ML19256F853
| ML19256F853 | |
| Person / Time | |
|---|---|
| Site: | Crystal River, Bellefonte, Crane  |
| Issue date: | 10/25/1979 |
| From: | Office of Nuclear Reactor Regulation |
| To: | |
| Shared Package | |
| ML19256F850 | List: |
| References | |
| NUDOCS 7912260346 | |
| Download: ML19256F853 (8) | |
Text
. Primary System Pertur a I.
Introduction-B&k' plants empioy a once through st.c generator (OI5G) design, rather than U-tube steam generators which are used in other pressurized water Each steam generator nas approxima.aly 15,000 vertical straight reactors.
603-505"F tnd exiting
' tubes, with the primary coolant entering the top at the bottom at about 555 F.
Primary coolant flows down inside the steam generator tubes, while the secondary coolant flows up from the bottom on The secondary coolant turns to. steam about the shell side of the OTSG, the remeining lengtn of tne steam generat:r being used to half way c:, with supernest tne s. ear.
in the stear spa e of the OT5G, Tne secondary-side heat transfer coefficient, Inis resf.~.s in a heat is mg:n less than tnat in the botto liquid section.
transfer rate from the primary syste; which is quite ser.sitive to the liquid If a fee: water in:rease aver. o::urs, the levei in the stear generators.
This liquid-vapor interf ace rises, increasing the overall neat transfer.
decreases the outlet temperature below 555 F and initia.es an over:coling C
By contrast, event, which can lead to primary syster cecressurizatier..
if a feedwater decrease event o::urs, the overall hea transfer decreases, the outlet primary temperature increases, and a pressurization transient ensues.
In either of tnese cases, the res;0nse cf tne primary syste: pressure and,
- ressuri:er level
.c a chan;e in mais fee:.ater fio ra e (or tempera ure)
Innse ra;i" primary sys".er -'*ess#e :.anpes due
- .o is om:aratively ra:ic.
Si'"e i. Es sys te" "s en.siti vi ty" and is
- rtr, pes ir, fee:-7.te- ;cnditi rs s t on-1638 293 D**
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. unique to the B&W OTSG design.
Following the incident at Three Mile Island, various a:tions were taken to increase the reliability of the auxil#ary feedwater systems and improve plant System modifications to increase ne reliability of the A?W transient response.
However, use of AFW results may have resulted in more frequent AFW initiation.
in introduction of cold (100 F vs. 400 F) feedwater into the mo This may act to enhance system sensitivity.
upper section of the steam generators.
Further system modifications provide control-grade reactor trips based on While se:ondary syste. malfun:: ions, such as turbine or feecwater pump trip.
these reactor trips do serve to reduce under:coling feedwater transients by redu:ing reactor power promptly followin; !.0MFW, they may amplify subsequ over:coling.
A reexamination was made of small break and loss of feedwa This resulted in a modificafion of operator procedures for dealing plants.
with a small break, which include prompt RCP trip and raising the water le B:th these in the steam generators to (95%) to prom:te naturai circulation.
actions are taken when a prescribed low pressure set point is reached in the rea: tor coolant system and for antic.ipated transients such as loss of feedw these actions may amplify undesirable primary system responses.
In at:ition to the pest-TMI changes dis:essed above, a:tions were also tak
-educe tne :hailenges to tne power c:erated relief valve (PORV) by rai Knile these point and lowerin; the higr cressure rea: tor trip.
tne F3RV se:
a::i:r.s r, ave neer, su::essful i reducin; tne frecuen:y of PGRV operation, they j
i a w %
have resulted in an increased number of reactor trips.
the reac'cr will' n'ow trip for transients it previously would have ridden through by ICS and' PORV cperation.
5 While The staff is concerned by the inherent responsiveness of B&W OT5G desi n.
some specific instances are presented in the next section of this paper, the s It is felt that good design practice concerns are also of a general nature.
and maintenance of the defense-in-depth concept, requires a stable well-behave To a large part, meticulous operator attention and prompt canual action sys tem.-
is used' on these plants to compensate for the system sensitivity, rather than any inherent design features.
The staff believes that the general stability of the B&W piant control systems OTSG feedwater perturbations be should be improved, and that plant response t:
dampened.
II.
