ML20055E349
| ML20055E349 | |
| Person / Time | |
|---|---|
| Site: | Millstone  |
| Issue date: | 06/15/1990 |
| From: | Murley T Office of Nuclear Reactor Regulation |
| To: | Mroczka E CONNECTICUT YANKEE ATOMIC POWER CO. |
| References | |
| GL-89-16, TAC-74872, NUDOCS 9007110301 | |
| Download: ML20055E349 (35) | |
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June-15, 1990 y
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' Docket No. 50-245 1
Mr. Edward J. Mroczka Senior Vice President Nuclear Engineering and Operations
~ l Connecticut Yankee Atomic Power Company Northeast Nuclear Energy Company Post Office Box 270 Hartford, Connecticut 06141-0270 j
De'ar Mr. Mrotzka:
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BUBJECT:
STAFF'S BACKFIT ANALYSES FOR MILLSTONE NUCLEAR POWER STATION, UNIT 1'REGARDING INSTALLATION OF A HARDENED WETWELL VENT t
(GENERICLETTER89-16)(TACNO.74872) i j
In'SECY 89-017, " Mark-1 Containment Performance Improvement Program," of l
January 23, 1989, the staff. demonstrated that hardened wetwell venting 0
capabilities at Mark I containments would prevent the majority of severe accident = sequences involving loss of decay heat removal capability (TW sequences) from resulting in core melt. The staff also demonstrated that i
. venting through a hardened vent path from suppression pool airspace would Esignificantly. mitigate the risks to public. health and safety, because u
-substantial amounts of fission products released by a core melt would be trapped in the suppression pool and would not be available for release to the environment. Some benefits-are also expected because of the prevention.
1 of severe accident sequences.other than TW sequences from resulting in-core I
melt. - Based 'on the analyses-in-SECY 89-017, the staff informed the Commission that the generic installation of hardened. vent capabilities at y
Mark I containments would provide significant:added benefits resulting from a reduction of severe. accident risks to public health and safety.
On July:11, 1989, the Commission responded to the staff recommendations in SECY 89-017 and directed the staff to implement, on a generic basis, the-q installation-ofhardened;ventcapabilitiesatboilingwaterreactors(BWRs) i with Mark I containments. Accordingly, on Se)tember 1, 1989, the staff' H
issued Generic Letter 89-16 (GL 89-16).
In t1at letter, the staff urged-the affected licensees to voluntarily install hardened vent capabilities at I
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their Mark I containments using the provisions of the Commission's rules in j
If the licensees chose not to install the hardened vent
- capability on a voluntary basis, the staff requested in GL 89-16 that the
.. licensees provide their plant-specific estimates of costs of installation of hardened vent capabilities. The licensees'were informed that the staff cwould use the cost data to perform plant-specific backfit analyses, and to determine if hardened vent installations could be imposed as backfits in accordance with the Commission's backfit rule in 10 CFR 50.109.
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. Mr. Edward J. Mroczka June 15, 1990
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. By.. letter of October 30, 1989, you responded to GL 89-16 indicating that you had decided not to commit to install hard vent capabilities on a voluntary basis until this issue has been prioritized using the ISAP process, due for completion by December 15, 1990.
You also provided the staff with,
l c plant-specific cost estimates for modifications at the Millstone Nuclear Power l
Station, Unit 1 (Millstone, Unit 1).
Following the receipt of your October 30, 1989, letter, the staff initiated plant-specific backfit analyses for Millstone, Unit 1.
In its analyses, the staff used the plant-specific cost estimates that you provided. The staff estimated the benefits of venting by determining the reductions in core damage frequencies (CDFs) for only the TW sequences.
The benefits were calculated by using-the results of the probabilistic risk assessments (PRAs) for BWRs with Mark I containments and isolation condenser systems (ICS) similar to Millstone, Unit 1.
The staff then adjusted the analyses to account for recent advances l
L in the PRA methodology (NUREG-1150). The results of the staff's analyses
.showed that for TW sequences alone, the overall CDF for Millstone, Unit 1 can be reduced by 1.4 E-5 per reactor year. The credit for the operation of the-105.was included in the analyses. The analyses were adjusted to account for the power levels of Millstone, Unit 1, and the density of population surrounding the Millstone, Unit 1 site. The staff has calculated that for TW sequences l
alone, the operation of the vent would avert the expected radiological exposure to public by 35.1 man-rem per reactor year. Using 21 years of remaining plant life for Millstone, Unit 1, the staff has estimated an averted radiological population exposure of 671 man-rem per million dollars for Millstone, Unit 1.
-The preceding results of the staff analyses demonstrate.that hardened vent capabilities would provide significant benefits in the expected reduction in radiological exposure risks posed by TW sequences.
1 The staff has also calculated the averted costs to clean the site surroundings and to replace the lost power, that would be associated with severe accidents involving TW sequences. Assuming that the modification costs would~be offset by the averted costs of cleaning the site surroundings and replacing power, the staff estimates that 819 man-rem will be averted per million dollars for Millstone, Unit 1.
I The staff has considered but not-quantified the reduction in risks posed by (1) severe accidents other than TW sequences, and (2) scrubbing of the fission products in the suppression pool for accident sequences that result in significant damage to the core. These benefits along with the benefits of reduction in CDF caused by TW sequences provide justification to support the backfit in.accordance with the Commission's rule in 10 CFR 50.109. A copy of the staff's supporting plant-specific analyses for Millstone, Unit 1 is enclosed for your information.
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g Mr. Edward J. Mroczka
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June 15, 1990 u
' In light of th'e staff's' b'ackfit analyses, the staff urges th'at.you' reconsider your decision and comit to install a hardened vent capabilityJ at Millstone, l
in Unit'1. You are requested to inform the staff of your_ intent within 30 days of receipt of this letter.' You may implement your commitment under the provisions of_ the Commission's rules in 10 CFR 50.59, provided that the modifications are in' place by January 1993.. In the absence of such a comitment, the staff intends to pursue the imposition of-this backfit under 1
the provisions of the Commission's backfit rule in.10 CFR 50.109.
Sincerely, j
original signed by
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Thomas E. Murley, Director
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Office of Nuclear Reactor Regulation J
Enclosure:
Plant-Specific Backfit. Analyses for Millstone Unit 1-cc w/ enclosure:
See next page l
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..L Mr. Edward J; Mroczka Millstone Nuclear Power Station i
Ncrtteest Nuclear Energy Company Ur.it ho. 1 cc:
Gerald Garfield, Esquire R. M. Kacich, Manager Day, Berry and Howard Ceneration Facilities Licensing s.
Counselors at Law Northeast Utilities Service Company City Place Post Office Box 270 Hartford, Connecticut 06103-3499 Hartford, Connecticut 06141-0270 l
- 11. D. Romberg, Vice President D. O. Nordquist Nuclear Operations Manager cf Quality Assurance Northeast Utilities Service Company Northeast Nuclear Energy Company Post Office Box 270 Post Office Box 270 Hartford, Connecticut 06141-0270 Hartford, Connecticut 06141-0270 Kevin McCarthy, Director Regional Administrator Radiation Control Unit Pegion !
Department of Environmertal Protection U. S. Fuclear Regulatory Connission State Office Building 475 Allendale Road Hartford, Connecticut 06106-King of Prussia, Pennsylvania 19406 Bradford S. Chase, Under Secretary First Selectmen Energy Division Town of Waterford i
Office of Policy and Management Hall of Records 00 k'est ington Street 200 Boston Post Road Hartfor.1, Connecticut 06106 ilaterford, Connecticut 06385 S. E..cace, Station Superintencent W
J. Raymond, Resident Inspector Hillstone huclear Power Station M111 stone' Nuclear Power Station
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-Northei st.l!uclear Energy Company c/o U. S. Nuclear Regulatcry Comission Post Office Box 128 Post Office Box 811 a
Waterford, Connecticut.06385 Niantic, Connecticut 06357 J. P. Stetz, Unit Superintendent 1
M111stc.te Unit No. 1 Northeast Nuclear Energy Company Post Offi e Box 128 Waterford, Connecticut 06385 j
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- f Plant-Specific' Analysis for the Millstone Nuclear Power Station,. Unit _1, Regarding Installation of a Hardened Vent l
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TABLE OF CONTENTS L
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1.0 Background.................................................. 1 2.0 Discussion.................................................
