ML20043G356
| ML20043G356 | |
| Person / Time | |
|---|---|
| Issue date: | 01/03/1990 |
| From: | NRC |
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| References | |
| NUDOCS 9006200185 | |
| Download: ML20043G356 (11) | |
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Generic Environmental Assessment and Findino of No Sicnificant Impact Relatina to the Installation of a Hardened Vent Capability from the Suppression Pool Airspace of'Boiljnc Water Reactors (BWRs) with Mark I containments s
January 3, 1990 I
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80 esang an. -
9006200185 900108 PDR TOPRP ENVGENE C
PNV
I TABLE OF CONTENTS I
1.0 INTRODUCTION
1.1 Description of Proposed Action.
I 1.2 Need for the liardened Vent 2
1.3 Alternatives Considered 2
5 2.0 RADIOACTIVE ~ WASTE 3.0 RADIOLOGICAL IMPACT ASSESSMENT 5
4.0 NON-RADIOLOGICAL IMPACT.
5 5.0 ACCIDENT CONSIDERATIONS 6
6 5.1 Design Basis Accidents.
5.2 Severe. Accidents.
6 7
6.0
SUMMARY
6.1 Alternative Use of Resources.
7 o6.2 Agencies and Persons Consulted.
7 7
7.0 FINDING OF NO SIGNIFICAN,T IMPACT
8.0 REFERENCES
9 4
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wn.
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,i 1.0 IllTRODUCTIOll 3.1 Des _g_tiption._of Proposed Action In SECY-87-297', dated December 8, 1987, the staf f presented to the Commission its pr'ogram plan to evaluate generic severe accident containment vulnerabilities in a program entitled the Containment Performance Improvement (CPI) program.
This effort is predicated on the conclusion that there may be generic severe accident challenges to each light water reactor (LWR) containment type that should be assessed to determine whether additional regulatory guidance or requirements concerning needed containment features is warranted.
The bases for the conclusion that such assessments are.
needed include the relatively large uncertainty in the ability of some LWR containments (e.g. Mark I) to successfully survive some severe accident challenges, as indicated by draf t liUREG-1150, dated June 19 89',
This effort is integrated closely with the Individual Plant Examination (IPE) program and is intended to focus on resolving hardware and procedural issues related to generic 3
containment challenges.
In SECY-89-017, dated January 23, 1989, the staff's the findings related to the Mark I CPI program were presented to the Commission.
On July 11, 1989', the Commission endorsed the provision of a hardened vent capability from the containment wetwell air space, and directed the staff to require a hardened vent capability for all Mark I plants for which it could be shown to be cost-effective.
The staf f has concluded that venting, if properly implemented, can have a significant benefit on plant risk.
The capability to vont has long been recognized as important in reducing risk from opera-
[
tion of BWR Mark I facilities due to loss of long term decay heat removal events. Controlled venting can prevent the long-term over-pressurization and failure of containment, the failure of ECCS pumps from inadequate not positive suction head, and re-closure of the ADS valves.
Venting of the containment is currently included in the BWR emergency operating procedures.
A vent path external to existing containment penetrations, currently exists for some Mark Is.
This vent path consists of a ductwork system which has a low design pressure of only a few psi.
Venting under high pressure conditions (either before or after core molt) may fail this
- ductwork, release the containment atmosphere into the reactor building, and potentially contaminate or damage equipment needed for accident recovery.
In addition, with the existing hardware and procedures at some plants, it may not be possible to open or to close the vont valves for some accident scenarios.
Therefore, venting via a sheet metal ductwork path, as currently implemented.
at some Mark I plants, is likely to greatly hamper or complicate post-accident recovery activities, and is, therefore, viewed by the staff as yielding reduced improvements to safety.
The staff continues to agree with this view unless the potential downsides of using the existing hardware are corrected.
-2
- 1. 2 Need for the liardened Vent A hard pipe vent capable of withstanding the anticipated severe accident pressure loadings would eliminate the disadvantages of using a vent path with ductwork.
The Commission concurs and directed the staf f on July 11, 1989' to proceed.,with the imposition of a hardened vent capability for each BWR with a Mark I contain-ment where a' plant specific-backfit analysis supports such a backfit.
