ML20059G956
| ML20059G956 | |
| Person / Time | |
|---|---|
| Site: | 05200001 |
| Issue date: | 11/03/1993 |
| From: | Fox J GENERAL ELECTRIC CO. |
| To: | Poslusny C Office of Nuclear Reactor Regulation |
| References | |
| NUDOCS 9311090208 | |
| Download: ML20059G956 (9) | |
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GENuclear Energy Genera!Dectre Comrey 175 Curtner Avenue, San Jr:se. CA 95125 November 3,1993 Docket No.52-001 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 ABWR Schedule - Primary Containment Pressure Control EPG-Iow Pressure Venting
Dear Chet:
Enclosed is a discussion paper on the subject EPG, which will be the topic of discussion for a NRC/GE conference call scheduled for November 4,1993.
Please provide copies of this transmittal to John Monninger and Mike Snodderly.
Sincerel,
c-1
. TW J ek Fox Advanced Reactor Programs cc:
Alan Beard (GE)
Norman Fletcher (DOE)
Cal Tang (GE)
JNF93-293 i
ORenen h
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i 9311090208-931103 47 PDR ADOCK 05200001 94 G
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ABWR "Early Venting" l
1 November 3, 1993 i
CONCERNS l
The NRC has raised a primary containment pressure control issue regarding the ABWR Emergency Procedure Guidelines (EPG) as follows:
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Since the ABWR has the Containment Overpressure System (COP), doesn't this I
obviate the need to vent the containment at low pressure?
r The main concerns expressed by the St.aff are:
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(1) Since the ABWR has no drywell to reactor building vacuum breakers, removing nitrogen from the containment by venting and then initiating containment sprays could cause the containment to implode.
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(2) For an isolation event with inadequate heat removal (e.g. the severe l
l accident TW sequence), the suppression pool will eventually saturate and steam j
will be the vented ef fluent.
If the reactor building HVAC system fails or'is isolated, then the reactor building could be filled with steam which threatens i
reactor building safety system equipment.
i Other concerns that were expressed by the Staff during-a conference call with j
GE on August 30, 1993 include:
i (3) The loss of nitrogen from venting: a) degrades conditions if sprays are l
subsequently initiated and, b) invalidates the EPG limit curves for other scenarios (e.g. DBA events).
i (4) Philosophically, venting should be a last measure to save the containment, j
not the first thing to do at the very early stage of an emergency.
j (5) The step which vents containment has no caution or direction on recovery actions.
l INTRODUCTION TO RESPONSE Prior to examining specific issues, it is appropriate to discuss the EPG intent regarding use of the vent and purge system for pressure control and to identify the equipment to be used for this purpose.
Inerting and de-inerting the primary containment requires use of the large. (22 inch) butterfly valves in the Atmospheric Control System (ACS).
For subsequent primary containment pressure control, only the ACS nitrogen makeup and bleed lines (2 inch).are used.
The concept of "early venting" conjures up opening large vent paths to I
the atmosphere.
To the contrary, use of the vent and purge, Standby Gas
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l Treatment (SGTS), and Reactor Building Heating, Venting and Air Conditioning (RBHVAC) systems in the EPG initial primary containment pressure control step l
intends use of the ACS pressure regulation features involving small lines and control valves with very limited flow areas.
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l BASIS FOR LOW PRESSURE VENTING l
The ABWR EPG Primary Containment Pressure Control (PC/P) step specifies-monitoring and control of primary containment pressure below the high drywell i
pressure scram setpoint (with s ue restrictions) using SGTS, RBHVAC, and the i
nitrogen vent and purge system.
The intent is to employ the systems normally used to control containment pressure.
This is the typical approach used in the initial steps of many sections of the EPG, where the first actions are to l
use normal operating systems to control the specific parameter.
Examples inclurle use.of drywell cooling to maintain drywell temperature below the LC0 or maximum normal operating temperature in DW/T, and use of j
condensate /feedwater to restore RPV water level in RC/L-2.
Simply stated, this step directs the operator to take actions that may already.
be underway prior to entry to the EPG to control primary containment pressure.
This assures that normal methods of control are employed first, before initiation of more complex (and sometimes more drastic) actions such as the subsequent containment spray and RPV depressurization steps in the primary containment pressure control guideline.
l These initial measures are typically prescribed within some relatively low normal operating range, after which more drastic actions are specified.
