ML17202U740

From kanterella
Jump to navigation Jump to search
Plant-Specific Analysis for Dresden Nuclear Power Station, Unit 3,Re Installation of Hardened Vent
ML17202U740
Person / Time
Site: Dresden  
Issue date: 06/15/1990
From:
NRC
To:
Shared Package
ML17202U737 List:
References
NUDOCS 9007170197
Download: ML17202U740 (35)
v • d • e


Text

{{#Wiki_filter:.* Plant-Specif ~c Analysis for the Dresden Nuclear Powe~ station, Unit*J, Regarding

n. ! 90071?oi97 PDR ADOCK p

Installation of a Hardened Vent J 900615 05000237 I PDR 1? * 'i --~ --' . Jr. f

TABLE OF CONTENTS 1.0 Background............................................. ~...... 2.0 Discussion.......................................... *... -........

2. 1 Safety Benefits................ ! ****************************

2.2 Reduction in Core Damage Frequency and Public Risk *.**.*.... 2.2.1 Plant Similarity Assessment *...**.**********.**.**... 2.2.2 Reduction in Core Damage Frequency **.************...*

2. 2. 3 Risk _Reduction.......................................

2~3 Cost-Benefit Analysis.......................................

2. 3.1 Cost Estimation...... *... **.........'.... *............... -

2

  • 3
  • 2 Value-Impact Assessment.*..*.**.***.*******.****.**....

2.4 Alternatives Considered and Impacts on Other Programs ******.

2. 5 Environmental Assessment.** * *.***.**... * *.******.************

3.0 Conclusions and Recommendations **..*.*.*********.************* 3.1 Rationale for the Recommendation...******.********.***.*....

4. O References. !I' *****.* ~ **********************.* ~ **.*,*.*.***.

ppendix A Regulatory Analysis on the Backfit of Hardened Vent 1 2 2 4 4 5

5.

6 6 '6 7 7 8 8 ],O

Plant-Specific Analysis- . for the Dresden Nuclear Power Station, Unit 3, Regarding Installation of a Hardened Vent 1*. o Background In SECY-87-297 (Reference 1), dated December 8, 1987, the Nuclear Regulatory Commission (NRC) staff presented to the Commission its program plan to evaluate generic severe accident containment vulnerabilities in a program entitled the Containment Performance .Improvement (CPI) program. The staff *began this effort with the premise 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 premise that such assessments are needed is based on the relatively large

  • uncertainty in.the *ability of some LWR containments (for example, Mark I) to successfully survive some severe accident.challenges, *as' indicated by NUREG-1150, dated June 1989 (Reference 2).

This effort -is integrated closely with the program for Individual Plant Examinatiori (IPE) an~ is intended to focus on resolving hardware* and. procedural issues concerning generic containment chai1enges!.In .. SECY-.89-017 (Reference 3), dated January 23, 1989, the staff presented its findings concerning the.Mark I CPI program to the ommission'.* One of the improvements that the staff recommended was* he installatiol) of a hardened vent capability. .'The staff c;oncluded that venting, if properly implemented, can-significantly reduce plant risk. This vent capability has long been recognized as important in reducing risk caused by loss of long-term decay 'heat removal events.

  • controlled venting can prevent the long-

. term 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 in the Automatic Depressurization System* (ADS). *Venting-of the containment

  • is currently included in the emerg*ency operating procedures for boiling water reactors (BWRs).

A vent path using existing 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 under high-pressure conditions created either before or after core-melt 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 vent valves for some accident scenarios. Therefore, venting through a sheet metal ductwork path, as currently implemented at some Mark I plants, is likely to hamper or complicate* post-accident recovery ~ctivities, and is, therefore, viewed by the staff as reducing the safety benefit. A hardened pipe vent capable f withstanding.the anticipated pressure loading of a severe accident o_uld eliminate this disadvantage. The Commission concurred with the staff's position and directed the staff 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, el'ect.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 requiring the installation of hardened vents. The staff issued Generic Letter (GL) 89-16 dated September 1, 1989 (Reference 5) to BWR licensees with Mark I containments: (1) to inform them of the direction given by the Commission regarding the hardened vent issue, (2) to provide them with a generic cost e~timate for the installation of a hardened vent and (3) to request that each licensee provide notification of its plan for addressing resolution of this issue. Moreover, the staff encouraged licensees to implement -the design changes to install the hardened vent. For those plants not electing.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 staff developed a program to meet t~e objectives of he Commission's directive.

This program plan contains the following ive tasks: (1) cost estimation, (2) plant similarity assessment (3) cost-bene*fit analysis, (4) environmental. assessment, and (5). imposition of requirements. 2.0 Discussion The purpose of this report is to document the results of the plant-specific backfit analysis performed by the staff for the Dresden Nuclear Power Station, Unit 3. This analysis compli_es with the backfit rule in 10 CFR 50.109 (~eference 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 implementation of the capability. 2.1 Safety Benefits The major benefit of a hardened vent is the reduction of both.the core damage frequency and public risks. Probabilistic Risk Assessment (PRA) studies for BWRs indicate that accidents initiated by transients dominate the total core damage frequency (CDF) in severe accident sequences. The principal accident sequences.for BWRs onsist of Loss of Long-Term Decay Heat Removal (TW), Station lackout (SBO), and Anticipated Transient Without Scram (ATWS). The ea.:c::tor _Safety study _(WASH-140Q) (Reference 7) indicated that TW is the-dominant accident sequence causing core damage at the Peach Bottom Atomic Power Station. Further, draft NUREG-1150 (Reference 2) indicates that SBO is the dominant contributor to core damage frequency at Peach Bottom. At Peach Bottom, *it was estimated that the TW frequency has been greatly reduced because of the successful implementation of containment venting procedures. This study indicates that venting, if properly implemented, 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 enhanced plant capabilities with _regard to both accident prevention and mitigation.

