ML20023C138
| ML20023C138 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 04/28/1983 |
| From: | Eisenhut D Office of Nuclear Reactor Regulation |
| To: | Ahearne J, Gilinsky V, Palladino N NRC COMMISSION (OCM) |
| References | |
| TASK-2.K.3.05, TASK-AS, TASK-BN83-034, TASK-BN83-34, TASK-BN83-34A, TASK-TM BN-83-034, BN-83-34, BN-83-34A, NUDOCS 8305110513 | |
| Download: ML20023C138 (4) | |
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O UNITED /7TEs V[7, [i -[ ' wg
- j Vc. NUCLEAR REGULAbRY COMMISSION ip < f n Assmoron. o. c. 20sss s 2.o; x h3 O((
Mit %2 81983 Docket No. 50-289 MEMORANDUM FOR:
Chairman Palladino Commissioner Gilinsky Commissioner Ahearne Commissioner Roberts Commissioner Asselstine FROM:
Darrell G. Eisenhut, Director, Division of Licensing, NRR
SUBJECT:
BOARD NOTIFICATION (BN-83-34A) THI-1 RESTART HEARING In accordance with the NRC procedures for Board Notification, the enclosed letter (H. Hukill (GPUN) to D. Eisenhut (NRC) dated March 31, 1983) is provided directly to the Commission.
The letter provides the GPUN response to our earlier letter of March 4, 1983 (BN-83-34) which provided criteria for resolution of TMI Action Item II.K.3.5 " Automatic Trip of Reactor Coolant Pumps (RCP)." GPUN intends to revise RCP trip criterinn from 1600 psig to 25 F indicated subcooling margin, and intends to implement the changes under 10 CFR 50.59. As described in our March 4,1983 letter, we will inspect the implementation of this item at TMI-l on a schedule en:;istent with that for other operating licensees. The ASLAB is being informed by copy of this notification.
Original signed b7 p-n c.
innhn?.
Darrell G. Eisenhut, Director Division of Licensing
Enclosure:
3/31/83 GPUN 1tr. to NRC cc w/ enclosure:
Dr. John H. Buck, ASLAB Judge Reginald L. Gotchy, ASLAB Christine N. Kohl, Esq., ASLAB Dr. Lawrence R. Quarles, ASLAB Judge Gary L. Edles, ASLAB OPE OGC B305110513 830428 PDRADOCKOSOOOg SECY EDO P
Parties to Hearing
Contact:
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OFFICIAL RECORD COPY usam mi-m nac ronu sia tio-noi nacu o24o
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o, UNITED STATES NUCLEAR REGULATORY COMMISSION g
j WASHINGTON, D. C. 20665
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088#4 DISTRIBUTION FOR BOARD N0TIFICATION'NO. BN-83-34A RE: TMI-l RESTART HEARING Docket File AE0D NRC PDR JThoma L PDR H0rnstein ORB #4 Rdg EBlackwood '
ORB #4 Memo File TMI Site Pouch 4
ORB #4 Board Notification File JGray HDenton JScinto ECase MCutchin DEisenhut EChristenbury ss Glainas BSnyder Dr. W. H. Jordan, ASLB JStolz Dr. John H, Buck, ASLAP, EW-529
. Dr. L. Little, ASLB JVan Vliet Judge Reginald L. Gotchy,, ASLAP, EW-529 RIngram Christine N. Kohl, Esq., "
MJambor-De, Lawrence R. Quarles, "
JHard Judge Gary J. Edfes., ASLAP, EW-529 Chairman, ASLAP, EW-529 ABennett-2 I. Smith, ASLB ACRS-16 (nailing labels provided by OELD)
Mr. Henry D. Hukill
~
Vice President RHartfield GPU Nuclear Corporation P. O. Box 480 I
Middletown, Pennsylvania 17057
,0IE i
PPAS MWilliams, ORB #4 THI-l Service ~ List (Attached)
TPoindexter L
JGoldberg D
GPU Nuclear Corporation 50-289, TMI-l Mr. R. J. Toole Jordan D. Cunningham, Esq.
Manager, TMI-l Fox, Farr and Cunningham GPU Nuclear Corporation 2320 North 2nd Street P. O. Box 480 Harrisburg, Pennsylvania 17110 Middletown, Pennsylvania 17057 Ms. Louise Bradford TMIA Board of Directors 1011 Green Street P.A.N.E.
