ML20058A810
| ML20058A810 | |
| Person / Time | |
|---|---|
| Site: | Crane  |
| Issue date: | 07/09/1982 |
| From: | Novak T Office of Nuclear Reactor Regulation |
| To: | NRC ATOMIC SAFETY & LICENSING APPEAL PANEL (ASLAP) |
| References | |
| TASK-AS, TASK-BN-82-65 BN--82-65, BN-82-65, NUDOCS 8207220034 | |
| Download: ML20058A810 (5) | |
Text
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( {) ' b [ h 1 dVL 0 91982 q
[.DISTRIBUTI0ft:Dockjt Filed DEisenhut RIngram PPAS NRC PDR Memo File RJacobs MWilliams File.
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Docket No. 50-289 JStolz RMattson ACRS OELD RHartfield, MPA MEMORANDUM FOR: Atomic Safety and Licensing Appeal Board for Three Mile-Island-l FROM:
Thomas M. Novak, Assistant Director for Operating Reactors, Division of Licensing, NRR
SUBJECT:
BOARD NOTIFICATION (BN-82-65) - Three Mile Island-l The NRR staff has recently received B&W small break LOCA calculations perfomed by the Los Alamos National Laboratory (Enclosure 1). Thege calculations indicate that for a certain small break size (.0129 ft ),
natural circulation was not restored during the long term recovery phase of the event. The NRC staff does not believe that the analyses indicate an immediate sr.fety pmblem since the core was calculated to be covered by water and adequately cooled. A similar primary system response has been pmviously noted in B&W break spectrum analyses for large breaks and small breaks which do not repressurize the reactor system.
This infomation is being provided to the Three Mile Island-l'ASLAB, since natural circulation, void fomation and small break LOCA analysis were among the issues on theThree Mile Island-l hearing records; and it represents new information related to the NRC staff analysis of post accident long term cooling following small breaks in the primary system.
No new safety issues are raised by this information and it does not change NRC positions taken in testimony on safety evaluations.
It is being transmitted to the boards and parties as supporting information relating to the natural circulation mode of heat removal in post accident long term core cooling.
The NRC staff has completed a preliminary analyses of this information but NRR has not conducted a detailed evaluation of the TRAC code.
Enclosed to this notification is a copy of the draft LANL report entitled "Small-break LOCA Recovery in Babcock & Wilcox Plants". We have also enclosed pre-liminary NRC staff comments on the draft report as Enclosure 2.
We have previously addressed these analyses in our recent response to Dr. Henry
$ers of the Subcommittee on Energy and the Environment and indicated that despite the absence of natural circulation flow, core cooling was Our 1.etter maintained by HPI injection and internal vent valve circulation.
to Dr. %ers addressing these analyses is provided as Enclosure 3.
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Thomas M. Novak, Assistant Director 8207220034 820709 PDR ADOCK 05000289 for Operating Reactors P
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NRC FORM 3tS (10-80) NRCM OMO OFFICIAL RECORD COPY esa,o.,m_w,..
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i
- g NUCLEAR REGULATORY COMMISSION g)g]g/l WASHINGTON, D. C. 20555 JJL O9 ES2
\\,j Docket No 50-289
\\
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MEMORANDUM FOR: Atomic Safety and Licensing Appeal Board for Three Mile-Island-1
\\
Thomas M. Novak, Assistant Director for Operating FROM:
3 1
Reactors, Division of Licensing, NRR
SUBJECT:
BOARD NOTIFICATION (BN-82-65) - Three Mile Island-l The NRR staff has recently received B&W small break LOCA calculations performed by the Losl Alamos Nstional Laboratory (Enclosure-1). Thege calculations indicate"that for a certain small break size (.0129 ft ),
natural circulation wasinot restored during the long term recovery phase of the event. The NRC staff does not belisve that the analyses indicate an immediate safety problem since the core was calculated to be covered by water and adequately cooled. A similar primary system response has been pmviously noted in B&W break spectrum analyses for large breaks and small breaks which do not repressurize the reactor system.
