ML20059J229
| ML20059J229 | |
| Person / Time | |
|---|---|
| Site: | Calvert Cliffs  |
| Issue date: | 11/04/1993 |
| From: | Denton R BALTIMORE GAS & ELECTRIC CO. |
| To: | NRC OFFICE OF INFORMATION RESOURCES MANAGEMENT (IRM) |
| References | |
| NUDOCS 9311120103 | |
| Download: ML20059J229 (16) | |
Text
L BALTIMORE '
l GAS AND ELECTRI i
1650 CALVERT CLIFFS PARKWAY . LUSBY, MARYLAND 20657-4702 ,
l ROBERT E. DENTON Vict PRESIDENT ,
NUCLE AR rNERGY use) no-4.ss ,
November 4,1993 U. S. Nuclear Regulatory Commission Washington, DC 20555 .
ATTENTION: Document Control Desk
SUBJECT:
Calvert Cliffs Nuclear Power Plant Unit Nos.1 & 2; Docket Nos. 50-317 & 50-318 Recuest for Amendment: Containment Vent /Hydrocen Purce Line
REFERENCES:
(a) Ixtter from Mr. D. H. Jaffe (NRC) to Mr. J. A. Tiernan (BG&E),'
dated February 20,1986, Issuance of Amendments Nos.115 and 98 (b) Safety Evaluation of BG&E's Calvert Cliffs Nuclear Power Plant Units 1 and 2, dated August 28,1972 (c) Ixtter from Mr. R. E. Denton (BG&E) to NRC Document Control Desk, dated May 6,1993, Control Room Habitability - Interim Engineering Analysis for Thyroid Dose $
Baltimore Gas and Electric (BG&E) Company hereby requests an amendment to its Operating .
Licen.e Nos. DPR-53 and DPR-69 for Calvert Cliffs Units 1 and 2, respectively,with the submittal of the following proposed changes to the Updated Final Safety Analysis Report (UFSAR). :
DESCRIPTION OF CIIANGE We have determined that a proposed design change to our containment vent / hydrogen purge system -
would constitute an unreviewed safety question, as defined in 10 CFR 50.59. We would like to remove an orifice plate in the containment vent / hydrogen purge line to allow greater flow through '
the line. This will restore full flow capability to the line and result in less time required to vent the containment. Analyses performed in support of this change indicate that the consequences of an accident previously evaluated in the safety analyis report would be increased above the maximum ,
hypothetical accident described in the UFSAR (Chapter 14.24). Therefore, per 10 CFR 50.59(2)(c),
we request the NRC review and approve this change through an amendment to our operating l licenses pursuant to 10 CFR 50.90.
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HACKGROUNI) ,
l The containment vent / hydrogen purge system is designed to operate as a containment vent dunng power operations and as a hydrogen purge during accident situations. 'As a containment vent, the l system is utilized to control containment pressure and airborne radioactivity within Technical j Specification limits. During accident situations, the system is designed to control hydrogen inside the i containment below 3.0% should both hydrogen recombiners fail to function properly. !
e Each containment has a separate but identical vent system. Penetration room exhaust fans in the l auxiliary building draw air through an in-containment moisture separator and an in-containment i motor-operated valve (MOV). The air is passed through the auxiliary building via the vent lines l which have an outside containment MOV, flow reducing orifice, a flow monitoring system, a motor- ]
operated butterDy valve, and a set of two high efficiency particulate air and two charcoal filters in i paralle! (the penetration room ventilation system filter bank). The air is then discharged by the fans I through the main plant vent. Vented air is replaced through a separate penetration. The use of this ;
system as a containment vent was approved in Technical Specification Amendments Nos.115 and 98 ;
(Reference a). l
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Upon receipt of a safety injection actuation signal (SIAS), containment radiation signal (CRS), or a !
high radiation signal, the in-containment and outside containment MOVs close. j As part of the approval for the use of these vahres as containment vent valves (Reference a), changes !
to the calculated offsite dose were also approved. The maximum hypothetical accident doses !