Harrisburg, Pennsylvania 17102 P. O. Box 268 Middletown, Pennsylvania 17057 Ms. Marjorie 14. Aamodt R. D. #5 Coatesville, Pennsylvania 19320
- Docketing and Service Section Earl B. Hoffman U. S. Nuclear Regulatory Commission Dauphin County Commissioner Washington, D. C.
20555 Dauphin County Courthouse Fmnt and Market Streets Chauncey Kepford Harrisburg, Pennsylvania 17101 Judith H. Johnsrud Environmental Coalition on Nuclear Power Union of Concerned Scientists 433 Orlando Avenue c/o - Harmon & Weiss State College, Pennsylvania 16801 1725 I Street, N. W.
Seite 506
- Judge Reginald L. Gotchy Washington, D. C.
20006 Atomic Safety & Licensing Appeal Board U.S. Nuclear Regulatory Commission Mr. Steven C. Sholly Washi.aton, DC 20555 Union of Concerned Scientists 1346 Connecticut Avenue, N. W.
J. B. Lieberman, Esq.
Dupont Circle Building, Suite 1101 Berlock, Israel & Lieberman Washington, D. C.
20036 26 Broadway New York, New York 10004 Regional Administrator,."
U. S. H. R. C., Region I 631 Park Avenue King of Prussia, Pennsylvania 19406
- Gary J. Edles, Chairman Atomic Safety & Licensing Appeal Board U.S. Nuclear Regulatory Commission.
ANGRY /TMI PIRC Washington, DC 20555 1037 Maclay Street Harrisburg, Pennsylvania 17103
- Or. John H. Buck Atomic Safety & Licensing Appeal Board John Levin, Esq.
U.S. Nuclear Regulatory Commission Pennsylvania Public Utilities Washington, DC 20555 Commission Box 3265 Harrisburg, Pennsylvania gl7120
1 i
l tru nucacar t,orporanon General Counsel Federal Emergency Management Agency Mr. Thomas Gerusky ATTN: Docket Clerk Bureau of Radiation Protection 1725 I Street, NW Department of Environmental Resources Washington, DC 20472 P. O. Box 2063 Harrisburg, Pennsylvania 17120 Karin W. Carter, Esq.
505 Executive House
'P. O. Box 2357 Harrisburg, Pennsylvania 17120 G. F. Trowbridge, Esq.
Dauphin County Office Emergency Shaw, Pittman, Potts & Trowbridge Preparedness 1800 M Street, N.W.
Court House, Room 7 -
Washington, D. C.
20036 Front & Market Streets Harrisburg, Pennsylvania 17101 Mr. E. G. Wallace Licensing Manager GPU Nuclear Corporation 100 Interpace Parkway Parsippany, New Jersey 07054 William S. Jordan, III, Esq.
Harmon & Weiss Ms. Lennie Prough U. S. N. R. C. - T!11 Site 1725 I Street, HY, Suite 506 P. O. Box 311 Washington, DC 20006 Middletown, Pennsylvania 17057 Ms. Virginia Southard, Chairman Citizens for a Safe Environment 264 Walton Street Lemoyne, Pennsylvania 17043 Mr. Robert B. Borsum Babcock & Wilcox Nuclear Power Generation Division,
Suite 220, 7910 Woodmont Avenue Bethesda, Maryland 20814 '
Mr. David D. Maxwell, Chairman Board of Supervisors Londonderry Township RFD#1 - Geyers Church Road Middletown, Pennsylvania 17057 Mr. C. W. Smyth Supervisor of Licensing Ti1I-l GPU Nuclear Corporation Regional Radiation Representative P. O. Box 480 EPA Reoion III Middletown, Pennsylvania 17057 Curtis Building (Sixth Floor) 6th and Walnut Streets Philadelphia, Pennsylvania 19106 Mr. Richard Conte Governor's Office of State Planning Senior Resident Inspector (TMI-1) and Development L
U.S.N.R.C.
