ML20217G981

Assessment of Spent Fuel Cooling, Presented at 970526-30 5th Intl Conference on Nuclear Engineering (ICONE5) in Nice, France

ML20217G981
Person / Time
Issue date:	05/26/1997
From:	Ibarra J, Lanik G, Ornstein H NRC OFFICE FOR ANALYSIS & EVALUATION OF OPERATIONAL DATA (AEOD)
To:
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NUDOCS 9804290249
Download: ML20217G981 (10)

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Tha 5th IntIrnatiens! C:nfiranca en Nucitar Enginnring ICONES May 26-30,1997 Nice, France ICONES-2045 ASSESSMENT OF SPENT FUEL COOLING Dr. Harold L. Ornstein Jose G. Ibarra William R. Jones George F. Lanik Sadanandan V. Pullani Office for Analysis and Evaluation of Operational Data U.S. Nuclear Regulatory Commission Mall Stop T-4 A9 Washington, D.C. 20555 l

l l AHSTRACT The paper presents the methodology, the findings, and the The "Susquehanna" 10 CFR 21 report postulated loss of SFP conclusions of a study that was done by the Nuclear Regulatory cooling resulting in boiling of tue SFP, failure of emergency core Commission's Office for Analysis and Evaluation of Operational cooling system (ECCS) and other equipment due to steam Data (AEOD) on loss of spent fuel pool cooling. The study releases and condensation of SFP vapors, reactor core heatup and involved an examination of spent fuel pool designs, operating damage, spent fuel heatup and damage, and large offsite experience, operating practices, and procedures. AEOD's work radioactivity releases.

I was augmented in the area of statistics and probabilistic risk assessment by experts from the Idaho National Engineering In early 1996, the U.S. Nuclear Regulatory Commission's

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Laboratory. Operating experience was integrated into a Executive Director for Operations directed the NRC's Office for probabilistic risk assessment to gain insight on the risks from Analysis and Evaluation of 0perational Data (AEOD) to perform spent fuel pools. an independent assessment of the likelihood and consequences of en extended loss of spent fuel pool (SFP) cooling. The study will be published soon as NRC report NUREG-1275, Vol.12.

1 INTRODUCTION The AEOD study:

In recent years there have been several instances in which the adequacy of spent fuel pool (SFP) cooling systems has been -

Developed generic configurations delineating SFP brought into question. For example, two contractors at equipment for a boiling-water reactor (BWR) and a Susquehanna Steam Electric Station plant, submitted a Title 10 pressurized-water reactor (PWR) and utilized these generic of the Code offedera/ Regulations (10 CFR) (Ref.1) Part 21 configurations to assess the loss of SFP cooling and rcport (Ref. 2) on the adequacy of SFP cooling at Susquehanna. inventory, j

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May 26-30,1997 Nice, France Reviewed and assessed 12 years of operaticnal experience 2.1 Loss of Spent Fuel Pool Coolant Inventory for both domestic reactors and foreign reactors with designs The primary pathways for loss of SFP coolant inventory can similar to that of the United States.

be broadly categorized as (1) loss through connected systems,

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(2) leakage through movable gates or seals, and (3) leakage Performed six site visits to gather information on SFP through or failure of the fuel pool or the fuel pool liner.

physical configuration, practices, and procedures; and conducted interviews with the authors of the 10 CFR 21 2.1.1 Consequences of Loss of Spent Fuel Pool Coolant l report to better understand their concems. Inventory i

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For a large loss of SFP inventory, the primary consequence i Reviewed applicable SFP regulations and the NRC Standard is potential uncovery of the stored fuel Given the unlik.ely Review Plan (SRP) for the acceptance criteria and applicable occurrence of a large leak at the bottom of the SFP structure,

Regulatory G. ides.

b: yond the available make-up capacity, the fuel could uncover

! tud heat up to the point of clad damage and release of fission Performed independent assessments of electrical systems, p 'oducts. The uncovery of the fuel would also result in extremely l

instrumentation, heat loads, . and radiation to better high radiation fields around the SFP area.

understand the role of these issues to loss of SFP cooling.