2 2.1 Safety Benefits.........................................
2 2.2 Reduction in Core Damage Frequency and Public' Risk....... -4 2.2.1 Plant-Similarity Assessment....................... 4 1
- 2. 2. 21 Reduction in Core Damage Frequency...............
5 2.2.3 Risk Reduction...................................
5 2.3 Cost-Benefit Analysis.
..................................-6 2.3.1 Cost Estimation..................................
6 2.3.2 Value-Impact Assessment........................... 6 2.4 Alternatives Considered and Impacts on Other Programs...
7 2.5 Environmental Assessment................................
7 3.0 Conclusions and Recommendations............................
8 3.1 Rationale for the Recommendation........................
8 4.0 References.................................................. 10
. Appendix A Regulatory Analysis on the Backfit of Hardened Vent li l
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s Plant-specific Analysis or the Millstone-Nuclear Power Station, Unit 1, Regarding y
?.1 Installation of a Hardened Vent
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_1. 0 Backaround
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LIn SECY-87-297 (Reference 1), dated December 8, 1987, the Nuclear R2gulatory Commission (NRC) staff presented to the Commission its j
program plan to evaluate generic severe accident containment-1
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vulnerabilities in a program entitled the Containment Performance Inprovement.(CPI) program.
The staff began this effort with the premiseithat there may be generic severe accident challenges to each
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light-water reactor (LWR) containment type that'should be assessed to dotermine whether additional regulatory guidance or requirements t
ccncerning-needed containment features.is warranted.
The premise that l
L cuch assessments are needed is based on the relatively large l
L uncertainty in the ability of-some LMR containments (for example, Mark I)uto snacessfully survive some severe accident challenges, as iindicated by NUREG-1150, dated June 1989 (Reference 2).
This effort
-10 integrated closely with the program for Individual Plant:
Examination (IPE) and is-intended to focus on resolving hardware and j
procedural issues concerning. generic containment challenges.
In SECY-L
- 89-017 (Reference 3), dated January 23, 1989, the staff presented its l
findingc concerning the Mark I CPI program to the Commission.
One of the improvements that the staff recommended was the installation of a i
- hcrdened vent capability.
.The. staff concluded that venting, if properly implemented, can oignificantly reduce plant-risk.
This vent capability has'long been rccognizedrastimportant in. reducing risk caused by. loss of long-term decay heat' removal. events.- Controlled venting can preventothe long-torm over-pressurization and eventual failure of containment, the failure:of Emergency Core Cooling System (ECCS) pumps caused by inadequate net positive suction head, and the re-closure of-the valves j
D in the Automatic Depressurization System-(ADS).
Venting of the ccntainment is currently included in the emergency operating procedures for boiling water reactors (BWRs).
A vent path using oxisting containment penetrations currently exists in all. Mark I plants.
This vent path generally consists of a system of sheet metal ductwork that-has a low design pressure of only.a few psi.
Venting a
- i under high pressure conditions created either before or after core colt-may fail this.ductwork, release the containment atmosphere into the reactor building, and potentially contaminate or damage equipment y
i nceded-for accident recovery.
In addition, with the existing hardware-ond procedures at some plants, it may not be possible to open or to
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close the vent valves for.some accident scenarios.
Therefore, venting
~through a sheet metal ductwork path, as currently implemented at some 1 l
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Mnrk I plants, is likely to hamper or complicate post-accident rceovery activities, and is, therefore, viewed by the staff as rcducing the safety benefit.
A hardened pipe vent capable of withstanding the anticipated pressure loading of a severe accident would eliminate this disadvantage.
The Commission concurred with the staff's position and directed the otaff on July 11, 1989 (Reference 4) to begin imposing a hardened vent capability on a plant-specific basis for each BWR with a Mark I containment.
For licensees who, on their own initiative, elect to incorporate this plant improvement, the staff was directed to consider installation of a hardened vent under the provisions of 10 CFR 50.59.
For the other licensees who do not intend to install a hardened vent voluntarily, the staff was to perform a plant-specific backfit analysis for each of these Mark I plants to evaluate the efficacy of rcquiring the installation of hardened vents, l
The staf f issued Generic Letter (GL) 89-16 dated September 1, 1909 l
(Reference 5) to BWR licensees with Mark I containments: (1) to inform them of the direction given by the Commission regarding the hardaned vant issue, (2) to provide them with a generic cost estimate for the installation of a hardened vent and (3) to request.that each licensee provide notification of its plan for addressing resolution of this l
issue.
Moreover, the staff encouraged licensees to implement the dssign changes to install the hardened vent.
For those plants not olecting to voluntarily install hardened vents, the staff requested in GL 89-16 that the licensees provide a cost estimate for installation of the hardened vent. In response to the Commission's directives, the Otaff developed a program to meet the objectives of the Commission's directive.
This program plan contains the following five tasks: (1) cost estimation, (2) plant similarity assessment (3) cost-benefit analysis, (4) environmental assessment, and (5) imposition of rcquirements.
2.0 Discussion The purpose of this report is to document the results of the plant-cpecific backfit analysis performed by the staff for the Millstone Nuclear Power Station, Unit 1.
This analysis complies with the backfit rule in 10 CFR 50.109 (Reference 6) and includes an assessment of the safety benefits, an estimate of the reduction in core damage frequency and public risk, and a cost-benefit analysis.
From the results of this analysis, the staff concludes that the installation of a hardened vent capability will substantially increase public safety and that the results of the cost-benefit analysis support the l
inplementation of the capability.
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2.1 Safety Benefits j
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The major benefit of a hardened vent is the reduction of both-the core dtmage frequency and public risks.
Probabilistic Risk Assessment (PRA) studies for BIIRs: indicate that~ accidents initiated-by transients q
'dcminate the total core damage frequency (CDF) in severe accident l'
- z. ? ocquences.
The prancipal accident sequences for BWRs consist of Loss j
1 of Long-Term Decay Heat Removal (TW), Station Blackout (SBO), and L.
Anticipated Tranwient Without Scram (ATWS).
The Reactor Safety Study
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(WASH-1400)r (Reference 7) indicated that ind is the dominant accident.
ocquence causing' core damage at the Peach Bottom Atomic Power Station, j
Further, draft NUREG-1150 (Reference 2) indicates that SBO is the dominant contributor to core damage' frequency at Peach Bottom.
At Pcach Bottom, it was estimated that the TW frequency has been greatly i
rcduced because of the successful implementation _of containment svcnting procedures. This study indicates that venting, if properly-icplemented, can significantly increase safety.
In SECY.89-017, the staff concluded on a. generic basis for Mark I
. plants that the proposed hardened vent capability would provide 3
onhanced plent capabilities with regard to both accident prevention and mitigation.
A core melt, combined with reactor vessel rupture and i
containment failure, would release significant amounts of-fission products to the environment. 'The addition of a hardened vent (1) prevents the majority of loss of long-term decay heat removal capability sequences (TW) from resulting in core melt, and (2) aitigates the consequences of residual sequences involving core melt where venting through the suppression pool is found necessary.
The TW ccquences are initiated by transient events and are followed by
" failure of long-term decay heat removal, the containment fails from i
overpressurization and causes the subsequent core melt.