In the case of primary containment pressure control, venting is terminated when the primary containment pressure cannot be maintained below the high drywell i
pressure scram setpoint (if not already automatically isolated).
i Any entry condition to the primary containment control guideline requires parallel execution of all primary containment control EPG sections (SP/T, DW/T, PC/P, SP/L, and PC/H).
Eliminating part of the normal pressure control capability (i.e. venting) because of a single entry condition (e.g. high suppression pool temperature) is not appropriate.
l VENTING ACTIONS AND SYSTEM CAPABILITIES Step PC/P in the ABWR EPG is as follows:
Monitor and control primary containment pressure below the high drywell pressure scram setpoint using the following systems:
o SGTS, RBHVAC, and nitrogen vent and purge only if containment pressure is less than the SGTS and RBHVAC design pressure; use containment vent and purge operating procedures.
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When primary containment pressure cannot be maintained below the high drywell scram pressure scram setpoint, or the offsite radioactivity l
release rate reaches the offsite release rate LCO, isolate the primary containment vent and purge.
The conditions postulated in the severe accident TW sequence are that SRVs'are discharging steam at decay heat levels to the suppression pool with adequate' core cooling and drywell cooling being provided, but without suppression pool i
cooling.
Under these conditions, the suppression pool level and temperature i
will rise and the containment pressure will gradually increase.
Initial EPG i
actions will be to vantrol pool level while tryino to establish pool cooling.
Failing that, containment pressure would rise and actions to control i
containment pressure would be initiated.
i At several locations in the EPGs, the use of a specific procedure is invoked when a particular system is initiated.
This is done to assure that the system is operated correctly in situations where the system operating ' procedure is j
too complicated to be addressed in the top level (EPG) procedure. PC/P is a case in point where the direction is given to use the containment vent and i
purge operating procedure.
It is intended that the operating procedure will i
appropriately address priorities for use of systems and components of the vent and purge, SGlS, and RBHVAC.
In addition, these procedures will specifically j
address vent paths, valve lineups, system operation and limitations (pressure,-
temperature, moisture, radioactivity, etc.) for use of-these systems for venting the containment.
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While such detailed venting procedures have not been developed yet for ABWR, it is anticipated that the venting at low containment pressure would follow l
f the philosophy of the BWROG EPGs as documented in Appendix B to EPG Rev. 4.
This invokes use of normal containment pressure control systems.
LAt the "early venting" step, containment pressure is slightly above the normal range (still below the high drywell pressure scram setpoint), so the venting action would be limited to use of the drywell bleed line in the Atmospheric Control System (ACS).
This bleed line is in parallel with the -large upstream exhaust j
butterfly valve used to inert or de-inert the drywell.
This design feature i
provides the pressure relief path if containment pressure reduction is needed i
during normal operation. This feature is also used to bleed nitrogen from the containment during normal heatup. The drywell bleed line is manually operable from the control room.
Flow through the bleed line will be directed i
preferentially through the SGTS but also can be exhausted through the reactor j
building HVAC (RBHVAC), and will be monitored by the SGTS (or RBHVAC) flow and radiation instrumentation.
All ACS primary containment isolation valves, j
including the bleed line valve, are automatically closed if high radiation is i
detected in the exhaust flow.
l The ACS supplies makeup nitrogen to the containment through a common makeup line with separate branches to the drywell and wetwell.
Each of these l
branches has a valve which is open during normal operation.
These branches i
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' provide an interconnecting path between the wetwell and drywell which equalizes the pressure between the two primary containment volumes.
Increasing pressure in the wetwell would be relieved through this pathway, maintaining the drywell and wetwell pressures nearly equal.
Venting through the drywell bleed line will reduce the overall containment pressure.
This action assumes the absence of an isolation signal, or if previously present, the clearing of the signal and a reset of the isolation logic.
The large butterfly valves used to exhaust the containment atmosphere in.the inerting or deinerting process would not be opened at this step in the EPG J
unless necessary to use the RBHVAC to discharge the bleed line flow.
In this situation the second (downstream) butterfly valve must be open for access to the RBHVAC and the upstream butterfly valve would remain closed.
These butterfly valves are used for inerting and deinerting the primary containment and are not considered a part of the normal operating systems for containment pressure control.
However, the downstream butterfly valve could be opened if necessary to discharge the bleed flow through the RBHVAC instead of through the preferred SGTS path.
I EARLY VENTING PHILOSOPHY As discussed in the sections above, venting at Step PC/P is not a departure from normal operating procedures.
