ML20147H660

Informs of Preliminary Results of Review of BWROG Emergency Procedure & Severe Accident Guidelines & to Identify Areas Where Addl Info & Discussion W/Bwrog Necessary

ML20147H660
Person / Time
Issue date:	04/02/1997
From:	Matthews D NRC (Affiliation Not Assigned)
To:	Donovan K CENTERIOR ENERGY, GENERAL ELECTRIC CO.
References
PROJECT-691 FACA, NUDOCS 9704040182
Download: ML20147H660 (8)

Text

y mang g & UNITED STATES s :!

t j NUCLEAR REGULATORY COMMISSION WASHINGTON. D.C. 20666 0001

% April 2, 1997 Mr. Kevin P. Donovan, Chairman Boiling Water Reactor Owners' Group Centerior Energy Perry Power Plant MC A210 4 P. O. Box 97 Perry, OH 44081

SUBJECT:

BWROG EMERGENCY PROCEDURE AND SEVERE ACCIDENT GUIDELINES

Dear Mr. Donovan:

By letter dated August 29, 1996, the Boiling Water Reactor Ouiers Group (BWROG) submitted Revision 0 of the BWROG Emergency Procedure and Severe Accident Guidelines (EP/ SAG) and associated support documents, for information. Consistent with the understanding set forth in our May 27, 1994, letter to S. LaBruna, the Nuclear Regulatory Commission (NRC) staff has performed a high-level review of this document. The purpose of this letter is to inform you of the preliminary results of the review, and to identify those

, areas where additional information and discussion with the BWROG is considered necessary to complete the review.

The staff's evaluation focussed on: (1) the transition from the emergency procedure guidelines (EPGs) to the severe accident guidelines (SAGS), (2) the adequacy of the SAG as stand-alone guidance, and (3) the consistency of the

! SAG strategies with the current understanding of BWR severe accident

progression and phenomenology. Specific assessments were made of the SAG provisions for containment flooding, use of drywell spray, containment venting, protection against Mark I liner failure, and prevention of recriticality upon reflood of a damaged core. Changes to the EPGs not directly related to the SAG, and basic assumptior.s for deriving several of the limit curves were also assessed.

Based on its limited review, the staff considers the EP/ SAG document to be effective in establishing the basic goals that the plant-specific accident management guidance should be designed to achieve, such as submerging core and core debris, averting reactor vessel bottom head failure, promoting fission product removal within containment, and preventing uncontrolled releases from containment. However, a number of areas were identified where the revised guidelines appear to reduce safety margins and increase consequences relative to Revision 4 of the EPGs. These items are summarized in the enclosure for further consideration by the BWROG.

I propose that a working-level meeting be arranged at earliest convenience to discuss the rationale and underlying analyses for the guidance in the areas of concern. The objective of the meeting should be to provide sufficient information t xgsqQve the majority of the concerns, and to identify paths to s resolutio@ YeAMning issues such that the review can be completed by 970404o182 970402 PDR TOPRP EMVGENE Q'3c' h

g (Gus anuan to accutacar cagns. FILES 9 74f

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i mid-1997. This would minimize the impact of any resulting changes to the

EP/ SAG or implementing guidance on utility accident management implementation

schedules.

If you have any questions concerning this matter, please contact the project manager, Jim Wilson, at (301) 415-1108.

Sincerely, Original Signed- By: l David B. Matthews, Chief  !

Generic Issues and Environmental  :

Projects Branch  :

Office of Nuclear Reactor Regulation- i Project No. 691  ;

Enclosure:

As stated cc w/ encl: See next page e

i DISTRIBUTION: .

Central File PGEB r/f OGC GHolahan RPalla MSlosson PUBLIC ACRS TMartin CBerlinger JKudrick LChandler 4 Alevin GThomas JLyons LPhillips Document Name: BWROGEPG. SAG *See Previous Concurrences /

OFC PGEB M SC:PGEB C:SCSB C:SRXB C:PSENf NAME JWilso[:sw RArchitzel CBerlinger* JLyons* DMabews DATE '4/K./97 4/J /97 3/31/97 4/01/97 4/1/97 0FFICIAL RECORD COPY

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mid-1997. This would minimize the impact of any resulting changes to the EP/ SAG or implementing guidance on utility accident management implementation ,

schedules. '

If you have any questions concerning this matter, please contact the project manager, Jim Wilson, at (301) 415-1108.