A core melt, combined with reactor vessel rupture and 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) mitigates the consequences of residual. sequences involving core melt where venting through the suppression pool is found necessary. -The ,Tw sequences are initiated by transient events and are followed by failure of long-term decay heat removal; the. containment *fails-from overpressurizatio~ and causes the subsequent core melt. The _ installation of a h~rdened vent will increase the survivability of containment, reduce the likelihood of a core.melt from TW sequences, nd therefore reduce the risks to the public. For other sequences

  • here core melt occurs before containment failure, venting could be effective in delaying containment failure and in mitigating the

.release of fissipn products beause venting through the suppression pool would provide significant scrubbing of particulate and volatil.e releases. In a BWR, containment venting.is currently included in the emergency . operating procedures. The existing vent path* generally consists of ductwork ranging in pressure capability down to design pressure of only a few psi for most Mark I plants.- The _low design-pressure ductwork is inadequate for accoinmodating the high containment pressure following a severe accident. Consequently, ven~ing under severe accident conditions could result in failure of the ductwork and a direct release of radioactivity into the reactor building. The

  • discharge of high-temperature gases over an extended period of time may threaten the availability or performance of safety-related equipment.

If substantial fuel damage has occurred, the discharge of hydrogen could cause hydrogen burns (or detonations) inside the reactor building. Electrical cables, motor operators on valves, relays, and control room components may fail under these environmental conditions. Adverse environmental conditions would complicate entry into the reactor building. This environment.of high temperature and perhaps radiation could hamper recovery efforts by preventing personnel from entering into the reactor building if ystems needed to terminate the accident need repair. As a result, hen relying ori the existing ductwork, the benefits of containment enti_ng a,re significantly uncertain. Therefore, hardening the.vent ath to withstand the anticipated pressure loading during a severe accident would eliminate this disadvantage while retaining all the benefits of containment venting. Because of the reduced core melt frequency, reduced fission product releases, and possible reduction or elimination of a significant

  • containment failure mode, the staff concluded that the safety benefits of venting are significant, and further improvement can be achieved by installing hardened vents.

In Reference a, the staff estimated the benefits in the reduction in CDF and in offsite risk, which are discussed in the following sections. 2.2 Reduction in Core Damage Frequency and Public Risk

  • To estimate the plant-specific reduction** in CDF, all Mark I plants were categorized into several groups based on the similarity of the design features that are important to the accident sequences that.

could be affected by the installation of a hardened vent. In performing_ 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 ~draft NUREG/CR-5225 (Refereilce 9), the three accident sequences hat were identified as being affected by venting are: (1) Loss of. Long-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) t~rough its effect on TW

  • sequences.

In* the TW sequence, failure to remove decay-heat following a transient will c~use the gradual pressurization of the containment. The containment may fail from overpressurization and . subsequently may lead to a core melt. 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 failure and subsequent core melt. The design features important to this sequence are the systems used for decay heat removal and containment cooling. The reduction in CDF for the_ TW sequence of each Mark I plant resulting from the installation of the hardened vent was estimated by the staff in Reference 8. To account for similarity in design, all .Mark I plants were.grouped according to the design of their decay heat removal *and containment cooling systems*- factors important in assessing the frequency of TW sequences. In determining the groups by examining individual plant features in simplified piping and instrument diagrams,.the staff studied the differences between the RHR systems, isolation condensers, power conversion system, and ervice water systems for all Mark I plants. In addition, the staff tudied the available PRAs and fail.ure probabilities of related components to identify any major differences and similarities in terms of CDF affected by the hardened vent capability. After careful study of the available PRAs, the staff categorized the Mark I plants into the following four groups: (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 t.rains, 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 (4) Plants with isolation condensers. 2.2.2 Reduction in Core Damage Frequency -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 vent would reduce 90 percent of these sequences. The estimates of CDF reduction conservatively consider only the TW

  • sequences, and therefore, the benefits on the SBO and ATWS sequences re not included.
  • For.Dresden 3,* the reduction in CDF was estimated using the PRA results of* a plant with similar design features.

The credit of the Isolatiori Condenser System (ICS) being used as the decay heat removal* system was included. To be consistent with the failure frequency assumed in NlJlIBG-1150, the staff incorporated several changes into the referenced PRA *. With these phanges, the staff calculated that venting would produce a reduction in CDF from TW sequences of l.4E-5 . per reactor year. More detailed information of this analysis is given in Reference 8. 2.2.3 Risk Reduction Installation of a hardened vent capability will reduce the CDF and will result in reduction in the populsation* dose that would be . associated with TW sequences. The estimate of the reduction in. population dose for Dresden 3 was calculated by multiplying the reduction in CDF estimated for Dresden 3 by a scaling factor to convert the Peach Bottompopulation dose to the Dresden 3 population dose. The scaling factor was obtained from NUREG/CR-2723 (Reference

10) for Dresden 3 plant-specific reactor power and p~pulation density.

The Peach Bottom population dose from TW sequences was derived using the insights from NUREG-1150. The resulting reduction in the population dose for Dresden Jdue to reduction in CDF for TW -s-

sequences was estimated to be 3.6E6 man-rem. The averted population os_e for Dresc;ien 3 was calculated by multiplying the reduction. in CDF by 3.6E6 man-rem to give 50.2 man-rem per reactor year. For the 21 years of operation remaining, the estimated total averted dose is 1055 man-rem. In addition, consideration of a likely 20-year operating life extension will increase the estimated total: averted .dose to 2060 man-rem. The averted occupational health risk resulting from the installation of the proposed hardened vent system is discussed and calculated in Section 4.1.2.2 of Appendix A. The estimated occupational risk is approximately one to two percent of the public health risk and is not considered to be a significant contributor. Therefore, the occupational health exposures are not further considered in the cost-benefit 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 used the *risk-re~uction 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 16,.all;_ Mark.I licensees except four (with five plants) indicated *

  • th~_t.,~~~y. intend to install the hardened vent under the provisions *.of 10 !CFR SO.* 59..