The installation of a hardened vent will increase the survivability of u
p containment, reduce the likelihood of a-core melt from int sequences, onnd therefore reduce the risks to the public.
For other sequences where core melt occurs,before containment failure, venting could-be t
.cffective in delaying containment failure and in mitigating the
~i rolease:of fission products because venting through the suppression pool would provide significant scrubbing of particulate and volatile roleases.
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In a BWR, containment venting is currently included in the emergency l
cperating procedures.- The existing vent path generally consists of L
ductwork ranging in pressure capability down to design pressure of cnly a few psi for most' Mark I plants.
The low design-pressure ductwork is inadequate for accommodating the high containment-pressure t
following a severe accident.
consequently, venting under severe t
cccident conditions could' result in failure of the ductwork and a direct ~ release of radioactivity into the reactor building.
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discharge of high-temperature-gases over an extended period of time nay threaten the availability or performance of safety-related L,
cquipment.
If substantial fuel damage has occurred, the discharge of r l u
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l' hydrogen could cause hydrogen burns (or detonations) inside the L
rcactor building.
Electrical cables, motor operators on valves, l
- rolays, and control room components may fail under these environmental
.c:nditions.
Adverse environmental conditions would complicate entry
- ipto the reactor building., This environment of high. temperature and perhaps radiation could hamper recovery efforts by: preventing L
personnel from entering into'the reactor building if systemssneeded to t
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tcrainate the accident need: repair.
As a result, when relying on the cxisting ductwork, the benefits of containment venting are.
'oignificantly uncertain. Therefore, hardening the vent path to withstand the anticipated pressure loading during a severe accident J
l would eliminate this disadvantage while retaining all the benefits of-i centainment venting.
Because of the reduced core melt frequency, reduced fission product releases, and-possibleireduction or elimination of-a significant
' containment failure' mode, the staff concluded that the safety benefits
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of venting are significant, and further improvement can be achieved by
-l installing hardened vents.
In Reference 8, the staff estimated the l
i Lb;nefits in the reduction in CDF and in offsite risk, which are discussed in the following sections.
-2.2 Reduction in Core Damace Frecuency and Public Risk To, estimate the plant-specific reduction in CDF, all Mark I plants tware categorized into several groups based on the similarity of the d0 sign features that are important to the accident sequences.that
-could'be affected by the installation of a hardened vent.
In
.porforming the analysis, the staff used existing Mark I PRAs along-
~with-the plant similarity assessment to estimate the-reduction in CDF for each group of plants.
The analysis includes only the change in the core melt frequency for the TW sequence.
2.2.1 Plant Similarity Assessment In, draft NUREG/CR-5225-(Reference 9), the_three accident sequences 4
'that were identified as'being affected by venting are: (1) Loss of lLong-Term Decay Heat Removal (TW), (2) Anticipated Transient Without Scram (ATWS), and'(3) Station Blackout (SBO).
Among these sequences, the addition of-a hardened. vent'was found to produce the greatest reduction in core damage' frequency (CDF) through its effect on TW ccquences.
In the TW sequence, failure to remove decay heat following c transient will cause the gradual pressurization of the containment.
The containment may fail.from overpressurization and subsequently may i
-1 cad to a core.nelt.- In this sequence, venting can be used to allow
=the, removal of long-term decay heat from the containment through pool boiling and, therefore,' reduce the likelihood of containment fa'ture end subsequent core melt.- The design features important to this ccquence are the systems used for' decay heat removal and containment
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s The reduction in CDF for the TW sequence of each Mark I plant rcsulting from the installation of the hardened vent was estimated by the staff in Reference =8.
To account for similarity in design, all Merk I plants were' grouped according to the design of their decay heat o
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ircmoval and containment cooling systems - factors important in k'
cssessing the frequency of TW sequences.
In determining the groups by examining individual plant features in simplified piping and e
instrument diagrams, the staff studied the differences between the RHR
.cystems, isolation condensers, power conversion system,.and service
. water systems for all: Mark I plants.
In addition, the staff studied
.the available FRAs and failure probabilities of related componcnts to identify any major differences and similarities in terms of CDP affected by the hardened vent capability.
After careful study of the svailable PRAs, the staff categorized the Mark I plants into the
'following four groups -
U (1) Plants with a residual heat removal (RHR) system consisting of two trains,-with two RHR heat exchangers and two RHR pumps per
- train, (2) Plants.with an RHR' consisting of two trains, with one RHR heat exchanger and two RHR pumps per train, (3) Plants with an'RHR consisting of two trains, with one RHR heat exchanger and one RHR pump per train, and l
(4) Plants with-isolation condensers.
2.2.2.
Reduction in Core Damace Frecuency To estimate the-reduction in CDF from the installation of a hardened vent capability, the staff looked into the sequences that require the failure of containment cooling for core damage, and assumed that using hardened.vant would reduce 90 percent of these sequences. The ostimates of CDF reduction conservatively consider only the TW Leequences, and.therefore, the benefits on the SBo and ATWS sequences are not included.
For Millstone 1, the reduction in CDF was estimated using the PRA results of Millstone l.
The credit of the Isolation Condenser System (ICS) being used-as the decay heat removal syatem was included.
To be consistent with the failure frequency assumed in NUREG-1150, the staff
. incorporated several changes into the Millstone 1 PRA.
With these changes, the staff calculated that venting would produce a reduction in CDF from TW sequences of 1.4E-5 per reactor year.
More detailed information of this analysis is given in Reference 8.
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l 2.2.3 Risk Reduction Installation of a hardened vent capability will reduce the CDF and will result in reduction in the population dose that would be l
ccsociated with TW sequences.
The estimate of the reduction in population dose for Millstone 1 was calculated by multiplying the rcduction in CDF estimated for Millstone 1 by a scaling factor to L*
convert the Peach Bottom population dose to the Milltone 1 population dose.
The scaling factor was obtained from NUREG/CR-2723 (Reference i
- 10) for Millstone 1 plant-specific reactor power and population density.
The Peach Bottom population dose from TW sequences was darived using the insights from NUREG-1150.
The resulting reduction in the population dose for Millstone 1 due to reduction in CDP for TW scquences was estimated to be 2.5E6 man-rem. The averted population dose for Millstone 1 was calculated by multiplying the reduction in CDF Dy 2.5E6 man-rem to give 35.1 man-rem per reactor year.
For the
'l 21 years of operation remaining, the estimated total averted dose is 738 man-rem.
(Consideration of a 20-year operating life extension will increase the estimated total averted dose to 1441 man-rem.)
The averted occupational health risk resulting from the installation of the proposed hardened vent system is discussed and calculeted in Ssetion 4.1.2.2 of Appendix A.
The estimated occupational risk is approximately one to two percent of the public :.ealth risk and is not considered to be a significant contributor.
Therefore, the occupational health exposures are not further considered in the cost-bonefit analysis, 2.3 Cost-Benefit Analysis The method used to calculate the cost-benefit ratio is described in NUREG/CR-3568 (Reference 11), and the plant-specific data were considered.
The staff obtained plant-specific cost estimates provided by the licensee from the response to Generic Letter (GL) 89-16 and uced the risk-reduction data discussed above in Section 2.2.3 to calculate the value-impact ratio in man-rem saved per million dollars.
2.3.1 Cost Estimation GL 89-16 requested licensees to provide the staff with plant-specific cost estimates for installing a hardened vent.
In response to GL 89-16, all Mark I licensees except four (with five plants) indicated that they intend to install the hardened vent under the provisions of 10 CFR 50.59.
Millstone 1 is one of the five Mark I plants.
Northeast Nuclear Energy Company (the licensee) has decided not to voluntarily install the hardened vent capability.
By letter dated October 30, 1989 - -
. -~ _ _. - - -.