This infomation is being pmvided to the,Three Mile Island-l'ASLAB, since natural circulation, void fomation and small break LOCA analysis were among the issues on theThree Mile' Island-l hearing records; and it represents new infomation related to the NRC staff analysis of post accident long term cooling following small breaks in the primary system.
No new safety issues are raised by this information and it does not change NRC positions taken in testimony on safety evaluations.
It is being transmitted to the boards and parties as supporting infomation relating to the natural circulation mode of heat removal in post accident long term core cooling.
The NRC staff has compl'eted a preliminary analyses of this information but NRR has not conducted a detailed evaluation of the TRAC code.
Enclosed to this notification is a copy of the draft LANL report entitled "Small-break LOCA Recovery in Sabcock & Wilcox Plants". We have also enclosed pre-lininary NRC staff comments on the draft report as, Enclosure 2.
We have previously addressed these analyses in our recent response to Dr. Henry Myers of the Subcommittee on Energy and the Environment and indicated that despite the absence of natural circulation flow,' core cooling was maintained by HPI injection and internal vent valve circulation. Our letter to Dr. $ers addressing these analyses is provided as Enclosure 3.
j l
/
)
W Thomas M. Novak, Assistant Director for Operating Reactors Division of Licensing Enclosures :
As Stated cc w/ enclosures: See next page
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e
y l-ENCLOSURE 1
~
SMALL-BREAX LOCA RECOVERY IN BABC0CX & WILCOX PLANTS by N. S. DeMuth I.
INTRODUCTION Analyse s of loss-o f-cool ant transients smay cold-leg breaks in Babcock and Wilcox (B&W) plants indicate tha a r circulation in the primary loops may be interrupted by steam accumu t 9 in the " candy canes" (hot-leg U-bend).
The purpose of these analyses was evaluate alternative strategies for recovery from small
-aak loss-of-coolant accidents (LOCA) and I
for reestablishing natural-c.tulatio Calculations of a small-break os fo t accident were perfonned with the TRAC computer code, version F,
for a 0.0445-m (1.75-i n. )-di a:n.
cold-leg break.
The break size for hs study' was selected to provide conditions for loss o circulatio ooliag, while the primary pressure o
remained relatively gh (
bye the setpoint for accumulator injection).
Figure 1 shows a TRAC
_ diaoram for the B&W lowred-loop model based on owuee 7
the BB2 design.
Loop presents the loop with the col'd-leg break and includes t a leg with t ressurizer (labeled PRIZER in the figure)y th steam
- ator, tw cold legs--one intact and one with the 0.00155-m 2
(0.01 t ) break.
Each loop-A cold leg includes a loop seal, a pump, and a conne 1 for
$ high-pressure injection system (HPIS).
The reactor coolant pum r
deled using the LOFT pump characteristics built into TRAC m e e.
but scaled wit -
pump data. The break is located in one cold leg between the HPIS connection and the vessel.
Loop B represents the unbroken loop and is similar to loop Iexcept that there is no break or pressurizer and the two cold legs are combined to increase calculational efficiency.
The small break in the cold leg was modeled using a TEE component (No. 35 in Fig. 1).
A BREAX component at atmospheric pressure was attached to the end of the side tube, and the flow out the break was computed using the critical flow model in the released version (Mod 0) of TRAC-PF1.
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,The secondary side of each steam generator is attached to the mair, feedwa,ter inlet, the auxiliary feedwater inlet, and a long pipe to the steam outlet with a side connection to a. safety valve that vents to the atmosphere.
We represented the safety valve with a FILL component in Witch the mass flow out is controlled by the steam pressure.
Geometry and other t data were obtained from the Oconee Final Safety Analysis Report 3 (FS d other data sources for the Oconee-1 plant.
The model for the auxilia fee
+er system allows flow to enter near the top of the steam generators, and t low of auxiliary feedwater is controlled based on the water level in the secon es.
The vessel ms modeled with four azi al
- segments, tw radial segments, and seven levels.
The seven levele clude M 1cwer plenum, three active core levels, tw levels in the upper p1 ermit the vent valves to be above the hot-and cold-leg connections, an upper head.
The model includes connecti,ons from the upper head to each leg to simulate the upper-head circulation observed by in their flow tes s.
