NRC Generic Letter 1981-21: Difference between revisions

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{{#Wiki_filter:UNITED STATES0 \NUCLEAR REGULATORY COMMISSIONWASHINGTON, D. C. 20555MAY 5 1981TO ALL LICENSEES OF OPERATING PRESSURIZED WATER NUCLEAR POWER REACTORS ANDAPPLICANTS FOR OPERATING LICENSES (EXCEPT FOR ST. LUCIE, UNIT NO. 1)Gentlemen:t'
{{#Wiki_filter:UNITED STATES0 \NUCLEAR REGULATORY COMMISSIONWASHINGTON, D. C. 20555MAY 5 1981TO ALL LICENSEES OF OPERATING PRESSURIZED WATER NUCLEAR POWER REACTORS ANDAPPLICANTS FOR OPERATING LICENSES (EXCEPT FOR ST. LUCIE, UNIT NO. 1)Gentlemen:t' SUBJECT: NATURAL CIRCULATION COOLDOWN(Generic Letter No. 8121-)On June 11, 1980, the St. Lucie Plant, Unit No. 1, was forced to cool downon natural circulation as a result of a component cooling water malfunction.During the cooldown process, abnormally rapid increases in pressurizer levelwere observed. Subsequent analyses have confirmed that these abnormal levelincreases were produced by flashing of liquid in the upper head of the reactorvessel, forcing water out of the vessel and into the pressurizer. A morecomplete description of the event and circumstances involved is provided iny-' the enclosure which includes a letter sent to the PWR NSSS vendors solicitingtheir opinions and comments on the significance of the event and phenomenon> 0 pin general.>' Based on our review of the event to date, we believe that core cooling was-never lost during the St. Lucie, Unit No. 1 event. That specific event doesnot constitute a direct safety concern. We have, however, identified two areasof concern applicable to all pressurized water reactors requiring prompt action:1. The Unacceptability of Vessel Voiding During Anticipated Cooldownoenditions INatura Circulation Due to LOSS Ot Osite Power, Coss ofPumps, etc.)Cooldown with a significant steam void in the vessel requires controllinga "two pressurizer" system, which is an undesirable challenge to theoperator. In fact, we are not aware of any training facilities (simulators)today which would allow an operator "hands on" experience in practicingsuch control. Moreover, it is our opinion that any significant vesselvoiding produced during controlled cooldown conditions increases thesusceptibility of the plant to more serious accidents. For these reasonsreactor vessel voiding during controlled natural circulation cooldownsshould be avoided.3§At47) UfJo2.67 /410.
 
SUBJECT: NATURAL CIRCULATION COOLDOWN(Generic Letter No. 8121-)On June 11, 1980, the St. Lucie Plant, Unit No. 1, was forced to cool downon natural circulation as a result of a component cooling water malfunction.During the cooldown process, abnormally rapid increases in pressurizer levelwere observed. Subsequent analyses have confirmed that these abnormal levelincreases were produced by flashing of liquid in the upper head of the reactorvessel, forcing water out of the vessel and into the pressurizer. A morecomplete description of the event and circumstances involved is provided iny-' the enclosure which includes a letter sent to the PWR NSSS vendors solicitingtheir opinions and comments on the significance of the event and phenomenon> 0 pin general.>' Based on our review of the event to date, we believe that core cooling was-never lost during the St. Lucie, Unit No. 1 event. That specific event doesnot constitute a direct safety concern. We have, however, identified two areasof concern applicable to all pressurized water reactors requiring prompt action:1. The Unacceptability of Vessel Voiding During Anticipated Cooldownoenditions INatura Circulation Due to LOSS Ot Osite Power, Coss ofPumps, etc.)Cooldown with a significant steam void in the vessel requires controllinga "two pressurizer" system, which is an undesirable challenge to theoperator. In fact, we are not aware of any training facilities (simulators)today which would allow an operator "hands on" experience in practicingsuch control. Moreover, it is our opinion that any significant vesselvoiding produced during controlled cooldown conditions increases thesusceptibility of the plant to more serious accidents. For these reasonsreactor vessel voiding during controlled natural circulation cooldownsshould be avoided.3§At47) UfJo2.67 /410.


-2-As described in the enclosure, vessel voiding at St. Lucie, Unit No. 1,was caused by the operator reducing system pressure such that thecorresponding saturation temperature dropped to the temperature of therelatively stagnant fluid in the reactor vessel upper head. Presently,primary system cooldown rates are based on vessel structural integrityconsiderations and do not explicitly consider avoiding production ofsignificant steam voids in the vessel. Moreover, cooldown rates arebased on fluid temperatures measured in the primary piping. As the St. LucieUnit No. I event has shown, these measured temperatures can in fact be onthe order of 100 degrees Fahrenheit or more lower than the upper head fluidtemperature, and, therefore, not indicative of the saturation pressure ofall fluid in the primary system.Under conditions which require cooldown on natural circulation and whenrapid depressurization is not necessary there may be a number of ways toavoid reactor vessel voiding. For example, a low cooldown rate can bespecified, coupled with "holding" the plant at intermediate conditions toallow the fluid in the upper vessel to equilibrate with the rest of theprimary system. However, avoidance of vessel voiding by lower primarysystem cooldown rates can increase the time required to achieve shutdowncooling entry conditions and thus increase the time auxiliary feedwateris depended upon to remove decay heat (specifically, for the loss-of-offsite power case). Thus, supplies of condensate-grade auxiliary feed-water must be considered if cooldown times are extended.2. Failure of the Operator to Have Prior Knowledge and Training for ThisEventThe cause of initial surges in pressurizer level at St. Lucie, Unit No. 1,was not immediately recognized or understood by the operator. Weattribute this to the fact that long-term natural circulation cooldownunder the specific circumstances of the event was never explicitlyanalysed by the NSSS vendor from the standpoint of trying to recognize aphenomenon such as that which occurred at St. Lucie, Unit No. 1. In theSt. Lucie event, the operator ultimately recognized the cause.of thelevel surges and was able to maintain control of the plant. Our concern,however, is the possibility of an operator taking incorrect action in aneffort to correct for an unknown event or unrecognized phenomena.We believe that proper procedures and training can provide the necessaryguidance to the operators both to avoid reactor vessel voiding as well asrecognize it when, and if, it occurs during controlled natural circulationcooldown. We are not sure if such procedures and training are in place atpressurized water reactor facilities.
-2-As described in the enclosure, vessel voiding at St. Lucie, Unit No. 1,was caused by the operator reducing system pressure such that thecorresponding saturation temperature dropped to the temperature of therelatively stagnant fluid in the reactor vessel upper head. Presently,primary system cooldown rates are based on vessel structural integrityconsiderations and do not explicitly consider avoiding production ofsignificant steam voids in the vessel. Moreover, cooldown rates arebased on fluid temperatures measured in the primary piping. As the St. LucieUnit No. I event has shown, these measured temperatures can in fact be onthe order of 100 degrees Fahrenheit or more lower than the upper head fluidtemperature, and, therefore, not indicative of the saturation pressure ofall fluid in the primary system.Under conditions which require cooldown on natural circulation and whenrapid depressurization is not necessary there may be a number of ways toavoid reactor vessel voiding. For example, a low cooldown rate can bespecified, coupled with "holding" the plant at intermediate conditions toallow the fluid in the upper vessel to equilibrate with the rest of theprimary system. However, avoidance of vessel voiding by lower primarysystem cooldown rates can increase the time required to achieve shutdowncooling entry conditions and thus increase the time auxiliary feedwateris depended upon to remove decay heat (specifically, for the loss-of-offsite power case). Thus, supplies of condensate-grade auxiliary feed-water must be considered if cooldown times are extended.2. Failure of the Operator to Have Prior Knowledge and Training for ThisEventThe cause of initial surges in pressurizer level at St. Lucie, Unit No. 1,was not immediately recognized or understood by the operator. Weattribute this to the fact that long-term natural circulation cooldownunder the specific circumstances of the event was never explicitlyanalysed by the NSSS vendor from the standpoint of trying to recognize aphenomenon such as that which occurred at St. Lucie, Unit No. 1. In theSt. Lucie event, the operator ultimately recognized the cause.of thelevel surges and was able to maintain control of the plant. Our concern,however, is the possibility of an operator taking incorrect action in aneffort to correct for an unknown event or unrecognized phenomena.We believe that proper procedures and training can provide the necessaryguidance to the operators both to avoid reactor vessel voiding as well asrecognize it when, and if, it occurs during controlled natural circulationcooldown. We are not sure if such procedures and training are in place atpressurized water reactor facilities.