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A more likely sequence would be a loss ofinventory through Contracted with Idaho National Engineering Laboratory a gate or seal which would terminate when the level reached the (INEL) to review existing risk analyses and use risk elevation of the leak. Then, due to the decreased inventory of l assessment techniques to evaluate the risk of losing SFP water in the SFP and the loss of suction to the SFP cooling cooling and coolant inventory, system, the remaining water in the pool would boil away until the fuel was uncovered. Unless corrective actions were taken, the 2 SPENT FUEL COOLING final consequences would be similar to loss of SFP coolant A survey of SFPs indicates that a wide variety of inventory described above.

configurations exists. Since most plants were built prior to issuance of specific NRC regulatory guidance, diverse designs Loss of SFP coolant inventory events for which correcove

- would be expected. For purposes of this study, loss of spent fuel actions are taken prior to the severe consequences described cooling is considered to include subcategories of loss of SFP above have the potential for other problems. Even a minor loss coolant inventory and loss of SFP cooling; this convention will of SFP coolant inventory can lead to loss of SFP cooling because be used throughout. Potential problems with SFP coolant the lower SFP level causes loss of suction to the SFP cooling {

inventory and SFP cooling which can lead to loss of spent fuel system, Losses of SFP coolant inventory may produce flooding l cooling are discussed. The potential consequences of loss of or environmental problems in other areas of the plant. Ventilation spent fuel cooling are considered. Once the problems have been and drain systems can transport water and steam to other parts of identified, possible approaches to prevention and response to loss the plant and impact emergency equipment. A significant amount of spent fuel cooling situations are described. of water vapor may be generated either by direct boiling or i evaporation from the SFP. Various SFP equipment and '

Figure 1 shows a " generic" PWR SFP and Figure 2 shows a ventilation configurations may allow the water vapor to

" generic" BWR SFP. accumulate on and cause SFP cooling equipment to fail, further 3 exacerbating the loss ofinventory, j The following discussion considers potential scenarios which 4 can lead to loss of spent fuel cooling due to (1) loss of SFP Where the SFP area atmospheric water vapor can be  !

coolant inventory sufficient to interrupt heat transfer to the transported to areas which house other equipment important to cooling system or result in uncovery of the fuel and (2) failure of safety, that equipment may be affected. This potential problem is  !

the SFP cooling system pumps and heat exchangers to transfer important in some multiunit sites during and immediately l heat from the pool to the ultimate heat sink. Figure 3 is a following full core offloads, where the fuel pool atmospheric schematic classification of the types of events which could lead water vapor from the unit refueling can be transported to areas to loss of spent fuel cooling.

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May 26-30,1997 Nice, France housing safety equipment for the unit operating at or near full Preventing a loss of SFP coolant inventory due to gate seal power. In this situation, this transport could cause equipment failures or cavity seal failures relies on correct installation and required for a safe shutdown of the operating unit to be damaged testing of the seals, and testing and control of the air supply for or to fail. nis issue is discussed in Section 5.2. Most plants have the inflatable seals. Better seal performance could be achieved by sufficient flood protection, ventilation, and equipment separation seal replacement at intervals consistent with manufacturers so that this scenario is not a problem. Ilowever, according to a recommendations or when inspection of seals shows evidence of recent survey assessment by NRC's Office of Nuclear Reactor aging, cracking, or tearing.

. Regulation (NRR), eight multiunit sites may be susceptible to this scenario.

%c response to loss ofinventory events depends, first of all.

on timely discovery of the event by the operator. The rate ofloss l 2.2 Loss of Spent Fuel Pool Cooling of SFP coolant inventory can vary greatly depending on the Figure 3 also represents potential causes ofloss of cooling cause; for example, water level drop from a reactor cavity seal to the SFP. Cooling can be lost by loss of SFP cooling flow or due failure can be quite rapid. The reduction in level during these to an ineffective SFP heat sink. Losses of SFP cooling system events is usually discovered either by direct observation by flow can occur due to several mechanisms including: loss of operations staffin the spent fuel area or due to alarm actuation in

. electrical power to the SFP cooling pumps, pump failure, loss of- the control room. Reliable and accurate instruments and suction due to loss oflevel, flow blockage or diversion in the SFP annunciators can alert the operator to a SFP event. If the operators cooling system. Losses of heat sink can occur due to operation are aware of a SFP event in a timely manner, the large volume of with less than the required SFP cooling system complement or water in the SFP will usually allow sufficient opportunity for with heat loads in the SFP in excess of the SFP cooling system operator response to diagnose and correct the problem.

design capability.

