ML20199G783
| ML20199G783 | |
| Person / Time | |
|---|---|
| Issue date: | 07/20/1998 |
| From: | Essig T NRC (Affiliation Not Assigned) |
| To: | Rausch T COMMONWEALTH EDISON CO. |
| References | |
| PROJECT-691 NUDOCS 9901250019 | |
| Download: ML20199G783 (10) | |
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.] p urg y UNITED STATES
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{o RUCLEAR REGULATORY COMMISSION
~f WASHINGTON, D.C. 20666 4001
+9 ,o July 20,1998 Mr. Thomas J. Rausch, Chairman Boiling Water Reactor Owners' Group Nuclear Fuel Services Commonwealth Edison Company 1400 Opus Place,4th Floor ETWill Downers Grove, IL 60515
SUBJECT:
BWROG EMERGENCY PROCEDURE AND SEVERE ACCIDENT GUIDELINES
Dear Mr. Rausch:
By letter dated August 29,1996, the Boiling Water Reactor Owners' Group (BWROG) provided Revision 0 of the Emergency Procedure and Severe Accident Guidelines (EP/ SAG) to the NRC staff for information. The staff provided the results of its limited review of the EP/ SAG document in a letter to the BWROG dated April 2,1997, and identified several areas where the guidelines appear to reduce safety margins and increase consequences relative to Revision 4 of the Emergency Procedure Guidelines (EPGs). The letter also identified additional information andjustification considered necessary to resolve these concems. The BWROG provided responses to the staff's concems in a letter dated January 9,1998, together with a l copy of Revision 1 of the EP/ SAG. i The staff has reviewed the additionalinformation provided by the BWROG to determine the degree to which it resolves the issues raised in the April 2,1997, letter. Based on the 1 responses and the supporting materialin the appendices to the EP/ SAG, the staff considers most of the items to be adequately addressed, and the actions specified in the guidelines to be reasonable. These include issues related to the following areas of the guidelines:
e bypms of radiation interlocks e cold and hot shutdown boron weights e reactor pressure vessel RPV water level for depressurization and containment flooding e sequence of operator actions taken to control containment pressure e use of drywell spray in conjunction with venting o effect of venting on drywell spray initiation limits e emergency depressurization after exceeding the pressure suppression pressure limit in addition, the EP/ SAG includes modifications to the EPGs, Revision 4, to address reactor core thermal-hydraulic instabilities that were approved by the staff in its safety evaluation transmitted by letter dated June 6,1996, from D. Matthews, NRC, to K. Donovan, Centerior Energy.
Despite further clarification by the BWROG, the staff is still concemed about several other aspects of the guidance provided in the EP/ SAG, specifically:
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j , Mr. Thomas J. Rausch o climination of guidance regarding alternate boron injection methods
.r e increased flexibility in venting the containment, particularly where venting is permitted irrespective of dose e elimination of the previously established priority of wetwell venting over drywell venting
- containment pressure for drywell spray termination
- use of extemal water sources following RPV breach
- e overly-conservative assumptions recommended for establishing certain limit curves which could defeat the purpose of actions directed elsewhere in the guidelines e lack of emphasis on the importance of establishing a drywell pool orior to reactor vessel breach to reduce the threat of liner melt-through l
l In general, the staff believes that the guidance in these areas provides unwarranted latitude to I utility and plant staff responsible for developing and/or executing the plant-specific EP/ SAG, and does not go far enough in addressing insights regarding melt-through of the containment shellin Mark l containments. You are requested to consider the staff's views and expectations l regarding licensee implementation of the generic guidance that are provided in the Enclosure and notify the staff when you can meet to discuss them.
In order to schedule a meeting with the staff, or if you have any questions about this request, please contact the NRC project manager, James H. Wilson, at (301) 415-1108.