-3 -Consequently, we request that you promptly review your current plant operationsin light of the St. Lucie, Unit No. 1 event and the discussions above andimplement, as necessary, procedures and training which will enable operatorsto avoid (if possible), recognize and properly react to reactor vessel voidingduring natural circulation cooldown.We conclude that the actions described above should be completed as soon asthey reasonably can be (i.e., within 6 months for operating reactors). Inaddition, so that we may determine whether your license should be amended toincorporate these actions as requirements, licensees of operating pressurizedwater reactors are requested, pursuant to §50.54(f), to furnish, within 6months of receipt of this letter, an assessment of your facility proceduresand training program with respect to the matters described above. Yourassessment should include:1. a demonstration (e.g. analysis and/or test) that controlled natural circula-tion cooldown from operating conditions to cold shutdown conditions, conductedin accordance with your procedure, should not result in reactor vesselvoiding;2. verification that supplies of condensate-grade auxiliary feedwater aresufficient to support your cooldown method; and3. a description of your training program and the provisions of your procedures(e.g. limited cooldown rate, response to rapid change in pressurizer level)that deal with prevention or mitigation of reactor vessel voiding.Applicants for operating licensees are requested to implement the subjectprocedures and training and provide the requested assessment within 6 monthsof receipt of this letter or 4 months prior to the staff's scheduled issuanceof its operating license Safety Evaluation Report, whichever is later.Please refer to this letter in your response.This request for information was approved by OMB under a blanket clearancenumber R0072 which expires December 31, 1981. Comments on burden and duplicationmay be directed to the Office of Management and Budget, Reports Management,Room 3208, New Executive Office Building, Washington, D.C. 20503.S\ncerely,.i'seDivision o LicensingOffice of Nuclear Reactor Regulation
-3 -Consequently, we request that you promptly review your current plant operationsin light of the St. Lucie, Unit No. 1 event and the discussions above andimplement, as necessary, procedures and training which will enable operatorsto avoid (if possible), recognize and properly react to reactor vessel voidingduring natural circulation cooldown.We conclude that the actions described above should be completed as soon asthey reasonably can be (i.e., within 6 months for operating reactors). Inaddition, so that we may determine whether your license should be amended toincorporate these actions as requirements, licensees of operating pressurizedwater reactors are requested, pursuant to §50.54(f), to furnish, within 6months of receipt of this letter, an assessment of your facility proceduresand training program with respect to the matters described above. Yourassessment should include:1. a demonstration (e.g. analysis and/or test) that controlled natural circula-tion cooldown from operating conditions to cold shutdown conditions, conductedin accordance with your procedure, should not result in reactor vesselvoiding;2. verification that supplies of condensate-grade auxiliary feedwater aresufficient to support your cooldown method; and3. a description of your training program and the provisions of your procedures(e.g. limited cooldown rate, response to rapid change in pressurizer level)that deal with prevention or mitigation of reactor vessel voiding.Applicants for operating licensees are requested to implement the subjectprocedures and training and provide the requested assessment within 6 monthsof receipt of this letter or 4 months prior to the staff's scheduled issuanceof its operating license Safety Evaluation Report, whichever is later.Please refer to this letter in your response.This request for information was approved by OMB under a blanket clearancenumber R0072 which expires December 31, 1981. Comments on burden and duplicationmay be directed to the Office of Management and Budget, Reports Management,Room 3208, New Executive Office Building, Washington, D.C. 20503.S\ncerely,.i'seDivision o LicensingOffice of Nuclear Reactor RegulationEnclosure: As statedcc: Service list  
 
£WI?.11[ESTA1ES ,Enclosure 1NUCLEAR RLGULATOR:Y CC),.Iil'(,:j.ASus:GIOM. D. C. 7055NA %U 12 1BOLetter sent to PWR NSSS Vendors:Westinghouse, Combustion Engineering andBabcock and WilcoxDear Mr.SUBJECT: VOID FORIATION IN VESSEL HEAD DURING ST. LUCIE NATURALCIRCULATION COOLDOWN EVENT OF 6/11/80On June 11, 1980, the St. Lucie reactor was shutdown due to a loss ofcomponent cooling water to the reactor coolant pump seals. This alsorequired shutdown of the reactor coolant pumps and c:oldown Was accomplishedby natural circulation.At approximately 4 hours into the event, charging flow, which Was initiallybeing divided between the cold legs and the auxiliary pressurizer spray, 1..asdiverted entirely to the auxiliary spray to enhance the depressurization andreduce the system pressure on the pump seals. At this time, abnormallyrapid increases in pressurizer level were observed which could not beexplained by the charging flow rate alone. Detailed evaluation andfollow-up analyses by the licensee and NSSS supplier have indicated thata steam void was probably formed in the upper head region of the reactorvessel and displaced water from the vessel into the pressurizer.Continued alternating realignment of charging flow between the cold legsand auxiliary spray line produced a 'saw-tooth" pressurizer level behavior.Relevant information and data available to the staff to date are providedin the enclosure.It has been postulated that the steam void in the upper vessel was producedwhen the system pressure dropped below the saturation pressure correspondingto the temperature of the fluid in the upper head. Because the measured hotand cold leg temperatures at the time of voiding here highly subcooled(-2000F), it appears -that the fluid. in the upper head was much hotter, relatively-stagnant, and in-poor communication with the fluid exiting the core and in theupper plenum. In addition, stored heat in the upper head structures mostlikely contributed to the voiding.Because of the unexpected occurrence of the void, the failure of the operatorsto immediately recognize the void formnation and take corrective action, andthe question of whether such void formation is properly accounted for in safety a -:.d * -;analyses (Chapter 15), we have sent a list of que~tions documuenting ourconcerns to the licensee. These questions are also provided in the-enclosure for your information.We are presently evaluating the need to pursue this issue generically withall PUR licensees. Prior to taking any definitive action however, we aresoliciting your technical opinion and advise regarding the potential forvoid formation under similar circumstances in USSS's designed by you.Specifically, we need to know if you can justify why the voiding phenomenoncannot occur in USSS's designed by you (or can confirm that such phenomenacan be properly predicted by your transient analysis models), and if it canoccur, is properly accounted for in operating procedures (e.g.. cooldowmrates), operator guidelines, and operator training (including the simulator)The urgency of this matter requires you advise us within fifteen (15)working days after receipt of this letter whether a supplemental informationsubmittal by you on the subject would preclude the need to expeditiouslypursue this issue generically with your customers.tu..binal Signd b1'sui S. MCA .Paul S. Check, Assistant Director forPlant SystemsDivision of Systems IntegrationOffice of Nuclear Reactor RegulationI : :. .-e. ...i_..-aI  
===Enclosure:===
IPra arc&UNITED STATESNUCLEAR REGULATORY COMMISSIONs XWASHINGTON. 0. C. 2CmJune 25, 1980***Docket No. 50-335LICENSEE: Florida Power & Light Company (FP&L)FACILITY: St. Lucie Unit No. 1SUBJECT: SUMMARY OF MEETING WITH FP&L AND COMBUSTION ENGINEERING (CE) REGARDINGST. LUCIE UNIT NO. 1 COOLDOWN ON NATURAL CIRCULATIONOn June 20, 1980 a meeting was held in Bethesda, MD regarding the June 11,reactor trip and cooldown on natural circulation at St. Lucie Unit No. 1.Significant points discussed are summarized below. Enclosure 1 is a list ofattendees.DiscussionA brief chronology of the event was presented with traces of parametersenclosed as Enclosure 2. The plant experienced a complete loss of componentcooling water (CCW) to the reactor coolant pumps (RCP). This loss was due toa direct short on the terminal board for a solenoid operated CCW common returnvalve from the RCP's. Plant procedures allow ten minutes to correctthe problem before securing the RCP's. Subsequently, the reactor was trippedand the RCP's secured. About three minutes after shutdown one RCP was runbriefly to aid natural circulation flow. The licensee stated that intermittentRCP leakoff flow alarms were actuated and that subcooled margin remained withinthe range 65 to 180'F subcooled during the cooldown.The licensee indicated that a steam bubble had formed in the head of the reactorvessel during cooldown. This was indicated by abnormal pressurizer level changesduring charging operations and was apparently due to elevated temperatures inthe hydraulically stagnant area of the upper head.The possibility of steam bubble formation was confirmed with an analysis by CEusing an in-house model. This model decoupled the vessel head region from therest of the reactor vessel with respect to temperature. As a result of theanalysis CE issued an information bulletin (Enclosure 2) warning of the longvessel head cooldown times which may be required (10-15 hours) and recommendingactions to maintain plant conditions until the vessel head has cooled should abubble form.The licensee indicated that while in the shutdown cooling mode of operation someprimary coolant may have leaked to the refueling water tank through LPSI lB pump'srecirculation line. The isolation valve in this line was found not to becompletely shut. The licensee stated that there were no indications of radio-activity release. While on shutdown cooling LPSI Pump lB was operatingin the injection mode with its recirc line open.
As statedcc: Service list  
£WI?.11[ESTA1ES ,Enclosure 1NUCLEAR RLGULATOR:Y CC),.Iil'(,:j.ASus:GIOM. D. C. 7055NA %U 12 1BOLetter sent to PWR NSSS Vendors:Westinghouse, Combustion Engineering andBabcock and WilcoxDear Mr.
 