Response to loss of SFP cooling requires effective 2.2.1 Consequences of Loss of SFP Cooling instrumentation, procedures and training. Most operating An extended loss of SFP cooling would resuh in heat up and . situations would allow a relatively long time to respond to such boil off of SFP coolant inventory and eventual uncovery of the an event. Ilowever, following a full core offload, the SFP could stored fuel in the unlikely event that no corrective actions were heat up to near boiling in a few hours. Operators would attempt taken. This would result in high levels of radiation in the SFP area to restore cooling either by correcting any problems with the SFP and deny personnel access. Clad failure and radiation release cooling system, or by initiating operation of backup cooling l could be the final outcome. Ilowever, losses of cooling pose less systems,if available. '

hazard than loss ofinventory because loss of cooling does not pose the immediate threat of fuel uncovery. No fuel damage is As with prevention and response to SFP coolant inventory likely until the fuel is uncovered. events, prevention and response to loss ef SFP cooling is also largely dependent on configuration control and human During an extended loss of SFP cooling, water vapor may be performance. The primary concern is to maintain electrical power l generated either by direct boiling or evaporation from the SFP. to the equipment involved in SFP cooling. I Various SFP equipment and ventilation configurations may allow the water vapor to condense and accumulate in locations which 3 OPERATING EXPERIENCE  ;

could affect other equiprNut. All the potential impacts that apply Operating experience with SFP loss of coolant inventory and to the situation describu Mye for loss of SFP coolant inventory loss of cooling was reviewed. The primary source ofinformation

'leadmg to generation of steam and water vapor which is was licensee event reports (LERs) from 1984 through early 1996, transported to other parts of the plant applies to the extended loss screened from the Sequence Coding and Search System. In some of SFP cooling, cases, events before 1984 were included due to sparse data for some types of events. Additional information sources included 2.3 Presenting and Responding to Spent Fuel Pool Events event notifications made in accordance with 10 CFR 50.72, NRC There are no systems available for automatic response to a Inspection Reports, NRC regional morning reports, NRC i loss of SFP coolant inventory or loss of SFP cooling, preliminary notifications, and industry cotamunications. More Consequently, operator actions form the basis for preventing and than 700 separate sources ofinformation were reviewed. This responding to a loss of spent fuel cooling. screening process resulted in about 260 events related to SFPs.  ;

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Tha 5th int:rnitienil Canfarrnca on Nucinr Enginnring ICONES I May 26-30,1997 Nice, France Table 1 is a summary of these SFP events listing the number of Table 2 Loss of Coolant Inventory Events events of each type under the two main categories (loss of SFP coolant inventory and loss of SFP cooling). That table indicates that numerous precursor events were found during the study. Type of Event Actual Precursor These precursor conditions represent potential losses of SFP coolant inventory or loss of SFP cooling given the condition nnected Systems 20 12 which did occur plus other postulated failures.

Configuration Control 16 2 Table 1 Spent Fuel Pool Events Siphoning 3 1 PWR Transfer Tube 1 1 Piping 0 Type of Event Actual Precursor 1

Piping Seismic Design 0 7 SFP Inventory 33 M Gates and Seals 10 $

Connected Systems 20 12 Cavity Seals 0 6 l Gates and Seals 10 8 Gate Seals 10 2 Structure or Liner 8 35 Pool Structure or Liner .3 M SFP Cooling M 22 Liner Leaks 7 1 Cooling Flow Load Drops 1 32 50 20 Pool Seismic Design 0 2 Ileat Sink 6 2 The operating events obtained in this study provide a Using the number of events found during this study over a reasonable representation of experience with SFPs. However, period of about 12 years for which level drops could be during discussions with operations staff, a number of additional quantified, the frequency ofloss ofinventory events in which loss events were discovered which provide insights mto problems with ofmore than I foot occurred can be estimated to be on the order SFPs. While these events have been included in this study, they ofless than I per 100 reactor years.