Sincerely,
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Original Signed By Thomas H. Essig. Acting Chief Generic issues and Environmental Projects Branch Division of Reactor Program Management Office of Nuclear Reactor Regulation Project No. 691
Enclosure:
As stated cc w/ enclosure: See next page DISTRIBUTION:
Docket File TCollins PUBLIC MChatterton PGEB r/f AUlses JRoe DMatthews TEssig MMalloy e JWilson DOCUMENT NAME: G:UHWitBWROGEPG. SAG *see previous concurrence l
l OFFICE PM.PGEB g (A)SC:PGEB DSSA/SCSB/NRR b (A)BC:PGE%
JHWilsh h TEssig 'd NAME MMallhb CBerlinger DATE 7 M98 / 7GO /98 7/20 /98 7 M/98 OFFICIAL RF CORD COPY l
.r Project No. 691 Boiling Water Reactor Owners Group cc: Thomas J. Rausch, Chairman W. Glenn Warren Boiling Water Reactor Owners' Group Southem Nuclear / Georgia Power Commonwealth Edison Company E.I. Hatch Nuclear Plant Nuclear Fuel Services PO Box 1295 M/C B052 1400 Opus Place,4th Floor ETWill Birmingham, AL 35201 Downers Grove,IL 60515 Carl D. Terry Dennis B. Townsend Vice President, Nuclear Engineering GE Nuclear Energy Niagara Mohawk Power Corporation M/C 182 Nine Mile Point-2 175 Curtner Avenue PO Box 63 San Jose, CA 95125 Lycoming, NY 13093 Drew B. Fetters Thomas A. Green PECO Energy GE Nuclear Energy Nuclear Group Headquarters Mail Code 182 MC 62C-3 175 Curtner Avenue 965 Chesterbrook Blvd. San Jose, CA 95125 Wayne, PA 19087 John Hosmer John Kelly Commonwealth Edison New York Power Authority Executive Towers,4th Floor 14th Floor Mail Stop 14K 1400 Opus Place Centroplex Building Downers Grove,IL 60515 123 Main Street White Plains, NY 10601 George T. Jones Pennsylvania Power & Light MC AS-1 Two North Ninth Street Allentown, PA 18101 Lewis H. Sumner Southem Nuclear / Georgia Power E. l. Hatch Nuclear Power Plant 40 inverness Parkway PO Box 1295 Birmingham,GA 35201
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r 3 NRC Staff Views on BWROG Emergency Procedure and Severe Accident Guidelines Guidance Reaardina Alternate Boron iniection Methods in Revision 4 of the Emergency Procedure Guidelines (EPGs), licensees were encouraged to identify attemate methods of boron injection, and as a result, many licensees developed procedures for altemate boron injection. In the new Severe Accident Guidelines (SAG), the BWROG purposely decided to omit any direction for attemate and extended boron injection based on a "BWR Principle" that recriticality is very unlikely upon flooding a standing core in which control blades have melted but fuel rods have not. The BWROG also claimed that use of attemate methods generally requires access to areas that may be uninhabitable under conditions requiring containment flooding, and may interfere with use of reactor pressure vessel (RPV) injection systems for primary containment flooding. However, the BWROG guidance provides that if a plant decides that attemate boron injection methods are necessary, plant-specific procedures can be included.
The staff disagrees with the BWROG position that guidance for attemate boron injection methods is unnecessary. The staff notes that alternate boron injection methods could be beneficial in two types of accidents: (1) anticipated transient without scram (ATWS) events in which the initial boron inventory is depleted, and (2) degraded core events in which the standby liquid control system is unavailable. BWROG arguments that attemate methods may interfere with primary flooding are not relevant in ATWS events since these events do not require containment flooding. Furthermore, the probability of recriticality during a degraded core accident is an area of considerable uncertainty, and should not be a basis for eliminating reasonable measures to deal with potential challenges to the reactor coolant system (RCS). In the staffs view, licensees remain responsible for considering and incorporating methods for attemate or extended boron injection in plant-specific guidance. Where licensees have already incorporated guidance on attemate boron injection in their plant-specific procedures, this guidance should not be eliminated without adequate plant specific justification.
Contabment Ventina The Emergency Procedure and Severe Accident Guidelines (EP/ SAG) permit containment venting, irrespective of the offsite radioactivity release, for several purposes: (1) to maintain containment pressure below the primary containment pressure limit, (2) to maintain hydrogen and oxygen concentrations below deflagration limits, (3) to reduce containment back-pressure in order to facilitate containment flooding or reactor pressure vessel (RPV) injection, and (4) to maintain suppression chamber pressure below the pressure suppression pressure (PSP) limit thereby assuring containment integrity in the event of RPV breach. The guidance regarding venting for the first two purposes is essentially unchanged from that in EPG, Revision 4, and is addressed in the staffs previous safety evaluation. Venting for the last two purposes represents an expansion in the scope of accidents for which venting would be permitted, relative to EPG Revision 4. In addition, the guidelines eliminate the provision in EPG, Revision 4 that drywell venting will not be undertaken unless the suppression chamber could ENCLOSURE l
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not be vented. In the staff's view, containment venting in the presence of a significant source term should remain a last resort action, and should be carried out only after a deliberate evaluation of the consequences of venting relative to the option of not venting. This is l
discussed further in the sections below. ;
Venting to Facilitate Primary Containment Flooding or RPV injection Containment venting is permitted if it will facilitate containment flooding (EP/ SAG Guideline RC/F-1 and Guidelines RC/F-3 through -6) or restoration of RPV water level above top of active fuel (Guideline RC/F-2). Appendix B of the EP/ SAG indicates that venting is appropriate if it can appreciably shorten the time required to submerge core debris, irrespective of the resulting offsite radioactivity release rate, even if containment pressure limits are not challenged. The document states that the potential radioactivity release may not be warranted if venting will only nominally increase the flooding rate, but specific criteria for making this judgement are not provided. In response to a staff comment, the BWROG indicated that the effects of containment venting on flooding rate must be weighed against projected dose rates of the resulting radioactivity release in determining whether to vent containment to facilitate containment flooding. To make this decision, the emergency response organization (ERO) is expected to consider pump curves of the associated pumps, desired pressure reduction, expected duration of venting to achieve the desired pressure reduction, and the projected dose rates which would result from the expected venting. This information is both plant-specific and event-specific. For a given plant condition, what may be appropriate in some events may be inappropriate in other events simply due to differences in meteorological conditions or protective actions taken by the state or local agencies.