SUBJECT: VOID FORIATION IN VESSEL HEAD DURING ST. LUCIE NATURALCIRCULATION COOLDOWN EVENT OF 6/11/80On June 11, 1980, the St. Lucie reactor was shutdown due to a loss ofcomponent cooling water to the reactor coolant pump seals. This alsorequired shutdown of the reactor coolant pumps and c:oldown Was accomplishedby natural circulation.At approximately 4 hours into the event, charging flow, which Was initiallybeing divided between the cold legs and the auxiliary pressurizer spray, 1..asdiverted entirely to the auxiliary spray to enhance the depressurization andreduce the system pressure on the pump seals. At this time, abnormallyrapid increases in pressurizer level were observed which could not beexplained by the charging flow rate alone. Detailed evaluation andfollow-up analyses by the licensee and NSSS supplier have indicated thata steam void was probably formed in the upper head region of the reactorvessel and displaced water from the vessel into the pressurizer.Continued alternating realignment of charging flow between the cold legsand auxiliary spray line produced a 'saw-tooth" pressurizer level behavior.Relevant information and data available to the staff to date are providedin the enclosure.It has been postulated that the steam void in the upper vessel was producedwhen the system pressure dropped below the saturation pressure correspondingto the temperature of the fluid in the upper head. Because the measured hotand cold leg temperatures at the time of voiding here highly subcooled(-2000F), it appears -that the fluid. in the upper head was much hotter, relatively-stagnant, and in-poor communication with the fluid exiting the core and in theupper plenum. In addition, stored heat in the upper head structures mostlikely contributed to the voiding.Because of the unexpected occurrence of the void, the failure of the operatorsto immediately recognize the void formnation and take corrective action, andthe question of whether such void formation is properly accounted for in safety a -:.d * -;analyses (Chapter 15), we have sent a list of que~tions documuenting ourconcerns to the licensee. These questions are also provided in the-enclosure for your information.We are presently evaluating the need to pursue this issue generically withall PUR licensees. Prior to taking any definitive action however, we aresoliciting your technical opinion and advise regarding the potential forvoid formation under similar circumstances in USSS's designed by you.Specifically, we need to know if you can justify why the voiding phenomenoncannot occur in USSS's designed by you (or can confirm that such phenomenacan be properly predicted by your transient analysis models), and if it canoccur, is properly accounted for in operating procedures (e.g.. cooldowmrates), operator guidelines, and operator training (including the simulator)The urgency of this matter requires you advise us within fifteen (15)working days after receipt of this letter whether a supplemental informationsubmittal by you on the subject would preclude the need to expeditiouslypursue this issue generically with your customers.tu..binal Signd b1'sui S. MCA .Paul S. Check, Assistant Director forPlant SystemsDivision of Systems IntegrationOffice of Nuclear Reactor RegulationI : :. .-e. ...i_..-aI IPra arc&UNITED STATESNUCLEAR REGULATORY COMMISSIONs XWASHINGTON. 0. C. 2CmJune 25, 1980***Docket No. 50-335LICENSEE: Florida Power & Light Company (FP&L)FACILITY: St. Lucie Unit No. 1
 
SUBJECT: SUMMARY OF MEETING WITH FP&L AND COMBUSTION ENGINEERING (CE) REGARDINGST. LUCIE UNIT NO. 1 COOLDOWN ON NATURAL CIRCULATIONOn June 20, 1980 a meeting was held in Bethesda, MD regarding the June 11,reactor trip and cooldown on natural circulation at St. Lucie Unit No. 1.Significant points discussed are summarized below. Enclosure 1 is a list ofattendees.DiscussionA brief chronology of the event was presented with traces of parametersenclosed as Enclosure 2. The plant experienced a complete loss of componentcooling water (CCW) to the reactor coolant pumps (RCP). This loss was due toa direct short on the terminal board for a solenoid operated CCW common returnvalve from the RCP's. Plant procedures allow ten minutes to correctthe problem before securing the RCP's. Subsequently, the reactor was trippedand the RCP's secured. About three minutes after shutdown one RCP was runbriefly to aid natural circulation flow. The licensee stated that intermittentRCP leakoff flow alarms were actuated and that subcooled margin remained withinthe range 65 to 180'F subcooled during the cooldown.The licensee indicated that a steam bubble had formed in the head of the reactorvessel during cooldown. This was indicated by abnormal pressurizer level changesduring charging operations and was apparently due to elevated temperatures inthe hydraulically stagnant area of the upper head.The possibility of steam bubble formation was confirmed with an analysis by CEusing an in-house model. This model decoupled the vessel head region from therest of the reactor vessel with respect to temperature. As a result of theanalysis CE issued an information bulletin (Enclosure 2) warning of the longvessel head cooldown times which may be required (10-15 hours) and recommendingactions to maintain plant conditions until the vessel head has cooled should abubble form.The licensee indicated that while in the shutdown cooling mode of operation someprimary coolant may have leaked to the refueling water tank through LPSI lB pump'srecirculation line. The isolation valve in this line was found not to becompletely shut. The licensee stated that there were no indications of radio-activity release. While on shutdown cooling LPSI Pump lB was operatingin the injection mode with its recirc line open.
 
St. Lucie Unit No-2-For corrective actions the licensee has-modified its procedures to includethe CE guidelines and installed backup means (additional air supplies andmanual tachet) of opening the CCW containment isolation valves.ConclusionsThe NRC staff indicated the need for more detailed analyses of the naturalcirculation cooldown including consideration of secondary makeup supplies.In addition, the impact of steam bubble formation and its effects on depres-surization rates may have to be considered in the analysis of other transients.This information wilt be requested of the licensee by separate letter and doesnot have to be resolved prior to restart.The NRC's Office of Inspection and Enforcement will further review thelicensee's corrective actions as well as the acceptability of operating onetrain of the shutdown cooling.system In the injection mode with the othertrain operating In the shutdown cooling mode prior to plant restart.Chris C. Nelson, Project ManagerOperating Reactors Branch 13Division of Licensing