(Reference 12), the licensee of Millstone 1 responded to GL 89-16 with 2 cost estimate of 1.1 million dollars for the installation of a hordened vent, and incremental costs of $600,000 for an AC-independent power source.
2.3.2 Value-Imoact Assessment
.The.value-impact ratio is calculated in the regulatory analysis (Appendix A) using the method described in NUREG/CR-3568 (Reference
- 11) to support the backfit decision.
The benefits to public risk
'raduction in man-rem were calculated in Section 2.2.3.
The averted population dose for Millstone 1 was calculated in Section 2.2.3 to be l
-35.1 man-rem per reactor year.
For the 21 years of operation rcmaining, the estimated total averted man-rem is 738.
The cost of installation of the hardened vent capability was estimated in Section 2.3.1 as 1.1 million dollars.
The value-impact ratio, not including the averted onsite cost, is calculated to be 671 man-rem saved per i
sillion dollars.
The averted cost associated with prevention and mitigation of an accident can be discussed as five separate costa: replacement power, cleanup, onsite occupational health impacts, offsite health impacts,.
y cnd onsite property damage.
The details of each of these items are discussed in Appendix A section 4.1.2.2.
If the savings of $199,556 to Millstone 1 from accident avoidance (cleanup, repair of onsite
' damages, and replacement power) were included, the overall value-
' impact ratio would be 819 man-rem saved per million
' dollars.
(Consideration of a 20-year operating life extension will increase the value-impact ratio to 1644 man-rem saved per million
' dollars.)
2.4 Alternatives Considered and Imoacts on Other Procrams Other alternatives considered and their associated value-impact ratios are discussed in Section 3.0 and 4.0 of the Regulatory Analysis in Appendix A, Regulatory Analysis.
The effect of the addition of the hardened vent capability on other requirements including IPE, Improved Plant Operations (IPO), Severe Accident Research Program (SARP),
c External Events, and Accident Management are discussed in Section 4.2 of Appendix A.
A summary of the compliance to'the backfit rule (10 CFR 50.109(c)) is also included in Attachment 1 to Appendix-A.
2.5 Environmental Assessment
.The' staff performed a generic environmental assessment (EA) concerning the installation of the hardened vent at Mark I plants.
Concurrent with this plant-specific analysis, a draft EA is being sent out for public comments.
In the draft EA, the staff concluded that the installation of a hardened vent capability will have no significant
radiological or non-radiological impact on the environment.
The installation,of the hardened vent capability will prevent and mitigate severe accidents.
During normal plant operations or design-l basis accidents, the hardened vent will not be used, and therefore, J
will not result in any changes in amounts of radioactivity. released to the atmosphere from the plant.
Venting during severe accidents will reduce the CDF end will reduce the radiological environmental risks.
For venting sequences, the hardened vent connected to the plant stack could reduce dose consequences more ef fectively by approximately a factor of two than venting through the ductwork.
This reduction is J
due to a greater effectiveness of atmospheric dispersion resulting from a controlled elevated release compared to an uncontrolled ground j
lovel release from ductwork.
Furthermore, venting through the i
cuppression pool would provide scrubbing of non-noble-gas fission products with an effective decontamination factor of about 100.
The i
cddition of a hardened vent will greatly reduce the occupational doses
.for personnel that need to enter and work in the reactor building and that could be exposed to the containment environment.
1
'The staff has concluded that this generic EA' applies to Millstone 1 and the installation of the hardened vent will, therefeore, reduce dose consequences and will not result in an adverse environmental impact.
Plant-specific design features will have an effect on the degree of the environmental benefits, but not on'the conclusion concerning no significant environmental impact.
3.0 Conclusions and Recommendations Based on the safety benefits discussed in Sections 2.1, 2.2, and 2.3 for Millstone 1 and in SECY 89-017 for generic Mark I plants and
. supported by the plant-specific cost-benefit analysis, the staff b311 eves that the installation of a hardened wetwell vent at Millstone 1-is warranted.
3.1 Rationale for the Recommendation In SECY-89-017, the staff concluded on a generic basis for Mark I plants that the proposed hardened vent capability'would provide enhanced plant capabilities with regard to both accident prevention 1
and mitigation.
The addition of a hardened vent (1) prevents the majority of TW sequences froc resulting in core melt, and (2) sitigates the consequencas of residual sequences involving core melt where venting through the suppression pool is found to be necessary.
In TW sequences, the containment fails before the core melt occurs; therefore, significant releases could result.
A core melt, combined
.with a reactor vessel and containment failure, would release oignificant amounts of fission products to the environment.
The curvivability of the containment, which acts as the last barrier for
, i
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4 cnLuncontrolled release of radiation, would increase-with. venting..
1 tly' reduces thetlikelihood of
.'The installation of a hardened vent greao core melt from TW sequences and therefore reduc lpublic.
For other sequences.where core melt is predicted,. venting
-.could be effective'in delaying containment failure and in mitigating
,the. release of fission products. Although venting of the containment r
'io-currently included in BWR emergency operating procedures, it a.,
ganerally uses ductwork with a low design pressure.
Venting under.
.u high-pressure severe accident conditions could' fail this ductwork, Oh
~ralease the containment atmosphere into the reactor! building, and d: mage equipment, or contaminate equipment needed'for accident recovery.
Venting through this ductwork will probably hamper or ccmplicate post-accident recovery activities, and is therefore-viewed e
co reducing the' safety benefit.
The installation of a reliable-herdened wetwell vent allows for controlled venting through a path:
4 lwith significant scrubbing of fission products to the plant stack and w:uld prevent damage to equipment needed~for accident recovery.
With the installationlof the hardened vent capability, the staff cotimated that the total plant CDF for Millstone 1 can be reduced by-1.4E-5 per reactor year because of the reduction in the probability of 1N sequences.
Implementation of the proposed hardened vent nodification will significantly reduce'the total risk to the health
'ond safety of the public.- The averted population dose lof 35.1 man-rsa per reactor year was calculated for Millstone 1 from the installation of= hardened vent capability.
For-21 years of remaining 4
operating life.the total averted population dose would be 738' man-rca.
If the averted cost associated with an accident is included, the cniculated value-impact ratio for Millstone 1 is 819 man-rem saved'per
- nillion dollars.
-(Consideration of a/20-year operating life extension
- wil14 increase the total averted population' dose to 1441 man-rem and the: calculated value-impact ratio to 1644 man-rem saved per million dollars.
This demonstrates additional benefits for the' installation L
of'the' hardened-vent capability-in the event;of a license extension.)
. Additional benefits of venting, not quantified, include' source term reduction and the delay in containment failure for some of the ccenarios that lead to core melt.
Based on both the qualitative and quantitative benefits discussed e
L horein and'the supporting plant-specific cost-benefit. analysis, the otaff believes that there.will be a substantial < increase in the oves otection of-the public health and safety'by implementing the
- har, t capability for Millstone 1.
'Therefore, the staff bel-
.t this backfit is justified.
l~.
e
. p
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f 4.0 References l
- 1..
SECY-87-297,'U.S. NRC, " Mark I Containment Performance Program l
Plan,"
V.
Stello to NRC Commissioners, December 8, 1987.
2.
NUREG-1150, Second Draft, U.S. NRC, " Severe Accident Risks: An-1 Assessment for Five U.S. Nuclear Power Plants," June 1989.
. 3.
SECY-89-017, U.S. NRC, " Mark I Containment Performance
= Improvement Program,." V. Stallo to NRC Commissioners, January 23,'1989.
4.
Memorandum from S. J. Chilk to V. Stello, "SECY-89-017 - Mark I Containment Performance Improvement Program," July 11, 1989, 5.
U.S. NRC, Generic Letter 89-16, " Installation of a Hardened Wetwell Vent," September 1, 1989.
j J
6.'
Backfit Rule, Code of Federal Regulation, 10 CFR 50.109.