These connections are needed because TRAC, with only asf al ring in the upper plenum, models poorly the flow up out of th h the center of the upper plenum, into the upper head with turni utward, and back down into the upper plenum.
Yent valves connections r accurIulator injection into the
~
downcomer are modele 6 of the y el.
l The response he p t following a small-break LOCA was simulated to evaluate the effecti s d the following operator actions in restoring 7
natural circulation cool (1) cooling and depressurizing by venting steam from the enerator daries, (2) " bumping" the main. coolant pumps, (3) ve. g steao om the upper elevations of the hot legs (candy canes) and (4)
-ting a po ion of the flow from the HpIS into the candy canes.
The 1
1 e ffects differf a cooldown rates on recovery of natural circulation flows w re asses i
veral calculations.
In the following section, w will I
analyze the p t response for three scenarios:
(1) no operator-initiated cooldown; (2) operator-initiated cooldown without injection from the core flood tanks (accumulators);
and (3) operator-initiated cooldown with accumulator injection.
i, /
g, II. - TRAC CALCULATIONS OF SMALL-BREAX LOCA RECOVERY A.
Small-Break LOCA Mthout 9perator-[nitiated booldown
'We modeled this transient starting from a steady-state powr level of 2568 MW with the break assumed to occur at the beginning of the transient. At 60.6 s the reactor and main feedwater pumps wre ped on a
low-primary-pressure signal of 13.1 MPa (1900 psia).
The re.
was by an insertion of negative reactivity (-0.0535 hk/k),
d subsequent powr level was computed from the coupled reactor kinetics and ay heat formulations in TRAC.
The reactor kinetics equ tions included re ivity feedback from fuel and c:oderator temperature ch s.
Injection of auxiliary feedwater into the upper regions of the stea,.
nerat began 30 s after the reactor and main feedwater pumps wre tripped.
The emergency core cooling system (ECCS).
ctivated at 81.5 s by a low primary pressure of 11.135 MPa (1615 psia), and HPIS began delivering flow 35 s later'(at 116.5 s).
reactor coolant umps wre tripped at 3
131.5 s (50 s after activation of th As shown in Fig. 2, the primar r
reased rapidly until 150 s, at sich time the HPIS began deliverin out 50 kg/s of cold (305.4 X) water to the cold legs.
Thereafter, the pr. ry, pressure decreased slowly as the flow of heated wate e break wa removing more heat than was being generated, and th wr r duced by fission product decay was decreasing.
For this calculation, om the HPIS was divided equally between loops A and B, and the loop-A snow [in Fig. 3, was divided equally betwen the split cold The aux 11 feedwater refilled the steam generators to 507, of the
...a 1 ting range in about 10 min. and was controlled to maintain this 1 vel for th emainder of the transient.
The auxiliary feedwater flow to both am gene a! ors is shown-in Fig. 4.
The r
.tions in the top of the hot legs are shown in Fig. 5.
At about '500 s the transient, the loop-A candy cane began to empty of l
liquid, and by 800 s, it was filled with vapor.
By 2300 s the upper i
elevations of the loop-B hot leg wre filled with saturated vapor, and. natural circulation flow in this loop stopped.
The flows in the loop-A and -B cold legs are shown in Fig. 6.
After natural circulation stops in the loop-A cold leg (at 500 s), the flow in the split cold legs (Al and A2) is in opposite directions, even though the loop seals wre modeled individually.
Natural -
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High-Pressure-Injection flow for Loop-A split old legs. --,--
so (s0 LID) LOOP-A AUX F/W 25-(oAsa) Loop-a Aux r/W
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Auxiliary feedwater fl w rolled on the 50% level.
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l Vapor fractions in the upper elevations of the hot legs..
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Cold-leg flows xits.
circulation flows in the loop-B cold ceasedu(at 2100 s) when the candy e
cane on that loop fill steam.
Cessation of ural
((onvection accompanied by the loss of pressure di fferential between comer and the upper plenum allowd the vent valves in the vessel
-+ 7150 s, as shown in Fig. 7.
With the vent valves open, the vessel a our elevations of the hot legs were virtually isolate team gener tors, and the core was being cooled by flow from the W11ch ed with steam flowing through the vent valves into the downc.
l The gra flow out the break is compared in Fig. 8 with the total inflow from IS, and Fig. 9 shows the' net mass loss (the difference betwen the break flow and HPIS inflow).