===Enclosures:===
St. Lucie Unit No-2-For corrective actions the licensee has-modified its procedures to includethe CE guidelines and installed backup means (additional air supplies andmanual tachet) of opening the CCW containment isolation valves.ConclusionsThe NRC staff indicated the need for more detailed analyses of the naturalcirculation cooldown including consideration of secondary makeup supplies.In addition, the impact of steam bubble formation and its effects on depres-surization rates may have to be considered in the analysis of other transients.This information wilt be requested of the licensee by separate letter and doesnot have to be resolved prior to restart.The NRC's Office of Inspection and Enforcement will further review thelicensee's corrective actions as well as the acceptability of operating onetrain of the shutdown cooling.system In the injection mode with the othertrain operating In the shutdown cooling mode prior to plant restart.Chris C. Nelson, Project ManagerOperating Reactors Branch 13Division of LicensingEnclosures:1. List of Attendees2. Chronology of Events,Evaluation, Actions Taken& Summarycc w/enclosures:See last page IEnclosure IJUNE 20, 1980 MEETING WITH FP&L -ST. LUCIE UNIT NO. 1 -ATTENDEESNRCChris C. NelsonThemis SpeisBrian SheronEd JordanEdward BlackwoodJerry MazetisKris ParczewsklPhil MatthewsGeorge LanikE. V. InbroR. A. ClarkJ. S. Cresst2l1Monte ConnerChang LiThomas R. WolfH. F. ConradCECharlie BrinkmanR. S. TurkR. S. DaleasJ. C. MouhtonR. E. WolfFP&LC. M. WethyJ. A. DeMastryH. N. Paduano I.1:TA.!IT IN4'4LR. ; !j 'I'* i i IN HE fla ! it. :* i w.i!:hlIIII I fit1jj* ji liii'IIII::.i IIII 1.Is..atRiiII:IiiiiII'Ih:1iiiI:I;I*Ij::iii:..II:: ..lsue e1f*.I.I''' 'Ii -Il:it!I;.: I -I ; .!I -,I; II,I;:::iir. ii. I'Iiit,'I*pir.iiA!.,*I'i IP:WthI i** I"II'I'I I I,j1L.~ II i1:i ..~*.:' lii;:11i~i~iIj.6.I.LI;.:.11': 5;::1 .21:-.. -... .m.. ..... -...... .-:.-- A.14-.m'''II-%%ftii.1;.;I'I1;Ii~"i!I WII~llrl IiaIIli',l;..Ill...........II : iI:; I ..,:! :,.;a... i.r-,1:! I.- .!I-1-.!I:,'It.ii1.:t::a.:..a.1:2:i:.1':Ii.II,.1::I:'I! ii.5:..I.II!MI;1.1lil.V.:L.W:11vAl;I......I.I
1. List of Attendees2. Chronology of Events,Evaluation, Actions Taken& Summarycc w/enclosures:See last page IEnclosure IJUNE 20, 1980 MEETING WITH FP&L -ST. LUCIE UNIT NO. 1 -ATTENDEESNRCChris C. NelsonThemis SpeisBrian SheronEd JordanEdward BlackwoodJerry MazetisKris ParczewsklPhil MatthewsGeorge LanikE. V. InbroR. A. ClarkJ. S. Cresst2l1Monte ConnerChang LiThomas R. WolfH. F. ConradCECharlie BrinkmanR. S. TurkR. S. DaleasJ. C. MouhtonR. E. WolfFP&LC. M. WethyJ. A. DeMastryH. N. Paduano I.1:TA.!IT IN4'4LR. ; !j 'I'* i i IN HE fla ! it. :* i w.i!:hlIIII I fit1jj* ji liii'IIII::.i IIII 1.Is..atRiiII:IiiiiII'Ih:1iiiI:I;I*Ij::iii:..II:: ..lsue e1f*.I.I''' 'Ii -Il:it!I;.: I -I ; .!I -,I; II,I;:::iir. ii. I'Iiit,'I*pir.iiA!.,*I'i IP:WthI i** I"II'I'I I I,j1L.~ II i1:i ..~*.:' lii;:11i~i~iIj.6.I.LI;.:.11': 5;::1 .21:-.. -... .m.. ..... -...... .-:.-- A.14-.m'''II-%%ftii.1;.;I'I1;Ii~"i!I WII~llrl IiaIIli',l;..Ill...........II : iI:; I ..,:! :,.;a... i.r-,1:! I.- .!I-1-.!I:,'It.ii1.:t::a.:..a.1:2:i:.1':Ii.II,.1::I:'I! ii.5:..I.II!MI;1.1lil.V.:L.W:11vAl;I......I.I
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'I _1e L or6-4.-depressuri1140 to 69I v....-. -...- -.f1'*.:.w:-i:'H,a: 7'.f .7 T7(I..:.7'.'* -. -:..-_. ...=:. _:-:::. -:... ..__ _:.:e *_.-: =._ __: -=:... ..:.::.. : ..: .-r_ __... __.._.., __.-i -._1.. _. ........ .__::: -r:..... ..-:l-r--I .::-r_. t ....I2 '.!'. :I=. --.z_"I :'-__ I; ..Z. .:r..-_=l-' -l--1- B--:-r_ r _-I4: IV.._.:1 .-IF4-"*.{s"Y~~I K .I.I:64 wi!) pumlps 1 r -Ca&stop feed CI0 psig 2 chg pumps leistop letdownhlV[I-1 401 14l ,-- -J.1II-- 7.' ' --: .:Irt feedchg pump.down((a--1.,_.rl_ s_ I._I, .-r.l __ =l: _ ,_=1 L=l _ = Xr -X=.g.__:1_sa4'a'N) -.::.I* z.1 l 1 w -- 1=.' 7::7.7 '-.=I I --. .. -.Z-.:L: _..-:.1_. .:.1-'L I_I=.'i3-I..-aj"M.;:. ;Ln'.1: I.,..Z-tl:.1; L.,-: =:1. I .-.A 0-- n..z.:: I -.;-t;; il.- I-.-7..:.I :=m. -"It.:.I.;;. -20 -II.- -A .,IJ-' I ..-I ...- t_.I-:snl---M. ' .,-:I .I.--- .. ..- ,- _ ..=-:1,, _, _, _, _,_ u ..w.i 7.W.j:101I. -,: I 1.1;- -.!-. I ----7:..-r--J ;:..., t. I.".I '71 :,--:A'. :1 L!I ..C.:.I ..'ito.-.ILi Ii.....I -ba- :I.I 111Z4II3:IS.11 .r.,q .--, F ".;:I -... J I.I.:;I- %1.,.-.1II.' L I1"'..I(I.... .._ l ___ _J. t __IIII I04000500TIME0233 03000600070008000900% PRESSURIZER LEVEL11 JUNE 1980 Level Increase -/chargils 11K 8K.. _.@_Mr1:-EL , ' .Lng to aux spray start LPSI IAinjection toRCS from RWTdrawbubbleLevel Decrease-lcharging to RCS loopsr .>: 1 _.._ .p 7.U : .;n: o.:l- :..--secureLPSI IA-Iin PZRr- -----Ostart 2nd charging pumplI stop 2nd chargindR PUmP..illI Ill))0I *10h1:.. .I :.t--1Jik.7!.1 -K.m~K .rI- .rIl...,,.--.1- .1 :l:.z5 '}'1.'I.-l~1- '-I *-I nl-r--li}li: , zt:j. t tJILIL XEa*Y-~I -_ _.--ii-'. *II I3 -1Co..,. __ _,w,lIIL:'S.%iB;)'''rf_ __f_.- -.I .:1 --_-!' .';3.I II I---.7[ fI .-F'.1.:_:1.:. -I ._:.,,IS *eI *1100 i §X oor-1000110012001300140015001600ltOO1A00  
'I _1e L or6-4.-depressuri1140 to 69I v....-. -...- -.f1'*.:.w:-i:'H,a: 7'.f .7 T7(I..:.7'.'* -. -:..-_. ...=:. _:-:::. -:... ..__ _:.:e *_.-: =._ __: -=:... ..:.::.. : ..: .-r_ __... __.._.., __.-i -._1.. _. ........ .__::: -r:..... ..-:l-r--I .::-r_. t ....I2 '.!'. :I=. --.z_"I :'-__ I; ..Z. .:r..-_=l-' -l--1- B--:-r_ r _-I4: IV.._.:1 .-IF4-"*.{s"Y~~I K .I.I:64 wi!) pumlps 1 r -Ca&stop feed CI0 psig 2 chg pumps leistop letdownhlV[I-1 401 14l ,-- -J.1II-- 7.' ' --: .:Irt feedchg pump.down((a--1.,_.rl_ s_ I._I, .-r.l __ =l: _ ,_=1 L=l _ = Xr -X=.g.__:1_sa4'a'N) -.::.I* z.1 l 1 w -- 1=.' 7::7.7 '-.=I I --. .. -.Z-.:L: _..-:.1_. .:.1-'L I_I=.'i3-I..-aj"M.;:. ;Ln'.1: I.,..Z-tl:.1; L.,-: =:1. I .-.A 0-- n..z.:: I -.;-t;; il.- I-.-7..:.I :=m. -"It.:.I.;;. -20 -II.- -A .,IJ-' I ..-I ...- t_.I-:snl---M. ' .,-:I .I.--- .. ..- ,- _ ..=-:1,, _, _, _, _,_ u ..w.i 7.W.j:101I. -,: I 1.1;- -.!-. I ----7:..-r--J ;:..., t. I.".I '71 :,--:A'. :1 L!I ..C.:.I ..'ito.-.ILi Ii.....I -ba- :I.I 111Z4II3:IS.11 .r.,q .--, F ".;:I -... J I.I.:;I- %1.,.-.1II.' L I1"'..I(I.... .._ l ___ _J. t __IIII I04000500TIME0233 03000600070008000900% PRESSURIZER LEVEL11 JUNE 1980  
Level Increase -/chargils 11K 8K.. _.@_Mr1:-EL , ' .Lng to aux spray start LPSI IAinjection toRCS from RWTdrawbubbleLevel Decrease-lcharging to RCS loopsr .>: 1 _.._ .p 7.U : .;n: o.:l- :..--secureLPSI IA-Iin PZRr- -----Ostart 2nd charging pumplI stop 2nd chargindR PUmP..illI Ill))0I *10h1:.. .I :.t--1Jik.7!.1 -K.m~K .rI- .rIl...,,.--.1- .1 :l:.z5 '}'1.'I.-l~1- '-I *-I nl-r--li}li: , zt:j. t tJILIL XEa*Y-~I -_ _.--ii-'. *II I3 -1Co..,. __ _,w,lIIL:'S.%iB;)'''rf_ __f_.- -.I .:1 --_-!' .';3.I II I---.7[ fI .-F'.1.:_:1.:. -I ._:.,,IS *eI *1100 i §X oor-1000110012001300140015001600ltOO1A00  
-a I___II Foui LkLiA SYSTEMSVAGSU OrMOWA-00A0-3IALI-AII-j 1FAA* ADVISORY CONCIRMING A TECHNICAL DEVELOPMENT 1aLATED TO TIIAPPLICATION OX OPERATION OF NUCLEAR PLANT EoutrutaT SUMLIED BYCOMMUIT X E1GINKERING.un '.0, I1ILtMURAL CIRCULATICMO COOLOWOINTRODUCT1ON: On June 12. 19M, C-E was Wnfored by FPSL that wvexpented variationsin prissurlzer level here observed while attempting to reduce plant pressure duringa natural circulation cooldown at St. Lucit 1. The variations were determined tohave resulted from the expansion and collapse of a stama void in the Reactor Vessl (RY)head region. This bulletin provides details currently known about this avnt and ap-propriate operational guidance for utilities with operating C-E XSns.OISCUSSt0x: Following a loss of Coponent CoOTIng Water. to the Rs. St. Lucie Iiced an expeditious natural circulation cooldoin to cotd shutdown conditions.RCS depressurizatlo+n via pressurizer auxi tary spray was initiated approzittely 3.5hours after the reactor trip..- Oring depressurization, saturation condttions werereachd in the RY head region which Tflited further depressurization until the headregion had cooled off. Under natural circulation flow conditions there was aininElcoolant flow in the upper head region and effectively all cooling was by heat loss to-the ambient or by reactor coolant surge flow through the head region. An RC pressureappropriate for initiating shutdown cooling (SCS) was attained approximately 9 hoursfollowing the reactor trip. Ouring this event, the con region was always significantlysubcooled.In the event of a natural circulation cooldowu it should be realized that cooling oftha vessel head region ezy be limiting in establishing SCS enry conditions. Coolingof the RY head region my require as auch as 10 to IS hurs. Perfodic attemts to^eA--i-% -AI44-ri can ha mxAs Whilt wm~tihfnim fiiv f inicains af v4idn3a;rE U4 Lw cx-E w 4*w J. s*~ *w* C."ww.",..__,_..,-IIfor7Ation. When SCS entry conditions art achieved, the shutdown coling systse mybealigned in accordance with operating procedures.Buring depressurizatton. forcation of a stma void In the RY head wuTd be indicated by arapid increase in pressurizer level while delivering auxiliary spWrayr, k (.4 bdecrease in pressurizer level when charging is realitned to the RCS looP The respotmes are. indicative of an expanding void in the R;J head as the pressurizer issprayed and collapse of the steam void In the RY head as charging is directed tthe loop.
-a I___II Foui LkLiA SYSTEMSVAGSU OrMOWA-00A0-3IALI-AII-j 1FAA* ADVISORY CONCIRMING A TECHNICAL DEVELOPMENT 1aLATED TO TIIAPPLICATION OX OPERATION OF NUCLEAR PLANT EoutrutaT SUMLIED BYCOMMUIT X E1GINKERING.un '.0, I1ILtMURAL CIRCULATICMO COOLOWOINTRODUCT1ON: On June 12. 19M, C-E was Wnfored by FPSL that wvexpented variationsin prissurlzer level here observed while attempting to reduce plant pressure duringa natural circulation cooldown at St. Lucit 1. The variations were determined tohave resulted from the expansion and collapse of a stama void in the Reactor Vessl (RY)head region. This bulletin provides details currently known about this avnt and ap-propriate operational guidance for utilities with operating C-E XSns.OISCUSSt0x: Following a loss of Coponent CoOTIng Water. to the Rs. St. Lucie Iiced an expeditious natural circulation cooldoin to cotd shutdown conditions.RCS depressurizatlo+n via pressurizer auxi tary spray was initiated approzittely 3.5hours after the reactor trip..- Oring depressurization, saturation condttions werereachd in the RY head region which Tflited further depressurization until the headregion had cooled off. Under natural circulation flow conditions there was aininElcoolant flow in the upper head region and effectively all cooling was by heat loss to-the ambient or by reactor coolant surge flow through the head region. An RC pressureappropriate for initiating shutdown cooling (SCS) was attained approximately 9 hoursfollowing the reactor trip. Ouring this event, the con region was always significantlysubcooled.In the event of a natural circulation cooldowu it should be realized that cooling oftha vessel head region ezy be limiting in establishing SCS enry conditions. Coolingof the RY head region my require as auch as 10 to IS hurs. Perfodic attemts to^eA--i-% -AI44-ri can ha mxAs Whilt wm~tihfnim fiiv f inicains af v4idn3a;rE U4 Lw cx-E w 4*w J. s*~ *w* C."ww.",..__,_..,-IIfor7Ation. When SCS entry conditions art achieved, the shutdown coling systse mybealigned in accordance with operating procedures.Buring depressurizatton. forcation of a stma void In the RY head wuTd be indicated by arapid increase in pressurizer level while delivering auxiliary spWrayr, k (.4 bdecrease in pressurizer level when charging is realitned to the RCS looP The respotmes are. indicative of an expanding void in the R;J head as the pressurizer issprayed and collapse of the steam void In the RY head as charging is directed tthe loop.