(UFSAR, Chapter 14.24) were revised to include venting of the containment at the initiation of the j accident. This assumption increased the offsite dose above that described in the UFSAR, and NRC
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approval of the change was requested and granted. Use of the valves as containment vents began in June 1991 and the UFSAR has been updated to indicate the approved increase in offsite dose.
t Venting is accomplished through the four inch containment vent / hydrogen purge lines. Due to a !
calculational error in the offsite dose calculation (discussed in Attachment 2), we installed an orifice plate with a one inch opening in the vent line. This plate limited the flow through the line in order to !
maintain the offsite dose at its previously approved levels. As a result, it now takes approximately i 7 times longer to vent the containment than it did with a four inch line (28 hours3.240741e-4 days <br />0.00778 hours <br />4.62963e-5 weeks <br />1.0654e-5 months <br /> versus 4 hours4.62963e-5 days <br />0.00111 hours <br />6.613757e-6 weeks <br />1.522e-6 months <br />). In j addition, venting now occurs over three operating shifts instead of being completely contained within i one shift. Venting is a manually controlled operation in that it requires operator attention (the [
operator opens and closes the valves from the control room). Stretching the venting over three shifts !
introduces the possibility of additional human error into the venting process. Another consideration j is that the probability of an accident occurring during venting decreases with decreased vent time. j Reducing the venting time will not increase the number of times we need to vent because the starting i and ending conditions for venting remain the same. Therefore, the total amount of time the containment vent is open is decreased.
We now wish to remove the orifice plate and restore full flow through the vent line. This will reduce !
the time the valves must remain open to accomplish normal containment venting. In order to remove the orifice plate, we reanalyzed the maximum hypothetical accident doses accounting for the .!
full flow through the vent line at the beginning of the accident. The results of this analysis indicated j that this change would increase the consequences of a previously evaluated accident, therefore, we !
are requesting NRC approval prior to implementing the change.
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l SAFl?IY ANALYSIS i By increasing the flow through the containment vent at the initiation of an accident, the amount of !
radionuclides released to the atmosphere is also increased. This increase has a direct impact on the i results of the maximum hypothetical accident described in UFSAR Chapter 14.24. We have also l included the dose due to leakage from the Refueling Water Tank in accordance with Information Notice 91-56. A reanalysis of the maximum hypothetical accident was performed and is described in .
Attachment (1). Note that there were changes in the analytical methods, dose reduction assumptions ,
and correction of calculational errors. These account for most of the differences in the offsite dose f between the original analyses and the revised one. These changes are described in Attachment (2). -
The maximum hypothetical accident is the limiting case for offsite dose calculations. The other ;
accident scenarios in Chapter 14 were evaluated to determine if the change in vent flow rate would '
impact them as well. No other accident scenario in Chapter 14 is affected by the proposed change.
In addition, as part of the Control Room IIVAC Upgrade Project, we will be re-evaluating the whole .
j body and thyroid dose to the operators during this type of release. Our interim evaluation 1 (Reference c) does not account for the containment vent / hydrogen purge line. The final analysis of !
Control Room operator dose will consider the source term release described in this letter. ;
3 The results of the analysis show the exclusion area and low population zone doses are much less than !
j the limits of 10 CFR 100. This hypothetical acciden* :nvolves a greater fission product release than !
that which would result from any credible accident scenario. t
] j Calculated Previously 10 CFR +
4 Offsite Dose Approved Dose Part 100 :
l Attachment (1) UFSAR 14.24 Limits I (Rem) (Rem) (Rem) !
1 i Exclusion Area ;
Thyroid 118 124.9 300 ;
Whole Body 10.6 3.0 25 i Imw Population Zonc !
3 Thyroid 39.3 33 300 l j Whole Body 2.7 0.8 25 '
0 !
As shown above, the whole body doses increased by approximately 3.5 times the previously approved I
- dose. The result is a whole body dose which is approximately 40% of the exclusion area limit (25 Rem) and 10% of the low population zcme limit (25 Rem). As described in Attachment (2), most of the increase in whole body dose is due to a calculational error in the original contr.inment vent / hydrogen purge analysis. The low population zone thyroid dose increased slightly from the l previously approved dose. Most of this increase is attributable to the addition of dose due to leakage from the Refueling Water Tank (.in accordance with Information Notice 9156). The exclusion area 3
thyroid dose decreased from the previously approved dose.
i
Document Control Desk November 4,1993 Page 4 In the performance of this calculation, changes in calculational methodology and assumptions were made. Because the changes in some of the key dose reduction assumptions were in the conservative direction, the dose was expected to increase. Often these increases were offset by methodology changes. The change from the results reported in the UFSAR (Chapter 14.24) to the results given in this latest calculation do not change the conclusion stated in the UFSAR that this result is less than the guidelines given in 10 CFR Part 100. In approving our operating license (Reference b), the NRC calculated thyroid and whole body dose at the exclusion boundary and the low population zone boundary. The results stated in the Safety Evaluation Report are 110 Rem thyroid and 4 Rem whole body (exclusion area), and 80 Rem thyroid and 3 Rem whole body (Iow population zone). Our results are similar to the results stated in the SER.