The staff believes that the guidance regarding venting to facilitate containment flooding or RPV injection provides unnecessary latitude to utility and plant staff responsible for developing and/or executing the plant-specific EP/ SAG. Further controls on venting should be established to assure that venting operations are performed only after careful consideration of the need for venting versus the projected offsite consequences of venting. Consistent with the BWROG response, which indicates that these decisions are plant- and event-specific, the staff expects each licensee to develop supplementary guidance and criteria for use by ERO staff in determining whether venting, that is otherwise permitted by the generic guidelines, is appropriate given plant-specific and event-specific considerations. The supplementary guidance should provide reasonable assurance that venting will not lead to an increase in consequences relative to the option of not venting when these considerations are taken into account.
Venting to Maintain Suppression Chamber Pressure Below PSP The guidelines direct the operators to vent containment to remain below the PSP limit if breach of the RPV is imminent and pressure suppression capability is functional (RC/F-5). The PSP limit is used to ensure that containment pressurization from RPV breach does not exceed the primary containment pressure limit (PCPL). Use of PSP is based on the assumption that
, containment pressurization at reactor vessel breach (with the RPV depressurized) would be l comparable to that following a design basis loss-of-coolant-accident (LOCA).
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- o The staff questioned the BWROG rationale for venting at this critical time and for using PSP as the criterion for initiating venting in lieu of more realistic estimates of containment pressurization. The staff view is that, for sequences in which the RPV is depressurized, containment pressurization at RPV breach would be substantially less than the pressurization associated with a design basis LOCA. Therefore, guidance to maintain pressure below PSP may result in unnecessary venting. The BWROG response stated that the consequences of failure to maintain the containment pressure suppression function under these conditions may be a loss of primary containment integrity and an uncontrolled release greater than what might otherwise occur. The BWROG did not provide additionaljustification for using PSP as the criterion for venting.
The staff agrees that controlled releases associated with venting to preserve the pressure suppression function are preferable to containment failure and uncontrolled releases which might occur if the pressure suppression function is compromised. However, an acceptable basis for using PSP as the criterion for venting for depressurized sequences has not been provided by the BWROG. In developing plant-specific guidance, licensees should consider the margins to PCPL for realistic estimates of containment pressurization at RPV breach, and justify that the criterion selected for initiating containment venting is appropriate and not unduly conservative.
Priority of Wetwell Over Drywell Venting in the EPGs, Revision 4, the suppression pool vent is given first priority for containment venting, thereby achieving the benefit of pool scrubbing. Drywell venting would not generally be undertaken unless the suppression chamber could not be vented. In the revised guidelines, the directions specifying vent path preferenc :s and restrictions with respect to the use of suppression chamber and drywell vents have been deleted. Appendix B indicates that such detailed instructions for venting the primary containment are beyond the scope of the generic guidelines and should be provided through plant-specific operating procedures.
In the staff's view, the venting priorities established in Revision 4 should be unchanged by this revision. The staff considers it incumbent upon the licensee to continue to ensure that full !
advantage is taken of the benefits of suppression pool scrubbing when feasible. For example, l plants with a hardened wetwell vent path through the secondary containment should continue to place priority on this vent path over drywell vent paths. The staff expects these priorities to be i reflected in the plant-specific operating procedures for venting.