~ 7.-
WASH-1400, U.S. NRC " Reactor Safety Study," October 1975.
8.-
Memorandum from Brian W. Sheron to Ashok C.
Thadani, October 19, 1989, " Reduction in Risk From the Addition of Hardened Vents in BWR Mark I Reactors."
l 9.
NUREG/CR-5225, draft,_"An Overview of Boiling. Water Reactor j
Mark I containment Venting Risk Implications," October 1988.
l 10 ~ '.
NUREG/CR-2723, " Estimates of the Financial Consequences of-I Nuclear Power Reactor Accidents," September 1982.
11.
NUREG/CR-3568, "A Handbook-for Value-Impact Assessment," -
December 1983.
j l
- 12. ;
Letter from E.J. Mroczka (Northeast Utilities) -to U.S. NRC, Octobe.r 30,.1989, " Millstone Nuclear Power Station Unit: No.1 1
Response to Generic Letter 89-16, Installation of a. Hardened Wetwell Vent."
- l a )
- \\j, f.s&.,
1 T
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s.-
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Appendix-A q i" 1;-,,
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L{
MARK I PIANT-SPECIFIC ENHANCED VENTING CAPABILITY REGUIATORY ANALYSIS 4
FOR MILLSTONE NUCLEAR POWER STATION, UNIT 1.
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o TABLE OF CONTENTS 1.0 STATEMENT OF THE PROBLEM..............................
A-1 A-2 2.0 OBJLCTIVES.............................................
3.0 ALTERNATIVE RESOLUTIONS...............................
A-2 3.1 Alternative (i)
A-2 3.2 Alternative (ii)
A-3 3.3 Alternative (iii)
A-4 4.0 CONSEQUENCES..........................................
A-4 4.1 Costs and Benefits of Alternative Resolutions....
A-4 4.1.1 Alternative (i)
A-4 4.1.2 Alternative (ii)
A-5 4.1.2.1 Valuet Risk Reduction Estimates....
A-5 4.1.2.2 Impacts: Cost Estimates............
A-5 4.1.2.3 Value-Impact Ratio.................
A-7 4.1.3 Alternative (iii)
A-7 4.1.3.1 Values Risk Reduction Estimates....
A-7 4.1.3.2 Impacts: Cost Estimates............
A-7 4.1.3.3 Value-Impact Ratio.................
A-8 4.2 Impacts on other Requirements..................... A-12 4.3 Constraints...................................... A-12 5.0 DECISION RATIONALE.................................... A-12 5.1 Commission's Safety Goal......................... A-12 6.' 0 IMPLEMENTAT I ON........................................ A-12 6.1 Schedule for Implementation...................... A-12
7.0 REFERENCES
............................................ A-14 ATTACKMENT 1 To APPENDIX A - BACKFIT RULE ANALYSIS......... A-15 l
i 4
(
o f
Mark I Plant-Specific Enhanced Venting Capability Regulatory Analysis 1.0
$TATEMENT OF THE PROBLEM In SECY-89-017 dated January 23, 1989 (Reference 1), the staff presented its findings concerning the Mark I containment Performance Improvement (CPI) program to the Commission.
One of the improvement that the staff recommended was the installation of hardened vent capability.
The Commission concurred with the staff's position and directed the staff to proceed with the imposition of a hardened vent capability for each boiling water reactor (BWR) with a Mark I containment where a plant-specific backfit analysis supports such a backfit.
The General Electric Company has designed and constructed several BWR configurations with three basic containment designs designated as Mark, I, Mark II, and Mark III.
Probabilistic Risk Assessment (PRA) studies have been performed for a number of BWRs with Mark I containments.
Although these PRA studies do not show the BWR Mark I plants to be risk outliers as a class relative to other plant designs, they do suggest that the Mark I containment could be challenged by a large scale core melt accident, primarily due to its smaller size.
- However, estimates of the probability of containment failure under such conditions are based on calculations of complex accident conditions 1
that contain significant uncertainty.
Draft NUREG-1150 (Reference 2) evaluated the dominant accident sequences for five plants, one of which was a BWR Mark I.
The dominant accident sequences were identified as station blackout (SBO),
which includes the loss of all AC and DC powers and anticipated transient without scram (ATWS). This list would have included the loss of long-term decay heat removal (TW) except that, for the particular plant being reviewed, the likelihood of this sequence was considered to be greatly reduced because of assumed successful venting of the containment.
While the TW sequence was not considered in NUREG-1150 to be a dominant sequence for the plant reviewed, it can be a significant contributor to overall plant risk for Mark I plants in general.
(The June 1989 version of draft NUREG-1150 reported similar results for the Peach Bottom Atomic Power Station as were reported in the February 1987 edition.)
All BWRs with Mark I containments have a capability to vent the i
containment with various size. lines.
The largest lines usually are associated with the vant and purge system used to inert and de-inert containment.
Venting of containment as an accident mitigative action is permitted in the Emergency operating Procedures (EOPs).
In part, A-1
the existing vene path uses sheetmetal ductwork from the containment isolation valves through the standby gas treatment system (SGTS) to the plant stack.
The sheetmetal ductwork is usually designed for low pressure and is expected to fail under severe accident pressures.
Failure of the ductwork would introduce the containment atmosphere to the reactor building.
This could result in harsh enviranaental conditions that would complicate operator accident recovery actions within the reactor building and could cause failure of equipment within the reactor building.
Tha hard pipe vent would be designed to withstand severe accident pressures, and, thus, would not fail during a TW event thereby alleviating the harsh environmental concerns in the reactor building.
This regulatory analysis studied the costs and benefits of installing a hardened vent capability at BWRs with Mark I containments.
2.0 OEJECTIVES i
The staff objective is to reduce the overall risk in BWR Mark I plants by pursuing a balanced approach using accident prevention and accident mitigation.
Most recent PRA studies indicate that TW is an important contributor to BWR Mark I risk.
The balanced approach includes (1) accident prevention - those features or measures that should reduce the likelihood of an accident occurring or measures that the operating staff can use to control the course of an accident and return the plant to a controlled, safe state, and (2) accident mitigation - those features or measures that can reduce the magnitude of radioactive i
releases to the environment during an accident.
Although the staff considered the quantification aspects of both accident prevention and mitigation, tais regulatory analysis only quantified the preventive aspects.
The proposed hardened vent capability would provide enhanced plant capabilities and procedures concerning both accident prevention and mitigation.
3.0 ALTERNATIVE RESOLUTIONS Plant modifications to the containment venting capability are being proposed to reduce the probability of or to mitigate the consequences of a severe core melt accident.
The proposed modification consists of I
installation of a hard pipe from the existing wetwell ventilation 1
penetration, bypassing the ductwork to the standby gas treatment system, and going to the plant stack.
The ventilation penetration is the 18-to 24-inch penetration normally used as part of the vent and purge system for deinerting the containment.
For the proposed modifications, the new components need not be safety-grade or safety-related.
However, no failure of the modified system or non-safety-related component is to adversely affect any safety-related structure, system, or component required for coping with A-2 1
design-basis accidents.
3.1 Alternative (1)
?
This alternative is the no-action option, that is, to leave the existing venting capability unaltered.
The existing venting capability vents the containment through the existing ductwork from the suppression pool to the SGTS.
The ductwork design pressure is usually a few psid or less (Reference 3).
consequently, venting under severe accident conditions could cause failure of the ductwork and a direct release into the reactor building.
The discharge of high-temperature gases over an extended period of time may pose a threat to the availability or performance of safety-related equipment.
The discharge of hydrogen could result in hydrogen burns (or detonations) inside the reactor building.
Electrical cables, motor operators on valves, relays, and control room components may fall under these environmental conditions.
Adverse environmental conditions would complicate entry irio the reactor building.
calculations from a venting study during an anticipated transient without scram (ATWs) indicate a severe environment would be present in the reactor building during venting operations (Reference 4).