The vont isolation valves should also be remotely operable from the control room.
This capability, in conjunction with proper operating procedures and operator training would result in greatly reducing the probability of core melt in the event of a lose af long term decay heat removal.
Use of the containment vent to prevent a core _ melt accident, the most likely way in which it would be used, would result in the release of very low levels of radioactivity associated with the reactor coolant activity.
In the unlikely-event of a core melt accident, venting of the wetwell airspace would provide a scrubbed venting path to reduce the release of particulate fission products to the environment.
Venting has been estimated to reduce the likelihood of late containment over-pressure failure and to reduce offsite consequences for severe accident scenarios in which the containment shell does not fail for other reasons.
Failure of the shell due to core debris attack (shell melt through) would reduce the benefits from venting in that it would rel' case fission prudacts directly into the reactor building. However, significant scrubbing of particulate releases would take place even if shell failure occurred, but was delayed for a period of a few hours (by the addition of water atop the molten core debris, for example).
1.3 Alternatives Considered With the goal of preventing or delaying containment failure due to over-pressurization, several alternatives were considered.
The containment pressure could be relieved using the existing ductwork vent path (the "No Action" op. ion); the existing ductwork path could be replaced with a hard pipe path; a hard pipe path to an external filter could be installed; an alternate means of removing l
the decay heat either from the reactor or from containment could be installed; or not permitting the venting of containment.
Each of these alternatives is discussed below.
1.
Existina Ductwork Vent Path (No Action Option)
This includes venting the condnment through the existing ductwork from the suppression pr a to the standby gas treatment system 5
(SGTS).
Ductwork deeign pressure is usually a few psid or less.
Consequently, venting could result in failure of the duct,wo W an~d~
a direct release into the reactor building.
The discharge of high l
l
s s temperature steam and other gases over an extended pt>riod of time may pose a threat to the availability or performance of safety related equipme'nt.
Electrical cables, motor operators _on valves, and relays may be subject to failure under these environmental conditions.
Adverse environuental conditions would complicate entry into the reactor building.
Calculations from a study which examined venting during an anticipated transient without scram (ATWS) sequence indicated that a
severe environment (high temperature and radiation) would be present in the reactor building during venting.6 The discharge of hydrogen (under core melt conditions) could result in hydrogen burns (or detonations) inside the reactor building.
This environment could hamper recovery ef forts by preventing personnel access into the reactor building if repair of systems needed to terminate the accident are necessary. Since this approach not,only jeopardizes personnel, but jeopardizes the ability to regain control of the f acility given the accident, this is not considered a reasonable alternative.
2.
Installation of Hard Pipe in lieu of Ductwork The installation of a hard pipe in lieu of the ductwork from the wetwell airspace (bypassing the SGTS) to the plant stack would include additional isolation valves (to isolate the ductwork path from the hard pipe vent path) and would include radiation monitor (s) to monitor any offsite releases, Given a loss of long term decay heat removal accident, this alternative would prevent failure of the vent path inside the reactor building and would result in an elevated release, which could red 6ce (by approximately a factor of two depending on meteorology) the nearby of fsite conse-quences.
Since the vent path is not expected to fail inside the reactor building, personnel would be able to repair equipment and to perform other plant recovery activities.
Furthermore, there would be no harsh environme'ntal conditions to degrade or f ail other equipment.
All potential releases through the vent will be scrubbed by the suppression. pool water which will significantly reduce the parti-culate fission products by one to two orders of magnitude, but would have no effect on the noble gases.
The effectiveness of the scrubbing is expected to vary in accordance with the temperature of the water, but as long as there is water present, all releases to the vent will pass through the water with some retention of particulates.
Use of the hard pipo vent could prevent or delay core degradation, for those accident scenarios where containment failure results in core degradation.
This is estimated to reduce the total core damage frequency per reactor-year by the specific values shown in Table 1 for each Mark I.'
The risk reduction in man-roms por reactor year is also shown in Table 1.