It sh,uld be noted that for this change, the safety implications are about the same as before the change. Given that Operations needs a reasonable vent path, the issue becomes one of probability versus consequences. With the valve open for a shorter period of time, the probability of an accident occurring during containment venting is decreased. And although the consequences are increased, the originally intended limits are not being pressed. Therefore, there is no significant change to the margin of safety provided by the current plant configuration.
DETERMINATION OF SIGNIFICANT IIAZARDS:
This proposed change has been evaluated against the standards in 10 CFR 50.92 and has been determined to involve no significant hazards considerations, in that operation of the facility in accordance with the proposed amendment would not:
- 1. involve a significant increase in the probability or consequences of an accident previously evaluated; or This proposed change does not involve an increase in the probability of an accident j presiously evaluated. The removal of the orifice plate will decrease the amount of time the ;
containment vent / hydrogen purge line is open to accomplish containment venting. This {
decreases the probability of occurrence of a maximum hypothetical accident while venting. !
In addition, this proposed change would not affect any precursors to any of the accidents in j the Updated Final Safety Analysis Report.
Ilowever, this proposed change does involve an increase in the consequences of an accident previously evaluated. In the worst case event, a maximum hypothetical accident, the ,
exclusion area whole body dose and the low population zone thyroid and whole body doses ;
j increased above the previously approved doses. This increase is not significant. In fact, the !
offsite doses presented here are similar to those reported in the NRC's Safety Evaluation
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Report (Reference b). Other accident scenarios were evaluated to determine if this change i
would impact them as well. No other accident scenario is impacted by this change.
Therefore, removal of the orifice plate in the containment vent / hydrogen purge line does not l involve a significant increase in the probability or consequences of an accident previously
, evaluated. ,
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- 2. create the possibility of a new or different type of accident fmm any accident previously ;
evaluated; or . .i The proposed change affects a previously evaluated accident, but creates no new or different ,
type of accident. The equipment required to mitigate the consequences of an accident would i continue to be operable. We are not proposing to alter the function of any equipment or j have it operate differently than it was designed to operate. In fact, we are restoring the ;
containment vent / hydrogen purge line to full flow capability by removal of an orifice plate, i Therefore, this change does not create the possibility of a new or different type of accident [
from any accident previously evaluated. i
- 3. involve a significant reduction in a margin ofsafety.
The margin of safety defined by 10 CFR Part 100 has not been significantly reduced. There will be an increase in the exclusion area and low population zone doses, but the total dose is still significantly less than the guidelines given in 10 CFR Part 100. Additionally, the NRC has previously calculated offsite dose for Calvert Cliffs (Reference b) using assumptions :
similar to those used for the analysis performed in support of this request. The NRC's results l are similar to those obtained from our calculations. The increase in dose does not affect any t conclusions stated in the NRC's SER. Therefore, the margin of safety has not been significantly reduced. !
Moreover, the Commission has provided guidance concerning the application of standards m 10 CFR 50.92 by providing certain examples (March 6,1986 51FR7751) of amendments that are >
considered not likely to involve a significant hazards consideration. This proposed change is very similar to exampic (vi) in the Federal Register Notice, in that this change results in an increase in the consequences of a previously analyzed accident but the results of this change are clearly within all acceptable criteria. The criteria for maximum hypothetical accident results is found in Standard ,
Review Plan Section 15.6.5.Section II.1 provides exposure guidelines for offsite dose calculations. !
The guidelines given are 150 Rem to the thyroid and 20 Rem to the whole body. The results we have ,
calculated clearly fall within these acceptance criteria. Therefore, based on the information contained in this submittal, we believe that this change does not result in a significant hazard. 6 ENVII(ONNIENTAL ASSESSSIENT During normal plant operation, the need exists for containment purge. For example, during startup, the pressure inside containment increases due to the heatup of the containment atmosphere. This ,
pressure must be relieved to maintain margin to the 4 psi engineered safety features actuation setpoint. By removing the orifice plate in the containment vent / hydrogen purge line, we will be restoring full flow through the line. Normal environmental releases, including those from this vent :
pathway, are controlled by our Technical Specifications. Therefore, with regard to normal !
environmental releases, there will be no additional environmental impact and, therefore, no irreversible consequences resulting from the removal of the orifice plate.
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The ervironmental impact of allowing removal of the orifice plate in the containment vent / hydrogen purge line results from consideration of a maximum hypothetical accident initiated when the vent is !
open to the atmosphere. We have evaluated the radiological consequences of a hypothetical ,
accident while the vent lines are open. The results of our evaluation are described in Attachment 1. ;
It should be noted that the dose estimates represent an extreme upper bound because the release from the containment was assumed to contain fission products derived from a uniform mixing in the !