ATTN: Coordinator, Pennsylvania P. O. Box 311 State Clearinghouse Middletown, Pennsylvania 17057 P. O. Box 1323 Harrisburg, Pennsylvania 17120
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e t GPU Nuclear Corporation tuclear
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Middletown, Pennsylvania 17057 717 944-7621 TELEX 64-2386 Writer's Direct Dial Number:
March 31,1983 5211-83-017 i
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Office of Nuclear Reactor Regulation Attn:
D. G, Eisenhut, Director Division of' Licensing U. S. Nuclear Regulatory Ccmmission Washington, D.C.
20555
Dear Sir:
Three Mile Island Nuclear Station, Unit 1 (TMI-1)
Operating License No. pFR-50 l
Docket No. 50-289 RCS Trip on 25 Subcooling Margin In response to your letter of March 4, 1983, GPUN intends to revise RCP trip criterion from 1600 psig ESAS to 25 F indicated subcooling margin by June 1, 1983. discusses the basis for a reduction in subcooling from 500F to 25 F developed from plant specific error analyses. Enclosure 2 provides 0
the basis for RC pump trip on subcooling margin. addresses RC pump operation' criterion for normal, transient and accident conditions.
These changes are in compliance with the requirements of 10 CFR 50.46 and improvr$ the plant safety margins for certain non-LOCA events.
- Further, these changes do not involve a change to Technical Specifications or an unrevis ed safety question and are, therefore, being implemented under*
Sincerely, t
9
'H)D.Hukill Director, TMI-l HDH:LWH:vjf Enclosures cc:
R. C. Haynes J. F. Stolz N
Mt / u t:/ W' ~I u IW
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GPU Nuclear Corporation is a subsidiary of tne G=neral Puti;c Utihties Ccrporation
r Encicsure 1 25'F Subcooling Margin 1
e
===1.
Background===
Since.the accident at TMI-2 the NRC and utilities recognized the need to [
maintain ~ ~ adequate subcooling margin.
The NRC, through but1stins and NUREGs required each PWR Ligensee to assure that adequate subcooling margtn was maintained and in the long term to install a saturation margin monitor. Through coordl* nation with the utilities a minimum margin of 50*F was established. For B&W units, the 50*F was based on an actual margin of 5'F (which allows for the differences in loop temperature of the highest elevation and the point where the hot leg temperature is measured) with a. 45"F instrument error.
The 50*F subcooling margin became the determlM criterion for allowing throttilng.of HPl ll.
Anal ysis GPUN'has performed plant specific ca culations of instrument string error using the Root Mean Square (RMS) method f or subcooling margin at pr'escures greater than 300 psig. The calculations show that during normal and accident conditions (i.e., SB LOCA's), the maximum Instrument string erro-is -18.7'F (+ 21.7'F).
With the addition of the 5'F physical configurition margin, a. 25*F subcooling margin is justif, led. During a Large Break LOCA the subcooling margin monitor is not required for RC pump trip. Further, HPI operation criteria is noT dependent on the saturation margin monitor but rather LPI cperation.
In order to provide i
specific guidance to the operatcr, GPUN proposes that the 25'F subcooling margin monitor criteria be used and alarmed.
ill.
Discussion Since the fall of 1979 analyses performed by G?l:5 and Ei.W have demonstrated that liPI ~ initiation and throttling based on subcooling.argin is adequate to ensure subcocling during the three main events of interes:.
a.
Smali Break LOCA Events i
he reduction in subcooling margin to 25*F during SS LOCA events allows bette plant control during system recovery by allowing' HPI throttling l
sooner.. The lower subcooling margin allcws a broeder control band which j
permits. the operator to maintain conditlens with greater margins ~to possible' overcooling conditions.
The SB LOCA events are bounded by the analysis discussed-In Sectiom II (Containment Temperature - 245*F, 4 R).
Containment Pressure -<30 psig, RH - 100% anftose,-.5 x.1.0 SteamGeneratorTuj.RuptureEvents-b.
s l-
l 8
The prima y to secondary leak rate during single and multiple tube ruptures is a function of primary to secondary c1f f erential pressure.
The dif ferential pressure is minimized with, reduced subcooling margin and by j
primary depressurization operating the RC pumps. Figure 1 illustrates That by changing from a 50*F subcooling margin (with pumps of t) to a 25'F subcooling margin (with pumps on) a 50% reduction in the amount of RCS leakaDe is attainable. Reduced integrated leakage will, as a consequence, reduce the dose to individuals on and off site,
- c.. Overcooling Transients During overcooling events a reduced subcooling margin provides an increased operating band making it easier for the operator to stay within the, ~~
presssure temperature Iimits.