were not imtially captured by the study's event review proctss, primarily because some relevant events are below the reporting 3.2 Spent Fuel Pool Cooling threshold required by SRC regulations. Fifty-six events found during the operating experience review involved actual losses of SFP cooling. There were 22 3.1 Loss of Spent Fuel Pool Coolant Inventory precursor events which when coupled with additional failures or About 38 events involved actual loss of SFP coolant or postulated events could result in losses of SFP cooling. Table 3 refueling water. There were about 55 precursor events. Table 2 provides a summary of the numbers and types ofloss of SFP pr vides some details about loss of SFP coolant inventory events. cooling events. Figures 6 and 7 provide an overview of the loss Figures 4 and 5 provide an overview of the SFP loss of coolant of SFP cooling events for which temperature increase and inventory events for which level drops and duration times could duration could be quantified. These figures indicate that the losses be quantified. These figures show that SFP losses of coolant of SFP cooling are infrequent. Ilowever, some events have lasted inventory have been infrequent. Ilowever, several events have for significant time periods and four events have resulted in l lasted more than 12 hours1.388889e-4 days <br />0.00333 hours <br />1.984127e-5 weeks <br />4.566e-6 months <br /> and about 10 events have resulted in temperature increases of more that 20 'F. The low number of level decreases of more than 1 foot before the event was events found with small temperature increases may be due to a terminated. The low number of events found with smaller level lack of reporting of such events.

changes may be due to a lack of reporting of such events.

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Tha 5th Intsrnational Confarance on Nucisar Enf,inscring ICONE5 May 26-30,1997 Nice, France NUMBER OF OCCURRENCES 8 NUMBER OF OCCURRENCES 10 7

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<1 1 TO 4 4 TO 8 8 TO 24 > 24 <3 3 To 12 12 To 60' 60 DLJRATION (HRS)

LEVEL DECREASE (INCHES)

Figure 4 Loss ofInventory Duration Figure 5 Loss of Inventory Lesels 1

Table 3 Loss of Cooling Esent Figure 8 shows the history of full core offloadmg times at I Nine Mile Point Unit 2. Figure 9 shows the ranges of calculated

.t .ype of E,sent Actual Precursor times available to initiate boiling at Nine Mile Point Unit 2. For operation with the SFP gates out, the licensee's conservative Cooling Flow calculations estimated the ame to initiate boiling reduced from 51 EQ 10 hours1.157407e-4 days <br />0.00278 hours <br />1.653439e-5 weeks <br />3.805e-6 months <br /> during the first refueling outage to 24.2 hours2.314815e-5 days <br />5.555556e-4 hours <br />3.306878e-6 weeks <br />7.61e-7 months <br /> during the SI P Pumps 39 8 founh refueling outage. For operation with the SFP gates Configuration Control nstalled, the licensee's conservative calculations estimated the 1 0 Loss of Pump Suction 4 0 time tc, initiate boiling reduced from 17.6 hours6.944444e-5 days <br />0.00167 hours <br />9.920635e-6 weeks <br />2.283e-6 months <br /> to 8.4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />.

Flow Blockage 1 0 Similarly, during a visit to the South Texas plant, AEOD learned Single SFP Pump Failure 5 12 h hWim WoM fm a e MW g estimated that the initiation of boiling could begin approximately licat Sink 6 """# # "E # #" " # # '# "

2 performed by NRR indicated that, if a full core had to be ofiloaded during midcycle, boiling could begin about 2 to 3 hours3.472222e-5 days <br />8.333333e-4 hours <br />4.960317e-6 weeks <br />1.1415e-6 months <br /> after losing SFP cooling.

