ML20082L278
| ML20082L278 | |
| Person / Time | |
|---|---|
| Site: | Catawba  |
| Issue date: | 04/10/1995 |
| From: | Rehn D DUKE POWER CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| Shared Package | |
| ML20082L282 | List: |
| References | |
| NUDOCS 9504210086 | |
| Download: ML20082L278 (24) | |
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, Duke 1%ver Cornpany D. L REM Catawba Nuclear Generation Deparvnent Vice President 4800 ConcordRoad (803)8314205 Ollice York SC29745 (803)8313426 Fax DUKEPOWER April 10,1995 U.S.NuclearRegulatory Commission ATTN: Document ControlDesk Washington, D.C. 20555
Subject:
Catawba Nuclear Station, Units 1 and 2 Docket Nos. 50-413 and 50-414 Proposed. Technical Specifications (TS) Changes
- (TS 4.6.4.3a and Bases Section 3/4.6.4)
Hydrogen Mitigation System 7
Gentlemen:
Pursuant to 10CFR50.4 and 10CFR50.90, attached are license amendment requests to Appendix A, Technical SpaciAca+iaan, of Facdity Operating Licenses NPF-35 and NPF-52 for Catawba Nuclear Station Units 1 and 2, respectively. The requested amendments allow removal of the hydrogen igniters in the incore instrument tunnel for each train of the Hydrogen Mitigation System. As stated in the C .ws, the affected igniters are not needed to mitigate the effects of hydrogen generation and release to containment during a degraded core accident.
Attachment I contains a background and description of the enclosed amendment request. Attachment 2 contains the required jueineatian and safety evaluation. Pursuant to 10CFR50.91, Attachment 3 provides the analysis performed in accordard:e with the standards contained in 10CFR50.92 which concludes that the requested amendments do not involve a significant hazards consideration.
' Anachment 3 also contains an environmental impact analysis for the requested amendments.
Attachment 4 contains the marked-up Technical Specification amendment pages for Catawba. Duke Power Company is forwarding a copy of this amendment request package to the appropriate South Carolina state official Should there be any questions conceming this amendment request or should additional information be required, please call L.J. Rudy at (803) 831-3084.
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Document Control Desk Page 2 April 10,1995 Very truly yours, D.L. Rehn ,
UR/s Attachments
, xc(W/ Attachments):
! S.D. Ebneter, Regional Administrator RegionII R.J. Freudenberger, Senior Resident Inspector R.E. Martin, Senior Project Manager ONRR
. Max Batavia, Chief Bureau ofRadiological Health, SC American NuclearInsurers M&M Nuclear Consultants INPO Records Center i
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b Document ControlDesk Page 3 April 10,1995 i
D.L. Rehn, being duly sworn, states that he is Vice President of Duke Power Company; that he is authorized on the part of said Company to sign and file with the Nuclear Regulatory Commission this revison to the Catawba Nuclear Station License Nos. NPF-35 and NPF-52 and that all statements'and matters set forth therein are true and correct to the best ofhis knowledge.
sA D.L. Rehn, Vice President t
l Subscrital and swom to before me this 10th day of Ap% 1995.
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ATFACIIMENT 1 BACKGROUND AND DESCRIPTION OF AMENDMENT REQUEST
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Background
The Hydrogen Mitigation (Eini) System is designed to remove hydrogen from the containment atmosphere following a severe accident condition. The system was installed as a post-TMI ;
requirement for ice condenser units to mitigate the effects of hydrogen generation and release to containment which might occur during a beyond-design basis accident. The EHM System was installed in containment as a result of 10CFR50.44 requirements for a hydrogen control system capable of handling, without loss of containment integrity, an amount of hydrogen equivalent to that generhted from a metal-water reaction invoMng up to 75% of the fuel cladding. Per 10CFR50.44, the system l was also designed to meet the following requirements: I
- 1) Seismic category I, i
- 2) Two independent redundant trains,
- 3) Each train supplied from Class IE vital buses, and j
- 4) Must remain operable under post-LOCA conditions. )
l The EIni System protects the containment from overpressurization caused by potential uncontrolled hydrogen combustion during post-accident conditions. The system at Catawba consists of 72 glow plug ignition devices (igniters) located throughout containment in dead-ended compartments and other j areas where hydrogen gas pockets are most likely to form. The system ensures buming in a controlled manner as hydrogen is released, instead of allowing it to be ignited at high concentrations by a random ignition source. The system is divided into Trains A and B, with each location containing an igniter from each train. Of these 72 igniters,12 are located in upper containment,46 are located in lower containment,12 are located in the upper plenum of the ice condenser, and 2 are located in the region of the reactor cavity in the incore instmment tunnel (EHM-1 and Elmi-2).