* TheSe indicattons May be prid by an indication of letdon flow in excessof charging flow If the Pressurizer level control syste has beei In utoatic duringthe cooldwm. Upon indtcatton of a steao void the following actios are recrendedtn order to maintain plant conditions until the RY head has Cooled:1. tsolate letIO.2. Stop further RCS cooldmiu until Indications of RV head steas void cease.3. Energize pressurizer heaters to limit rate of depressurizatimo.4. Operate char dgre and auxiliary spray to raintain pressurizer levl bet:e -30% and 705 tn order to l imit void expansion.STATUS: The event is currently being reviewed. Additional operator gufdance willSInorporated Into. C-C tratntng programs and forwarded to utilfty operating staffsas appropriate.ThE AMXSAM MAW OpWL S7V*W C~ftACr P TH6 APPUCA.W.E FIANF. A A EWHS To You" OcgzArocU. As A MRISM. ANO San= OP6A4TM OF YOUR PLANT 4 C0&TLVT IWv.9in YOURM a A!Ln*wOMaa'LfTY. API N aVRSV MAXI PA=CHME M~r WTTMU 04rG 40MjSOai I "W UPGruTON MAYruTiLE O 0*1.7 WIrMTH m UftM MT7A3f0~ THAT 04 MAZt NO 1A*PAMiTZ 0* FWmUZMfTAYV*.ILXP"62S0 ON MWUISO. POCLM04P TNUE WAR .U~t5 CF P67hma Poo AFAiWV~mAwwcw o WIOMMAXT.ABILJTY.* Wi" R EPCr TO THt ACC.IACV. C04PZTZfM CM LDEP1ULKU CW ThE NWON"aT C*4IMIDin TMe VULLRTTU. A0 T" T C4 D4 .A1M. ^M VOI AW ., r ALL L A MlY. W E@" EftS CM O rw ..AS A WESULT OP YOU -- OF mS 1___ATM_CeF0012MarI}}
* TheSe indicattons May be prid by an indication of letdon flow in excessof charging flow If the Pressurizer level control syste has beei In utoatic duringthe cooldwm. Upon indtcatton of a steao void the following actios are recrendedtn order to maintain plant conditions until the RY head has Cooled:1. tsolate letIO.2. Stop further RCS cooldmiu until Indications of RV head steas void cease.3. Energize pressurizer heaters to limit rate of depressurizatimo.4. Operate char dgre and auxiliary spray to raintain pressurizer levl bet:e -30% and 705 tn order to l imit void expansion.STATUS: The event is currently being reviewed. Additional operator gufdance willSInorporated Into. C-C tratntng programs and forwarded to utilfty operating staffsas appropriate.ThE AMXSAM MAW OpWL S7V*W C~ftACr P TH6 APPUCA.W.E FIANF. A A EWHS To You" OcgzArocU. As A MRISM. ANO San= OP6A4TM OF YOUR PLANT 4 C0&TLVT IWv.9in YOURM a A!Ln*wOMaa'LfTY. API N aVRSV MAXI PA=CHME M~r WTTMU 04rG 40MjSOai I "W UPGruTON MAYruTiLE O 0*1.7 WIrMTH m UftM MT7A3f0~ THAT 04 MAZt NO 1A*PAMiTZ 0* FWmUZMfTAYV*.ILXP"62S0 ON MWUISO. POCLM04P TNUE WAR .U~t5 CF P67hma Poo AFAiWV~mAwwcw o WIOMMAXT.ABILJTY.* Wi" R EPCr TO THt ACC.IACV. C04PZTZfM CM LDEP1ULKU CW ThE NWON"aT C*4IMIDin TMe VULLRTTU. A0 T" T C4 D4 .A1M. ^M VOI AW ., r ALL L A MlY. W E@" EftS CM O rw ..AS A WESULT OP YOU -- OF mS 1___ATM_CeF0012MarI  
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NRC Generic Letter 1981-021: Natural Circulation Cooldown
ML031080586
Person / Time
Site: Beaver Valley, Millstone, Calvert Cliffs, Davis Besse, Salem, Oconee, Mcguire, Palisades, Palo Verde, Indian Point, Fermi, Kewaunee, Catawba, Harris, Wolf Creek, Saint Lucie, Point Beach, Watts Bar, Sequoyah, Byron, Arkansas Nuclear, Braidwood, Summer, Prairie Island, Columbia, Surry, North Anna, Turkey Point, Crystal River, Haddam Neck, Ginna, Diablo Canyon, Callaway, Waterford, Farley, Robinson, South Texas, San Onofre, Cook, Comanche Peak, Yankee Rowe, Maine Yankee, Rancho Seco, Zion, Midland, Bellefonte, Fort Calhoun, McGuire, 05000000, Zimmer, 05000398, 05000380, Trojan, Marble Hill, Crane
Issue date: 05/05/1981
From: Eisenhut D G
Office of Nuclear Reactor Regulation
To:
References
GL-81-021, NUDOCS 8105140267
Download: ML031080586 (12)
v • d • e