For the first 75 min of the transient, the primary was slowly emptying, as the break flow exceeded the inflow from the HPIS.V After about 75 min, the HpIS flow nearly equals the break flow -- W111 cutt # attributes this to the increase in " void fraction of
+
the f,1uid reaching the break".
Throughout the transient the fluid entering the break contained a mixture of vapor and subcooled liquid.
The vapor fraction of this mixture varied greatly depending upon the direction of flow in the split cold leg (A2), and this markedly influenced the mass flow out the 7
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Vapor velocity through vent flow from upper plenum to downcomer is positive).
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Fig. 8.
Comparison of break and HPIS flows.
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u break.
When flow we
. inately fr. the vessel (downcomer) toward the break, the vapor etion as relative y large and the liquid subcooling small, so that flo t
break was reduced.
When the flow us largely subcooled liquid from e"r' < ana HPI inlet) toward the vessel, the fluid entering the break had a vapor fraction, and the mass flow out the break increas this tr sient the core remained covere'd witt ifquid (Fig.
and fuel-rod temperatures remaincJ below their st:4dy-state value 9 11).
l The ults this analysis with version PF1 of TRAC compared favorably with results ed with version PD2 and reported in Ref. 4.
The transients l
di ffered in initial and boundary conditions and in the setpoints for actuating safety systems.
We modeled a small brgak of 0.0343-m (1.75-in) equivalent diameter, Wiile the transient of Ref. A had a break of 0.0343-m (1.35-in) equivalent diameter. An investigation of the critical-flow model in the released version (Mod 0) of TRAC-PF1 has shown that this model under-predicts the mass flux at the break by about 30%, so that the break flow in the TRAC-PF1 calculations corresponds to an effective diameter of 0.039 m (1.54 in).
Other differences in primary response characteristics may be '
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Fig. 11.
Maximum average fuel rod temperature.
i a+tributed to differences in trip setpoints and boundary conditions, such as the distribution of HPIS flow among the cold legs.
To test the effectiveness of operator actions to cool and depressurize the plant, the calculations were restarted with the steam ge tors being
~
cooled slowly by venting steam from the relief or dump valves wo scenarios wre considered.
In the first, the operator isolates the
+ ors before starting cooldown, and in the second scenario the accumulators beg jecting den the primary pressure drops below the accumulator pressure.
B.
Small-Break LOCA Recovery Without Accumulator / Objection The small-break LOCA calculations descri '
in e preceding section wre restarted at 3480 s and now included o tor ions to increase the steam generator levels from 50 to SE% of the operating range.
This level increase required about 500 s, during dich the steam-generator secondaries wre cooled by the inflow of auxiliary feed r.
After the steam generators filled to 95% of the no Sqating range, venting of steam from the atmospheric relief valves began.
he se flow was controlled by the pressure in the steam generator second ex such that at 6 MPa the steam flow was about 1% (8 kg/s) of the full-p r flow 1778 kg/s).
As the steam generator secondaries the steam w us reduced to 4 kg/s at 2 MPa and 1 kg/s at 1 MP The xiliary feedwater flow, shown in Fig.12, was controlled to maintai e
level until 3480 s and the 95% level thereafter.
When the calculat rr 2 -aMarted at 3480 s, the upper elevations of the primary, including the ssurizer, the upper plenum, the hot-leg candy canes a ermost cel s in the steam generators, wre filled with ste a..
igures 1 d 14 show the vapor fractions in cells 4, 5 and 6 of the loop-A loop-B 0 am generators, respectively.
At 3480 s, cell 6 in the loop-A s gene or has a vapor fraction of 0.2, dereas in the loop-B o
steam genera 11 6 is filled with vapor. As shown in Fig.1, cell 6 is the highest cel in the steam generators from dich heat can be transferred to the fluid in the secondary.
During filling to the 95% level, the vapor fractions in the upper cells of the primary increased.
Howyer, after secondary cooling by discharging steam started, the primary-liquid level rose to fill cell 6 in both steam generators.