The EHM System is manually actuated. Each circuit is manually energized by remotely closing its respective 600-volt AC contactor from the control room. Once energized, the igniters will bum off any hydrogen accumulating within the containment until they are manually deenergized.
Igniters EHM-1 and EHM-2 have always posed a problem from a radiation dose perspective. Since they are located in the incore instrument tunnel, which is a high radiation area, periodic maintenance of these particular igniters represents an ALARA concern. The purpose of these proposed amendments is to allow the deletion ofigniters EHM-1 and EHM-2 for both units. TS 3.6.4.3 and 4.6.4.3 govem the operability of the EIBf System at Catawba.
Description of Amendment Request TS 4.6.4.3a is modified by changing the phrase " . at least 35 of 36 igniters . " to " . at least 34 of 35 igniters . " This will allow the igniters in the incore instrument tunnel (1 per train for each unit) to be deleted and will maintain the existing technical specification allowance of one inoperable igniter for each train.
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. l TS Bases section 3/4.6.4 is correspondingly modified to reflect the above described change.
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ATTACIIMENT 2 JUSTIFICATION AND SAFETY EVALUATION l
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l l Justification and Safety Evaluation -
An evaluation has been performed to determine whether or not igniters EHM-1 and EHM-2 at Catawba are required for hydrogen control.
i Computer Codes Two computer codes were utilized for predicting hydrogen behavior in containment during degraded core accidents. They are HECTR (Hydrogen Event Containment Transient Responses) and MAAP l (Modular Accident Analysis Program). Both codes permit modeling of the containment as a set of !
compartments connected by junctions. The HECTR model used in this evaluation is a six-compartment model. These compartments are in addition to the four default compartments in the ice ;
I condenser model ofHECTR. The compartments are:
- 1) Upper compartment l
- 2) Ice condenser upper plenum
- 3) Ice condenserlower plenum
- 4) Lower compartment
- 5) Dead-ended region
- 6) Reactor cavity 7-10) Lowest to highest regions ofice bed Figure 1 is a simplified diagram of the Catawba ice condenser containment. Figure 2 is a representation of the model used in the containment response analysis for HECTR.
The MAAP containment model is divided as follows (see Figure 3 for representation of the MAAP containment model):
- 1) Upper compartment
- 2) Ice condenser upper plenum
- 3) Ice condenser lower plenum and ice bed region
- 4) Lower compartment
- 5) Dead-ended region
- 6) Reactor cavity The biggest difference in the compartmentalization between the two codes is that in HECTR, the ice condenser is divided into more compartments than it is in MAAP.
Catawba Plant Model The reactor cavity compartment of the containment includes the volume within the biological shield wall where the reactor vessel is located, down into the region below the reactor vessel and up through l
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the incore instmment tunnel (see Figure 1). The igniters for the reactor cavity are positioned at the two locationsidentified in Figure 4.
There are two flowjunctions between the lower containment and the reactor cavity companment. The first flow area, the area around the reactor coolant piping penetrations of the biological shield wall, is approximately 40 sq fl. The second flow area, at the end of the incore instmment tunnel, is approximately 20 sq ft. The lower compartment is the most likely place for a LOCA to occur in the containment. For LOCAs occurring in the lower compartment of containment, an evaluation of the potential hydrogen concentrations in the cavity was made, considering both one flowjunction and two flowjunctions. It is expected that the two flowjunctions will develop a flow loop between the cavity and lower companments. However, a one flow junction model was evaluated to account for obstructions that may inhibit this flow loop from developing. For completeness, the analysis also evaluated the effects on hydrogen concentrations in the unlikely event of a LOCA occurring in the cavity compartment.
Accident Seauences Evaluated l The calculational method evaluated a broad range of severe accidents to determine the hydrogen behavior in the reactor cavity. In "An Analysis of Hydrogen Control hieasures at hicGuire Nuclear Station", Revision 15, dated Afarch 1993, a series of four accident sequences was evaluated using l HECTR, for licensing suppon of Catawba Unit 1. These accident sequences were relevant to this
- evaluation, since they were utilized in the past for addressing hydrogen issues related to 10CFR50.44 l requirements under severe accident conditions. However, the original HECTR models used in the l Afarch 1993 analysis did not model the reactor cavity as a separate companment. After making the l necessary changes to the original HECTR input files to include the reactor cavity compartment, all four I accident sequences were reanalyzed. The results of the four HECTR mns showed that the cavity hydrogen behavior was similar in all four cases. Therefore, it was not necessary to reanalyze all four accident sequences using hfAAP. Hence, the accident sequence chosen for the hfAAP mn was a small break LOCA with failure of emergency core cooling, since this is representative of a severe accident.