Dresden 3 is one of the five Mark I plants.: ; Commonwealth Edison (the licerisee) has de~ided not to voluntarily install the hardened vent capability< By letter dated October 30, 1989 (Reference 12), the. licensee ofiDresden 3 responded to GL 89-16 with a cost estimate of 1.0 million dollars *for the installation of *a hardened vent, and incremental costs of $500,000 for an AC-independent power source. 2.3.2 Value-Impact Assessment The value-impact ratio is calculated in the regulatory analysis (Appendix A) using the method described in NUIU:G/CR-3568 (Reference

11) to support the backfit decision.

The benefits to public risk reduction in man-rem were calculated in Section 2.2.3. The averted population dose f o'r Dresden 3 was calculated in Section 2. 2. 3 to be 50.2 man-rem per reactor year. For the 21 years of operation remaining, the estimated total averted man-rem is 1055. The cost of installation of the hardened vent capability was estimated in Section 2.3.1 as 1.0 million dollars. The value-impact ratio, not including ~he averted onsite cost, is calculated to be 1055 man-rem saved per illion dollars. 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 healtp impacts, and onsite property damage. The details of each of these items are discussed in Appendix A Section 4.1.2.2. If the savings of $208,414 to Dresden 3 from accident

  • avoidance (cleanup, repair of onsite damages, and replacement power) were included, the overall value-impact ratio would be 1333 man-rem saved per million dollars.

Consideration of a likely 20-year operating life extension will increase the value-impact ratio to 2688 man-rem saved per million dollars. 2.4 Alternatives Considered and Impacts on Other Programs 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), External Events, and Accident Management are discussed in Section 4.2 of Appendix A. A summary of the compliance* to the ba~kfit rule (10 CFR 50.109(c) ). is also included in Attachment 1 to Appendix A. ~.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-basis accidents, the hardened vent will not *be used, and therefore, 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 and will.reduce the radiological environmental risks. For venting sequences, the hardened vent connected to the plant stack could reduce dose consequences more effectively by approximately a.factor of two than venting through the ductwork. This reduction is due to a greater effectiveness of atmospheric dispersion resulting from a controlled elevated release compared to an uncontrolled ground level release from ductwork. Furthermore, venting through the suppression pool would provide scrubbing of non-noble-gas fission products with an effective decontamination factor of about 100. The addition of a hardened vent will greatly reduce he occupational doses for personnel that need to enter and work in he reactor building and that could be exposed to the contairunent environment. The staff has concluded that this generic EA applies to Dresden 3 and the installation.of the hardened vent will, therefore, reduce dose consequences and will not result in an adverse environment.aL 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 Dresden 3 and in SECY 89-017 for generic Mark I plants and supported by the plant-specific cost-benefit analysis, the staff believes that the installation of a hardened wetwell vent at Dresden 3 is warranted. 3.1 Rationale for the Recommendation ~n 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 and mitigation~ 'The addition of a hardened vent.(l) prevents the majority of TW sequences from resulting in core melt, and (2) itigates the consequences of residual sequences involving core melt here 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.* significant amounts of fission products to the environment. The

  • survivability of the containment, which acts as the last barrier for an uncontrolled release of radiation, would increase with venting.

The installation of 'a hardened vent greatly reduces the likelihood of a core melt from TW sequences and therefore reduces the risks to the public. For other sequences where core melt is predicted, venting cquld be effective in delaying containment failure and in mitigating the release of fission products. Although venting of the containment is currently included in BWR emergency operating procedures, it generally uses ductwork with a low design pressure. Venting under high-pressure severe accident conditions could fail this ductwork, release the containment atmosphere into the reactor building, and damage equipment, or contaminate equipment needed for accident recovery. Venting through this ductwork will probably hamper or complicate post-aqc:i,dent recovery activities, and is therefore viewed as reducing the safety benefit. The installation of a reliable hardened wetwell vent allows for controlled venting through a path with significant scrubbing of fission products to the plant stack and . would prevent damage to equipment needed for accident recovery. With the installation of the hardened vent capability, the staff stimated that the total plant CDF for Dresden 3 can be reduced by .4E-5 per reactor year because of the reduction in the probability_ .. of.;TW :sequences. Implementation of the proposed hardened vent modification will significantly reduce the total risk to the health an~ safety of the public. The averted population dose of 50.2 man-rem per reactor year was calculated for Dresden 3 from th~ installation of hardened vent capability. For 21 years of.remaining operating life the total averted population dose would be 1055 man-rem. If the averted cost associated with an accident is included, the calculated value-impact ratio for Dresden 3 is 1333 man-rem saved per million dollars. In addition, consideration of a likely 20*-year oper-ating life extension will increase the total averted population dose to 2060 man-rem and the calculated value-impact ratio to 2688. man-rem saved per million dollars, which demonstrate additional benefits for the installation of the hardened vent capability. Additional benefits of venting, not quantified, include source term reduction and the delay in containment failure for some of the scenarios that lead to core melt. Based on both the qualitative and quantitative benefits discussed herein and the supporting plant-specific cost-benefit analysis, the

  • staff believes that there will be a substantial increase in the overall.protection. of the public hea1th and safety by implementing the hardened vent capability for Dresden 3.

Therefore, the staff believes that th:i,s backfit is 'justified. .9 References

1.