UNITED STATES0 \NUCLEAR REGULATORY COMMISSIONWASHINGTON, D. C. 20555MAY 5 1981TO ALL LICENSEES OF OPERATING PRESSURIZED WATER NUCLEAR POWER REACTORS ANDAPPLICANTS FOR OPERATING LICENSES (EXCEPT FOR ST. LUCIE, UNIT NO. 1)Gentlemen:t' SUBJECT: NATURAL CIRCULATION COOLDOWN(Generic Letter No. 8121-)On June 11, 1980, the St. Lucie Plant, Unit No. 1, was forced to cool downon natural circulation as a result of a component cooling water malfunction.During the cooldown process, abnormally rapid increases in pressurizer levelwere observed. Subsequent analyses have confirmed that these abnormal levelincreases were produced by flashing of liquid in the upper head of the reactorvessel, forcing water out of the vessel and into the pressurizer. A morecomplete description of the event and circumstances involved is provided iny-' the enclosure which includes a letter sent to the PWR NSSS vendors solicitingtheir opinions and comments on the significance of the event and phenomenon> 0 pin general.>' Based on our review of the event to date, we believe that core cooling was-never lost during the St. Lucie, Unit No. 1 event. That specific event doesnot constitute a direct safety concern. We have, however, identified two areasof concern applicable to all pressurized water reactors requiring prompt action:1. The Unacceptability of Vessel Voiding During Anticipated Cooldownoenditions INatura Circulation Due to LOSS Ot Osite Power, Coss ofPumps, etc.)Cooldown with a significant steam void in the vessel requires controllinga "two pressurizer" system, which is an undesirable challenge to theoperator. In fact, we are not aware of any training facilities (simulators)today which would allow an operator "hands on" experience in practicingsuch control. Moreover, it is our opinion that any significant vesselvoiding produced during controlled cooldown conditions increases thesusceptibility of the plant to more serious accidents. For these reasonsreactor vessel voiding during controlled natural circulation cooldownsshould be avoided.3§At47) UfJo2.67 /410.

-2-As described in the enclosure, vessel voiding at St. Lucie, Unit No. 1,was caused by the operator reducing system pressure such that thecorresponding saturation temperature dropped to the temperature of therelatively stagnant fluid in the reactor vessel upper head. Presently,primary system cooldown rates are based on vessel structural integrityconsiderations and do not explicitly consider avoiding production ofsignificant steam voids in the vessel. Moreover, cooldown rates arebased on fluid temperatures measured in the primary piping. As the St. LucieUnit No. I event has shown, these measured temperatures can in fact be onthe order of 100 degrees Fahrenheit or more lower than the upper head fluidtemperature, and, therefore, not indicative of the saturation pressure ofall fluid in the primary system.Under conditions which require cooldown on natural circulation and whenrapid depressurization is not necessary there may be a number of ways toavoid reactor vessel voiding. For example, a low cooldown rate can bespecified, coupled with "holding" the plant at intermediate conditions toallow the fluid in the upper vessel to equilibrate with the rest of theprimary system. However, avoidance of vessel voiding by lower primarysystem cooldown rates can increase the time required to achieve shutdowncooling entry conditions and thus increase the time auxiliary feedwateris depended upon to remove decay heat (specifically, for the loss-of-offsite power case). Thus, supplies of condensate-grade auxiliary feed-water must be considered if cooldown times are extended.2. Failure of the Operator to Have Prior Knowledge and Training for ThisEventThe cause of initial surges in pressurizer level at St. Lucie, Unit No. 1,was not immediately recognized or understood by the operator. Weattribute this to the fact that long-term natural circulation cooldownunder the specific circumstances of the event was never explicitlyanalysed by the NSSS vendor from the standpoint of trying to recognize aphenomenon such as that which occurred at St. Lucie, Unit No. 1. In theSt. Lucie event, the operator ultimately recognized the cause.of thelevel surges and was able to maintain control of the plant. Our concern,however, is the possibility of an operator taking incorrect action in aneffort to correct for an unknown event or unrecognized phenomena.We believe that proper procedures and training can provide the necessaryguidance to the operators both to avoid reactor vessel voiding as well asrecognize it when, and if, it occurs during controlled natural circulationcooldown. We are not sure if such procedures and training are in place atpressurized water reactor facilities.