I Containment Pressure for Drvvmil Sorav Terminat!on i 1
in Revision 4 of the EPGs, the operator is instructed to terminate drywell spray if drywell pressure drops below the high drywell pressure scram setpoint (2 psig). In contrast, the EP/ SAG permit dryweli sprays to be operated entil containment pressure reaches O psig. This change was made in order to permit continued spray cperation for fission product scrubbing if the containment has failed or is at low pressure. Sprays are terminated prior to the containment reaching atmospheric pressure in order to avoid drawing a vacuum and introducing air into containment via the pressure suppression chamber vacuum breakers. This is necessary to maintain containment pressure above the negative design pressures of the
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drywell and suppression chamber, and to maintain the inerted status of the Mark I and 11 containments.
The basis for using the scram setpoint in Revision 4 of the EPGs was that it provided margin in preventing negative containment pressure and deinerting. The staff expressed concem with the reduction in this margin and how licensees would protect against negative contai vnent pressure, especially given the reliance on manual operator action and the potential for instrument error. This concem is heightened by the fset that the containment may be vented prior to or during spray operation. The BWROG indicated that evaporative cooling can create a rapid pressure and temperature reduction rate but is limited to the initial pressure reduction following spray initiation. Continued pressure reduction after the initial evaporative cooling pressure drop is due to convective cooling and produces a much lower pressure and temperature reduction rate that is easily controlled by operators. Shortly after sprays are initiated as permitted by the drywell spray initiation limit (DWSIL), the evaporative cooling phase I of pressure reduction will be completed and, by definition of the DWSIL, the pressure will be at or above the drywell high pressure scram setpoint. The containment atmosphere will be saturated and the subsequent convective cooling phase willinvolve much slower rates of atmosphere temperature and pressure reduction. With respect to instrument error, the BWROG argued that compensation for such error is not ingrained within the EP/ SAG because of their symptomatic nature, and because this could create conditions where an assumed instrument error could be conservative in one area but non-conservative in another area.
Rather, the operators are expected to compare available parameter indications and determine which appears to be the most reliab!s based on existing plant conditions.
The staff considers the BWROG justification for reducing the drywell pressure for terminating drywell spray to be acceptable. However, the BWROG response does not fully address staff ,
concems regarding measures to protect agairat reverse pressurization and deinerting.
Consistent with the response, if a licensee chooses to implement this provision of the EP/ SAG, the licensee should develop and implement reliable measures to protect against reverse pressurization and deinerting. This should include providing guidance and training to operators conceming the reliability and accuracy of containment pressure instrumentation under design basis and severe accident conditions, and appropriate use of this or other instrumentation (such as vacuum breaker position sensors) to prevent reverse pressurization and deinerthy.
Use of External Water Sources Followina RPV Breach Guideline RC/F-1 instructs the operators to maximize RPV injection from extemal sources following RPV breach in order to flood containment, provide cooling to fuel and core debris remaining in the RPV, and direct water to relocated core debris outside the RPV. The priority on extemal water sources is maintained until the containment water level reaches top of active fuel.
Maximizing injection from extemal water sources for the duration of the event can lead to earlier venting and a net increase in offsite consequences in certain containments and scenarios. For example, in Mark I containments continued injection from extemal sources will compress non-condensible gases in the containment airspace and necessitate drywell venting prior to reaching top of active fuel. (For plants equipped with a passively actuated hardened vent 1
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(rupture disk), the hardened vent may need to be isolated to prevent system actuation and flow of liquid into the vent line.) As such, maximizing injection from extemal water sources minimizes the time to containment vaMing and the time available for fission product decay and removal. A decision to defer or reduce the rate of containment flooding at an appropriate point during the flood up process could substantially delay the time to venting and reduce the associated offsite risk.
l The staff considers the guidance to maximize injection from extemal water sources to be j appropriate up to the point at which the ex-vessel core debris is submerged. However, in '
contrast to the BWROG guidance to maximize injection from external sources for the duration of the event, the staff recommends a more flexible approach where the need for and rate of containment flooding is reassessed during flood up. With the risk r;f containment liner melt-through ameliorated by submerging the ex-vessel core debris, the rate of containment flooding can potentially be reduced without compromising the long term objective of cooling the fuel and core debris remaining in the RPV. Because of the link between containment flooding and containment venting, the staff expects licensees to structure and implement their plant-specific guidance in such a way that the rate of containment flooding and its impact on containment venting is reassessed after ex-vessel core debris is submerged. The decision to proceed with l flooding should be evaluated based on plant- and sequence-specific considerations, and l
coordinated with offsite protective measures.
Plant-Soecific Limit Curves it is expected that each utility will calculate plant-specific limit curves during conversion of the generic EP/ SAG to plant-specific guidelines. Sample limit curves are provided in the generic guidelines to support this process. According to the BWROG, the limit curves in the generic guidelines are based on bounding assumptions in order to address all mechanistically possible events without having to diagnose the event.