For all other transient-and accident conditions, there is no reduction in the saf ety margin or consequences of an accident as described in the FSAR.
Mitigation of LOCA, tube rupture end overcooling events is not dependent on subcooling margin as a signal to initiate automatic plant protection. Since HPl is not automatically initiated for any events analyzed in the FSAR besides LOCA, tube rupture and, overcooling events; HPI throttling does not affect the consequences as analyzed in the FSAR (See Table 1).
l IV.
Conclusion
~
Maintaining a minimum of 25*F Indicded subccoling margin assures that the RCS
~
is subcocied during normal, transient and accident conditions.. Therefore, HPl initiation and throttling may be based on 25'F subccoling margin without a reduction in p l ant saf ety.
Furthermore, a reduction in subcooling margin from 50'F to 25'F reduces the primary to secondary leak rate, integrated leakage, dosy rate and Integrated dose resulting from a tube rupture event enc, therefore, increases plant safety.
V.
References:
1.
tRC letter dated 23,1979 "l4eting Minutes fer October 12, 1979".
2.
Letter from Warren J. Hall to Darrell G. Eisenh;;t dated March 31, 1982, Question 1.13.
l l
Er.cicsure 2 RCP Trip I.
Background
i Based on~1nitial analyses received from PWR vendors, NRC concluded in NUREG 0623 that delayed trip of reactor coolants pumps during a small break LOCA can lead to predicted fuel cladding temperatures in excess of current IIcensing limits.
B&W examined what would happen if the reactor coolant pumps were tripped at some t!'me into'the accident when the system void fractica was high. They arbitra.rily assurwd that the pumps were tripped when the system vold fraction was 905 At' the time of pump trip the liquid that was previously dispersed around the primary evstem through pumping action now collapsec down to low points of the primary system such as the bottom of the vesse! and steam generators. This resulted in significant uncovery of the reacter core, due to an insufficient amount of liquid being available to provide acceptable core cooling. Due to design features as well as temporal behavior of system void fraction, B&W concluded that unacceptable consequences would result f rom delayed reacter coolant pump trip only for a range-of smal1 breaks (.025 to 0.25 f t') and'a range of trip delay times after accident initiation.
Based en these findings,'a meeting of utility vendors and ownurs was held with lGC in September 1979.
At this meeting it was agreed that the 1600 psis ESAS signal ;rrovided timely Control Room Indication for manual action to prevent possible voli: ling scenaries.
s 11.
Analysis in a recent B&W'g analysis, various RCP tripping schemes wers investigated.
They included saturation margin, coincidence ESAS/Tsat, low low pressure and vold fraction.
The saturation margin scheme i.a's chcsen by GPUN since this signal results in fewer pump trips during ove.cocling events and if The saturation margin is suitably low, in no pump trips during design, basis tube rupture events or very small break LOCA's. The reselts cf the analysis centered around a review of void fraction for pumps on/6.ff ccndiTiens.
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The above schematics ill usTrate that the system wit ! Initially be less voiced as well as at a lower pressure with pumps running.
These results cccur because the continued operation of the RC pumps keeps the coolant circulating throughout the loops providing better steam generetcr heat transf er and lower hot leg temperatures. The lower system pressure with the pumps running during the initial phases of the transient results in a decreased leak flow and thus a lower vol d fraction. However, the continuous cperation of The RC pumps results in lower quality discharge through the break wnich eventually of f sets the, decreased pressure ef f ect on the leak flows.
Thus, the fluid in the primary!
system ultimately evolves to a high void fraction for certain break sizes as n' result of the continued RC pump operation. The studies done by B&W show that the crossover point between the pumps on and off cases occurs at 40% system vold frpetion independent of the break size.
. ~.
Loss of subcooling margin in the hot leg occurs well before the RCS void fraction can become large enough to threaten core. uncovery. Therefore, the use of the subcooling margin is an acceptable alternative to 1600 psig ESAS.
Ill.
Discussion Since the f all of 1979, analyses per' formed ty GFUN anc ELW have demcnstrated that Reactor Coolant Pump trip on subcooling margin acccmpt ishes the erigingi objective of tripping RCP's without-decreasing safety margin fer the three main events of interest.
a.