Using the number of events found during this study over a period of about 12 years for which temperature and duration 3.4 Operating Experience Review Findings could be quantified, the frequency of loss of SFP cooling events 1.osses of SFP or refueling water inventory are dominated by in w hich a temperature increase of more than 20 *F occurred can events involving system or SFP configuration control problems be estimated to be on the order of about 2 to 3 per 1000 reactor due to human error. The second most prevalent cause ofloss of years. SFP inventory is leaking inflatable gate seals generally due to loss of air to the seats because of human error. Losses ofinventory 3.3 Shortening Refueling Outage Times from SFP gates due to leaking inflatable gate seals have generally Review of operating experience has shown that in an effort been of greater magnitude than those due to configuration control to minimize refueling outage times, many plants perfonn full core problems. Loss of inventory due to configuration control ofiloads early in their outages. The effect of such practices is to problems is more easily controlled by the operations staff than reduce the time available to recover from a loss of SFP cooling leaks from gates. Ilowever, configuration control problems seem esent. AEOD discussions with the engineering manager of Nine to have taken longer to diagnose.

Nile Point Umt 2 provided good insight to the effect this practice has upon reducing the time available until boiling begins.

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The 5th International Conference on Nuclear Engineering ICONES May 26-30,1997 Nice, France NUMBER OF OCCURRENCES NUMBER OF OCCURRENCES l 25 20 22 16 1@ 32 HRS 15 1@ 30 HRS 15 1@ 24 HRS 10 to 10 6

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y_ nn 24 2 to i 1 2 3 4 1 2 3 4 REFUEL OUTAGE NUMBER REFUEL OUTAGE NUMBER Figure 8 History of Full Core Offloading Figure 9 Reduced Time to Boil Pool leakage events do not appear to have caused problems frame and associated temperature rises. The events were evenly with long-term losses of spent fuel cooling. Inadvertent distributed between BWRs and PWRs.

movement of heavier than allowed loads over SFPs is continuing even though the NRC has taken steps to reduce this problem. While conditions have been reported suggesting the possibility of SFP boiling affecting other plant equipment The most prevalent type ofloss of cooling events involved important to safety, operating experience does not provide loss of electrical power to the SFP cooling pumps, generally due insights into what is apparently a very compi:x issue.

to human error. He few losses of SFP cooling due to loss of SFP heat exchanger cooling were also generally due to human error. Operating experience provides only limited insight into Both types of events resulted in losses of about the same time instrumentation problems. Several loss oflevel events have taken place while level instrumentation was inoperable or level 7

i Tha 5th intirn tisnil Csnfiranca en Nucliar Enginnring ICONES May 26-30,1997 Nice, France annunciators were already actuated for other reasons. There have The newer plants have more of the better features such as been relatively few of these instrumentation problems captured by safety-related power, analog control room meters, more parameter this study. They represent concerns about how SFP indicators in the control room, more sources of water, and instrumentation is treated and regarded. generally better qualified equipment. Ilowever, some older pluts have made improvements by adding indicators or annunciators in Some ventilation events (damper problems, heater problems) the control room, and supplying safety-related power to the SFP could be potential areas of concern when coupled with postulated equipment. All of the sites visited are including the SFP system

) SFP events which could lead to radiation release. in the equipment covered by the Maintenance Rule.

Foreign operating experience appears to be consistent with 5 RISK ASSESSMENT that from U.S. plants. Operating experience suggests that losses Over the years, the SFP has not received the risk assessment of make-up capability are not very likely. attention that the reactor had because early analysis put the risk of l a SFP accident an order of magnitude below a reactor event.

l 4 OBSERVATIONS FROM THE SITE VISITS AND Therefore, the analyses done for the SFP were limited. Ilowever, INTERVIEWS in recent years several issues have required that certain aspects of Six site visits were conducted to gain understanding of the the SFP be studied further. INEL was contracted to review the licensees' SFP physical configurations, practices, and operating previous SFP risk assessments and to utilize the useful insights to procedures. Site selection was a cross sampling of the industry assess the current risk of SFP accidents. In addition to those risk that included BWRs and PWRs, large and small architect.

1 insights, INEL utilized the AEOD operating experience review, engineer designs, shared and single pools, old and new designs engineering analyses, site visits, and site interviews in assessing and all four U.S. nuclear steam supply system vendor designs. the likelihood of SFP events.