, containment atmosphere of the iodines and noble gases specified in TID-14844. Even though the i percentage increase in offsite doses is not small, the actual total doses are a fraction of the limits of !
10 CFR Part 100. In evaluating the impact of the increased doses, it is important to view these i results in light of the low probability of the accident. This change does not significantly affect the risk i of any dominant accident scerario and the effect on overall risk of accident at this facility is . {
insignificant. i SCIIEDULE f
This change is requested to be approved and issued by April 15, 1994. However, issuance of this !
i amendment is not currently identified as havmg an impact on outage completion or continued plant l operation. !
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j SAFETY COMMflTEE REVIEW i i
This proposed change to the Operating License and our determination of significant hazards have l been reviewed by our Plant Operations and Off-Site Safety Review Committees, and they have !
concluded that implementation of these changes will not result in an undue risk to the health and !
safety of the public. i l
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i Should you have any questions regarding this matter, we will be pleased to discuss them with you.
urs, c 3A ,
STATE OF' MARYLAND :
- TO WIT :
COUNTY OF CALVERT :
I hereby certify that on the M day of _ ,1993 , the subscriber, a Notary Public of the State of Maryland in and for /'1/n ed, befor - ,
personally appeared Robert E. Denton, being duly sworn, and states that he is Vice Presiddnt of the j Baltimore Gas and Electric Company, a corporation of the State of Maryland; that he provides the foregoing response for the purposes therein set forth; that the statements made are true and correct to the best of his knowledge, information, and belief; and that he was authorized to provide the response on behalf of said Corporation.
WITNESS my Hand and Notarial Scal: W . 'll6 143-Notary Public
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My Commission Expires: nzdAq /, /19h
() - (JDate .
RED / PSF / dim.43jd ;
Attachments (1) Maximum Hypothetical Accident Analysis (2) Differences in the Maximum Hypothetical Accident Calculations - [
cc: D. A. Brune, Esquire J. E. Silberg, Esquire i R. A. Capra, NRC -
D. G. Mcdonald, Jr., NRC ;
T. T. Martin, NRC P. R. Wilson, NRC R. I. McLean, DNR i J. H. Walter, PSC r
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ATTACIIMENT (1)
MAXIMUM IIYPOTIIETICAL ACCIDENT ANALYSIS i
Itackcround Chapter 14.24 describes the maximum hypothetical accident for Calvert Cliffs. This accident i scenario is evaluated to determine compliance with the siting criteria given in 10 CFR Part 100. In i general, the maximum hypothetical accident (MHA) is a non-mechanistic scenario which evaluates the containment's capability to contain released radioisotopes. Safety system effectiveness is not ,
4 considered; the quantity of radioisotopes released to the containment atmosphere is dependent on the power level (MWt) of the reactor. The criteria for this release is established so that the magnitude of the release bounds all credible accident releases. Calvert ClitTs used the guidance given in Technical Information Document (TID) 14844 (referenced in 10 CFR 100.11 and Safety Guide 4) to determine an appropriate fission product release. The calculational guidance given in Safety Guide 4 was used during the development of the calculational assumptions except for the dose ,
conversion factors. The dose conversion factors were determined using the guidance given in
To determine the effectiveness of the containment, certain assumptions are made regarding the ;
leakage from containment. The containment is assumed to leak at the design Icakage rate (0.2% per .
day) at the design internal pressure (50 psig) for part of the accident period. The leakage is then !
l assumed to be reduced based, in part, on the reduction in internal containment pressure.
- Additionally, the containment vent line is assumed to be open at the start of the event. This ,
provides an additional leakage pathway during the time the valves are closing. This pathway is ,
included in the containment leakage assumptions. Operation of safety systems which affect the containment atmosphere (either pressure or radionuclide removal) is assumed with some restrictions. .
( In response to concerns raised in Information Notice 91-56, we have also evaluated potentialleakage paths from the containment sump to the atmosphere via the Refueling Water Storage Tank. These assumptions are described in more detail below.
Computer Code The Bechtel standard computer program LOCADOSE is a set of computer programs used to i calculate radionuclide transport and individual doses. Two modules of the program are required to calculate nodal and offsite doses: the Activity Transport Program; and the Dose Calculation :
Program. j The Activity Transport Program in LOCADOSE simulates the release of radioactivity from a user- l defined source (reactor core, reactor coolant system, damaged assembly, . . . ), via the manual l insertion ofisotopic activities or via the automatic generation of activities based on the reactor power j
, and the methodology of TID 14844. The code allows for separate iodine species (elemental, ,
particulate, organic), iodine spiking factors, group release fractions, and the calculation of time-dependent daughter activities.