The hard vent path ygulp also provide some risk reduction for those scenarios where core melt has occurred, although for the purposes of the regulatory
w
-4 analysis, this benefit was not included.
The estimated cost for
- installation of the hard pipo vent path has been estimated for each Mark I plant and is shown in Table 112, Therefore, this is considered a reasonabic and practical alterna-tive.
3.
Installation of Hard Pine Vent to External Filter System This alternative has all of the advantages of alternative 2 above with one improvement.
With the external filter, the amount of particulate removal would not be sensitive to conditions in the suppression pool.
The additional risk reduction for having an external filter system in addition to the suppression pool scrubbing has not been explicitly estimated but is expected to be small.
An external filter would not reduce the core damage frequency in comparison with thci hard pipe vent alone.
In i
addition, the external filter is not expected - to significantly affect offsite consequences.
In both cases, there would be no retention of noble gases.
External filters have been estimated to cost $10'million to $50 million for the Filtra design and about $5 million for the Multi-Venturi Scrubber System design.
Therefore, since the. benefit is very small compared to Alternative 2,
while the cost is very high, this is not considered a practical or reasonable alternative.
4.
Installation of Other Means of Decay Heat Removal In lieu of venting containment, an additional decay heat removal system could be provided to remove the heat from either the reactor or the containment, or a system which has not been previously accounted for could be used, such as the reactor water cleanup system.
Installation of a new system has been considered in NUREG-8
'1289 which is associated with Unresolved Safety Issue A-45, shutdown heat removal requirements.
The installation of a new decay heat removal system was not found to be cost beneficial in NUREG-1289.
The use of another, previously, unaccounted-for system was estimated to require unusual system piping line-ups, which if performed incorrectly or inappropriately, could reduce the likelihood of accident recovery with normal systems or create a new and unanalyzed accident sequence.'O Therefore, this is not considered a practical or reasonable alternative.
5.
No Ventinct of Containment This alternative would remove the guidance in Revision 4 of the Emergency Procedure Guidelines (EPGs) which instructs the operator to vent the containment under certain conditions.
Given the loss of long term decay heat removal capability (TW) sequence, without drywell failure, the containment has a relatively high probabijltL of failure f rom over-pressurization if venting does not take place.
l The ef fects of containment failure could have a significant effect i
i.
-S-on the ability to return the plant to a safe and controlled condition and would result in'an increase in risk."
Therefore, this is not considered a practical or reasonable alternative.
2.0 RADIOACTIVE WASTE Each plant contains radioactive waste treatment systems designed to collect and process the gaseous, liquid, and solid waste that might contain radioactive material.
The proposed installation of a hard pipe vent will not result in a change in any waste treatment system or their effluents under normal plant conditions or under design basis accident conditions.
3.0 RADIOLOGICAL IMPACT ASSESSMENT The small radiation dose associated with this proposed plant modification will not affect the licensee's, ability to maintain individual occupational doses within the limits of 10 CFR Part 20, and is expected to be as low as reasonably achievable.
Normal radiation control-procedures (NUREG-0800, U.S.
NRC 1981) and Regulatory Guide 8.8 (U.S. NRC 1978) will preclude any significant occupational radiation exposures.
Installation of the hard pipe vent path should not increase the radiation dose to operating personnel or the public. Any increased doses associated with the _tosting of the additional isolation valves should be minimal
- and, in most cases, insignificant.
Furthermore, use of the hard pipe should reduce exposure to operating personnel and to the public, should a TW event occur, by preventing contamination of the reactor building and by reducing the likelihood of containment failure.
1 Thus, we have concluded that the proposed installation of the hard i
pipe vent will not result in any significant long term increase in doses received by workers.
4.0 NON-RADIOLOGICAL IMPACT The amount of new piping (up to approximately 20 inches in diameter) and valves would be f abricated of fsite and brought onsite by standard sized trucks.
This is not expected to impact ' ter--
restrial resources not previously disturbed during the original construction.
No non-radiological effluents are expected to be affected by the installation of the hard pipe vent.
There is no anticipated change in the use of chemicals in conjunction with the hard pipe vent.
The proposed plant modification will not require any change to the NPDES permit.