No interlocks need be bypassed to execute this step; however, if some previous trip had occurred and had subsequently cleared, the logic must be reset before this action can be taken.
Venting at this step is limited to containment pressures over the range of normal operation to the high drywell pressure scram setpoint.
Venting capacity is limited by the small (2" diameter) ACS drywell bleed line.
This vent path can easily be isolated by closure of a single valve operated from the control room.
If this valve were to fail open, i solation of this vent path is accomplished by closure of other isolation valves in the ACS or SGTS.
Any i
normal containment isolation signal will automatically isolate this vent path.
In the operating plants, venting the containment under normal eperation occasionally occurs under conditions such as: a) bleedoff during normal heatup, b) bleedoff in the event of a nitrogen leak into the containment, and c) for some plants, to adjust primary to secondary containment differential pressure as the atmospheric pressure changes with storm weather front movement
,L through the local area.
Other transients or upset conditions where this EPG pressure control step might be used include a very small break or steam leak into the drywell which 4
would gradually pressurize the containment or a stuck open SRV which would j
gradually heat the suppression pool and eventually begin to pressurize the j
containment if not closed.
Maintaining this capability for the operator under the mild conditions where containment pressure has not reached the high drywell pressure trip is-considered essential for normal operation and appropriate for. use in the initial step of containment pressure control.
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LACK OF CAUTIONS OR RECOVERY PROCEDURES i
When written in the EPGs, a caution has a specific meaning and purpose.
The j
intent of a caution is to alert the operator of a possible condition or consequence of the action being taken, but not to change or delay the action.
Cautions in the EPGs fall into one of two categories: a) conditions may be such that an instrument can not be reliably used,- or b) under certain conditions, equipment could be damaged as a result of the actions being taken.
for early containment venting, the conditions are only slightly above the j
normal value, the operator is using normal equipment and procedures, and the criteria above is not met.
Therefore, no cautions have been identified which i
are appropriate at this early corrective action step.
A recovery procedure is used when the parameter being controlled (e.g.
containment pressure) has been restored to the normal range.
For this step the recovery procedure would specify actions to properly terminate the venting action when no longer needed for containment pressure control.
The appropriate plant specific recovery procedure is part of the normal pressure control steps in the vent and purge system operating procedure and is not included in the EPGs.
3 If this initial EPG action is not successful at controlling pressure, the operator action is clearly stated within the step to: "... isolate the primary containment vent and purge", and the operator is required to continue to the next step in the pressure control procedure.
I INADVERTENT SPRAY AND POTENTIAL FOR CONTAINMENT IMPLOSION j
Protection against implosion is provided by two means in the EPGs: 1) limiting the conditions for spray initiation, and 2) defining conditions to terminate sprays.
The override to Step PC/P-2 requires that if while executing the following steps, containment sprays have been initiated and suppression chamber or drywell pressure drops below the high drywell pressure ' scram setpoint, containment sprays are to be terminated.
This action addresses the concern about containment depressurization arising from convective cooling provided by the spray.
Step PC/P-2 of the ABWR EPG directs the operator to initiate containment sprays, but only under very limited conditions.
First, the suppression chamber pressure must exceed the Suppression Chamber Spray Initiation Pressure (SCSIP) [about 9.6 psig].
The step further requires that the sprays be initiated only if:
(1) suppression pool level is below the elevation of the bottom of suppression pool-to-lower-drywell vent, and (2) drywell temperature and pressure are within the Drywell Spray i
Initiation Limit (DSIL)
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Both the SCSIP and the DSIL limit the pressure at'which sprays can be initiated. The minimum pressure and temperature values for the DSIL are about l
3 psig - and 115 F, respectively.
Observing the DSIL and the override. assures 4
that the containment pressure will not drop below atmospheric pressure and the
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containment will not implode.
The higher SCSIP value initiates sprays before conditions for chugging _ are reached and provides additional margin for the j
evaporative cooling pressure drop which occurs at the onset of spraying.
I In the particular TW sequence postulated where suppression pool cooling is not available, it is likely that containment sprays are also not available.
The ABWR Residual Heat Removal (RHR) :.ystem is used both for suppression pool cooling and for containment sprays.
Most types of RHR failures postulated i
would incapacitate both the suppression pool cooling and containment spray modes of operation.
t If a containment pressure control error is postulated, inadvertent initiation l
of sprays can lead to excess depressurization whether or not the containment i
has been vented.