-3 -Consequently, we request that you promptly review your current plant operationsin light of the St. Lucie, Unit No. 1 event and the discussions above andimplement, as necessary, procedures and training which will enable operatorsto avoid (if possible), recognize and properly react to reactor vessel voidingduring natural circulation cooldown.We conclude that the actions described above should be completed as soon asthey reasonably can be (i.e., within 6 months for operating reactors). Inaddition, so that we may determine whether your license should be amended toincorporate these actions as requirements, licensees of operating pressurizedwater reactors are requested, pursuant to §50.54(f), to furnish, within 6months of receipt of this letter, an assessment of your facility proceduresand training program with respect to the matters described above. Yourassessment should include:1. a demonstration (e.g. analysis and/or test) that controlled natural circula-tion cooldown from operating conditions to cold shutdown conditions, conductedin accordance with your procedure, should not result in reactor vesselvoiding;2. verification that supplies of condensate-grade auxiliary feedwater aresufficient to support your cooldown method; and3. a description of your training program and the provisions of your procedures(e.g. limited cooldown rate, response to rapid change in pressurizer level)that deal with prevention or mitigation of reactor vessel voiding.Applicants for operating licensees are requested to implement the subjectprocedures and training and provide the requested assessment within 6 monthsof receipt of this letter or 4 months prior to the staff's scheduled issuanceof its operating license Safety Evaluation Report, whichever is later.Please refer to this letter in your response.This request for information was approved by OMB under a blanket clearancenumber R0072 which expires December 31, 1981. Comments on burden and duplicationmay be directed to the Office of Management and Budget, Reports Management,Room 3208, New Executive Office Building, Washington, D.C. 20503.S\ncerely,.i'seDivision o LicensingOffice of Nuclear Reactor RegulationEnclosure: As statedcc: Service list

£WI?.11[ESTA1ES ,Enclosure 1NUCLEAR RLGULATOR:Y CC),.Iil'(,:j.ASus:GIOM. D. C. 7055NA %U 12 1BOLetter sent to PWR NSSS Vendors:Westinghouse, Combustion Engineering andBabcock and WilcoxDear Mr.SUBJECT: VOID FORIATION IN VESSEL HEAD DURING ST. LUCIE NATURALCIRCULATION COOLDOWN EVENT OF 6/11/80On June 11, 1980, the St. Lucie reactor was shutdown due to a loss ofcomponent cooling water to the reactor coolant pump seals. This alsorequired shutdown of the reactor coolant pumps and c:oldown Was accomplishedby natural circulation.At approximately 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br /> into the event, charging flow, which Was initiallybeing divided between the cold legs and the auxiliary pressurizer spray, 1..asdiverted entirely to the auxiliary spray to enhance the depressurization andreduce the system pressure on the pump seals. At this time, abnormallyrapid increases in pressurizer level were observed which could not beexplained by the charging flow rate alone. Detailed evaluation andfollow-up analyses by the licensee and NSSS supplier have indicated thata steam void was probably formed in the upper head region of the reactorvessel and displaced water from the vessel into the pressurizer.Continued alternating realignment of charging flow between the cold legsand auxiliary spray line produced a 'saw-tooth" pressurizer level behavior.Relevant information and data available to the staff to date are providedin the enclosure.It has been postulated that the steam void in the upper vessel was producedwhen the system pressure dropped below the saturation pressure correspondingto the temperature of the fluid in the upper head. Because the measured hotand cold leg temperatures at the time of voiding here highly subcooled(-2000F), it appears -that the fluid. in the upper head was much hotter, relatively-stagnant, and in-poor communication with the fluid exiting the core and in theupper plenum. In addition, stored heat in the upper head structures mostlikely contributed to the voiding.Because of the unexpected occurrence of the void, the failure of the operatorsto immediately recognize the void formnation and take corrective action, andthe question of whether such void formation is properly accounted for in safety a -:.d * -;analyses (Chapter 15), we have sent a list of que~tions documuenting ourconcerns to the licensee. These questions are also provided in the-enclosure for your information.We are presently evaluating the need to pursue this issue generically withall PUR licensees. Prior to taking any definitive action however, we aresoliciting your technical opinion and advise regarding the potential forvoid formation under similar circumstances in USSS's designed by you.Specifically, we need to know if you can justify why the voiding phenomenoncannot occur in USSS's designed by you (or can confirm that such phenomenacan be properly predicted by your transient analysis models), and if it canoccur, is properly accounted for in operating procedures (e.g.. cooldowmrates), operator guidelines, and operator training (including the simulator)The urgency of this matter requires you advise us within fifteen (15)working days after receipt of this letter whether a supplemental informationsubmittal by you on the subject would preclude the need to expeditiouslypursue this issue generically with your customers.tu..binal Signd b1'sui S. MCA .Paul S. Check, Assistant Director forPlant SystemsDivision of Systems IntegrationOffice of Nuclear Reactor RegulationI : :. .-e. ...i_..-aI

IPra arc&UNITED STATESNUCLEAR REGULATORY COMMISSIONs XWASHINGTON. 0. C. 2CmJune 25, 1980***Docket No. 50-335LICENSEE: Florida Power & Light Company (FP&L)FACILITY: St. Lucie Unit No. 1SUBJECT: SUMMARY OF MEETING WITH FP&L AND COMBUSTION ENGINEERING (CE) REGARDINGST. LUCIE UNIT NO. 1 COOLDOWN ON NATURAL CIRCULATIONOn June 20, 1980 a meeting was held in Bethesda, MD regarding the June 11,reactor trip and cooldown on natural circulation at St. Lucie Unit No. 1.Significant points discussed are summarized below. Enclosure 1 is a list ofattendees.DiscussionA brief chronology of the event was presented with traces of parametersenclosed as Enclosure 2. The plant experienced a complete loss of componentcooling water (CCW) to the reactor coolant pumps (RCP). This loss was due toa direct short on the terminal board for a solenoid operated CCW common returnvalve from the RCP's. Plant procedures allow ten minutes to correctthe problem before securing the RCP's. Subsequently, the reactor was trippedand the RCP's secured. About three minutes after shutdown one RCP was runbriefly to aid natural circulation flow. The licensee stated that intermittentRCP leakoff flow alarms were actuated and that subcooled margin remained withinthe range 65 to 180'F subcooled during the cooldown.The licensee indicated that a steam bubble had formed in the head of the reactorvessel during cooldown. This was indicated by abnormal pressurizer level changesduring charging operations and was apparently due to elevated temperatures inthe hydraulically stagnant area of the upper head.The possibility of steam bubble formation was confirmed with an analysis by CEusing an in-house model. This model decoupled the vessel head region from therest of the reactor vessel with respect to temperature. As a result of theanalysis CE issued an information bulletin (Enclosure 2) warning of the longvessel head cooldown times which may be required (10-15 hours) and recommendingactions to maintain plant conditions until the vessel head has cooled should abubble form.The licensee indicated that while in the shutdown cooling mode of operation someprimary coolant may have leaked to the refueling water tank through LPSI lB pump'srecirculation line. The isolation valve in this line was found not to becompletely shut. The licensee stated that there were no indications of radio-activity release. While on shutdown cooling LPSI Pump lB was operatingin the injection mode with its recirc line open.

St. Lucie Unit No-2-For corrective actions the licensee has-modified its procedures to includethe CE guidelines and installed backup means (additional air supplies andmanual tachet) of opening the CCW containment isolation valves.ConclusionsThe NRC staff indicated the need for more detailed analyses of the naturalcirculation cooldown including consideration of secondary makeup supplies.In addition, the impact of steam bubble formation and its effects on depres-surization rates may have to be considered in the analysis of other transients.This information wilt be requested of the licensee by separate letter and doesnot have to be resolved prior to restart.The NRC's Office of Inspection and Enforcement will further review thelicensee's corrective actions as well as the acceptability of operating onetrain of the shutdown cooling.system In the injection mode with the othertrain operating In the shutdown cooling mode prior to plant restart.Chris C. Nelson, Project ManagerOperating Reactors Branch 13Division of LicensingEnclosures:1. List of Attendees2. Chronology of Events,Evaluation, Actions Taken& Summarycc w/enclosures:See last page IEnclosure IJUNE 20, 1980 MEETING WITH FP&L -ST. LUCIE UNIT NO. 1 -ATTENDEESNRCChris C. NelsonThemis SpeisBrian SheronEd JordanEdward BlackwoodJerry MazetisKris ParczewsklPhil MatthewsGeorge LanikE. V. InbroR. A. ClarkJ. S. Cresst2l1Monte ConnerChang LiThomas R. WolfH. F. ConradCECharlie BrinkmanR. S. TurkR. S. DaleasJ. C. MouhtonR. E. WolfFP&LC. M. WethyJ. A. DeMastryH. N. Paduano I.1:TA.!IT IN4'4LR. ; !j 'I'* i i IN HE fla ! it. :* i w.i!:hlIIII I fit1jj* ji liii'IIII::.i IIII 1.Is..atRiiII:IiiiiII'Ih:1iiiI:I;I*Ij::iii:..II:: ..lsue e1f*.I.I 'Ii -Il:it!I;.: I -I ; .!I -,I; II,I;:::iir. ii. I'Iiit,'I*pir.iiA!.,*I'i IP:WthI i** I"II'I'I I I,j1L.~ II i1:i ..~*.:' lii;:11i~i~iIj.6.I.LI;.:.11': 5;::1 .21:-.. -... .m.. ..... -...... .-:.-- A.14-.mII-%%ftii.1;.;I'I1;Ii~"i!I WII~llrl IiaIIli',l;..Ill...........II : iI:; I ..,:! :,.;a... i.r-,1:! I.- .!I-1-.!I:,'It.ii1.:t::a.:..a.1:2:i:.1':Ii.II,.1::I:'I! ii.5:..I.II!MI;1.1lil.V.:L.W:11vAl;I......I.I

