approximately 2% of all core damage sequences could lead to debr's entering the i
tunrtels.
However, in the event that it does, the tunnel steel will reach temperatures that may compromise its integrity. The h<:at transfer through the thin steel wall is so high that the water on the outside of the tunnel quickly goes into dryout, and the heat can no longer
'(
be removed at a rate suflicient to maintain the tunnelintegrity.
Failure of the tunnel wall at the lowest point will result in a fission product release path j
into the suppression pool. Ilowever, since everal meters ofwater would likely be present above the tunnel failure site fission products would be scrubbed and no containment -
l;
. bypass would result.
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23A6100 Rev. 3 ABWR standant sarery Analysis acport 9
00.0.1.5 References 00-1 Theofanous, T.G., Additon, S., Liu, C., KymilAinen, O., Angelini, S., and Salmassi,T. (1993), 'The Probability of in-Vessel Coolability and Retention of a Core Melt in the AP600", DOE Draft, September 1993.
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Material Properties used in Tunnel Integrity Analysis Steel kst 30 W/m0K Pst 8000 Kg/m3 C st 550J/KgoK P
ast 6.8x104 m2 s
/
Debris kc 8 W/m0K~
PC 8000 Kg/m3
Pc 500J/Kg K o
ac 1.9x10-6 3
m /s
,