LOCADOSE allows the modeling of up to nine internal compartments or nodes (sprayed containment, unsprayed containment, control room, sump, . . . ). The transport of fission products from one node to another is accomplished by the solution of a set of first-order differential equations, which include user.specified time-dependent production and removal terms (containment sprays, recirculation, plateout, purge and intake filters, atmospheric dispersion and natural decay) and which are evaluated using a matrix solution. The program also allows for variable purge valve i closure times, user-specified filter efficiencies, and an automatic spray cut-off option. The program also calculatus integrated activities within each node for each time step and the activity released to 1
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ATI'ACIIMENT 0) i MAXIMUM IIYPOTHETICAL ACCIDENT ANALYSIS i
the environment during each time step, by modeling the nodal leak rates and purge options. This l information is stored in a file called LOCATRAN to be read by the Dose Calculation Program. I The Dose Calculation Program utilizes the activities stored in the LOCATRAN file in conjunction with user input (atmospheric dispersion coefficients, breathing rates, occupancy factors) and a library - ;
of dose conversion factors to calculate time-dependent doses and dose rates. These doses are [
calculated at the internal nodes and at up to 20 offsite dose points assuming a semi-infinite cloud j model for immersion doses.
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Assumptions ,
- 1) The pre-accident thermal power rating is 2754 MWt (102% power). f
- 2) The fission product source term is based on guidance from TID 14844. This assumes that i 100% of the noble gases and 50% of the iodines of the total core inventory are released to '
the containment volume immediately after initiating the accident. They are assumed to be instantaneously and uniformly dispersed in the containment. Of the released iodine,50% is assumed to plate out on internal reactor surfaces, leaving only 25% of the total iodine available for leakage. The gaseous iodine is assumed to be present in the following forms; i 91% elemental. 4% organic, and 5% particulate. The effectiveness of the iodine removal l mechanisms are dependent on the form of the iodine being removed.
- 3) The containment free volume is assumed to be 2.035E+06 cubic feet (UFSAP ,
Chapter 14.17). This volume is divided into two regions representing the sprayed an ! 1 unsprayed portions of the containment. He dividing point is the 69' operating deck. Since
- Containment Spray Header No.1 is located at 182'7" and the inside diameter of the containment is 130', the sprayed volume is estimated to be 1.48E+06 cubic feet, or 72.7% of the containment volume. Thus the unsprayed volume, which is the difference between the !
net free containment volume and the sprayed volume, is 5.55E+05 cubic feet or 27.3% of l containment.
- 4) The containment leak rate is assumed to be 0.2% per day for the first 24 hours2.777778e-4 days <br />0.00667 hours <br />3.968254e-5 weeks <br />9.132e-6 months <br /> and 0.1% per ,
day for the duration of the accident per Safety Guide 4. Since the containment volume is l 2.035E+06 cubic feet, the containment leak rate is 2.8264 cfm for the first day and 1.4132 cfm for the duration of the event. The containment vent / hydrogen purge line is assumed to be in operation at the beginning of the event. This adds 1645 cfm to the ,
containment leak rate until the valve closes in 30 seconds (Technical Specification Table 3.6-1 gives a value of 15 seconds). The containment vent / hydrogen purge line connects to the penetration room exhaust system and passes through high efficiency particulate air (HEPA) and charcoal filters before being discharged to the plant vent. Credit is taken for these filters while the containment vent / hydrogen purge line is in operation. He air passing through the containment vent / hydrogen purge line is assumed to be filtered before being discharged to the plant vent.
- 5) All other containment leakage is assumed to be discharged directly to the atmosphere. There j
is no credit taken for leakage to the penetration room. This is conservative, because l approximately half of the containment penetrations terminate in the penetration room and j therefore, a large percentage of the leakage could be assumed to go into this room. The 3 ventilation system in the penetration room contains both HEPA and charcoal filters which l
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A'ITACllhfENT (1)
ATAX1h1Uh1 IIYPOTIIETICAL ACCIDENT ANALYSIS would be assumed to reduce the radionuclide concentrations in the room prior to discharging the air to the plant vent. His is a dose reduction method that is not credited.
- 6) Two of the three iodine filters in containment are assumed to operate at 20000 cfm. One filter starts at t=0, while the second is manually started n . - minutes. The elemental, organic and particulate iodine removal efficiencies are assumed to be 90%, 30% and 90%,
respectively.