Small Break LOCA' The change in parameters from low pressure ESAS actuation to loss of subcooling margin acccmplishes, the original c:JecTive of tripping the RCFs for small break LOCAs.
For the break sizes of ccncern, a less <cf 25'F subcooling,, margin will occur slightly before low RCS pressere actuation of ESAS. The cperator nas essentially the same time to trip the pumps at the onset of saturation as with the current ESAS actuation criteria. A review of the SB LOCA analysis indicates that the basis f r RC? trip was 20% void fraction.
b.
Steam Generator Tube Rupture Events Curing Stear. Generator Tube Ruptures in which min;. r subccolinc marcin is
~
maintained, continuous RC pump cperatien assures ex;eciTic:.s cecic:
>i n T
a minimum primary To seconcary dif f ererT's' ;ress.re.
This chance in
~
criteria ter RCP Trip will allow RCF's T: ;e ::e a sc 'cr a g eater spectrum cf Tube re;Tures (including ru; Tares beycnd T..e cesign tasis) anc to reduce the offsite doses for those events, c.
Overcooling Transients Fcr overcooling events in which the pressu-1:er dcas r.ct emety, Tne j
subcooling margin does not drop below 25'F even thca;;h RCS pressure is below the ESAS initation setpoint (160C psig).
When ecmsined witn our proposed RCP trip criteria, the RCP's vill remain cpPaTional, Thereby precluding void formation in the hot legs an: minimi:ing formation enc duration of voics in the reacter vessel heac.
Furthermere, RCP cperation allows continued use of tain Feedwater in lieu cf E ergency Feec=ater anc
. - 4, 9
the use of pressurizer spray to control RCS pressure during recovery frem the overcooling transient.
Table 1 illustrates thaT the 25'F-subccoling margin is not lost for any event in which f orcec RO flow is requirec for event mitigation. Therefore, the operato wculd nct be forced to terminate forced flow when it had previously been Taken credit for in FSAR analysis.
IV.
Alarm in addition, a Control Room alarm will be adjusted to annunciate if either subcoolIng margin monitor l'ndicates less than 25'F.
The plant computer also' independently computes pressure and temperature sataratien margin for logging, trcndIng and alarm.
.~
V.'
bbnclusion The change in RC-pump ' trip f rem low pressure ESAS actuation to loss of subcooling margin:
ensures RC' pumps are turned of f when required which oassures no decrease in safety.
permits RC pump oper'ation for seme evercooling events o
which increases plant control.
permits RC pump opetrrrion for mere tace re;ture events o(including those beyond design basis) which increases safety by decreasing leakage and expediting plant cccidown.-
VI.
Reference 1.
" Eval uation of Transient Behavior for Small Reactor Ccolant System Ereaks in 177 FA Plants", dated May 7,1979.
J l
e 2
- t
I
- TABLE 1 HPl Forced Flow 25'F S014 Accidont or For Event Assumed in Reache:i During Mitigation FSAR Event i
Transient ao
' ~~
Rod Withdraw ~al -
Power No Yes.
No Startup
.No Yes No Boron DIiution No Yas No Cold Water Addition No Yes
- ./A Loss of Flow No No N/A l
Dropped Rod No Yas Sc Loss of Elec. Load No
' No N/A Eleckout Yes No N/A Steamline Failure Yes See Oiscuss.
(Enc.*,Ill.c)
Tube Rupture
.Yes Yes Fuel Handling No N/A
/A R:d Ejection Yes Yes.
'as f
Feed ater Line Ereak No.
No
- .c i.asTe Gas Cecay Tank Rupture No N/A SS LOCA-Yes See Discuss.
See 21sc ss.
Large Ersak LOCA No No Yes
- Analyzed with and without RCP cperation.
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7IGUKI 1
iEffect of EC Purdp T:!p 0:fusrfon on integrated Systs:n Lesicage for Sir.gf e Ruptured Tub e 180 l, L
i 1
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1: 25.: SC, T JMPS ON 11: 50*F SC, PUMPS ON
~, _~j Ill: 50"F SC. PUMPS OFF
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RC Pump Operating Criteria 1
l.
RCP Operations - Trip A.
Set, points for RCP Trip
, ~.
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1.
Forced Flow Table 1 of enclosure 2 provides an accident / transient su=ary entrix for which forced flow is assumed.