SECY-87-297, U.S. NRC, "Mark I Containment Performance Program -p1an," V. Stello to NRC Commissioners, December 8, 1987.

2.

NUREG-1150, Second Draft, u.s. NRc,* "Severe Accident.Risks: An Assessment for Five U.S. Nuclear Power Plants," June 1989.*

3.

SECY-89-017, U.S. NRC, "Mark I Containment Performance Improvement Program, " v. Stello 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 li, 1989.

5.

U.S. NRC, Generic Letter 89-16, "Installation of a Hardened *' Wetwell -ven:t," September 1, 1989.

6.

Backfit Rule, -Code of Federal Regulation, 10 CFR 50.109.* .q. 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 I .Vents iri BWR Mark I Reactors." NUREG/CR-5225, draft~ "An Overvfew of Boiling Water R~actor Mark I-Containment Venting Risk Implications," October 1988.

10.

NUREG/CR-2723, "Estimates of the Financial Consequences of

11.

. Nuclear _Power React.or Accidents," September 1982. NUREG/CR-35~8,*-"A Handbook for Value-Impact Assessment, 11

  • December 1983 *. ~
12.

Letter from M. H. Richter (Commonweal th Edison) to U *. s. NRC, .October 30, 1989 "Dresden Station Units 2 and 3 Respon~e to Generic Letter 89-16." MARK I PLANT-SPECIFIC ENHANCED VENTING CAPABILITY REGULATORY ANALYSIS FOR DRESDEN NUCLEAR POWER STAT°ION, UNIT 3 Appendix A <2_ >

TABLE OF CONTENTS

-.... =1=. =o=*s=TATEMENT OF THE PROBLEM ******************************

A-1

2. 0 OBJECTIVES ************.. *. * * * * * * * * *.. * * * * * * * * * * * * * * * *. *
  • A-2
3. 0 ALTERNATIVE RESOLUTIONS *..*..************..***********

3.1 Alternative ( i)............ '*........................ 3.2 Alternative (ii)...*****.******..**.***..*********** 3.3 Alternative (iii) *******.***********..*..**.***.* ~.. A-2 A-2 A-3 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) ****p************************ A-5 4.1.2.1 Value: 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 Value: 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 c.onstraints............................... ~.. *........ A--12 .. 0 DECISION RATIONALE ******.**. * ****** ~ **************** * *** 5.1 Commission's Safety Goal * ** -*****. * ******************** 6.0 IMPLEMENTATION 6.1 Schedule for........................................ Implementation ****** ~.~ *.* ~ **.********* 7~0 REFERENCES................... ~........................ A-12 A-12 A-12 A-12 A-14 ATTACHMENT l TO APPENDIX A - BACKFIT RULE ANALYSIS.******** A-15

Mark I Plant-Specific Enhanced venting capability Regulatory Analysis 1.0 STATEMENT 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 sever~l 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 ~ontainments. Although these PRA studies do not show the BWR Mark I plants to be ~isk 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 ~ilure under such conditions are based on calculations of complex ccident conditions 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 power; and anticipated transient wi~hout scram (ATWS)

  • This list would have included the loss of long-term decay heat removal (TW) except that*, '

for the particular plant being rev.iewed, 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 containment with various size lines. The largest lines usually are associated with the* vent 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, the existing vent 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 A-1

ressure and is expected to fail under severe accident pressures. ailure of the ductwork would introduce the containment atmosph~re to the* reactor building. This could result in harsh environmental ~onditions that would complicate operator accident recovery actions within the reactor building and could cause failure of equipment within the reactor buildirig. The 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.concern~ in the reactor building. This regulatory analysis studied the costs arid benefits of installing a hardened vent capability at BWRs with Mark I containments. 2.0 OBJECTIVES 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 ris.k. 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 ~se 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 releases to* the environment during an accident. Although the staff considered the quantification aspects of both ~ccident prevention and mitigation, this regulatory analysis only

  • ~antified 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 capahility are being

  • proposed to reduce the* probability of or to mitigate the conse-quences of a severe core melt accident.
  • The proposed modification.
  • consists of installation of a hard pipe from the existing wetwell ventilation penetration, bypassing the-ductwork to the standby gas treatment system, and going to the plant stack.

The ventilation penetration is the 1a~ to 24-inch penetration normally used as part of the vent and purge system for deinerting.the containme~t.

  • 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 design-basis accidents. 3.1 Alternative Ci)

  • This alternative is the no-action option, that is, to leave the existing venting capability unaltered.

A-2

4t:he 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 reaQtor 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 fail under these environmental conditions. Adverse environmental conditions would complicate entry into the reactor building. Calculations from a_ venting study duri~g 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. ~.2 Alternative Cii> This alternative would involve the installation.of a hardened venting capability from the ~ontainment wetwell to the plant stack. he proposed venting improvement would provide a wetwell path to the lant stack capable of withstanding the anticipated environmental conditions of a severe accident..This propos~d modification would include the installation of har_d pipe from the outlet of an existing wetwell 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 radioactivity~ The emergency procedures would need to pe 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 building and would result in an elevated release. The elevated 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 building. *Furthermore, there would be no harsh environmental conditions to.degrade or fail other equipment. There is the possibility of inadvertent 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 A-3

aterial for those sequences where the drywell fails, such as from orium attack, once the drywell shell has failed. _3_!_:3:.. Alternative C iii) This alternative-would involve alternative (ii) plus the instal-

  • lation of an external filter system.