4
6-Therefore, the staff concludes that the non-radiological environ-mental impacts of installing a hard pipe vent will be insig-nificant.
5.0 ACCIDENT CONSIDERATIONS 5.1 Desion Basis Accidents The hard pipe vent, with proper operating procedures and operator
- training, should have no adverse effect on the likelihood of occurrence, progression, consequences, or termination of design basis accidents.
5.2 Severe Accidents The hard pipe vent will be ef fective primarily in preventing severe accidents, where the ability to vent containment helps to prevent events leading to core degradation and reactor vessel failure and in the prevention of containment failure due to over-pressurization following a core n'elt accident.
The dominant severe accident sequences in this category are the loss of long term decay heat removal capability (TW).
Some mitigation of other severe accident sequences is also expected due to suppression pool scrubbing.
Operation of the hardened vent system is expected to occur primarily prior to core degradation.
This would result in very low radioactivity releases associated with the normal reactor coolant.
The hard pipe vent would prevent failure of the vent path inside the reactor building and would result in an elevated release.
The elevated release could reduce the offsite consequences.
Since the vent path is not expected to fail inside the reactor building, personnel would be able to repair equipment and carry out other plant ' recovery activities.
Furthermore, there would be no harsh environmental conditions to degrade or fail other equipment in the reactor building.
All potential releases through the vent will be scrubbed by the suppression pool water which will significantly reduce particulate fission products released, but would have no ef fect on noble gases.
The ef fectiveness of the scrubbing is expected to vary in accor-dance with the temperature of the water, but as long as there is.
water present, all releases to the vent will pass through the water with some retention of particulates.
Therefore, the staff concludes that the reduction in the radiological risk of the plant's operation related to the installing the hard pipe vent can be significant.
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-7 6.0
SUMMARY
8 The staff's Safety Evaluation Report dated September 12, 1988 approved Revision' 4 of the Emergency procedure Guidelines (EPGs) which included the staff's approval for venting Boiling Water Reactor (BWR) Mark I containments.
This approval indicated that venting with the existing systems was acceptable to reduce the likelihood of core melt and in extremely rare cases to avoid uncontrolled releases.
Since the issuance of Revision 4 in March 1987, additional insights have been gained which indicate that a 8
venting strategy which includes a reasonably high likelihood of a breach in the vent path inside the reactor building could have a significant detrimental impact on personnel dose, potential plant recovery actions, and public risk when compared with the potential benefits of the proposed hard pipe vent path.
The increrantal occupational radiation dose for the proposed operation of the vent path is considered to be insignificant l
(unmeasurable) since the vent path would be operated from the control room.
The licensee should be able to keep small radiation doses associated with the installation of the hard pipe vent path within the limits of 10 CFR Part 20, and as low as is reasonably achievable.
Furthermore, the non-radiological impacts of the hard pipe vent path will be insignificant, and none of the alternatives g
4 are practical or reasonable.
6.1 Alternative Use of Resources This action does not involve the use of resources not previously considered in connection with the Nuclear Regulatory Commission's Final Environmental Statement, dated the Environmental Assessment issued 19_,
regarding the full term operating license, and the Environmental Report, dated 19__.
6.2 Acencies and Persons Consulted The NRC staf f is initiating this action based on research performed by the Office of Nuclear Regulatory Research and no other agencies or persons were consulted.
7.0 FINDING OF NO SIGNIFICANT IMPACT The staff has reviewed the plant-specific features in conjunction with the proposed hard pipe vent path modification relative to the requirements set forth in 10 CFR Part 51.
Based upon the l
environmental assessment, the staff has concluded that there are no significant radiological or non-radiological impacts associated i
I with the proposed action and that the proposed modification will l
not have a significant adverse effect on the quality of the human environment.
Therefore, the Commission has determined, pur5u~anI l
.a.
to 10 CFR 51.31, not to prepare an environmental impact statement for requiring the proposed plant modification.
Table 1 Potential Imtallation b
TW Frequency' Risk Reduction Costs
- Plant Name (per reactor-year).