In general, utilities are doing a good job of analyzing the 5.1 ' Risk Analysis for Spent Fuel Pool Cooling at SFP heat loads and heat up rates. Ilowever, control room Susquehanna Electric Power Station" operators are not always being made aware of the analysis and in October 1994, Battelle Pacific Northwest Laboratory results. This information could piove to be criticalin worst case (PNL) prepared a drafl report, " Risk Analysis for Spent Fuel Pool

. refueling outage conditions (e.g., full core offload and a very Cooling at Susquehanna Electric Power Station," (Ref,3) for short outage schedule). Some of the utilities are performing risk NRC's Risk Applications Branch of NRR. The report presented analysis as part of the outage planning. the results of PNL's analysis ofloss of SFP cooling events at the Susquehanna nuclear power plant, including estimates of the Some utilities have used lessons from operating experience likelihood for loss of SFP cooling, the near-boiling frequency and have done a very good job in correcting problems through (NBF), and order of magnitude estimates of core damage better analysis, good operator aids, training, and procedure frequency (CDF) attributed to SFP heat-up events.

revisions. Some utilities have a good system to evaluate industry experience. 5.2 Refinements of the Susquehanna Risk Assessment AEOD obtained technical assistance in the area of risk The site visits identified events where connected systems assessment from INEL. INEL reviewed the PNL Susquehanna could have caused loss of SFP coolant inventory. Many events PRA, assessed the adequacy of the risk analysis, and addressed such as draindowns are not being reported through the standard the adequacy and reasonableness of the assumptions made. INEL mechanisms that would allow for the standard analysis of the perfonned refmements of the PNL study, including modifications events. Therefore, the actual frequency of draindowns is higher of the initiating event frequencies using AEOD's operational than is typically assigned in the risk analysis. The site visits also event database, to cover a full spectrum ofloss of SFP inventory identified that little attention is paid to the antisiphon devices. events, including catastrophic seal failure (Ref. 4). The results of Very few sites performed testing or had analysis on the efficacy their analysis are shown in Table 4. The analysis found the NBF j of the antisiphon devices. for the Susquehanna plant after implementing the 10 CFR 21 improvements was SE-5/ year, which is approximately twice that found by PNL.

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Th3 Sth Int;rn tinn11 C:nf:ranca en Nucl:ar Enginnring i ICONES May 26-30,1997

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l Table 4 Near-Hoiling Frequencies (3) Effect of the Susquehanna 10 CFR 21 Report INEL PNL *E*"' " # ""* "" "' *### " # #

Susquehanna plant as it existed at the time of the 10 CFR 21 Total NBF report and after corrective actions were taken revealed that the 5 E-5 2E5 improvements that were made m the areas ofinstrumentation, LOOP 3 E-5 1E5 accident response procedures, operator training, and shutdown operations reduced the estimated NBF.

Inventory Losses 2 E-5 1E6 Improvements in instrumentation consisted of providing reliable SFP level and temperature monitoring instruments in the control room.

The dominant event mitiators were LOOP and SFP inventory Improvements in operations and accident response losses including configuration control errors and seal failures. procedures involved:

Due to the hmited time and resources available, INEL did not extend the analysis to include a quantitative estimate of the CDF. -

ventilation system isolation installation of drains in the standby gas treatment system 5.2.1 Qualitatise Results -

utilization of the RilR system of the operating unit to The SFP PRAs which were done by PNL and INEL were cool the SFP specifically for the Susquehanna plant. Many features of the -

venfication that removal of cask storage pit gates results design and operation of Susquehanna are unique, consequently in effective heat transfer between the SFPs the results of the PNL and INEL analyses cannot be applied directly to other plants. Nonetheless, there are certain qualitative (4) Dominant accident sequences insights that have been learned from those studies which may have generic applications. For example: For the Susquehanna plant, the PNL analysis found that the accident sequences which were the largest contributors to NBF (1) Effect of defueled unit upon operating unit were extended LOOP, and LOCA. The extended LOOP is a dominant contributor because at the Susquehanna station the SFP The analyses showed that for a dual unit BWR, it is possible cooling system pumps are not on die emergency busses. The for a problem with SFP cooling at a shutdown unit to affect the original accident scenario raised in the 10 CFR 21 report did not adjacent operating unit. The accident scenario postulated in the appear to be a significant contributor to NBF. The INEL study Susquehanna 10 CFR 21 repon was found to be a credible event, found the dominant contributors to NBF were LOOP and SFP but less likely than other events. inventory loss.