The proposed venting improvement includes the hard pipe vent discussed in alternative (ii) plus the installation of an external filter system, such as the Filtra system or the Multi Venturi Scrubbing System (MVSS). This external filter-would be installed outside of the 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. No significant additional risk reduction was estimated to result from an external filter system 'n addition to the suppression pool scrubbing. Since all particulate eleases through the hardened vent (alternative ii) are scrubbed, the external filter will only provide minimal additional scrubbing. The* external filter provides no 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 releases of radioactive material for those sequences where the drywell fails, such as from corium attack, once the drywell shell has failed*. 4.0 CONSEQUENCES 4.1 Costs and Benefits of Alternative Resolutions The staff used -available PRAs to estimate the incremental benefit of 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 C_DF, but not the total CDF from internal events (Reference 6).

4.1.1 Alternative Ci> This alternative wouid be to take no action. Since it is expected hat the ductwork would fail if the containment were vented at high A-4

ressure, this approach would not only jeopardize personnel, but also he ability to regain control of the facility during the accident *. Furthermore, based on a generic regulatory analysis (Reference 1) the ~~~i.ssion 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 Ciil 4.1.2.1 Value: 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 3.6E6.man-rem, this represents a risk reduction in man-rem per reactor year of 50.2. 4.1.2.2, Impacts: Cost Estimates The estimated cost for installation of the hard pipe vent path is

1. O million dolla~s (Reference 7) **

The avert:ed cost associated w.i th prevention and mitigation of an acc.ident 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 avert~ng plant damage and cleanup, the reduction in accident frequency was multiplied by the discounted costs of onsite property. The following equations from NUREG/CR-3568 {Reference 8) were used to make this calculation: Vop = NdFU U = {C/m).[ {e*rt> /r2] [ 1-e*r<t<f>*t>]. {l-e-nn) where: {cited values are from Table.2) .. vop N dF u c t{f) t {i) r m = =

= = = = = value of avoided onsite property damage {$) number of affected facilities = 1 reduction in accident frequency = 1.4E-5 /RY present value of onsite property damage {$) cleanup and repair costs = $1.0 billion years remaining until end of plant life = 21 years before reactor begins operation = O discount rate = 10% period of.time over which damage costs are paid out {recovery period in years) = 10 Using these values, :the present value of avoided onsite property damage is estimated to be $77,660. A-5

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 r~placement 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 w~th the installation of the proposed hardened vent system is expressed as total man-rem of-avoided exposure. The following equations from NUREG/CR-3568 were used to make this calculation:

  • N T-R where:

- value of public health risk avoided for net-benefit method ($) = number of affected reactors = 1 = average remaining lifetime of affected facilities (years) = 21 = avoided. public dose per _reactor-year (man-rem/RY) = 50.2 = monetary equivalent of unit dose ($/man-rem) = $1000 Using these values, the avoided public health. exposure of 1.055 _.- million dollars is obtained for Dresden 3. Considering a possible 20-year operating life extension, the value of avoided public health exposure is 2. 06 million dollars.. 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 were used.to-make this calculation:

  • vOHA = NT (DOA x R) where:

N T R =value of occupational-heaith risk due to accidents avoided ($) =number-of affected reactors (reactors)= 1 = average remaining lifetime of affected facilities (years) avoided occupational dose per reactor year (Man-Rem/Reactor-Year) = monetary value of unit dose ($/Man-Rem)=$1000/Man-rem A-6

~ There are two types of occupational exposure related to accidents,

  • immediate and long-term.

The first occurs at the time of the accident and during the immediate management of the emergency. The .*c:csecond 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 specif i~d 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-rem. The multiplication of* 21,000 man-rem by the reduction in CDF, l.4E-5 per reactor year, produces the avoided occupational dose per reactor year, D~. Using these values, the present value of avoided occupational 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 1055 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 1333 man-rem averted per million dollars. Considering a likely 20-year operating life extension, the overall value~impact ratio would be 2688 man-rem averted per million dollars. 4.1.3 Alternative Ciii) .4.1.3.1 Value: Risk Reduction Estimates This alternative would provide minor additional particulate scrubbing for the hard ve11t. However, because all particulate releases will have been scrubbed by the suppression pool, the improvem~nt over-alternative (ii) could be minimal. 4.1.3.2 Impacts: Cost Estimates 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. Using the same equations given in alternative (ii), the present valu~ of the estim~ted avoided onsite damage to property is $77,660. Similarly, the estimated replacement power cost is $168

  • million per year.

Thus, the estimated avoided damage to onsite property and the replacement power is $208,414. A-7

  • 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 $1.055 million.

~ 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 replacemerit power) were included, the overall value-impact ratio would be 182 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 d9llars 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. A-8

able 1 - Cost Benefits of Alternatives (i)-(iii) (man-rem averted per million dollars) Alternative (i) - do nothing Alternative (ii) - hard pipe venting for the remaining life with 20-year life extension Alternative (iii) - hard pipe venting + MVSS external filter A-9 0 1333 2688 182

    • 1 I\\

- Bukflt An1ly1h for Proposed Hirdened Yent*t1p1bllity for: Dresden] t-------------------------------------------------------------------------~-------------------~---------------------------------------------*---------.::1---------+ IAI - 181 ICI IOI IEI IFI 161 IHI Ill m IKI Ill '"I 1111 IOI I Phnt* D1t1 of Y11n* of Popul 1tl on,. 'Strip' Gron !!in-RH "an-RH Instill Y1lue~ --~-------------------------------------------------. Yd -I 1p I 6raup Co11trchl Opentlon 10-50 1ilt1I Ctlf . hctor DoH per RY Sned Cash lip.ct 11 VOii Vph Yoh1 Repl hr 4' Yap

  • 1/Yop&RP I *o.