Re:ent Feedwa er Transients the staff met with the B&k licensees to discuss recent 0n August 23, 1979 One aspect which is of interest is the relationship of feecwater transients.
In at least one the operator.to the functioning of the main feedwater system.
instance an operator manually opened.a block valve in series with a control va This resulted in an overfeed condition.
(partly open but thought to be closed).
In several re:ent events the feed flow was reduced to the point where the Subsecuent overfeed reduced pressure to below rea:: r tripped on hign pressure.
is:: :si, wnere HPI was initia.ed, rea:. r :::iant pumps tripped, and auxiliary
- ne ::p cf ne stear generators, which in:reased f ee: ate-flow introduced int:
ne severi y of the :::1:own transient.
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It appears that in many cases the main feedwater control system
~
i nts.
quickly enough or is not sufficiently stable to meet feedwater requ r
- ditions, Rather, the system will often oscillate fro: underfeed to overfeed cen i i One causing a reactor trip and sometimes a high pressure injecticn in transients und'esirable element of this lack of stability is that overcooling (decrease in on.the primary side proceed very much like a small break LO~A Thus, for a certain period of time the operators pressurizer level and pressure).
i nt. The same may not know whether tney are having a LOCA or an overcool ng ev 1 'sys tem.
This type of behavior can be initiated by the n:rmal reactor contro failure of a control-
.was demonstrated by a December.1975 event at 0:: nee, where ESF actuation.
re: order led to reactor trip, a feedwater transient, anc grade T,yg d in the A partial list of recent 5&W transients and their effects is conta Appendix to tnis report.
Dressuri:er Level indicator _
Role of the III.
A major area of concern arising from the B&W OT5G sensitivi y Several 5&W feedwater transients have led cf pressurizer level indication.
Most no.atbie was a November 1977 incide loss of pressurizer level indication.
The arrival at Davis Sesse where level indication was lost for sev tor year, but rate for this event appears to be on _the order of.1.2 per reac i
d feed-could be on the in:rease due to the potential for more reactor tr p fications.
This is of water transients resulting from post-TMI-2 system modi thereby creatin; concern because an over:coling event could empty the pressuri le; wnich may interrupt tne p tential for forming a steam bubble in the het Tne staff feels that RCP tri; :n low :ressure.
natural cir:uiation, followin:
- nase naturai circulation are somewhat One un:ertainties asso:iated with tw:
nign for an event with a re:urren0e interVaI :f a few years.
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Additionally, the staff believes that good design practice and adherence to, the defense-in-depth concept,'would require that plant operators be aware of A lW-level off-scale reading the reactor's status during expected transients.
on pressurizer level makes it impossible for the operators to assess system inventory and more difficult to differentiate between an accident and an The staff feels that the frequency with which this excessive cooldown transient.
1 situation occurs is undesirable.
Some concerns also exist with regard to the operation of the 'pressurizet heate Nonsafety grade control circuitry trips the when loss of level takes place.
If these n:nsafety grade cutoffs heaters off when pressurizer level is low.
This situation has should fail, the heaters would be kept on while uncovered.
the f ailure point, as happened the potential of overheating the pressurizer ::
with a test reactor at Idaho Falls.
IV.
Role of ICS-MFW_
The ICS appears to paly a significant role in the plant's feedwater response.
~
However, review of The staff is currently reviewing an FMEA study on the ICS.
operating experience suggests that the ICS often is a contributor to feedwate In some cases the ICS appeared inadequate to provide sufficient transients.
Some of the utility des:riptions of feedwater plant control and stability.
23, 1979) transients (as sumnarized in the minutes of a meeting on August The following emphasized the role of the operator in operating the M W syste=.
se:uen:e illustrates the type of event and syster response which the staff feel
- uld p tentially o::ur, l.
F.eactor at 100% power.
F,ea: tor trip, from aroitrary cause (does n: matter).
2.
Plant stabili:es in hot shutcoan, f:r a few minutes, heat reje: tion 3.
0; c
?n%" ( artd/ "
n a*V dur.: Vaives).
163h 2hf
w, pressurizer level shrinks, pressure reaches.160 psi, P.5 actuates; RCP tripped; AFW on.
(Possible RCP seal failure).
Operator manually controls AFW (possibly MFW instead or in addition, if 5.