Cavity Comoartment with One Flow Junction For a cavity considered to have only one flow junction between lower and cavity compartments, the cavity is a true dead-ended compartment. Since the cavity is dead ended due to the one flow path, the hydrogen concentration in the cavity will have a tendency to approach the concentration in the lower compartment over time. This is because the source of hydrogen for the cavity compartment is coming i from the lower companment. Flow from the lower companment into the cavity compartment is created due to the lower compartment being at a higher pressure than the cavity during the blowdown.
l The HECTR and hfAAP mns show that the hydrogen concentration in the cavity is maintained below 4% by volume throughout the accident sequence when one flowjunction is modeled (see Figures 5 and 6).
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. 1 Cavity Compartment with Two Flow Junctions l
l For a cavity considered to have two flow jurctions between lower and cavity companments, the cavity would not be dead ended. There will likely ee some type of flow loop established between the lower I and casity compartments. The gases near the reactor vessel will be hotter than the gases at the ;
i enclosure opening, which will tend to set up a flow loop between the lower and cavity companments. j This is the case with the MAAP mn, which shows that there is better mixing between the lower and l cavity companments with the two flow paths modeled (see Figures 8 and 9). The HECTR model does I
not include the heating of the gases near the reactor vessel; therefore, this flow loop does not develop (see Figurc 7). The flow of gases between the lower and cavity companments will tend to allow both compartment gas concentrations to approach equilibrium more quicidy when this flow circulation develops. 1 Therefore, the gases in the cavity, during a degraded core accident sequence involving a LOCA in the lower compartment of containment, will be well mixed with the lower compartment gases. Since hydrogen o a lighter gas than steam or air, and since the cavity is at the lowest possible location in the containment, hydrogen accumulation will not occur.
LOCA in Reactor Cavity In the event of a LOCA occurring in the reactor cavity companment, the hydrogen / steam / water combination coming out of the break is the source of fluids for the cavity companment. During the blowdown of the LOCA, the air that was present in the cavity will be pushed out by the pressurization of the c:avity relative to the rest of containment. Initially, what comes out of the break is steam, l followed by hydrogen and steam. The cavity gas space will be a non-combustible mixture of hydrogen and steam.
Once emergency core cooling is restored, if the injected flow into the reactor vessel comes out of the break as mostly steam with hydrogen, the cavity gas space will continue to be a mixture of steam and l hydrogen with the concentration of each governed by the mixture exiting the break.
l If the ECCS flow comes out of the break. as mostly liquid with hydrogen, then the cavity will fill with water, pushing all gases out into the rest of containment. Even with the igniters present, this will immerse the igniters, making them ineffective.
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Therefore, in either case, what will enter the cavity through the break during the ECCS injection is a combination of hydrogen and steam / water. Throughout such an accident sequence, there is no combustible mixture available for a burn to occur in the cavity. This means that at no time after the blowdown is there enough oxygen present in the cavity to permit combustion.
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1 Hydrogen Bumsin the Cavity i
The hydrogen concentration in the reactor cavity compartment will follow the lower companment concentration. This is due to the igniters in the lower compartment maintaining the hydrogen '
concentration in the lower compartment below flammable limits. These evaluations are supported by HECTR and MAAP results which produced no burns in the cavity throughout the accident sequences.
If hydrogen is burning in the lower compartment, then the cavity hydrogen concentration will be maintained by the lower compartment since this is the source of hydrogen for the cavity. The hydrogen concentration in the cavity will not reach the lower flammability limit.
In NUREG/CR-4993, "A Standard Problem for HECTR-MAAP Comparison: Incomplete Burning",
August 1988, a HECTR-MAAP comparison was made using multicompartment models that included a reactor cavity compartment. Based on the results in this study, all burns occurred in the lower compartment and propagated up through the ice-condenser using a bum limit of 6% by volume of hydrogen. The only exception was when the bum limit was increased to 8% by volume of hydrogen, the burns initiated in the upper plenum of the ice condenser and propagated down through the ice bed and then back up into the upper compartment dome region. At no time did the hydrogen burn enter the lower compartment in this case. No mention of any propagation into the reactor cavity is discussed, which leads to the conclusion that the hydrogen concentration in the reactor cavity remained low throughout the accident sequence.
There is a high point in the incore instmment region (see Figure 1) next to the enclosure that is not a part of any flow path and therefore, may accumulate hydrogen due to buoyancy effects. Excessive accumulation could lead to detonable conditions if sufIicient oxygen is available. Detonations are flame fronts moving faster than the speed of sound, as opposed to deflagrations, which move much more
. slowly. Detonations are of concem due to the large impulse loads they produce on the containment shell, even though these loads are for very short time periods compared to deflagrations.