- 7) Two containment cooling units are assumed to operate post-LOCA with a flow of 55000 cfm each.
- 8) With minimum safeguards equipment in operation, one of two containment spray pumps are assumed to operate at 180 cfm. The spray delay time after containment spray actuation signal (CSAS) actuation is 60 seconds. The delay time for CSAS actuation is assumed to be 30 seconds.
- 9) The site boundary atmospheric dispersion coefficient is 1.3E-04 sec/m3, while the 0-2hr, 2-24hr, and 24-720hr low population zone (LPZ) atmospheric dispersion coefficients are 3.30E-5,2.20E-6, and 5.40E-7 sec/m3, respectively (UFSAR Section 2.3).
- 10) The platcout removal coefficient of elemental iodine, by wall deposition due to the containment spray, is estimated by equations contained in Standard Review Plan 6.5.2, Revision 2. Assuming a total containment free volume of 2.035E+06 cubic feet and a total containment wetted surface area of 3.089E+05 ft"2 (UFSAR Table 14.16-1) yield a removal coefficient of 2.43/hr. The platcout removal of elemental iodine is limited by a maximum decontamination factor of 200, per the Standard Review Plan 6.5.2, Revision 2.
This value is reached about 1.1 hours1.157407e-5 days <br />2.777778e-4 hours <br />1.653439e-6 weeks <br />3.805e-7 months <br /> into the event. Platcout is assumed to occur in both the sprayed and unsprayed portion of the containment volume.
- 11) The spray removal coefficient for elemental iodine is estimated by the well-mixed droplet model given in the Standard Review Plan 6.5.2, Revision 2. This model yields a spray removal coefficient of 14.4/hr. The spray removal of elemental iodine is limited by a maximum decontamination factor of 13.7 (Standard Review Plan 6.5.2, Revision 2). He decontamination factor is reached about 13 minutes into the event.
- 12) The spray removal coefficient for particulate iodine is estimated by the Standard Review Plan 6.5.2, Revision 2. This yields a particulate spray removal coefficient of 3.34/hr until a DF=50 is reached for particulate iodine, after which particulate spray removal is conservatively assumed to cease.
- 13) The containment iodine removal filters are assumed to remain in operation for the entire duration of the event.
- 14) As discussed in Information Notice 91-56, there is a potential for an unmonitored release
, from the containment sump to the atmosphere through the Refueling Water Tank (RWT).
l During the recirculation phase, the sump water is recirculated through the ECCS pumps and l is assumed to leak through the two valves in series in the mini-flow recirculation line. This l leakage is assumed to go to the RWT where it is leaked to the atmosphere through the RWT vent. This leak path is unmonitored. We have calculated the contribution to the maximum hypothetical accident offsite doses due to this additional leakage. A leak rate of 0.2 gpm from the sump to the RWT was assumed. A TID-14844 source term was used to determine l
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ATTACIIMENT (I) :
MAXIMUM IIYPOTIIETICAL ACCIDENT ANALYSIS the amount of iodine present in the sump water. Ninety-one percent of the iodine was assumed to be in the form of elemental iodine per Regulatory Guide 1.4. A partition coefficient of 31.2 was calculated from equation 26 in NUREG-5732. Ieakage of RWT vapor is based on the maximum diurnal temperature variation in the tank. His calculation was performed separately from the MHA calculation, however, the assumptions are -
consistent and the results are additive.
i Summary of Results The results of the analysis show that the exclusion area and LPZ doses are much less than the limits of 10 CFR Part 100. His hypothetical accident involves a much greater fission product release than that which would result from any credible accident scenario.
t Dose Due to Dose Total Previously Containment DueTo Offsite Approved Dose Sources RWTIrakage Dose UFSAR 14.24 Rem) (Rem) (Rem) (Rem) t Exclusion Area (0-2 hours)
Thyroid 118 0.11 118 124.9 Whole Body 10.6 10.6 3.0 r Ixw Population Z(me (30 days) !
Thyroid 29.9 9.42 39.3 33 Whole Body 2.7 2.7 0.8 t
T t
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l NITACHMENT (2)
DIFFERENCES IN THE MAXIMUM HYPOTIIETICAL ACCIDENT CALCULATIONS Itackcround The currently approved Maximum Hypothetical Accident (MHA) calculation (including dose due to 4 containment venting) will be replaced with the calculation described in Attachment (1). If we were :
simply increasing flow through the containment vent / hydrogen purge line (by removal of the orifice t plate), the offsite dose would be expected to increase. However, differences between the ,
calculations have been identified which have caused the offsite dose to vary from what is expected.