(2) Uncertainties of core damage frequency estimates (5) Deviation from the modeled plant design The task of estimating the NBF appears to be amenable to Risk estimates from the SFP for *he Susquehanna plant may the use of PRA techniques. llow ever the task of estimating CDF be affected by changes planned for future refueling outages, is subject to very large uncertainties. PNL and INEL both which may represent major deviations from the models used by acknowledged that the methodology used for this task provided PNL and INEL Some of those anticipated changes are:

only " order of magnitude estimates."

operation without the SFP cross-tied for the future dry cask storage operations reduction of refueling outage from 55 days to 35 days partial core offloads taking place earlier in the outage 9

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l 7 Tha 5th inttrnatian:1 Csnfirinca en Nucl2:r Enginzring j ICONES J May 26-30,1997 Nice, France (6) Operatmg experience .

Efforts by utilities to reduce outage duration have resulted in full core officads occurnng earlier in outages. His increased ,

INEL found that SFP inventory losses such as draindowns fuel pool heat load reduces the time available to recover j or pneumatic seal failures may be important contributors to NBF from a loss of SFP cooling event early in the outage, at the Susquehanna plant. In previous PRAs such events were either not modeled or their occurrence frequency was assumed to .

The need for improved procedures and training for control be very low; once every 10,000 reactor years. l room operators to respond to SFP loss ofinventory and SFP loss of cooling events consistent with the time frames over 6 CONCLUSIONS which events can proceed, reco,,aizing the heat load and the The study's main conclusions are:

possibility of loss of inventory, should be evaluated on a

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l plant specific basis.

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Review of more than 12 years of operating experience determined that loss of SFP coolant inventory greater than +

The need for improvements to instrumentation and power 1 foot has occurred at a rate of about l per 100 reactor years. supplies to the SFP equipment to aid correct operator Loss of SFP eooling with a temperature increase greater than response to SFP events should be evaluated on a plant 20 *F has occurred at a rate of approximately 3 per 1000 specific basis.

reactor years. De consequences of these actual events have not been severe, liowever, events have resulted in loss of several feet of SFP coolant level and have gone on in excess 7 REFERENCES of 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br />. The primary cause of these events has been l human error. i

1. U.S. Code of Federal Regulations, Title 10 " Energy," i U.S. Government Printing Office, Washington, D.C., revised i Review of existing SFP risk assessments found that after periodically. j correction for several problems in the analyses, the relative risk due to loss of spent fuel cooling is low in comparison l

2. Lochbaum, D.A., and Prevatte, D.C., Letter to Martin, T.,

with the risk of events not involving the SFP. The review U.S. Nuclear Regulatory Commission, "Susquehanna Steam determined that the likelihood and consequences ofloss of Electric Station, Docket No. 50-387 License No. NPF-14, SFP cooling events are highly dependent on human 10 CFR Pan 21 Report of Substantial Safety Hazard,"

performance and individual plant design features. November 27,1992, ne need for specific corrective actions should be evaluated 3. Battelle Pacific Northwest Laboratory, Draft Report under for those plants where failures of reactor cavity seal or gate NRC Contract DE.AC96-76RLO 1830," Risk Analysis for seals, or ineffective antisiphon devices could potentially Spent Fuel Pool Cooling at Susquehanna Electric Power cause loss of SFP coolant inventory sufTicient to uncover the Station," October 1994.

fuel or endanger makeup capability.

4. Idaho !Cational Engineering Laboratory," Loss of Spent Fuel l ne need for improvements to configuration controls related Pool Cooling PRA: Model and Results," INEL-96/0334, to the SFP to prevent and'or miti gate SFP loss ofinventory September 1996.

events and loss of cooling events should be evaluated on a plant specific basis.

The need for plant modifications at some multiunit sites to t account for the potential effects of SFP boiling conditions on safe shutdown equipment for the operating unit, particularly during full core officads, shouhl be evaluated on a plant specific basis.

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