The staff is concemed that some of the assumptions for the limit curves are overly conservative and if implemented in their current form would serve to preclude effective use of the affected systems when needed under accident conditions., as illustrated in the following examples:
- The Drywell Spray Initiation Limit is calculated assuming 32'F spray water, no humidity in the drywell atmosphere, and no flow through the vacuum breakers. This would produce a curve that does not reflect typical conditions and that may be so restrictive that drywell sprays cannot be initiated when needed.
e The Minimum Zero injection Water Level is determined assurning that the reactor has been shutdown for ten minutes and that the power shape is the most limiting top-peaked. Assuming such a high decay power and a detrimental power shape is very conservative and would eliminate much of the opportun'ly for delaying the onset of core degradation.
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- The Minimum Drywell Spray Flow is defined as the lowest flow that ensures uniform spray distribution within the drywell. Although a full spray is desirable for fission product removal, the drywell sprays should be operated even if the minimum flow cannot be
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1J' o attained since the sprays can still be effective in providing water to the drywell floor to cool ex-vessel core debris.
Proper determination of plant-specific variables and careful preparation of plant-specific curvet is of particular importance if the mitigative actions specified by the guidelines are to be propcay carried out. In developing plant-specific limit curves, it is expected that licensees will scrutinize the assumptions inherent in the calculational approach for both applicability and efficacy rather than simply substituting plant-specific parameters into the calculational methodology for establishing the limit curves.
The EP/ SAG and Overview Document permit the use of prestaged, attemate curves which have been optimized or computed to bound only the existing conditions, if it is known that all bounding initial conditions do not exist. The guidance stipulates that all attemate optimized EP/ SAG limit curves will be prepared in advance of an event in order to utilize standard procedures for engineering calculations and obtain a disciplined verification and approval before the attemate curve is provided to the control room operating crew. The use of optimized limit curves is considered by the BWROG to be beyond the scope of the generic guidelines, and this decision if left up to the utihty. In view of the potential impact that conservative assumptions in the sample limit curves could have on guideline effectiveness, the staff encourages licensees to develop attemate optimized curves based upon more realistic limits appropriate for their particular plant, and to have these curves available to supplement the sample curves provided with the EP/ SAG.
Drvwell Floodina to Prevent Liner Melt throuch The SAG includes instructions that are expected to result in filling the lower portion of the drywell prior to reactor vessel breach. Specifically, operators are instructed to actuate drywell sprays prior to reaching a radiation level which requires a General Emergency. This radiation level is intended to be representative of 20 percent fuel clad damage and would be reached well before core debris could be released from the RPV. Other calls for the initiation of drywell sprays would likely be issued from other sections of the Containment and Radioactivity Release Control Guidelines, although th3s is not certain.
The staff notes that neither the EP/ SAG, supporting appendices, or the Overview Document include any discussion of the threat of Mark I liner failure due to release of debris onto a dry cor,tainment floor. For example, the BWR principles regarding ex-vessel debris cooling that are descdbed in the Overview Document do not include any consideration of the need to have water on tne drywell floor before debris is released from the reactor vessel. Fwthermore, the severe accident guideline lesson plans developed through the industry's Region 3 SAG Project do not emphasize this point. Although operation of the drywell sprays is specified by the SAG at a radiation level that would be reached before core debris could breach the RPV, spray actuation would be precluded until restrictions on spray operation (e.g., the drywell spray initiation limit) have been satisfied. Use of realistic rather than bounding assumptions in developing limit curves will minimize the potential to proceed to RPV breach without prior actuation of drywell sprays. Nevertheless, given the significance of liner melt-through as a containment failure mechanism, the staff considers it prudent for licensees with Mark l l containments to provide instruction to ERO staff concerning the importance of establishing r
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water pool on the drywell floor prior to RPV breach and maintaining the debris covered to prevent liner melt-through. This instruction can be provided in the form of more detailed ,
guidance or through the ERO training program. '
ADS Inhibit I The instruction to manually inhibit automatic initiation of the Automatic Depressurization System (ADS) in non-ATWS sequences was incorporated in Revision 4 of the EPGs and has been retained in the EP/ SAG. The staff's review of Individual Plant Examination (IPE) submittals shows this action to be a major contributor to core damage frequency for BWRs (see Chapters 11 and 13 of NUREG-1560). The January 9,1998 BWROG response provides rationale for the generic guidance, and offers several arguments as to why this strategy is preferable to allowing automatic actuation to occur. Although several of these arguments appear valid, we are unable to discount this important IPE insight without a more comprehensive assessment.
Principal contributor: R. L. Palla, Jr.
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