Sincerely David B. Matthews, Chief Generic Issues and Environmental Projects Branch Office of Nuclear Reactor Regulation Project No. 691

Enclosure:

As stated cc w/ encl: See next page i

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, t Aspects of BWROG Emergency Procedure and Severe Accident Management Guidelines Requiring Further Evaluation

Bypass of Radiological Interlocks l

, 1. The ATWS/ Stability EPG SER dated June 6, 1996, stated that for ATWS events which lead to severe core damage, the adequacy of maintaining the radiation lock in a bypass condition should be considered within the accident management studies. Appendix B indicates that bypassing the i

main steam line and off-gas high radiation interlocks when the main i condenser is available could reduce the overall risk to the public l during ATWS events (8-6-37). The specific studies performed to justify  !

this position should be identified. 1 Shutdown Boron Weight I

2. In the revised guidelines, Cold Shutdown Boron Weight has been changed I from 700 pounds to 213 pounds (D-5-58), and Hot Shutdown Boron Weight has been changed from 364 pounds to 77 pounds (D-13-46). The revised calculations are in Appendix C, which was not supplied. The analyses ,

justifying these changes should be provided. j RPV Water Level for Steam Cooling

3. The RPV water level at which the need for emergency RPV depressurization or steam cooling has been changed from TAF to MSCRWL (D-9-3). The BWROG ,

should submit the analyses performed to support this change.

Containment Venting

4. Steps PC/P1, P2 and P3 are performed concurrently in the revised guidelines, thereby permitting venting prior to taking other actions that could control containment pressure without compromising containment integrity, such as use of sprays (D-6-54). It is unclear why this change is needed, especially since it could result in an increase in offsite consequences relative to the sequential approach in EPG Rev. 4.

5. In a design basis large-break LOCA, reactor vessel water level cannot be maintained above TAF and entry into the SAGS would be required even though core damage is not imminent or expected. Venting would subsequently be called for in the SAGS, potentially violating the plant's licensing basis. Justification for the EP/ SAG transition criteria and the subsequent containment flooding and venting instructions should be provided for design basis events.

6. The priority of wetwell over drywell venting embodied in EPG Rev. 4 has been eliminated in the revised guidelines on the basis that concurrent venting from the drywell and wetwell may be appropriate in some evolutions (D-6-65). Further justification for this change in philosophy is needed since: (1) current guidance does not prohibit concurrent venting, and (2) the change could result in an increase in consequences. Additional guidance is also recommended to assure that Enclosure

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[t plant-specific procedures maximize suppression pool scrubbing, e.g., by

! specifying that priority should continue to be given to use of the j wetwell vent.

i 7. The guidelines permit containment venting in response to the first j

appearance of hydrogen in containment if releases are expected to remain '

below the technical specification limit (B-7-63, 81 However, in a degraded core situation, fission product release can). be expected to i

' follow the release of hydrogen in containment, necessitating closure of the vent It would appear prudent to forego venting for small

! quantities of hydrogen unless it can be established that the plant conditions are stable, core cooling has not been compromised, and

, fission product releases from the fuel are not imminent. Guidance and limitations on initiating venting in situations involving potential core

damage should be clarified.

! 8. Venting the containment irrespective of the offsite radioactivity release rate is permitted if it will facilitate primary containment flooding or RPV injection (e.g., B-15-44). Appendix B indicates that l vent.ing is appropriate if it can appreciably shorten the time required

' to submerge core debris, but that the associated radioactivity release may not be warranted if venting will only nominally increase flow or will not significantly affect the fill time. Additional guidance / criteria on making such judgements is needed to assure that this action will not lead to an unwarranted increase in consequences when implemented by utilities. The applicability of this provision to containments that have a long fill time regardless of back-pressure should also be addressed.

9. Venting is mandated to remain below the Pressure Suppression Pressure (PSP) if. breach of the reactor vessel is imminent and pressure suppression capability is functional (B-15-85). Use of PSP is based on the assumption that containment pressurization at reactor vessel breach (with the RPV depressurized) would be comparable to that following a design basis LOCA. The rationale for venting at this critical time and for use of PSP as the criterion for initiating venting (in lieu of more realistic pressurization calculations) requires further justification given the conservatisms in this approach and the potential for increased consequences.

Use of Sprays

10. Drywell and suppression pool sprays are now terminated before containment pressure reaches O psig instead of the high drywell pressure scram setpoint, in order to permit use of sprays at low pressure or if the containment has failed. The basis for using the scram setpoint in EPG Rev. 4 was that it provided margin in preventing negative containment pressure and deinerting. Discuss the rationale for the apparent reduction in margin in the EP/ SAG, and how licensees are expected to protect against the threat of deinerting, especially considering instrument error.

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i 11.- In PC/G-3, the guidelines require drywell spray operation after rather than before initiating drywell venting on the basis that a deflagration l following'ssray initiation could damage the (closed) vent valves and preclude su) sequent venting (8-7-80). The underlying analysis and its

i. applicability to the population of BWRs should be described since the

! challenge to valves may not justify the increased consequences

! associated with delaying spray initiation. The rationale for not L requiring drywell spray actuation (to scrub fission products) prior to drywell venting in PC/G-2 should also be provided, since releases above the LCO could be permitted under this step.