Thereafter, the transient increases in vapor fraction resulted in intemittent fluctuations in the primary liquid
{
1evel.
These fluctuations could be caused by the wak coupling betwen subcooled primary itquid in the steam generators and the saturated steam / water --
s.
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Fig. 13.
Vapor fractions in the loop-A steam generator primary.
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14.
Vapor fractions in lo generator primary.
mixture in the vessel.
As the steam erators cooled, the density of the primary liquid increased, and the heav e liquid uintemittently settled into the cold legs thereby saturated id from the hot legs.
The vapor fr - ions d the candy canes are shown in Fig.15.
During filling and cooling steam generators, slugs of hot fluid wre drawn through the hot legs in e ste[m generators.
As further evidence of this inte mitte rculation, t ss flows in the cold legs at the pump exits in Fig. 1 aica hat these flow transients are in the proper (positive) dire n.
After y tural circulation ceased in loop A (at 800 s) and before it cea in loop h (at 2100 s) flow in the split cold legs of loop A was predominat a
from the vessel in loop A2 (containing the break) and toward the (positive) in loop A1.
Betwen the time that natural circulation ceased (2100 s) and the cooling of the steam generators began (3480 s), the direction of the flows in the split cold legs of loop A was changing, such that when flow in loop Al was toward the vessel, the flow in loop A2 was away from the vessel.
, After secondary cooling was initiated, flow in loop A2 was toward the vessel and break, Wille the flow in loop Al was toward the loop seals.
The direction of flow and amount of subcooling in the split cold leg containing
. '.7
s
\\
t U
u
('OLID) Lo0P-A Tor or CucY cme (DASH) Lo0P-e Top or CecY Cat t-l I
i i
%n g
1 M-i i <v s
I w
M-I 1
I i
u-i t
U-Y-
C test asm anos ae'so sees esce psion sees Tint (e) e Yapor fractions in the up of the hot legs.
A.
o 007 L)oP-4 tsAss Flow aATE c-gLooP-At asAss rtow AATE O
-asse
'"-A?
hiAss Flow R ATI j
\\
ses-
-asse
_S
.m
^
w j
c f
A,,,
~
.mos u
l s.kh}.in -- -- -
r 5
,1 J y{g/
e
. T V 31 ~~
7 -
-eco -~
e noe soon aeos
.o noon Fees seas
.soe il6c (e)
Fig. 16.
l.oop flows in the cold legs at the pcap exits. _ _
1
the break had a marked effect on the mass flow out the break.
The fluid flowfng from the vessel toward the break contained a two-phase steam / water mixture, so that the mass flux -through the break was lowr than for the predominately subcooled liquid " flowing from the loop seals toward the break and vessel.
The liquid and saturation temperatures in the col containing the break (A2) are shown in Fig.17.
After secondary coo is initiated, the liquid in the cold leg containing the break is about 5 su led, s ile the liquid in the other cold leg (A1) is nearly at saturation condi s.
The pressure, flow and temperature oscillations that occur after cessa n of natural circulation flow in loop-B are discussed her in Appendix A.
The small-break transient simulating o r or i rvention to cool and depressurize the plant was carried out to 2.7 D
g this time, the total flow supplied by the HPIS reached its maximum val about 55 kg/s, as shown in Fig.18.
After cooldown began, the liquid ma ppifed by the HPIS exceeded the mass lost through the eak, so that by th end of the transient the primary contained nearly the s f fluid as den the small break was initiated; this is evident from 20.
However, owing to the liquid shrinkage during cooldown, the s am volume in the upper plenwn and candy canes remains relatively unchang Thus, the flow from the HPIS is able to make up the sst through e break, but not the decrease in if quid volume due he i L ased density.
The vapor fractions are shown in e
Fig. 21 for the uppe and in Fig. 22 for the upper cells in the downcomer.
(Cell 6 cont tne Tent valves and cell 5 the hot-and cold-leg connection In the upper num and downcomer, the vapor fractions reached their
.um es dile the steam generators are filling and did not decr significa t y thereafter.
rimary i d secondary pressures in the loop-A steam generator are shown in 23 d the pressurizer level is shown in Fig. 24.