(man-romp /rv) j[million)
Browns Ferry 1 2.3 E-05 32 7 0.75 Browns Ferry 2 2.3 E-05 32.7 0.75 Browns Ferry 3 2.3 T"05 32.7 0.75 Brunswick 1 4.5 Oc05 44.0 0.75 Brunswick 2 4.5 E-05 44.0 0.75 45.6 0.75 Cooper 4.5 E-05 Dresden 2 1.4 E-05 50.2 1.00 Dresden 3 1.4 E-05 50.2 1.00 Duane Arnold 4.5 E-05 55.0 0.75 Fermi 2 4.5 E-05 192.4 0.75 Fitzpatrick 4.5 E-05 65.5 0.68 Hatch 1 4.5 E-05 39.2 0.75 Hatch 2 4.5 E-05 39.2 0.75
-Hope Jreek 6.3 E-05 281.9 0.75 Mi); stone 1 1.4 E-05 35.1 1.10 Monticello 4.5 E-05 33.9 0.75 Nine Mile Point 1 1.4 E605 15.3 0.75 Oyster Creek -
1.4 E-05 55.4 1.50 Peach Bottom 2 3.6 E-06 15.5 0.75 Peach Bottom 3 3.6 E-06 15.5 0.75 Pilgrim 2.3 E-05 31.2 0.75 Quad Cities 1 4.5 E-05' 94.1 0.75 Quad Cities 2 4.5 E-05 94.1 0.75 Vermont Yankee 2.3 E-05 28.9 0.75 Notes: a) Taken from Table 2,
Column C,
from the Mark I Plant-Specific Enhanced Ventina Capability Reculatory Analysis b) Taken from Table 2,
Column F,
from the Mark I Plant-Specific Enhanced Ventina Capability Reculatory Analysis c) Taken from Table 2,
Column H,
from the Mark I Plant-Specific Enhanced Ventina Capability Reaulatory Analysis
,e 9-o 9-8.0 REPERENCES 1.
SECY-87-297, U.S. NRC, Mark I Containment Performance Procram Plan, V. Stollo to NRC Commissioners, December 8, 1987.
2.
NUREG-1150, U.S. NRC, Severe Accident Risks: An Assessment fqr Five__U.S. Nuclear Power Plants, dated June 1989.
.3.
SECY-89-017, U.S.
- NRC, Mark I
Containment Performance Improvement Procram, V.
Stello to NRC Commissioners,.7anuary 2 '1, 1989.
4.
Memorandum from S. J. Chilk to V. Stello, "SECY-89-0.7 - Mark I Containment Performance Improvement Program"', July 11, 1989.
5.
NUREG/CR-5225, U.S. NRC, An Overview of BWR Marl _ I Colttainment Ventina Risk Implications, November 1988.
6.
Harring, R.M.," Containment Venting as a Mitigation Technique for BWR Mark I Plant ATWS", 1986 Water Reactor Safety Meetina.
Gaithersburo. Maryland, October 1986.
7.
Memorandum.from M. Cunningham to W.
D.
Dockner, Reduction in Risk from the Addition of Hardened Vents in BWR Mark I Reactors, October 19, 1989.
8.
NUREG-1289, U.S.
- NRC, Reaulatory and Backfit Analysis:
Unresolved Safety Issue A-45.
Shutdown Decay Heat Removal Brequirements, November 1988.
9.
Letter from A. C. Thadani to D. Grace, Chairman, BWROG, Safety Emeroency Procedure Evpluation of "BWR Owner's Group Guidelines Revision 4. " NEDO-31331. March 1987, September 12, 1988.
10.
Letter from J. Dallman to J. Ridgely, A Preliminary Assessment of BWR Mark II Containment Challences. Failure Modes, and Potential Improvements in Performance, May 10, 1989, 11.
NUREG/CR
- 5225, Addendum 1,
An Overview of BWR Marl I
Containment Ventina Risk Implications, an Evaluation of.
Potential Mark I Containment Improvements, June 1989.
12 Memorandum J.G.
Partlow to T.E. Murley, " Licensees' Responses to Generic Letter 89-16 Related to Installation of Hardened I
Vent", dated November 9, 1989.
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