Phnt K11t Op1r1tlonl Re11lnlng

  • 1197012

]

  • 1SST114 IP82t013 ICtEI IAtfl lt"15H 16/HI 7

7 7 IS per Ytul Repl Pwr 16/H-NI I ---- ---------- I 4 Dr11d111 l 1971 21 6,305,000 I. 40£-05 0. 83411304 3. 59E+06 50.2 1055.1 1.00 1055 m,660 SI 1055, 140 tb,174 m8,m,200 s208,414 1m I _4 Dmden l 1971 41 6,305,000 l.40E-05 0.8346304 3.59E+06 50.2 2060.0 1.00 2060 $87,030 S2,060,035 s12,os4 s168,m,200 sm,s61 2688 t--------------------------'--------------------------------------------------------.---------------------------------------------------------------------------------* An1ly1h Dlte-> 16-Apr-90 Tilt*> 11113 II Sourn1 1ISKRC llURE6-l~O, dlted llmh 1989 21 Sourm USICRC llllRES-0348 4 Discount rite: 10 1 31 Sources llnonndu1 frDI 8.W. Slleron, d1ted October 19," 1989, to A.C. lh1d1nl, 'Reduction in Rist rra1 the Addi tlon of Hardentd Vtnh in* 811R "*rt I Reactors' 41 Sourttl USllRC llllRE6/CR-272l, dated Septe1ber 1982 le1upt: Hopt Creek * *IS1lt1f/PBAPSltlllWth-hc'"'lth-slll _ 51 Sourm Generic letter 189-16 1 d1ted Seph1btr 11 19891 'ln1hlhtlon of Hud Wetwell Vent'

6) Sauret: ln1t1lhtion co1h frDI 1t1or1ndu1 frDI J.6. Plrtlow to J.E. "urhy, dlted Novuber 9, 1989,

'liunnn' RespoHH to 6eneric letter 89-16 Rthttd to lnshlhtlon of Hardened llehtll Vent' 71 Sauret: USKRC llURE6/CR-l568, d1ted Decelber 1983, p1gt1 3.11-3.12, l.29-3.ll, 3.16-3.18 81 lhe nu1ben in th1 colu1n titles refer ta souru of infor11tion nu1ber 1bovt. 91 lht letter in bnckeh, 1111, 1r1 the colu1n Identifiers 1nd the lethrs in br1eteh, IC*El, are the rquition1 using th1 colu1n. identlfler1 for references. Tht 'Strip'. flctor ii the 1ultd 11n-rt1 SSH nu1ber frDI the Strip Report dhidtd by thr 1i1il1r nu1ber for Pe11cb BottDI Unit 2 to 1ecount for the site differences. 101 *eg*tht nulber1 in Coh1tn IOI indluh th1t the on11te cash 11ceed tht ln1hlhtlon costs. Therefore the proposed 1adiflcation eueed1 the tl000/11n-rN crihrh ind 11y be i1posed. 111 Va,

  • Hlat of noldtd aa11t1 'roptrty du191 ltl Vpll *Hin of pllllUc b11lth rilt naldltl far 11t-.. n1flt.*thod ltl Valla
  • nla of ac:c.,1tlan11l ht1m rlat RI to ucldtat **oldtd m

\\I

able 3*- Estimated Replacement.Power Costs (in dollars per day) ~- - Reactor Name Dresden 3 Year Est. Cost MWe Licensed 1985$ . 794 1971 $372,000 Est. Cost Est~ Cost 1989$ $461*, 280 (per year) $168,367,200 Notes: l:NUREG/CR-4012 (Table S.1) provides replacement power costs for all plants on per plant/season basis for 1987-1991. 2:The inflation rate used is 6 percent/year, and the. discount rate used is 10 percent/year. A-11

  • .. ~

.2 Impacts on Other Requirements There are six programs related to severe accidents: Individual Plant Examination (IPE), Containment Performance Improveinent (the topic of th:..rs:--:regulatory analysis), Improved Plant Operations, Severe Accident Research Program, External Events, and Accident Management. Each of the five programs related to Containment Performance Improvement (CPI) will be discussed briefly in Item 3 of Attachment 1, Backfit Rule Analysis. ~.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 Management of Plant-Specific Backfitting of Nuclear Power Plarits", which is based on the backfit rule (10 CFR 50.109), as published by the.Commission on September 20, 1985, and the provisions of.10 CFR 50 Appendix O, 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

  • vailable from the NUREG-1150 program and from existing PRAs.. *
  • he best estimate of the contribution of TW to the*. total plant CDF expressed in events per reactor year for Dre~den 3 is L4E-5.

Implementation of the proposed hardened venting capability will cause TW to be a minor contrib~tor to the total CDF and will significantly reduce the total* risk to the*health and safety to the_pu~lic. 5.1

  • Commission's Safety-*Goal On August 4, 1986, the Commission published in the Federal Register a policy statement on "Safety Goals for the Operations of Nuclear Power Plants" (51 FR 28044); This policy statement focuses on the risks to the public from nuclear power plant operation and establishes goals that broadly de.fine 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 Implementation The licensee may reconsider its position on the installation of the hardened vent under the p*rovisions of 10 CFR 50. 59. Without the licensee's commitment, the staff intends to pursue an order after 30 ays of its receipt of this analysis, requiring this backfit under A-12

he provision of 10 CFR 50.109. Within 60 days after issuance of the ackfit order, the licensee will be required to submit to the NRC a schedule for implementing any necessary equipment and procedural modifications to meet the performance goals and to provide adequate de-f*ense-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 achieve a reduction in the risk from TW. Shorter or less flexible schedules would be unnecessarily burdensome. A-13

~.O REFERENCES

1.

SECY-89-017, "Mark I Containment Performance Improvement

**. *Program," January 23, -1989.

2.

NUREG-1150, (Draft), "Reactor Risk Reference Document*;" February 1987.