Even though the thermal ing betwen the fluid in the vessel and the liquid in the steam generators was weak, the primary temperature and pressure followed those of the secondary, tile most of the decay powr was removed by heating and discharging through the break the water supplied by the HPIS.
The primary cooldown and the wak thermal coupling betwen the vessel (and ' hot legs) and the steam generators (and cold legs) is illustrated in Fig. 25, sich shows the loop-B hot-leg and cold-leg temperatures.
The primary temperature decreased from 550 K den the steam generators began filling to the 95'. level to 460 K at the end of the transient. The average
1 1
ene (SOLID) sAftstafloss 7twERATM (DASH) LioulD TDdPERATuet een.
n een n
5 i
b R
I:O' l
s li i
I: n 2
i!
!!:i!! s..'i 9
/
ii Q
ti
!jl1[iis %'1 e
e s,L4f n t e '
)j., '.
n.s f-
.ee.
Me anos soon.eos soon soon snee mese e
mas Tsw (e)
Fig. 17.
Liquid and saturation te tures adjacent to the break.
(seuo) Loor ms rtow man (nasH) Loor-was rtow aATE e
t
}
-s-7-
_7 i
1 3
=
o
~~8 e.
s e
=
I l
. - =
3 1
-*d 1
.f
..u 1
>..n u.
- Uy^ 1,f 'f ;,
-e soon asse moo soon eene me esos een mens e
noe
-=
flE (s)
Fig. 18.
liigh-pressure injection flows in loop A.
l 1.
emesse (508.10) MiGM-MICS$UIt[ INJECTION FLOW agenes (nasa) rLow tmouca sacAx g
.g w.e g
.eacea,g e.
a
/
Q.
f-4 5====-
f./.*
5 5
,w 7*
5
.e 6
anse som som soon y some new e
Tsuc(s)
Fig. 19.
Comparison of reak and HPIS flow y
a 3
5 4
I i
f.
eneen-l s.
s.
umeo r
W i see o-
/
s 7
y.sose-y
- nooo. y l
2 i
s
- wooo 8 y semo-
>l 3
g
./
.-2mos y e
-coco mooo sooo.ooo soon esso reoo soon soon sooo e
soo Tiht (s) l l
Fig. 20.
Primary begins gaining mass after operator-initiated filling and cooling of steam generators. --
~
W 3
5 5
5 5
5 E
E y
.c.
m i
,e1 1
x
$! Y / j,J).J: -l.
l i
i 50Llo=LIVEL 5 I
z M.
3 oAs w vtt s i: '
a f
i w
u.
s 2
!t I
f/
~
3.1 e
s'so sees aio. A mios some noe m'
mens Time (e)
Vapor fractions-1enum.
I L,
,\\ =
7
.-~(
y 34 j
{. i-i i
l g
t l scuo-Lrvrt s
=
Il O
t
~
I V JH u
i u-Y h I
\\l l
. n.
e e
sin esos aion.aos edeo mies noe some ones woes Time (e)
Fig. 22.
Vapor fractions in the downcomer.
I
l e-IMe'
~
(sa.io) tow-A sicos reimmy (DASH) LOOP-A $7CCM stCOCAny
~#'*
m..
. sos vE
{ and-I h
j
-sen L
'i sad-
.ooo
-ase e
== asas 7 ~ --;
u anoe== nom n
TI E (e) i.
Comparison of Loop-A pr ondary pressures.
r A,
r e-.
)I(souo) retsswim irva...
y s
,p
-s 1
5
-)
g 1.u 1.
-t anos seco som eooo== esso som mese e
see Ti w (s)
Fig. 24.
Pressurizer water level. -~:
y T
T F
W (SOLID) Loop-4 Hof LEG (oAsu) Looe-a ca.o Lrs ese-
.see m
~
E
^
g-.\\..
,1 :j.:
ase-j j
. ss W
a m.
1 I
a i
la l ii, C
%.'.)j:.;f;!i
!i
.ame
. i NI l-
~-
see-O3
.e "e
mis e asse ios ess noe some Taw (e) 7 i
5.