If systems that are needed to terminate the accident need repair, this environment (high temperature and radiation) could hamper recovery efforts by preventing personnel from entering into the reactor building.
3.2 Alternative (11)
This alternative would involve the installation of a hardened venting capability from the containment wetwell to the plant stack.
The proposed venting improvement would provide a wetwell path to the plant stack capable of withstanding the anticipated environmental conditions of a severe accident.
This proposed modification would include the installation of hard pipe from the outlet of an existing watwell vent outboard containment isolation valve to the base of the plant stack.
This pipe would be routed through a new isolation valve that would bypass the existing ductwork and the SGTS.
The hard pipe to the stack could contain a rupture disk to prevent inadvertent operation and release of rad.ioactivity.
The emergency procedures would need to be modified to provide appropriate instructions for the operator.
This alternative would mitigate the consequences of severe accidents by reducing the likelihood of core melt from the TW sequence.
All releases through the vent would pass through the suppression pool, and the particulates would be scrubbed.
During a loss of long-term decay heat removal accident, this alter-native would prevent failure of the vent path inside the reactor A-3 l
l
8 30 0
i s
building and would result in an elevated release.
The elevated i
release could reduce the offsite consequences.
Since the vent path should not fail inside of the reactor building, personnel could repair equipment and perform other plant recovery activities in the reactor l
building.
Furthermore, there would be no harsh environmental conditions to degrade or fail other equipment.
There is,the 1
l possibility of inadvertont operation of the vent that would release some radioactive material without any holdup time or filtration.
This alternative would not affect the releases of radioactive material for those sequences where the drywell fails, such as from corium attack, once the drywell shell has failed.
i 3.3 Alternative fili)
This alternative would involve alternative (ii) plus the installation of an external filter system.
The proposed venting improvement includes the hard pipe vent discussed i
in alternative (ii) plus the installation of an external filter system, such as the Tiltra system or the Multi Venturi scrubbing i
System (MVSS).
This external filter would be installed outside of the i
existing facilities.
A single external filter unit could be constructed to service multiple containments with proper isolation valves.
Both the Filtra and the MVSS systems do not rely on AC power to perform their intended functions.
Similar to alternative (ii), the emergency procedures would need to be modified to provide appropriate instructions for the operator.
This alternative would mitigate the consequences of a severe accident and could reduce the likelihood of core melt if the operator transfers suction of the injection pumps from the suppression pool to an alternate source of water, such as the condensate storage tank, before venting containment.
With the external filter, the amount of particulate removal of the external filter would not be sensitive to the conditions in the suppression pool.
lo significant additional risk reduction was estimated to result rom an external filter system in addition to the suppression pool se cubbing.
Since all particulate releases through the hardened f
vent (alternative 11) are scrubbed, the external filter will only provide sinimal additional scrubbing.
The external filter provides no l
additional benefit in core melt prevention although it would provide filtration and some holdup time for inadvertent operation of the vent.
Similar to alternative (ii), this alternative would not affect the relsases of radioactive material for those sequences where the drywell
' fails, such as from corium attack, once the drywell shell has failed.
t l
l 4.0 CONSEQUENCES 4
4.1 costs and Benefits of Alternative Resolutions The staff used available PRAs to estimate the incremental benefit of A-4 l
~... - - - -
o I
the three alternatives discussed in the following paragraphs.
The only accident sequence that is being considered for this analysis is the TW.
This is considered to be conservative since the alternatives could have a beneficial but small effect on other sequences (Reference 5). The staff estimated the change in the CDF, but not the total CDF from internal events (Reference 6).
4.1.1 Alternative fi)
This alternative would be to take no action.
Since it is expected that the ductwork would fail if the containment were vented at high pressure, this approach would not only jeopardize personnel, but also the ability to regain control of the facility during the accident.
Furthermore, based on a generic regulatory analysis (Reference 1) the i
commission instructed the staff to require hardened vent capability for plants for which it could be shown to be cost effective.
Therefore, based on the discussion below the no-action alternative is not recommended.
4.1.2 Alternative (111 4.1.2.1 Values Risk Reduction Estimates For those accident scenarios where containment failure results in core degradation and a severe accident, the approach using a hard pipe vent path could reduce or delay core degradation.
This is estimated to reduce the total core damage frequency per reactor year by 1.4E-5.
Corresponding to a release of 2.5E6 man-rem, this represents a risk reduction in man-rem per reactor year of 35.1.
4.1.2.2 Impacts: Cost Estimates The estimated cost for installation of the hard pipe vent path is 1.1 million dollars (Reference 7).
The averted cost associated with prevention and mitigation of an accident can be discussed as five separate costs:
replacement power, cleanup, onsite occupational health impacts, offsite health impacts, and onsite property damage.
To estimate the costs of averting plant damage and cleanup, the reduction in I
accident frequency was multiplied by the discounted costs of onsite property.
The following equations from NUREG/CR-3568 l
(Reference 8) were used to make this calculation V,,= NdFU U= (C/m) ( (e*"d))/r ) ( 1-e*'""I* " U I ) ( 1-e"*)
I 6
A-5 l
- - - - - ~
E.
wheret (cited values are from Table 2)
V,,
= value of avoided onsite property damage ($)
N-
= number of affected facilities = 1 dr
= reduction in accident frequency = 1.4E-5 /RY U
= present value of onsite property damage ($).
C
= cleanup and repair costs = $1.0 billion t(f) = years remaining until and of plant life = 21 t(1) = years before reactor begins operation = 0 r
= discount rate = 10%
a
= period of time over which damage costs are paid out (recovery period in years) = 10 Using these values, the present value of avoided onsite proparty damage is estimated to be $77,660.
Replacement power costs can be estimated using NUREG/CR-4012 (Reference 9), which lists the replacement power costs for each nuclear power reactor by season.
Using this information for only Mark I reactors averaged over the four years of projected data and escalated by six percent for 1989 dollars, the generic replacement power cost is $400,666 per day.
(The plant-specific replacement power cost is shown in Table 3.
NUREG-1109 (Reference 10) used a generic cost of $500,000 per day and compares favorably with NUREG/CR-4012.)
The change in public health risk associated with the instal-1ation of the proposed hardened vent system is expressed as total man-rem of avoided exposure.
The following spations from NUREG/CR-3568 were used to make this calculationt V, = NT (D, x R) p where:
V,,
= value of public health risk avoided for net-benefit method ($)
N
= number of affected reactors = 1 1
T
= average remaining lifetime of affect d facilities (years) = 21 D,
= avoided public dose per reactor-year man-ren/RY)
= 35.1 R
= monetary equivalent of unit dose ($/ man-ren)
= $1000 Using these values, the' avoided public health exposure of 0.738 million dollars is obtained for Millstone 1.
Considering a possible 20-year operating life extension, the value of avoided public health exposure is 1.441 million dollars.
A-6 i
,~
l-
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l i
The occupational health risk avoided because of the installation of the proposed hardened vent system is expressed as man-rem of avoided exposure.
The following equations from NUREG/CR-3568 c
were used to make this calculatnont Yea = NT(D, x R) wheret V,, = value of occupational health risk due to l
accidents avoided ($)
N
= number of affected reactors (reactors) = 1 T
= average remaining lifetime of affected facilities (years) = 21 D,
= avoided occupational dose per reactor year (Man-Rem / Reactor-Year)
[
i R
= monetary value of unit dose ($/ Man-Rem)
= $1000 / Man-Rea There are two types of occupational exposure related to accidents, immediate and long-term.
The first occurs at the time i
of the accident and during the immediate management of the emergency.
The second is a long-term exposure, presumably at significantly lower individual rates, associated with the cleanup and refurbishment of the damaged facility.