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 BWR Mark I Plant ATWS," 1986 Reactor Water Safety Meeting, Gaithersburg. Maryland, 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 from the Addi ti on of Harde'ned Vents in BWR Mark I Reactors, "

  • October 19, 1989.
7.

Letter. from M. H. R.ichter (Commonwealth Edison) to U.S. NRC, October 30,. "Dresden Station Units 2 and 3 Response to Generic Letter 89-16."

8.

NUREG/CR-3568, "A Handbook for Value-Impact Assessment,!** December 1983.

9.

NUREG/CR-4012, "Replacement Energy Costs for Nuclear Electricity-Generat~ng Units in the United States: 1987-1991, *" January *1987.

10.

NUREG-1109, "Regulatory/Backfit Analysis for the Resolution of Unresolved Sa~ety Issue A-44, Station Blackout," Jqne 1988.

11.

SE(!Y-88-147,* "Integration Plan "for Closure.of Severe Accident Issue~," May 25, 1988. 12.. Memorandum from' s. J. Chilk to v. Stello, "SECY-89-017 - Mark I Containment Performance Improvement Program," July 11, 1989. A-14

ATTACHMENT 1 TO APPENDIX A BACKFIT RULE ANALYSIS -:::Analysis and Determination That the Recommended Hard Pipe Vent Capability for Containment Performance Improvement Compl-ies with 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 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 Mark I containment design. Both deterministic and probabilistic analyses were performed to evaluate the loss*of long-term decay heat removal (TW) in challenging ontainment integrity and potential failure modes affecting the ikelihood 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 pipe vent capability* is not needed to provide adequate protection of the public heal th and safety_~

Rather, the proposed plant improvement will provide substantial cost-effective enhancement*to *Mark I_plal')t safety.

The.estimated benefit from implementing*the proposed 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 1055 and supports the conclusion of the Commission that implementation of the proposed 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 enhancement is l.O*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 emergency operating procedures, and 4) provide operator training.concerning mitigating the TW sequence.

he estimated value-impact ratio, not including accident avoidance A-15

osts, in terms of man-rems averted per million dollars is 1055. If he net 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 1333. If 20 years of life extension were included, the estimated overall value-impact in terms of man-rems averted per million dollars would be 2688. These values support proceeding with the proposed

  • hard pipe vent capability improvement.

Although the preceding quantitative value-impact analysis was one of th~ factors considered in evaluating the proposed improvements, other factors were considered as a part in the decision-making process. PRA studies performed for this issue have shown tha*t 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 -Yrom the reactor vessel, containment integrity may be significantly challenged under severe accident conditions. In general, active systems required for reactor and containment heat removal are

  • unavailable during the TW event. 'Therefore, the offsite risk is*

higher from a TW-event than it is from many other types of accidents. he containment;. integrity is primarily cha:J_lenged by over-pressure or 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 Dresden 3 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 paragr~phs 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. Analysis of 10 CFR 50.109Ccl Factors (1) Statement of the specific obiectives that the backfit is designed to achieve The objective of the proposed hard-pipe vent capability is to A-16

(! 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. (:i:):-:::c General description of the activity required by the licensee or applicant in order to complete the backfit To comply with the proposed improvement in containment venting, the lice.nsee will be required t.o: Evaluate the actual capability of the existing containment vent system to withstand the anticipated containment temperatures and pressures without failing any portion of the vent path to the plant stack. _ Evaluate the actual capability of the existing containment vent isolation valves to be opened and closed under anticipated containment pressures anq vent flow rates during severe accidents involving TW sequences. Determine the necessary plant modifications to ensure.a hard-pipe yent path will be available under TW events, develop a schedule for plant modification, and submit the schedule to the NRC within 60 days from *the issuance of the backfit order. Complete the necessary modifications by January, 1993. *.

  • The licensee will be requ~red 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 ReaQtor Owner's Group. (3) The potential safety impact of changes in plant or operational complexity. including the relationship to proposed and existing regulatory requirements The hardened vent capability to cope with the TW event should not add to plant or operational complexity, because the vent is normally closed and not operated during normal power_ operation *. Although this system does add some additional hardware to the plant, it is *a simple system. The containment performance improvement (CPI).program is related to implementation of the

  • Commission's Severe Accident Policy Statement. as defined in SECY-88-147 (Reference 11)~

In SECY-88-147 the staff described the various programs underway ~elated to closure of severe accident issues. Included among thes~ was the CPI program. Other programs described in SECY-88-147 are related to the CPI program as the following discussion indicates. Individual Plant Examination (IPE) The IPE.involves the formulation of an integrated and A-17

0 systematic approach to an examination of each nuclear power plant in operation or under construction for possible significant plant-specific risk contributors that might be missed without a systematic search. Supplement 1 to Generic Letter 88-20 requested that Mark I licensees include in their IPEs the proposed plant improvements identified in SECY 017, other than the hardened vent, namely operation of the enhanced automatic depressurization system, and alternative low-pressure water supply for injection into the reactor vessel and for containment sprays. The examination will carefully examine containment performance in striking a balance between accident prevention and consequence mitigation. The IPE program may require three to four years until the last plant has performed the IPE. Improved Plant Operations (IPO) The IPO includes consideration of continued improvements in the following areas: Systematic Assessment of Licensee Performance (SALP) program; regular reviews by senior NRC staff managers to identify and evaluate thos~ plants th~t may not be meeting NRC and industry standards -Of operating perfprmance; diagnostic team inspections; improved plant

  • Technical Specifications; improved operating procedures; expansion of the Emergency Operating Procedures (EOPs) *to include guidance on severe accident management strategies; industry's programs to reduce fransient and other challenges to engineered safety feature. systems; feedback from the IPE program of experience and improvements in operational areas, such as maintenance.and training; and continued research to evaluate the sensitivity of.risk to human errors, and the effectiveness of operational reliability methods to help identify potential problems early and prevent their.

occurrence. The IPO is related to the CPI progra~'s ~ecommendation since we recommend improved procedures and operator training to use the proposed hard*vent system. Severe Accident Research Program (SARP) The SARP was begun after the Three Mile Island, Unit 2, (TMI-

2) accident in Marchl979 to provide the Cominission and the NRC staff with the technical data *and analytical methodology needed to address severe accident issues.