Comparison of hot-leg and
+emperatures in loop B.
rate of primary cooling us 52 X/hr. (
/hr), dich compares favorably with the technical specifica n limit of 1 F/hr.
Fuel-rod temperatures shown 0
in Fig. 26 followd e
he liquid (
turation) temperature in the vessel and hot legs.
I rY. 'ac 6;ce for natural circulation was established Although a therm by cooling the steam ge
' ors our attempts to restore natural circulation
.by bumpin main coolant
.ps, by venting steam from the candy canes and by inj
+ ng co ter into the candy canes failed because the liquid volume in t rimary be; een 1 h and 2 h wuld not support natural circulation i
1 flows.
r 2 h, o actions to' restore natural circulation wre simulated because w s
the operators could continue cooling the core and depressurizing the set point of the low pressure injection and residual heat removal systems.
C.
Small-Break LOCA Recovery with Accumulator Injection To further test the effectiveness of the various strategies for recovery of natural circulation flows, the small-break LOCA calculations described in Section II.A. wre restarted at 2414 s, imediately after natural circulation flow in loop B ceased.
The initial operator action to increase the steam-generator level from 50 to 95% of the normal operating range was simulated, as ~. -.
l
.a.
.u.
E 8
y
,5
-a.e I
<l an.-.
..u.
5 l
aa-.
-.. l 3
1
,5
~
f u (.)
ig. 26.
Maximum averag rod temperature.
a shown in Fig. 27 and venting of ste
- r. the secondary side of both steam generators was begun at 3000 s to cool a d depressurize the plant.
The steam flow was controlled secondary p ures with the relief rates given in Sec. II.B. and th xil y-feedwater flow was throttled to maintain the level at 95% of no t1 operating range.
When the calculation was restarted at 2414 s, e>evations of the primary wre filled with a nnar steam as shown in Fig. 2 d the net fluid mass lost from the primary was 43000 k In the cident sequence described in Sec. II.B,' the net flui ss as 6 kg at 3480 s, den the operator began filling the steam gener to the S level.
Th imary d secondary pressures in the Loop-B steam generator are
~~
shown in F for this transient.
Primary-to-secondary heat transfer, established at 414 s den the steam generators begin filling, was maintained by venting steam at 3000 s.
The absence of effective thermal coupling between the steam generators and the saturated two-phase mixture in the upper plenum and hot legs resulted in the primary pressure remaining at 6 MPa for 3 min after primary-to-secondary heat transfer was established.
The sudden drop in pressure to 5 MPa (at 2600 s) was accompanied by slugs of hot fluid (Fig. 28) passing through the upper elevations of both hot-legs into the steam generators.
The pressure remained at 5 MPa for 8 min sile the steam L'
i l
s.
3 & W SadALL-SAEAX LoCA (S 4 FlLL. Cool. ACC. INJ.)
AUXILIARY FECDws1tA TLow RATES
\\
(sa.io) Looe-A Aux r/w as-
'oAsa) Looe-4 Aux r/w -.m
^
i l
m.
h R.
4, m
m-l f
m-I
~
e oe 1
G
.m L-s
=4 e
ese sin s as'a mes me sees asse Tim (e) 27.
Auxiliary feedwater flow Co 4t the 95% level after 2414 s.
s s
u u
T b
(souo) LN.
-A for or cAnn caw
('
)
(DASM) Loor-G TOP or CMcf CAW
)
{
" (* C y'
s
.i j
a l'
i i
s i
=
i u-i
,i
\\
.r
'e' e
nin noe aios a.se nao
.o=
- ios
=>o Ti w (e)
Fig. 28.
Vapor fractions in the upper elevations of the hot legs.
4 J 3 W sMALL-94(AK LoCA (s G FILL. Cool. ACC. INJ.)
Loop-a sTCAW clNCRAfot S** '
(souo) Loor-e stata remay I
(casa) Loor-e sTctm srcoseaar
~
T h-1 l ame'-
f
~
{
/ s, saa'-
.ees
' ' ~.,,
eee J
Tu (.)
. 29.
Primary and secondary pre s n loop-B steam generator.
generators filled to the 95% level ooling of the secondary by venting steam was initiated.