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'I _1e L or6-4.-depressuri1140 to 69I v....-. -...- -.f1'*.:.w:-i:'H,a: 7'.f .7 T7(I..:.7'.'* -. -:..-_. ...=:. _:-:::. -:... ..__ _:.:e *_.-: =._ __: -=:... ..:.::.. : ..: .-r_ __... __.._.., __.-i -._1.. _. ........ .__::: -r:..... ..-:l-r--I .::-r_. t ....I2 '.!'. :I=. --.z_"I :'-__ I; ..Z. .:r..-_=l-' -l--1- B--:-r_ r _-I4: IV.._.:1 .-IF4-"*.{s"Y~~I K .I.I:64 wi!) pumlps 1 r -Ca&stop feed CI0 psig 2 chg pumps leistop letdownhlV[I-1 401 14l ,-- -J.1II-- 7.' ' --: .:Irt feedchg pump.down((a--1.,_.rl_ s_ I._I, .-r.l __ =l: _ ,_=1 L=l _ = Xr -X=.g.__:1_sa4'a'N) -.::.I* z.1 l 1 w -- 1=.' 7::7.7 '-.=I I --. .. -.Z-.:L: _..-:.1_. .:.1-'L I_I=.'i3-I..-aj"M.;:. ;Ln'.1: I.,..Z-tl:.1; L.,-: =:1. I .-.A 0-- n..z.:: I -.;-t;; il.- I-.-7..:.I :=m. -"It.:.I.;;. -20 -II.- -A .,IJ-' I ..-I ...- t_.I-:snl---M. ' .,-:I .I.--- .. ..- ,- _ ..=-:1,, _, _, _, _,_ u ..w.i 7.W.j:101I. -,: I 1.1;- -.!-. I ----7:..-r--J ;:..., t. I.".I '71 :,--:A'. :1 L!I ..C.:.I ..'ito.-.ILi Ii.....I -ba- :I.I 111Z4II3:IS.11 .r.,q .--, F ".;:I -... J I.I.:;I- %1.,.-.1II.' L I1"'..I(I.... .._ l ___ _J. t __IIII I04000500TIME0233 03000600070008000900% PRESSURIZER LEVEL11 JUNE 1980

Level Increase -/chargils 11K 8K.. _.@_Mr1:-EL , ' .Lng to aux spray start LPSI IAinjection toRCS from RWTdrawbubbleLevel Decrease-lcharging to RCS loopsr .>: 1 _.._ .p 7.U : .;n: o.:l- :..--secureLPSI IA-Iin PZRr- -----Ostart 2nd charging pumplI stop 2nd chargindR PUmP..illI Ill))0I *10h1:.. .I :.t--1Jik.7!.1 -K.m~K .rI- .rIl...,,.--.1- .1 :l:.z5 '}'1.'I.-l~1- '-I *-I nl-r--li}li: , zt:j. t tJILIL XEa*Y-~I -_ _.--ii-'. *II I3 -1Co..,. __ _,w,lIIL:'S.%iB;)rf_ __f_.- -.I .:1 --_-!' .';3.I II I---.7[ fI .-F'.1.:_:1.:. -I ._:.,,IS *eI *1100 i §X oor-1000110012001300140015001600ltOO1A00

-a I___II Foui LkLiA SYSTEMSVAGSU OrMOWA-00A0-3IALI-AII-j 1FAA* ADVISORY CONCIRMING A TECHNICAL DEVELOPMENT 1aLATED TO TIIAPPLICATION OX OPERATION OF NUCLEAR PLANT EoutrutaT SUMLIED BYCOMMUIT X E1GINKERING.un '.0, I1ILtMURAL CIRCULATICMO COOLOWOINTRODUCT1ON: On June 12. 19M, C-E was Wnfored by FPSL that wvexpented variationsin prissurlzer level here observed while attempting to reduce plant pressure duringa natural circulation cooldown at St. Lucit 1. The variations were determined tohave resulted from the expansion and collapse of a stama void in the Reactor Vessl (RY)head region. This bulletin provides details currently known about this avnt and ap-propriate operational guidance for utilities with operating C-E XSns.OISCUSSt0x: Following a loss of Coponent CoOTIng Water. to the Rs. St. Lucie Iiced an expeditious natural circulation cooldoin to cotd shutdown conditions.RCS depressurizatlo+n via pressurizer auxi tary spray was initiated approzittely 3.5hours after the reactor trip..- Oring depressurization, saturation condttions werereachd in the RY head region which Tflited further depressurization until the headregion had cooled off. Under natural circulation flow conditions there was aininElcoolant flow in the upper head region and effectively all cooling was by heat loss to-the ambient or by reactor coolant surge flow through the head region. An RC pressureappropriate for initiating shutdown cooling (SCS) was attained approximately 9 hour1.041667e-4 days <br />0.0025 hours <br />1.488095e-5 weeks <br />3.4245e-6 months <br />sfollowing the reactor trip. Ouring this event, the con region was always significantlysubcooled.In the event of a natural circulation cooldowu it should be realized that cooling oftha vessel head region ezy be limiting in establishing SCS enry conditions. Coolingof the RY head region my require as auch as 10 to IS hurs. Perfodic attemts to^eA--i-% -AI44-ri can ha mxAs Whilt wm~tihfnim fiiv f inicains af v4idn3a;rE U4 Lw cx-E w 4*w J. s*~ *w* C."ww.",..__,_..,-IIfor7Ation. When SCS entry conditions art achieved, the shutdown coling systse mybealigned in accordance with operating procedures.Buring depressurizatton. forcation of a stma void In the RY head wuTd be indicated by arapid increase in pressurizer level while delivering auxiliary spWrayr, k (.4 bdecrease in pressurizer level when charging is realitned to the RCS looP The respotmes are. indicative of an expanding void in the R;J head as the pressurizer issprayed and collapse of the steam void In the RY head as charging is directed tthe loop.

  • TheSe indicattons May be prid by an indication of letdon flow in excessof charging flow If the Pressurizer level control syste has beei In utoatic duringthe cooldwm. Upon indtcatton of a steao void the following actios are recrendedtn order to maintain plant conditions until the RY head has Cooled:1. tsolate letIO.2. Stop further RCS cooldmiu until Indications of RV head steas void cease.3. Energize pressurizer heaters to limit rate of depressurizatimo.4. Operate char dgre and auxiliary spray to raintain pressurizer levl bet:e -30% and 705 tn order to l imit void expansion.STATUS: The event is currently being reviewed. Additional operator gufdance willSInorporated Into. C-C tratntng programs and forwarded to utilfty operating staffsas appropriate.ThE AMXSAM MAW OpWL S7V*W C~ftACr P TH6 APPUCA.W.E FIANF. A A EWHS To You" OcgzArocU. As A MRISM. ANO San= OP6A4TM OF YOUR PLANT 4 C0&TLVT IWv.9in YOURM a A!Ln*wOMaa'LfTY. API N aVRSV MAXI PA=CHME M~r WTTMU 04rG 40MjSOai I "W UPGruTON MAYruTiLE O 0*1.7 WIrMTH m UftM MT7A3f0~ THAT 04 MAZt NO 1A*PAMiTZ 0* FWmUZMfTAYV*.ILXP"62S0 ON MWUISO. POCLM04P TNUE WAR .U~t5 CF P67hma Poo AFAiWV~mAwwcw o WIOMMAXT.ABILJTY.* Wi" R EPCr TO THt ACC.IACV. C04PZTZfM CM LDEP1ULKU CW ThE NWON"aT C*4IMIDin TMe VULLRTTU. A0 T" T C4 D4 .A1M. ^M VOI AW ., r ALL L A MlY. W E@" EftS CM O rw ..AS A WESULT OP YOU -- OF mS 1___ATM_CeF0012MarI

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