MFW n': isolated such that OT5G level comes up to 95% of operating range.
This massive addition of cold water may lead to emptying of pressurizer and interruption of natural circulation (or, the het leg cay flash due to depressurization and interrupt natural circulation even if pressurizer
.does not empty).
transfer in OT5G; vaper'from core HPI delivers cold water,' no hea:
c.
leads to system repressuri:ation; steam may condense or PORY may lift.
No pump restart criteria available, circulation may not be reestablished.
7.
I-t appears th.at an upgraded safety quality ICS, which is designed to balance power to OTSG 1evel in a better f ashion, could reduce the sensitivity, illustrated in the above sequence.
Y, Role cf 5CCS and Auxiliary Feedwater it is known that some feedwater transients result in over:coling to the exten Traditionally, the operator isolates that the HPI a:tuation se point is rea:hed.
4 letdown and turns on an extra makeup pump f:llowing trip so as to avert this If this manual action is not perfomed cuickly enough, or if the actuation.
' severe, the HPI set point will be reached and the pum; cooldown transient is too Following pro:edures, the operator would then trip all ma automatically started.
based on the plant sympt:rns.
If
- c:iant pumps and utilize recovery procedures a small LOCA, he would then
- ne in:iden; was a:tually a feedwater even; and no:
When pressure has
- esu aciy go to tne loss of for:ed :ir:ula:icn cro:edures.
0
- ne coolant syster nas be: cme 5: F subcooled, the operat:r e::vered su:n :na:
One procler is the diffi:uity in cifferentiating between a s=a',
- ar secure HFI.
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The ::erator r:uld be forced break LOCA and an. excessive feedwater transient.
However, following the small to asse e a small 'LO".A until proven otherwise.
break procedures a'nd introdu:ing :old auxiliary fee:='ater, :ey increase the severity of an over:coling event.
Initiation f A. a' a.: delive y to the 05TG, r
especially if accompanied by filling to the hi;h ieve; re;; ired by new pro-Thus, the AFw cedures (95:;) will continue the cooldown and depressuri:ation.
system acts to increase the responsiveness of the rea.or to feedwater transients where excessive cooldown is o: urring.
VI.
Con:iusions The staff believes that the current B&W plants are overly res::nsive to feedwater transients be:ause of the OT5G design, :ressari:er si:in; and 50me of tne sensitivity also arises from PORY and high pressure trip set point.
inacequa:ies in the ICS to deal with expe:ted plant :er u-:itions.
Regardiess of the reasons, B&W plants are currently experie :ing a number The staff of feedwater transients which the staff feels are undesirable.
- plant sensitivity
-='"-a believes that modifications should be considerad ts nese events and thereby improve the defe.se-in-cap:. which will enhan:e tne safety of the plant.
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APPL.flDIX FLCilWAltil lllAf151[flT StittilARY DESClllPTI0li lilAfiSIEill DAIE I' AC II. I l Y h Pressure - 4 to 3 RCP, A-5/G underred'-72%
0/16/79 (0259 Reactor Trip on lil9 fleactor Trip on liigli Pressure - 3 IICP - A-S/G underfed - 45% P Cit-3 0/16/79 (1125) lleactor Trip on liigh Pressure
.3 RCP - A-S/G unde ~rfed - 40%Pw 0/17/79 (0706) lleactor Trip on liigh Pressure - 3 RCP - A S/G underfed - 26%
8/17/79 (1825)
Ileactor Trip on Low-Low level in both S/G - 10% Pwr.
8/02/79 (0202)
Turbine Trip - Antic.. Trip did not. work - Rx Trip on lli Press 0/13/79 (1749)
Arlo-1 lleactor Trip on Anti. Trip (L0fW) - 99% Pwr.
6/11/79 (0333)
Oconee-1 Reactor flanually Tripped when FWPT "10" Tripped 6/11/79(0752)
Itcactor 1 rip on liigh Pressure - feedwater oscillations - 10%
5/07/79 (0346)
Ileactor Trip on liigh Pressure - feedwater osclliations - 30%
Oconce-2 6/03/79 (2046)
Reactor Trip on Antic. Trip (L0fW) - 100% Pwr.
7/12/79 (1714)
Itantho Sci.o fl0fl0
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