The best estimate of the immediate occupational exposure as specified in NUREG/CR-
+
3568 is 1000 man-rem.
The best estimate of the long-term occupational exposure as specified in NUREG/CR-3568 is 20,000 man-rem.
This results in occupational exposure of 21,000 man-The multiplication of 21,000 man-rem by the reduction in rem.
CDF, 1.4E-5 per reactor year, produces the avoided occupational-dose per reactor year, D.
g Using these values, the present value of avoided occupational l
i l
health exposure was calculated to be $6,174, approximately one to two percent of the public health risk, and is not considered to be a significant contributor.
Therefore, the occupational health exposures will not be considered further.
4.1.2.3 Value-Impact Ratio The value-impact ratio, not including the costs of onsite accident avoidance, is 671 man-rem averted per million dollars.
If the savings to industry from accident avoidance (cleanup ea!
repair of onsite damages and roolacement power) were included, the overall value-impact ratio would be 819 man-rem averted per millisa de31ars.
Considering a 20-year operating life extension, the overall value-i-nact ratio would be 1644 man-rem averted per A-7 i
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million dollars.
i 4.1.3 Alternative flii) 4.1.3.1 Values Risk Reduction Estimates This alternative would provide minor additional par'ticulate scrubbing for the hard vent.
However, because all particulate releases will have been scrubbed by the suppression pool, the i
improvement over alternative (ii) could be minimal.
4.1.3.2 Impactst Cost Estimates j
External filters were estimated to cost $10 million to $50 million for the Filtra design and about $5 million for the Multi-Venturi Scrubber System design.
i Using the same equations given in alternative (ii), the present value of the estimated avoided onsite damage to property is
$77,660.
Similarly, the estimated replacement power cost is $157 million per year.
Thus, the estimated avoided damage to onsite property and the replacement power is $199,556.
The present value of the change in the estimated public health risk associated with the installation of the hard vent and the external filter is $0.738 million.
l 4.1.3.3 Value-Impact Ratio The overall value-impact ratio of this alternative is in terms of man-rem averted per million dollars.
If the savings to industry from accident avoidance (cleanup and repair of onsite damages and replacement power) were included, the overall value-impact ratio would be 125 man-rem averted per million dollars.
This is calculated from the value in Column G of Table 2 divided by the installation cost in Column H of Table 2 and added 5 million dollars for the MVSS design minus the value in Column N of Table 2.
- This alternative is not recommended because it does not provide substantial additional safety benefit over alternative (ii) and is not cost effective.
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{
(man-rem averted per million dollars) i
'1 Alternative (i)
- do nothing o
i Alternative (ii) - hard pipe venting for the remaining life 819 with 20-year life extension 1644 l
Alternative (iii) - hard pipe venting 125 i
+ NVSS external filter i
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tele 2 - Osc6 fit Analye6s for Presseed Rurepuse tout tapeility for Clisteme t 9
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11:13 Il Source: 59st aWEG-1330, ested naret tiet
' p 21 Seurte WW eWFG4308 31' Seurret assaramemo free 8.5. De en, ested ktdor 19, 1989, to A.C. Theempi.
tedetties to nest 4res tee asenties es agreened spots is am nort I spectes*
- i 49 Source: Sunt dWESMB-2T23, ested $setester 1982 testopts aspe treet e tsaleetfresP51evaatt-temstt-stil St Seurter seneret letter 059-16, estes 1setosher 1,1999, *testallatten el nure arteDll %Fet*
65 Sources lastelleties tests tres seenramese sees LE. Parties to T.E. merley, dated es ester 9, tie',
9 Wessees" Arnousers to Greerit letter Wr-te Reisted to testelleties of mereemed spearti seat' 71 Suurce: UK WESMR-35EB, ested Setoder 1983. pages 3.1t-3.17,3.29-3.31. 3.16-3.19 99 fee ouders to the totese titles ro6er to seerte et se$erestice easter ate e.
j_ p 99 fee letter le trattets, tal, are the teless idsstifiers amt the letters is testests, tr.sp, ye tte egmatics esieg the toisen 6dsstifiers ter eveerentes. Ite
- Strip
- fatter it the steled ese+ee $$11 esator frne tte strip i
Report eneierd by tee eieitar easter 9er Peact tuttee Ostt 2 to acteunt Ser the este es49erences.
p 199 togstive asumers le teless Igl teostate test the omsate tests setsof tee testelletien tests.
flerefore the propeesd asfification eetoves the Steetfeene tratoria and say te ispeerd.
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Table 3 - Estimated Replacement Power costs (in dollars per day)
Year Est. Cost Est. Cost Est. Cost Reactor'Name NWe Licensed 1985$
1989$
(per year) l Millstone 1 660 1971
$346,800
$430,032
$1.6,961,680
)
i Notes: 1:NUREG/CR-4012 (Table S.1) prcvides replacement p'swer costs for all plants on per plant / season bqsis for 19'J7-1991.
i 2 The inflation rate used is 6 percent / year, ar.d the discount rate used is 10 percent / year.
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4.2 lER4Ela_Qn other Recuirementa There are six programs related to severe accidents: Individual Plant Examination (IPE), Containment Performance Improvement (the topic of l
i.
this regulatory analysis), Improved Plant operations, Severe Accident i
Research Program, External Events, and Accident Management.
Each of the five programs related to Containment Performance Improvement (CPI) i will be discussed briefly in Item 3 of Attachment 1, Backfit Rule Analysis.
i 4.3 constraints The plant-specific imposition of a hardened vent is constrained by the guidelines of U.S. NRC Manual Chapter 0514, "NRC Program for F
Management of Plant-Specific Backfitting of Nuclear Power Plants",
which is based on the backfit rule (10 CFR 50.109), as published by I
the Commission on September 20, 1985, and the provisions of 10 CFR 50 Appendix 0, 10 CFR 50.54(f), and 10 CFR 2.204.
No other constraints have been identified that affect this program.
5.0 DECISION RATIONALE The evaluation of the CPI program included deterministic and probabilistic analyses.
Calculations to estimate the CDF and the consequences of the TW sequence were performed using information available from the NUREG-1150 program and from existing PRAs.
The best estimate of the contribution of TW to the total plant CDF expressed in events per reactor year for Millstone 1 is 1.4E-5.
Implementation of the proposed hardened venting capability will cause TW to be a minor contributor to the total CDF and will significantly I
reduce the total risk to the health and safety to the public.
5.1 commission's Safety Goal On August 4, 1986, the Commission published in the Federal Reaister a policy statement on " Safety Goals for the operations of Nuclear Power Plants" (51 FR 28044).
This policy stateuent focuses on the risks to the public from nuclear power plant operation and establishes goals that broadly define an acceptable level of radiological risk.
The discussion in the Regulatory Analysis of SECY 89-017 addressed the CPI program recommendation in light of these goals.
6.0 IMPLEMENTATION 6.1 Schedule for Inolementation The licensee may reconsider its position on the installation of the A-12 h
,-w--
- wwn,
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... s hardened vent under the provisions of 10 CFR 50.59.
Without the licensee's commitment, the staff intends to pursue an order after 30 i
days of its receipt of this analysis, requiring this backfit under the
)
provision of 10 CFR 50.109.
Within 60 days after issuance of the backfit order, the licensee will be required to submit to the NRC a i
schedule for implementing any necessary equipment and procedural modifications to meet the performance goals and to provide' adequate defense-in-depth.
All plant modifications are to be installed, procedures (including the decision making process for venting) revised, and operators trained not later than January 1993.
other schedules were considered; however, the staff believes the proposed implementation of the hard pipe vent capability can be largely performed with minimum interfacing with containment and engineered safety feature systems and thus with the plant online.
Therefore, the licensee can install the proposed modification without unnecessary financial burden for plant shutdown.