Dry nitrogen assumed in the development of the DSIL maximizes the evaporative pressure drop, so any "early venting" tends to produce a lower evaporative cooling pressure drop at the onset of sprays.
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STEAM THREAT TO SAFETY EQUIPMENT IN REACTOR BUILDING i
i It is possible that the "early venting" action could maintain containment l
pressure below the containment isolation pressure setpoint for an extended period of time, allowing heatup of the suppression pool to the saturation temperature.
It is further possible to postulate a condition that while the containment effluent is being discharged via the RBHVAC (not the preferred i
path), the exhaust fans fail or the RBHVAC exhaust line is isolated.
Under l
these postulated conditions, steam would flow back through the RBHVAC exhaust i
inlet ducts through the fire dampers into the reactor building.
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While conceivable, this scenario is extremely unlikely due to the following j
considerations:
- 1) It is probable that the 2" drywell bleed line in the Atmospheric f
Control System (ACS) is not capable of exhausting decay heat levels-of-steam fl ow.
As such, the containment pressure would increase to the high drywell pressure scram setpoint and containment isolation would occur.
The operator i
is also instructed to manually isolate all the vent lines at this pressure.
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- 2) If such a steaming scenario was in progress, reactor building area i
temperature increases would be observed and failing automatic action, the operator would isolate all systems discharging into the high temperature areas in accordance with the secondary containment control guideline (Step SC/T-3).
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- 3) If the RBHVAC exhaust line was isolated, it is likely that the l
containment would be isolated also, including the downstream ACS exhaust betterfly valve and the ACS drywell bleed line because of common isolation
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Failure of any of the components in the vent path is highly unlikely because
.l the RBHVAC is designed for volumetric flow rates that are much larger than the limited vent flow from the small drywell bleed line with a low driving l
(containment) pressure.
Containment pressure must be low (less-than the l
isolation setpoint pressure of 1.7 psig)~ to do the venting; otherwise the vent path would be isolated.
If an isolation failure is postulated in which a selected set of isolation valves close, such as closure of the RBHVAC isolation valves but not the ACS isolation valves, failure of any component-due to overpressure is still not likely.
Under these conditions the vent effluent would leak back through the RBHVAC exhaust inlet ducts through the fire dampers into the reactor building.
independent of the small likelihood of filling the reactor building with.
I steam, all the safety system equipment is designed and qualified for a steam
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environment at 15 psig for at least 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
Specifically, the High Pressure.
Core flooder (HPCF) system including the pump, motor, instruments, and control electrical equipment (including cables and sources of electricity) is designed-i and qualified for a steam environment at a temperature of 248 F at about 15 psig for at least 6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br />.
Therefore, even under these selected additional I
failure assumptions, it is very unlikely that such a failure would result in loss of the HPCF system.
NITROGEN LOSS AND POTENTIAL TO INVAllDATE EPG LIMITS The technical bases for the EPG curves and limits are described in detail 'in Appendix C to Revision 4 of the EPGs.
The curves and limits affected by the amount of nitrogen in the containment are the Heat Capacity Temperature Limit (HCTL), the Drywell Spray Initiation Limit (DSIL), and the Suppression Chamber Spray Initiation Pressure (SCSIP).
All of these assume a nitrogen atmosphere as part of the basis for the calculations.
The HCTL is used to assure that the final pressure at the end of a blowdown will not exceed the Primary l
Containment Pressure Limit.
The DSIL assures that the spray evaporative l
pressure drop does not exceed either the drywell-below-wetwell pressure i
capability or the high drywell pressure scram setpoint.
The SCSIP assures l
that the conditions for chugging in a LOCA event will be avoided if sprays are initiated.
In the development of these limits, consideration was given to all environmental conditions possible in the containment atmosphere and the limits are cased on worst case considerations.
Purging any amount of noncondensibles l
would make application of any of these limits more conservative.
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SUMMARY
AND CONCLUSION i
The pressure control action as specified in Step PC/P of the ABWR EPG has been I
characterized as "early venting".
A better description of this action is one i
of normal containment pressure control by means of a small bleed flow.
The l
EPG philosophy and the basis for containment pressure control have_been discussed along with the EPG intent of system equipment to be used for this purpose.
The identified Staff concerns have been evaluated in light lof this i
information and it has been concluded that the ABWR containment pressure control step is appropriate.
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