Note, specifically, that RCS flow is assumed for design basis, tube ruptures and that fo a 25'F subcooling cargin Trip setpcint RCP trip q
does not occur. Further, tripping RC pumps on 25'F subeccling margin reduces trips for non LOCA events for which continued RC ;urp c;eration is desiratie.,
i 2.
Volded System By tripping pumps on 25'F subcoct'rsg margin, cperaticn cf C purps in a voiced system is precluded.
3.
PORY Challenges
-T141-1 wlll use a~ manual operator RCP trip which avolcs inadvertent trips by
{
automatic systems.
Experience in The utility incustry since the Ti41 ~ accident has shown that inadvertent op'erator initiation has a very low probabi!!ty of Tripping RCP's on 25'F subcociing ma gin recuces s;uricus RCP occurrence.
trips, thereby reducing challenges to the PCRV f er non-LCCA events since pressurizer sprays is available to control RCS press.;re.
4 Hot Stagnant Fl uids The addition of 5'F margin to the Instrument e r:r calcu:at::. su-ar;:sc i.
enc!csure 1 of This letter provides suf ficient ear.i-prec:ece set.rati n conditions in the RCS cue to physical ic etlen c' T.e sensers. Curren plaat 3
cr anc e.c.a! c emergency prccedures discuss operatien in a v !:e:
voids that result from flashing. Operator tra'nt ; s;e:if:cally accresses'These procedures.
5.
RCP Services As described in section 2.1.1.5 of The TI51-1 Restar; Repert injection water te the RC pumps is capable of being restored fc!!ce.ing ::niai..enT isciatien.
In is Ics, tne injecTicn water
,Qhe event that the cooling water to the pumps prev i desnadequate cool i ng f or t he seals an: the p.:r; can be Operated indefinitely if The seal Injection is funcTicn!ng n:rra::y.
- 6,
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6.
ICC Instrumentation The saturation margin monitor which is part of the ICC instrumentation system will be used for indication of 25'F subcooling margin fer RCP trip.
B.
Justification for Manual RCP trip 1.
Conformance with 10CFR 50.46 A generic analysis has been performed by S&W which envelops TMi-1.
This; analysis is sumarized in Section iI of enclosure 1.
l 2.
Most Probable Best Estimate S}nce t are is no substantial change in time to trip RCP's for the worst case SB LOCA under tne 1600 psig criteria or the 25'F subcooli.ng margin and in the time 4
for the operator; to respond to the initiating signal, this analysis is not considered necessary.
4 C.
Other Considerations 1.
Instrumentation The saturation margin monitor and alarm ace addressed in secticn 2.1.16 cf the TMI-1 Restart Report. This Instrumentation wil l ::e u; graced to saf ety grade during the Cycle 6 refueling.
2.
Pump Restart Small break LOCA and.other emergency procedures will be updated to reestabi tsh the RCP restart criteria from 50*F fc 25'F.
3.
Training Training on RCP operation during transients and accicents is Integra! tc the training program on emergency procedures which incl. des simulater tral.* sing.
~
11.
Sumary of 10CFR 50.59 Evaluation 1.
The probability of occurrence er the cc.secus ces cf an accicent er mal function of equipment - impcrtant to saf e y 0 es t c:.s i y eva.l eated in the Safety Analysis Report has not'ir.c eased.
Enclosures ar.d 2 provide a summary analysis cf tnese e.e Ts
-f Ocr.cern anc Tatie
- address alI accidents eval ueTed in ne 75AR Cnapter 14.
In fact, by tripping RC Pumps on 25'F subcooling =argin, greater centrol and reduced consequences occur.
No additicnal e JIp5ent is acded by this change nor affected which would lead to a tal fur.ction.
2.
The possibility for en accident er malfunctica cf a dif ferer.t type than any evaluated previously in the Safety Analysis Report is nc?
created.
All credible events analyzed are er.veicpeo by existing FSAR events and no new unanalyzed events are creaTec.
i 3.
The margin of safety as defined in :ne basis of any Technical Specification is not reduced.
RCP trip a c pressure tempersrure limits t.
+
. o
' o 4
1 Tech $pec bases have been caref ufly evaluatec as Indicated i n Enclosures 1 and 2 with no resultir.g redetica in safety margin.
For certain events the safety margin bas, been increased.
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