Some information regarding detonations is available from NUREG/CR-5586, " Mitigation of Direct Containment Heating and Hydrogen Combustion Events in Ice Condenser Plants", October 1990.
This NUREG identifies the ice condenser as the most likely location for a detonable condition in a station blackout sequence. The NUREG also indicates that, ". . it has not even been established that detonations in the ice condenser constitute a significant threat to containment integrity." In addition, the NUREG states that, " . detonation of sufliciently small gas pockets would present little threat to containment integrity."
Additionally, NUREG/CR-5586 indicates that the effects of detonations can be reduced within " busy" volumes (free, unobstmeted spaces limited to less than a meter). Deformations of the s*metures within the volume will be an important energy absorption mechanism. Since the incore instmment region is a small volume relative to the ice condenser volume (which is the region of concem in the NUREG study) and is filled mostly with incore instmment tubes, the effects of detonations will be significantly reduced.
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l Therefore, it is maintained that: 1) It is unlikely that hydrogen concentrations will reach detonable levels in the region below the seal table, and 2) Considering the small volume of the region below the seal table, it is highly improbable that containment integrity will be affected.
In conclusion, based on the results of computer modeling, and the review ofresults of an external study ;
performed for a similar type containment, it is malatained that the hydrogen concentration will remain low in the cavity during degraded core accidents. This is due to the cavity hydrogen concentration being controlled by the lower compartment concentration, which is in turn maintained by the lower compartment igniters. Therefore, the two igniters located in the reactor cavity are not necessary for hydrogen control during degraded core accidents. The removal of these two igniters will not prevent Catawba from meeting 10CFR50.44 requirements.
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ATTACIIMENT3 NO SIGNIFICANT IIAZARDS CONSIDERATION DETERMINATION AND ENVIRONMENTAL IMPACT ANALYSIS
No Significant liazards Consideration Determination As required by 10CFR50.91, this analysis is provided concerning whether the requested amendments involve significant hazards considerations, as defined by 10CFR50.92. Standards for determination :
I that an amendment request involves no significant hazards considerations are if operation of the facility in accordance with the requested amendment would not: 1) Involve a significant increase in the probability or consequences of an accident previously evaluated; or 2) Create the possibility of a new or different kind of accident from any accident previously evaluated; or 3) Involve a significant reductionin a margin ofsafety.
The requested amendments allow the hydrogen igniters in the incore instrument tunnels for Units 1 and 2 to be deleted.
In 48FR14870, the Commission has set forth examples of amendments that are considered not likely to involve significant hazards considerations. Example (vi) describes a change which either may result in
, some increase to the probability or consequences of a previously-analyzed accident or may reduce in some way a safety margin, but where the results of the change are clearly within all acceptable criteria with respect to the system or component specified in the Standard Review Plan. In this case, the proposed change is even more conservative than the change described in example (vi) in that it does not result in any increase to the probability or consequences of a previously-analyzed accident and does not reduce any safety margin. In addition, the results of the change do not have any adverse impact upon any Standard Review Plan acceptance criteria.
Criterion 1 The requested amendments will not involve a significant increase in the probability or consequences of an accident previously evaluated. No impact upon accident probabilities will be created, since the EHM System is not an accident initiating system. In addition, it has been demonstrated that based on the results of computer analysis, and the review of results of an external study performed for a similar type containment, that hydrogen concentrations in the cavity during degraded core accidents will remain within acceptable limits. No imp =;t on the plant response to any accident will be created (either design basis or beyond-design basis).
Criterion 2 The requested amendments will not create the possibility of a new or different kind of accident from any accident previously evaluated. As stated previously, the EHM System is not an accident initiating system. No new accident causal mechanisms will be created as a result of deleting the affected igniters.
Plant operation will not be affected by the proposed amendments and no new failure modes will be created.
Criterion 3 The requested amendments will not involve a significant reduction in a margin of safety. No adverse impact upon any plant safety margins will be created. As shown previously, applicable computer analysis has successfully demonstrated that the affected igniters could be removed with no adverse consequences. No fission product barriers are being degraded. No change to the manner in which the units are operated is being made.
Based upon the preceding analyses, Duke Power Company concludes that the requested amendments !
do not involve a significant hazards consideration.
i EnvironmentalImnact Analysis i
The proposed amendments have been reviewed against the criteria of 10CFR51.22 for environmental considerations. The proposed amendments do not involve a significant hazards consideration, nor increase the types and amounts of effluents that may be released offsite, nor increase individual or cumulative occupational radiation exposures. Therefore, the proposed amendments meet the criteria l
given in 10CFR51.22(c)(9) for a categorical exclusion from the requirement for an Environmental
! Impact Statement.
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