ML17319A961

Background Info for Reactor Vessel Head Vent Operation.

ML17319A961
Person / Time
Site:	Cook
Issue date:	02/28/1981
From:	INDIANA MICHIGAN POWER CO. (FORMERLY INDIANA & MICHIG
To:
Shared Package
ML17319A959	List: ML17319A958 ML17319A960 ML17319A961 ML17319A962 ML17319A963 ML17331A772
References
PROC-810228, NUDOCS 8107210423
Download: ML17319A961 (15)

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BACKGROUND INFORMATION FOR REACTOR VESSEL HEAD VENT OPERATION REVISION 0 FEBRUARY 1981 8i072i0423 8i07i5 PDR ADOCK 05000315 P PDR

REACTOR VESSEL HEAD VENT OPERATION I. GENERAL DISCUSSION A. ~Pur ose The operator actions and precautions specified in this guideline are those instructions necessary for the removal of'gases from the reactor vessel head by operation of the reactor vessel head vent system.

8; Guideline Assumptions The specific design of the reac .or v sse'.',-ead vent sys-;ea used in this guideline includes a sirg!e cornec.'.ion to ~i:a vessel head with redundant flow paths and isolation valves

. extending from a common line. The common line includes a 3/8".orifice which limits the flowrate to within the makeup capability of the chemical and volume control system.. The redundant flow paths discharge to the reacto~ containment building. Note that those plants, which have vent systems installed which vent to the pressure~ er or th "r=ssur-.=er relief tank may desiro to modify or make 'd"i:icos -o :he existing guideline in order to incorporate the plant specific design.

2. Although the guideline does not require a reactor vessel level system it is recomnended that any venting operation be performed in con5unction with an accurate vessel level indication for both with and without reactor coolant pump operation. For plants without a level system, the performance of Appendix A, "RCS Gaseous Void Detection and Sizing" may provide an estimate of the total volume of gaseous voids in the RCS (other than the pressurizer).

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3. The reactor coolant system can be stabilized with a constant pressurizer level and adequate reactor coolant sub-cooling established. These conditions are required to ensure adequate core-cooling is maintained during the venting period.

4. The pressurizer level and pressure requirements throughout the guideline do not include error allocations due to an adverse contaihment environment. Therefore, it is assumed.-that containment temperature is near normal operating conditions.

5. Any venting operation must be p'rformed prior to the initiation-

'of safety injection flow throttl'ing during a POST-LOCA cooldown and depr essurizaticn "perat on. Sever.l g" Ide;inc actions require she initiation o safe-:.y ~n'ect".:-n ':- ressurizer level cannot be restore . The O'.ST-LOCA co .lccwn "nd d oresuri-zation operation begins to throttle safe-.y injecti"n .low and therefore full SI flow could not be delivered to the RCS when required.

6. The head vent system is not desi'gned for and should not.

be used as the primary means .o<<mitigate an inadequate t core cooling event. The vent flcwpa.h '.s not si:ed to proviCe"'his capability and should only be used in con'unction witn the Inadequate Core Cooling Guidelines.

C. ~S ~ot~ms 0

The following are symptoms that may indicate the presence of a gaseous void in the reactor coolant system. Note that any one or combination of symptoms may indicate that a void exists.

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For Plants with a RV level indication:

1. Reactor vessel level indication less than (plant specific) percent of span. The plant specific value shoul'd include an allowance for normal channel accuracy. If the level system does not include a reference '1eg temperature compensator an additional allowance should. be included for conditions

'han where the RCS temperature is not equal to the channel calibration ~

'V temperature.

For Plants ~ ith/without a RY level indication:

2. Variations from the norm-1 pressur.,zer pressure and level rosponse Cue to::oman char"ing and.s".ray',ng over=.".".",.s may not be observea ir a gaseous void e" ists n "he .-":CS.

The pressurizer level may decrease dur',ng a RCS pressurization from charging due to gaseous void .contraction and level may rise rapidly during a spraying operation due to a gaseous void expansion.