This program has provided input to the NUREG-1150 program and to the CPI program. Additional research i's being carried out to evaluate the need for and feasibility of core debris controls. Research will also confirm and quantify the benefits of having water in the qontainment to either scrub fission products or to prevent or delay shell melt by core debris. Accident Management A-18

c () The accident management program addresses certain preparatory and recovery measures that plant operating and technical staff can perform to prevent or significantly mitigate the consequences of a severe accident. This program includes the -~.* -following measures to be performed by the plant staff: 1) prevent core damage, 2) terminate the progress of core damage if it begins and retain the core within the reactor vessel, 3) failing that, maintain containment integrity as long as

  • possible, and 4) minimize the consequences of offsite releases.

The plant enhancement recommended by the CPI program would provide the accident management program with additional capability to achieve their goals by providing improved hardware with which to deal with a severe accident. The procedures for using the vent should be re-examined under the Acciden~ Management program. * (4) Whether the backfit is interim or final and, if interim, the justification for imposing the backfit on an interim basis

  • The proposed hardened-vent capability is not an interim measure.
  • 15)

Potential change in the risk to the public from the accidental offsite release of radioactive material Implementation of the proposed hardened-vent capability is expected to.result in an estimated risk reduction to the public of 1055 man-rem over tne remaining plant life. (6) Potential impact on radiological exposure of facility employees Although the reduction in occupational exposure caused by reduced CDF and associated post-accident cleanup and repair activities has not been.quantified,-it could be substantial if the hardened vent prevents contamination of the reactor building. The estimated total occupational exposure for installation of the hardened-vent path should be negligible. No increase in occupational exposure is expected from operation and maintenance of the hardened-vent system. In fact, if the vent is ever used, it should decrease the. risk to employees because of the reduced potential for vent path fa*ilure and the resulting reactpr building contamination. (7) Installation and continuing costs associated with the backfit, including the cost of facility downtime or the cost of construction delay .Because the plant can be operating during installation, there are no costs associated with construction delays. The hardened-vent path can be installed with the plant operating or during normal . plant outages. Thus, there are no costs associated with additional plant downtime. A-19

The estimated cost of the hardened vent system is 1.0 million dollars. - -{8-) The estimated burden on the NRC associated with the backfit and the availability of such resources With an estimated expenditure of 200 man-hours for review of the submittals, the estimated total cost for NRC review of industry submittals is $17,000. The staff will concentrate on the review of design criteria and the method to incorporate the venting into emergency operating procedures. (9) Consideration of important qualitative factors bearing on the need for the backfit at the particular facility The installation of the hardened vent will provide greater flexibility in managing accidents other than the TW events, and will provide defense in depth. (10) Statement affirming appropriate interoffice coordination related to the proposed backfit and the plan for implementation The licensee may reconside:r its position on the installation of the hardened vent under the provisions of 10 CFR 50.59. Without the licensee's commitment, the staff intends to pursue an order after 30 days, requiring -this backfit under the provision of 10 CFR 50*.109. The proposed backfit was developed as a cooperative effort between the Offices of Nuclear Regulatory Research (RES) and Nuclear Reactor Regulation (NRR) with consultation with the 0-ffice of General counsel. The implementation is being handled within the Nim. The staff considered implementation schedules consistent with the guidelines provided by the Commission (Reference 12). Within 60 days after issuance of the backfit order, the licensee is to provide to the NRC a schedule for implementing any equipment and procedural modifications necessary to meet the performance goals and to provide adequate defense-in-depth. All plant modifications are to be installed, procedures revised, and operators trained not later than January 1993. (11) Basis for requiring or permitting implementation on a. particular schedule Although other schedules were considered, *the staff believes the prqposed implementation of the hard pipe vent capability can be performed with minimum interfacing with containment and engineered safety feature systems and either with the plant online or during a normal refueling outage. Therefore, the staff believes the schedule 'is achievable without incurring unnecessary financial A-20

' I. C.,.

  • r

(... burden on the licensee for plant sliutdown. The schedule allows .reasonable time for the implementation of necessary hardware to

  • reduce the risk from TW and allows appropriate coordination with

-~E program. Shorter or less flexible schedules would be --unnecessarily burdensome. (12) Schedule for staff actions involved in implementatfon.and verification of implementation of the backfit, as appropriate The proposed backfit is to be installed under 10 CFR 50.59 for most o~ plants and, thus, will require minimal staff effort. Therefore, timely staff review will be expected. However, for those plants .. that choose not to implement the modifications under 10 CFR 50.59, more staff time and efforts will be involved. (13) Imoortance of the proposed backfit considered in light of other safety~related activities underway at the affected facility The proposed backfit should not directly involve any other safety- . related activities that may be underway at the a.ffected facility. (14) Statement of the consideration of the proposed plant-specific backf it as a potential generic backf it Initially, the staff proposed the in~tallation of hardened.vent as a generic backfit. The Commission directed.the staff to implement it as a plant-specific backfit considering the plant qifferences in risk reduction and benefits to.be gained from a generic backfit

  • A-21}}
Retrieved from "https://kanterella.com/w/index.php?title=ML17202U740&oldid=5128527"