Guidance on Alternate Limit Curves

12. The sample limit curves in the generic guidelines are based on conservative assumptions that may not be appropriate for establishing

plant-specific limits. Several examples are provided below. Statements l that alternate curves are not necessary to adequately meet the needs of

accident mitigation (e.g., in the Overview Document) should be replaced j with guidance indicating that replacement of the sample limit curves

provided with the EPGs and SAGS with limit curves based upon more

realistic values is authorized and encouraged.

The Drywell Spray Initiation Limit (DWSIL) 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 the drywell sprays cannot be initiated when needed, e.g., prior to reactor vessel breach.

The Minimum Zero Injection Water Level (MZIRWL) is determined  ;

assuming that the reactor has been shut down for ten rinutes 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 opportunity for delaying the onset of core degradation.

The Minimum Drywell Spray Flow (MDSF) is defined as the lowest flow that ensures uniform spray distribution within .the drywell.

Althcugh a full spray flow pattern is desirable for fission prodtet removal, the drywell sprays should be operated even if the MDSF cannot be attained since the sprays can still be effective in providing water to the drywell floor to cool ex-vessel core debris.

The ccnservative assumptions invoked during preparation of the Heat Capacity Temperature Limit (HCTL) and Minimum Debris Retention Injection Rate (MDRIR) curves (as described in the Overview Document) should be recognized by the utilities and alternate, more practical, versions should be developed and implemented.

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. 13. The Drywell Spray Initiation Limit (DWSIL) curve is based on the total i

mass of non-condensibles in the primary containment (B-17-10). This mass will change as a result of containment venting, invalidating use of the curve for subsequent decisions regarding spray actuation. Use of sprays while venting is in 3rogress could also result in deinerting (due to air ingression through tie vent) if sprays or vents are not secured when the containment reaches atmospheric pressure. Additional guidance is needed regarding the impact of venting on the DWSIL, and the use of sprays following venting. Development of additional DWSIL curves and/or procedures to cycle or throttle sprays may be viable options for addressing this concern. (The curve for PSP is affected by venting in a

similar manner.)

Boron Injection Systems

14. The guidelines lack appropriate guidance regarding alterna*,e boron injection methods (B-15-126), and contain an unnecessary step regarding terminating use of SLC (B-15-127). Specifically, the SAG should be modified to: (1) clarify that utilities are responsible for considering

' and incorporating methods for alternate or extended boron injection in plant-specific EP/ SAG (eliminate arguments in Appendix B against the use of alternate boron injection systems), and (2) eliminate the direction i

for securing SLC pumps if core debris has exited the reactor vessel, since continued operation would have no negative impact.

Mark I Liner Melt-Through

15. There is a lack of guidance in the SAG and Appendix B regarding the importance of establishing drywell pool / flow to prevent Mark I liner failure. Additional guidance should be provided, including a discussion of the beneficial role that a water pool on the drywell floor would play in preventing liner failure, and the importance of using the drywell sprays to create and maintain this pool.

Inhibiting Automatic Actuation of ADS

16. IPE results indicate a significant contribution to core damage frequency from events in which operators manually inhibit ADS in accordance with current E0Ps, but fail to manually initiate ADS later, as required (NUREG-1560, Vol 2, Sections 11.2.2.4 and 11.2.3.4). Justification for maintaining the strategy to prevent automatic initiation of ADS when low pressure pumps are running in non-ATWS situations (RC/L-2 and Cl-1) should be provided given this insight.

Emergency Depressurization After PSP Exceeded

17. The instruction in EPG step PC/P-2 that Emergency RPV Depressurization is required if suppression chamber pressure cannot be maintained below the PSP could challenge containment integrity if the action is taken after PSP is exceeded (B-7-36). Clarification on how operators should interpret this instruction should be provided, especially since the design leakage from the drywell to wetwell airspace would be sufficient to exceed PSP early in a small break LOCA.

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• Project No. 691 Boiling Water Reactor Owners Group cc: C. D. Terry Vice President, Nuclear Engineering

, Niagara Mohawk Power Corporation l

Nine Mile Point-2 1 PO Box 63 Lycoming, NY 13093 D. B. Fetters PECO Energy ,

. Nuclear Group Headquarters MC 62C-3 965 Chesterbrook Blvd. ,

j Wayne, PA 19087 '

L. A. England Entergy Operations Inc.

PO Box 31995 l Jackson, MS 39286 I i

K. K. Sedney i GE Nuclear Energy  :

175 Curtner Ave, M/C 182 ,

San Jose, CA 95125 T. J. Rausch Commonwealth Edison Company '

4 Nuclear Fuel Services 1400 Opus Place, 4th Floor ETWIII Downers Grove, IL 60515 i ,

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