3. An indication of reactor vessel head tempera'tures equal to or greater than saturation temperature warrants the presence of a steam bubble being generated in t.",e RCS.

'4. Gases in the reactor coolant system may result from several types of plant events. An accumulator tank" discharge or a core uncovery may result in non-condensible gases (e.g.

nitrogen and hydrogen) being trapped in the RCS. A rapid RCS cooldown may result in the vessel head temperature being greater than the primary saturation temperature and result in a steam bubble being developed. The operator should suspect the presence of gases in the RCS if any .of the above events occur.

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II. BASIS FOR SUBSEQUENT ACTIONS The CAUTION precedtng Step 'I earns the operator to matntatn the extsttng mode of core coo11ng durIng the performance of the subsequent actions. Tripping an operating reacto~ coolant pump could result in gases in the reactor coolant loops collecting in the steam generator U-tubes and may disturb natural circulation and primary to secondary heat transfer.'Starting reactor coolant pumps would disperse any gases already collected in the vessel head and make their removal more diffscult. Therefore, the existing status of the reactor coolant pumps should be maintained during the ventfng operation.

If possible and if conditions require so, then a reactor coolant pump should be started fo]lowing the compl. so>> of he venting operation.

The NOTE preceding Step identi.'es cer" n steps -(markea by an asterisk,'hich are not applicable if safety in~mection has been initiated and these steps should be skipped during the venting operatione

l. Once a gaseous void is detected or suspected in the RCS, then any changes being made to the prfnary system should be termfnated and a steady-state condition should be established. This step refers to events like a FCST-LOCA cooldown, a normal plant cooldown, or a plant recovery from z.'esign basis event. The intent is to aIIow the RCS to stabHfze so that the size and position of the void can be determined. Note that a normaI-pressurizer pressure control condition'may not be attainable due to the reasons stated in symptom 2.

2. The first action taken to remove any gases from the RCS fs to attempt to recombine any condensfble gases by increasing RCS pressure. This step may be slow acting and if a upward trend fs witnessed on the vessel level indicator then mafntafn this mode until the head is refilled or the upward trend stops-this step is successful in filling the head, then return to the appropriate operating instruction. If this step is not successful, then proceed to Step 4.

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The CAUTION a'lerts the operator that charging fiou may result in a sudden col'lapse of steam bubbles in the RCS and cause a rapid decrease in pressurizer level. The level should be restored by increasing the RCS makeup flowrate. If the increased makeup flow fails to restore level then safety injection should be initiated and the operator is to proceed to EOI-O, Immediate Actions and Diagnostics.

3. The venting operation will result in RCS gases being vented to the containment. Therefore the c'ontainment purge and exhaust system should be isolated to prevent the release of any radioactive gases to the environment. All available containment air circula-.icn ecuipment si.oui " ".e s;ar".ad o pr evens-. any hydirog n from for-irg a gas oockez and to ersure a re"r sant tive hydro".Ten concentrat';n is obtained in S: p 5.

4. Increasing the reactor coolant sub-cooling 50'F above the minimum

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plant specific value ensures that reactor coolant sub-cooling will be maintained over the entire range of RCS operating conditions if the venting operation is terminated following a 200 psi decrease in RCS pressure. The preferred method- of obtaining the additional sub-cooling is increasing RCS pressur tsince this ~ill aid in condensing any steam bubbles presen".. If the additioral 5C'F sub-cooling is already established then proceed to Step 5.

5. The actions of Appendix S "Venting Time Period" determine the maximum allcwable time period for venting which will limit the containment hydrogen concentration to less than 3 volume percent.

This limit is required to prevent a potentially explosive hydrogen concentration from being developed inside the containment.

(This step may not be applicable for plants which vent to the pressurizer or pressurizer relief tank.)

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• 6. Pressurizer level is increased to greater than 50" for the purpose of maintaining RCS mass inventory during the venting period.. The operator is instructed to isolate letdown in "

order to obtain the level increase. Letdown will remain isolated until the venting operation is complete.

a7. This step and the following CAUTION earns the operator that RCS. pressure will decrease during the venting and if in.itial pressure is near the low pressure safety injection actuation setpoint, then SI may.be automatically initiated during the venting. The operator is instructed:to block the low pressur SI actuation if and/or when the block permissive is ene. g zed to prevent an inadvent n. SI.