These differences can be attributed to changes in the calculational methodology, changes in the dose reduction mechanism assumptions, and correction of a previous calculational error. Discussion of the effect removal of the orifice plate has on offsite dose and the effect of calculational changes on >
the offsite dose are prosided below.
The calculational history of the maximum hypothetical accident is described here to proside understanding of the current calculation. Before Calvert Cliffs received its operating license for Unit 1, we submitted the results of a maximum hypothetical accident calculation to the NRC to be used as the basis for approval of our containment system. The NRC approved our assumptions and our containment system in Reference (b). 'nat analysis is described in Chapter 14.24 of the UFSAR. In 1985 we requested a Technical Specification change to allow the use of the hydrogen purge line for venting of the containment during normal operation. In support of that request, we submitted an evaluation of the increase in maximum hypothetical offsite dose due to the vent line being open at the beginning of an accident. It should be noted that, although many of the assumptions were similar, the analytical methods were substantially different than those used in Chapter 14.24. Despite ;
the differences, we combined the doses from the two calculations to produce a new maximum hypothetical accident dose. The NRC approved the new dose in Reference (a).
In support of adding a radiation closure signal to the containment vent / hydrogen purge valves, the calculation supporting the Technical Specification change (Reference 2) was reviewed. During this review, an error was discovered which affected the whole body dose. To maintain the dose within the ;
originally approved limits, an orifice plate was installed to limit the Dow from the containment i vent / hydrogen purge lines. Because of this error and the fact that different analytical methods had ;
been used to calculate the dose in Chapter 14.24 of the UFSAR from those used to calculate the ;
dose due to containment vent / hydrogen purge, we decided to recalculate the maximum hypothetical accident doses. A single calculation was performed which incorporated all of the assumptions and >
corrections identified during various reviews. The benefit of a single calculation is that it eliminates any differences in assumptions or calculational methodology and provides a consistent, repeatable >
basis for the answer. There are changes to the thyroid and whole body doses determined by this calculation as compared to the previously reported doses. We explain the quantitative and qualitative differences between the methodologies, assumptions and results from the various calculations. Note that the discussion of calculational changes below is broken down into the containment leakage calculation and the containment vent / hydrogen purge calculation.
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Chance in Orifice Sire l If we were to look at just changing the orifice size from one inch to four inches, it is clear that the offsite dose would increase. That increase is proportional to the increase in flow rate from the containment vent / hydrogen purge line. If we were to use the same calculational method for the containment vent / hydrogen purge line that was used in Reference (a), the thyroid dose would increase approximately five. fold, and the whole body dose would increase approximatel, three. fold.
All other things being equal, this would clearly result in an unreviewed safety question. However, the issue is not so clear because we are also changing the calculational method associated with the containment leakage calculation. Those changes are described below.
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i A'ITACIIMENT (2)
- DIITERENCES IN Tile MAXIMUM InTOTIIETICAL ACCIDENT CALCULATIONS Containment Leakace Calculation i
The containment leakage calculation considers containment leakage at the maximum allowed leak l rate. It does not include the leakage through the containment vent / hydrogen purge line. Hat ' i calculation is discussed separately.
The containment leakage calculation currently approved is described in Chapter 14.24 of the UFSAR. The proposed calculation is described in Attachment (1). In this section, we will discuss the containment leakage portion of the proposed calculation. The results for the two calculations are shown below. ;
Previously Calculated ;
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Approved Dose Offsite Dose UFSAR 14.24 Attachment (1)
Rem Rem Exclusion Area Thyroid 94 30.4 )
Whole Body 2.2 1.45 Irw Population Zone :
Thyroid 33 7.71 .
Whole Body 0.8 0.4 ,
i There are two factors which account for most of the differences in the resuhs: the filter and spray removal mechanisms and leakage of the iodine from the containment. These are discussed below.
The filter removal mechanisms were modelled using the following assumptions. Both models assumed the filters operate at 20,000 cfm each, with one initiating at 0 seconds and one at 20 minutes into the MIIA. The calculation described in Attachment (1) assumes 90% efficiency for elemental and particulate iodine and 30% efficiency for organic iodine for the containment filters. He I
approved Chapter 14.24 analysis assumes 90% efficiency for elemental iodine and 0% efficiency for organic and particulate iodine. Additionally, the iodine was partitioned as follows, Chapter 14.24 ,
assumes a 90% elemental, 0% particulate, and 10% organic iodine while the analysis in Attachment (1) assumes a 91%,5%, and 4% composition. This is the main difference between the approved Chapter 14.24 analysis and the analysis presented here and is not readily quantifiable. The removal mechanisms used in the calculation presented in Attachment (1) are described in assumptions 3,8,10,11, and 12 of the attachment.
leakage of the iodine from the containment was another area w'here major differences exist.