The schedule allows reasonable time for the implementation of necessary hardware to l
achieve a reduction in the risk from TW.
Shorter or less flexible schedules would he unnecessarily burdensome.
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7.0 EETERENCES 1.
SECY-89-017, " Mark I Containment Performance Improvement f
e Program," January 23, 1989.
2.
NUREG-1150, (Draft), " Reactor Risk Reference Document," February
]
1987.
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3.
NUREG/CR-5225, "An overview of Boiling Water Reactor Mark I Containment Venting Risk Implications," October 1988.
4.
- Harring, R.M., " Containment Venting as a Mitigation Technique for i
BWR Mark I Plant ATWS," 1986 Reactor Water Safety Meetina, Gaithersbura. Marv)and, October 1986.
5.
NUREG/CR-5225, Addendum 1, "An overview of Boiling Water Reactor Mark I containment Venting Risk Implications, An Evaluation of Potential Mark I Containment Improvements," June 1989.
6.
- Sheron, B.W., Memorandum to Thadani, A.C., " Reduction in Risk i
from the Addition of Hardened Vents in BWR Mark I Reactors,"
October 19, 1989.
7.
Letter from E.J. Mroczka (Northeast Utilities) to U.S. NRC, I
October 30, 1989, " Millstone Nuclear Power Station Unit No. 1 Response to Generic Letter 89-16, Installation of a Hardened Wetwell Vent."
i 8.
NUREG/CR-3568, "A Handbook for Value-Impact Assessment," December 1983.
I 9.
NUREG/CR-4012, " Replacement Energy Costs for Nuclear Electricity-Generating Units in the United States: 1987-1991," January 1987.
10.
NUREG-1109, " Regulatory /Backfit Analysis for the Resolution of Unresolved Safety Issue A-44, Station Blackout," June 1988.
11.
SECY-88-147, " Integration Plan for Closure of Severe Accident Issues," May 25, 1988.
12.
Memorandum from S. J. Chilk to V. Stallo, "SECY-89-017 - Mark I Containment Performance Improvement Program," July 11, 1989.
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AITACHMENT 1 TO APPENDIX A BACKFIT RULE ANALYSIS Analysis and Determination That the Recommended Hard Pipe Vent Capability for Containment Performance Improvement' Complies with i
the Backfit Rule 10 CFR 50.109 The Commission's regulations establish requirements for the design and testing of containment and containment cooling systems (10 CFR 50, Appendix A, General Design criteria 50, 52, 53, 54, 55, 56, and 57) with respect to design basis accident conditions.
As evidenced by the accident at TMI Unit 2, accidents could progress beyond design basis i
considerations and result in a severe accident.
Such an accident could challenge the integrity of containment.
Existing regulations do not explicitly require that nuclear power plant containments be designed to withstand severe accident conditions.
The staff and our consultants studied this issue as part of the severe accident program for the General Electric Company boiling water reactors (BWRs) with Mark I containments.
BWRs with Mark I containments were reviewed first because of the perceived susceptibility of the Mark I containments to failure based, in part, on the small containment volume of the Hark I containment design.
Both deterministic and probabilistic analyses were performed to evaluate the loss of long-term decay heat removal (TW) in challenging containment integrity and potential fa,ilure modes affecting the likelihood of core melt, reactor vessel failure, containment failure, and risk to the public health and safety.
The risk analysis shows that the risks from plants with Mark I containments are generally similar to the risks from plants with containments of other types.
In addition, the hardened pipo vent capability is not needed to provide
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adequate protection of the public health and safety.
Rather, the proposed plant improvement will provide substantial cost-effective enhancement to Mark I plant safety.
The estimated benefit from implementing the preposed hard pipe vent is a reduction in the frequency of core melt caused by TW and the associated reduction in risk of offsite radioactive releases.
The estimated risk reduction in terms of man-rem is 738 and supports the conclusion of the Commission that implementation of the proposed-i improvement provides a substantial improvement in the level of protection of the public health and safety.
The estimated cost to the licensee to implement the proposed safety l
enhancement is 1.1 million dollars. This cost would be primarily for the licensee to 1) assess the plant's capability, 2) install equipment to provide additional pressure relieving capability, 3) revise the esernancy operating procedures, and 4) provide operator training A-15
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concerning mitigating the TW sequence.
The estimated value-impact ratio, not including accident avoidance costs, in terms of man-rems averted per aillion dollars is 671.
If the not cost, which includes the cost savings from accident avoidance (i.e. cleanup and repair of onsite damages and replacement power following an accident), was included, the estimated overall value-impact in terms of man-rems averted per million dollars would be 819.
If 20 years of life extension were included, the estimated overall value-inpact in terms of man-rems averted per million dollars would be 1644.
These values support proceeding with the proposed hard pipe vent capability improvement.
Although the preceding quantitative value-impact analysis was or.e of the factors considered in evaluating the proposed improvemente, other factors were considered as a part in the decision-making precess.
PRA studies performed for this issue have shown that the loss of long-term decay heat removal (TW) events can be a significant contributor to core melt frequency.
With consideration of the conditional containment failure probability, TW events can provide an important contribution to reactor risk.
Although there are licensing requirements and guidance for providing a containment and support systems to contain any release of material from the reactor vessel, containment integrity may be significantly challenged under severe accident conditions.
In general, active systems required for reactor and containnsnt heat removal are unavailable during the TW event.
Therefore, the offsite risk is i
higher from a TW event than it is from many other types of accidents.
The containment integrity is primarily challenged by over-pressure for the TW events.
Under certain conditions, failure of the containment can also initiate core degradation.
The estimated frequency of core melt from TW events is directly proportional to the frequency of the initiating events.
The estimate of the TW frequency for Millstone 1 was partly based on information provided in draft NUREG-1150, " Severe Accident Risks: An Assessment for Five US Nuclear Power Plants," for the Peach Bottom Atomic Power station, Unit 2, and other available PRAs.
This is assumed to be a realistic estimate of the core melt frequency when compliance with 10 CFR 50.63, the Station Blackout Rule, has been achieved.
The factors discussed in the previous paragraphs support the determination that the additional defense-in-depth provided by the ability to cope with a TW event would substantially increase the overall protection of the public health and safety. Also, this increased protection will justify the direct and indirect costs of
' implementation.
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Analysis of 10 CTR 50.109(c) Factors (1)
Statement of the-soecific obiectives that the backfit is desianed to achieve j
The objective of the proposed hard-pipe vent capability is to reduce the risk from TW events by reducing the likelihood of core melt and to mitigate releases given a TW or other similar events leading to core melt.
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(2)
General descriotion of the activity resuired by the licensee or anolicant in order to comolate the backfit To comply with the proposed improvement in containment venting, the licensee will be required to Evaluate the actual capability of the-existing containment vent system to withstand the anticipated containment temperatures and pressures without failing any portion of i
I the vent path to the plant stack.
Evaluate the actual capability of the existing centainment
' tent isolation valves to be opened and closed under anticipated containment pressures and vent flow rates during severe accidents involving TW sequences.
Determine the necessary plant modifications to ensure a hard-pipe vent path will be available under TW events, develop a schedule for plant modification, and submit the l
schedule to the NRC within 60 days from the issuance of the backfit order.
Complete the necessary modifications by January, 1993.
l The licensee will be required to have the decision making process, the procedures and training to cope with and recover from a TW severe accident.
These procedures should conform to the Emergency procedure Guidelines of the Boiling Water Reactor L
Owner's Group.
(3)
The notential safety imoact of chances in olant or coerational comolexity, includina the relationshio to oronosed and i
j.
existina reaulatory reuuirementh l
l The hardened vent capability to cope with the TW event should not l '
add to plant or operational complexity, because the vent is l
normally closed and not operated during normal power operation.
Althouah this system does add some additional hardware to the plant, it is a simple system.
The containment performance A-17
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