• 8. Charging flow is incr sed .o maximum to limit the ne mass depletion of the RCS during the venting period. A se ond c~ar, r.'g',",

pimp should be started if it will provide additional make-.p flou. .

If the safety injection system is in;.operation then maintain the current plant configuration until the venting is complete.

The responses indicated in the NOTE will'provide the probable statJs of the RCS for those plants without a level sys:em.

maj also iden=ify the presence cf voids in the RCS other than the r eactor vessel head or pressurizer.

a) During a depressurization, any gaseous voids that exist in the RCS, other than the vessel hoad, will rapidly expand result. in an increase in the pressurizer level. 'nd b) .The orifice in the head vent discharge line is sized to limit water relief to within the make-up capability of a charging

'pump. Therefore, if no gases are present in the vessel head, then the vent flowrate wil.l match the charging flowrate and the pressurizer level will remain constant.

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c) The venting of gases will result in a rapid decrease fn pressurizer level due to the mass flowrate of the gases being greater than then the mass input being provided by the makeup flow.

9. Both isolation va1ves in one vent flowpath must be opened to initiate the venting operation. The NOTE instructs the

. operator to close both isolation valves in the flowpath ff..':

one or, both of the valves fail to open. 'The fsolatfon valves fn the redundant flowpath should then .be opened. Thfs prevents two flowpaths'being open if the failed valve suddenly opens.

10. The venting is 'terminated when the reactor vess=l head fs refilled or when the following criteria are me=.

b) lhe maximum time period allowable for venting whicn li..its ccntainment hydrogen concentration-to less than 3 volume percent (determined by Appendix B, "Venting Time Period" ).

c,d;e) These limiting conditions are consistent with the safety fngection re-initiation criteria of the emergency operating instructions. They provide suffjcient ope. ating margin for the venting an'd at the same time, provide i'm ts cn t.".e transient which ensures adequate'system control can be maintained. r f) Once the reactor vessel head is vented and refilled then water relief th~ough the vent path will begin. At this time, the rate of RCS depressurization and the rate of pressurizer level decrease should change any may even terminate. This may be used on an i'ndication that. the head has been 'refilled for those plants without a vessel level system.

Both isolation valves in the vent flowpath should be closed to terminate the venting.

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The CAUTION instructs the operator to maintain the RCS venting if loss of reactor coolant pump operation occurs. The venting is maintained to remove as much gas as possible from the vessel head during the RCP coastdown and onsought of natural'irculation.

This will minimize the amount of gas bubbles in the reactor coolant loops and steam generator U-tubes. The operator should refer to AOI-4 "Natural Circulation" to ensure adequate core coolin'g is being maintained.

• II. Nornial pressurizer pressure and level control is restored after the completion of the venting. A stable level and pressure should be maintained while, it is determined if further venting is reouired. I "12. If a 'gas bubble existed n the vessel head and the venting was terminated prior to the vessel head being comolezely ; f'.ll then *the operator should return to Step 4 and repeat the venting operation until the reactor vessel head has been completely ref i 1 I ed.

The NOTE alerts the operator that if~the time period for venting determined in Appendix 8 is met heft<re the vessel h ad '.s re-,illted,;

then the containment hydrogen concentration mus be reduced and a new venting period calcula:ed prior to performing additional .N venting. The hydrogen concentration could be reduced through the use,of the containment hydrogen re-combiners or by the purge

'.and exhaust system if radioactive gas concentrations are within limits. The new venting period will be based upon the reduced hydrogen concentration.

13. The operator should return to the appropriate operating instruction following the successful completion of venting 'the reactor vessel head. If, during subsequent actions, a gaseous bubble reforms in the vessel head, then the operator should return to Step I and repeat the venting operation.