Chapter 14.24 assumes that 50% of the containment leakage occurs through the penetration room ,
where it is passed through another iodine filter before being discharged to the atmosphere. Tne calculation described in Attachment (1) assumes that all containment leakage bypasses the penetration room and is released directly to the atmosphere.
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ATTACliMENT (t)
DIFFERENCES IN TIIE MAXIMUM IIYPOTIIEFICAL ACCIDENT CALCULATIONS The cumulative effect of these differences was to lower the offsite dose due to containment leakage from that originally reported in the UFSAR. This effect is mainly due to modelling changes which ,
allow for a more precise accounting of spray and plateout than was previously achievable. The differences in assumptions (especially the one concerning containment leakage through the penetration room) would have tended to increase the offsite dose.
Containment Vent /Ilvdrocen Purce Calculation [
This calculation addresses the contribution of the containment vent / hydrogen purge line to the offsite dose.
The containment vent / hydrogen purge calculation currently approved is described in Reference (a).-
The proposed calculation is described in Attachment (1). In this section, we will discuss the differences between the two calculations and how those differences affected the results. The- i answers for the calculations are given below.
Previously Calculated Approved Dose Offsite Dose Reference (a) Attachment (1)
Exclusion Area i Thyroid 30.9 87.2 Whole Body 0.8 9.11 ,
Low Population Zone f Thyroid (none 22.2 Whole Body reported) 2.3 There are three factors which account for most of the difTerences in the calculations. Changes to the '
source term, increased flow through the vent and a change in iodine removal mechamsms will be
- described below. ;
a ,
During review of the originally approved calculation, an error in the source term input was ,
discovered. This error resulted in the noble gas source term being knv by a factor of ten in the original calculation. The correction of that error results in a majority of the increase in the whole i body dose seen above. l The flow rate through the vent line was recalculated. He flow rate increased from !n8 cfm to 1645 cfm. This change increased both the thyroid and whole body dose.
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NITACilhf ENT (2)
DIFFERENCES IN TIIE AfAXIn1 Uni IIYPOTIIETICAL ACCIDENT CALCULATIONS i
Once again, the iodine removal mechanisms have been changed. Both calculations assumed no sprays or platcout for the first 30 seconds of the accident. The previous calculation assumed two filters operating during the event, with a 90% efficiency for elemental iodine. The revised calculation assumes only one filter operating with the efficiencies given in Attachment (1). Iodine chemical composition differed also. The previous calculation assumed 'a 90% elemental, 0%
particulate and 10% organic composition. The revised calculation assumed a 91%,5%, and 4%
composition. :
The cumulative effect of these three factors increased the calculated dose due to containment venting at the start of the accident. The calculational methodologies used in the two cases were i similar, and differences between the computer codes do not account for any significant differences in the answers.
Conclusion i When the containment leakage dose is added to the dose due to venting, the total doses are very similar for the two cases. (As noted earlier, a single calculation was run and the doses due to ;
containment leakage and containment venting were determined from that single calculation to allow us to make this comparison.) ,
Previously Approved Dose Containment Containment Leakage Venting Total iRem) (Remj (Rem)
Exclusion Area Thyroid 94 30.9 124.9 '
Whole Body 2.2 0.8 3.0 Low Population Zone !
Thyroid 33 (none 33 l Whole Body 0.8 reported) 0.8 1
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ATTACIIMENT (2]
DIFFERENCES IN TIIE MAXIMUM IIYPOTIIETICAL ACCIDENT CALCULATIONS Calculated Offsite Dose RWT Containment Containment Leakage Leakage Venting (Att.1) _ Total 1 (Rem) (Rem) (Rem) (Rem)
Exclusion Area Thyroid 30.4 87.2 0.11 118 Whole Body 1.45 9.11 10.6 Low Population Zone (30 days) j Thyroid 7.71 22.2 9.42 39.3 :
Whole Body 0.4 23 2.7 .
However, as described above, there are some differences in the maximum hypothetical accident dose calculations. Both changes in methodology and, more importantly, changes in assumptions account ,
for the change in the results. Because the changes in some of the dose reduction assumptions were ;
in the conservative direction, the dose should have increased. Often these increases were offset by-methodology changes. However, the different assumptions used led us to conclude that an unreviewed safety question existed concerning these calculations. Therefore, NRC review of the calculation is being requested.
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