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III. BASIS FOR APPENDIX A "RCS GASEOUS VOID DETECTION AND SIZING" If gases P

are present in the reactor coolant 'system, then the pressurizer pressure and level controls will not respond as they normally would. The total gas volume can be estimated by performing a routine pressurizer control operation and then comparing the expected results with. the actual results,,:This is the technique utilized in the fallowing steps.. If the safety injection system is in service, then the foll'owing steps are not applicable since normal pressurizer control will not be maintained.

1, The op"=rator is instructed to achievo =- .-.-able pressurizer pressure with normal controls being main a i.;ed.

2. System pressure and level are placed on .rend recorde. s "'.o achieve better accuracy for recording their valves. ;h cransie",-.

is not expected to exceed a 150 psi.or .'.1N of span charge in RCS conditions.

3.. These recordings will become the initial parameters in the follow'.ng calculation.

4. Letdown flow is isolated, pressurizer heaters are triooe",

and pressurizer spray is terminated to establish a condit".on where the pressurizer level will change only as a result of mass being injected into the RCS.

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5. The operator is instructed to allow the system pressure to increase 100 psi or the pressurizer level increase by 5 percent of span (pressurizer level is the preferred response). These conditions are obtained by a slow continuous charging rate.

6. These recordings will become the final parameters in the following calculation.'ebruary, 1981 Revision 0 Page 9

7. Normal RCS pressure and level controls are re-established and maintained until otherwise directed by steps of this instruction.

8. The operator is instructed to calculate the total volume of the pressurizer vapor space. The total cylindrical pressurizer vol.:me is the total volume of the pressurizer excluding the upper and lower sph'erical domes.

9. The total charged volume into the RCS is the difference between the (charging and seal injection flows) and the seal leakoff flow and then converted to cubic fe 't; An expected pressurizer level change can Le dei rmined the total charged volume in the preced'.rc s-.ep.

11. If the actual pressurizer level change is less than the expec.ed change (or if no level change was witnessed} then gaseous voids exist in the reactor coolant system. This is a result of the

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gaseous voids contracting when the pressure was increased by the charging flow. This will limit,or prevent a normal pressurizer, level increase. The void contractio;. may even be large enougr.

to cause ar. actual decrease in the pressuriz r level.

Step 12 should then be performed to estimate the total volume of the gas voids. in the RCS.

.12. The RCS void volume contraction, is equal to the change in pressurizer level converted to volume. Also the ratio of final void volume to initial void volume is equal to the ratio of initial RCS pressure to final RCS pressure. From these two equations the two unknown (initial and final RCS void volume) can be determined by inserting one equation into another. The if

.initial void volume calculated first and then fit into the volume/pressure ratio to determine the final void volume.

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IY . BASIS FOR APPEHOIX 8 YEHTING TIi"IE PERIOD'uring a core uncovery event, there exists the potential for a significant amount'of hydrogen generated in the core which could be trapped in the reactor vessel head and released to the containment atmosphere during the venting operation. The containment hydrogen concentration is limited to less than 4 volume percent to prevent a potential explosive mixture with oxygen, therefore, the amount of hydrogen that,.

can be vented to the containment is restricted. A maximum allowable time period for venting is determined to limit the containment hydrogen

..'oncentration.

1. The to.al containment volume in cubic feet is -.'rs= de:a~ ne" and then conv rted to stancard .e..oera..r'e and orossure c"ndit ons.

Ho.e "'..at the pressure ~erm for :ne convet sion is i..iy a" iicabie to sub-atmospheric containments and can be deleted  ;-'ar ='..e remaining :

plants.

'2, The containment hydrogen concentration, is then determined in volume percert'units. This value can be found by direct sampling or by hydrogen monitors. Sufficient time should be allowed for the air circulation equipment to mix the containm nt a--..osph re prio~ to sampling in or"er to determine a representa ive concentration. a The NOTE identifies to the operator that the containment hydrogen concentration will be insignificant if there has been no leakage from the RCS to the containment. The operator may assume the .

HZ concentration to be. 0 volume percent.

3. The maximum volume of hydrogen that can be vented is calculated which will limit the containment hydrogen concentration to less than 3 volume. percent.

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