The rapid drop pressure from 5 to 4.3 MPa that
~
occurred at 3100 s panied by s of hot fluid passing through the l
candy canes into ste Igenerators.
Continued cooling of the secondary produced a slow depr r
tion to 2.4 MPa (360 psta), den the calculation was terminated.-
7 When the primary sure fell below 4.27 MPa at 3290.s.
the accumu injecting liquid (Fig. 30) into the downcomer.
From this time il the ca lation was tenninated at 4800 s, the accumulators infected 3
about-
-. m (1
0 kg) of cold water.
The average rate of injection (8 kg/s) ng time would be equivalent to a 14% increase in the HPIS y
flow. Figur shows the outflow through the break compared with the inflow frca the HPIS alone and the total inflow from the HPIS and accumulators.
The I
l difference betwen the outflow and total inflow is shown in Fig. 32.
More l -
than half the mass lost out the break is recovered by inflow from the HPIS and 1
accumulators in the last 2400 s of the transient.
.While the primary was cooling and depressurizing, the vapor fractions in the upper elevations were not changing significantly.
This can be seen in 1
Figs. 28, 33, and 34, dich show the vapor fractions in the candy canes, the ~
O
~
S & W SMALL-SAIAx LoCA (S C rlLL. COOL. ACC. INJ.)
,ACCLMJLATot INJECTION r,
e.
~
e 3.
c R
no a
s.
I I
a s.
E g
}
s.
e B
e B
a e
a
-e e
.h.
.i
.m e n i \\hq.
Ta w (.)
V Liquid volume disc e r
. ula tors.
a.
S&
cAK LoCA ($
LL. Cool. ACC. INJ.)
joMorSatAx tcCs rLows I C 10W FL0w 3
I 1
- Act _tATOR INJCCT10W p ,.
3
,A
.s 3
=
p.. / -,
.c g
g C.
\\
/*
g sC
/,. *
-._ g 5
4 E
=5 w
./
5 e.
-4 e
Fig. 31.
Comparison of break, high-pressure injection and accumulator flows.
t.
3 & W SMALL-82(AX LoCA (5 C FILL. Cool. ACC. 8:!J.)
WASS Lo15 Frow PRlWARY e
sense
. sene m
essee g asene.
d I anes-9 8
4 sese-g
-., esse g I
I e..
- eeen
~ ~ * *
- e see see see seee noe asaa use esos 4
. 32.
Primary begins gaining
.a a f ter operator-initiated filling and cooling of steam a & w suAu-setAx L s c riu cool. Acc, swa.)
VolD IN I PCR PLENdid u
l Y
,ki'A.,,)\\
p, u.
~?*i !
l!
i
/
- C*~ f's%.j.& -
scuo=Ivn s cAsattvn s 2
e+
j 5
~
d f r^,
, ~
e.
l
- M sees noe a,e un eene een een e
en see see Time (e)
Fig. 33.
Vapor fractions in upper plenum decrease following accumulator injection.
t 1
w swALLM#tAK LoCA (s c FILL, Cool. Acc. INJ.)
8 at volD IN TH' oowNeowCR u
y
(
M
- ""*T'58" f-I
]
l scuo.ttvrt s g
f casa trvtt
[
3 u.
g g
"~
~
W 1
e -~.
e en on on am noe sooo ooe seee noe Time (s) 34.
f Vapor fractions in cells to cold legs decrease following accumulator inject o.
.?
upper plenum, and the downcomer, respe t vely.
Th% vent valves, Witch opened Wien natural circula lows cease t 2100 s, remained open allowing saturated fluid to fr
- he upper plenum to the downcomer.
The liquid an tu ton temperatures shown in Fig. 35 indicate that the primary temperature h~meoMO K in the last 2400 s of the transient.
This corresponds to an av e cooldown rate of 75 K/hr (135 F/hr), which is 0
someWiat than the technical specification limits.
Fuel-rod a
tempe
- res clo followed the core-liquid (saturation) temperatures.
When the c lation wasj "arminated, the primary pressure (2.4 MPa or 348 psia) and tempera (500 or 440"F) wre such that continued cooling and depressuriz the set point of the low-pressure injection (LPI) pumps could be demons rated without recovering natural circulation.
To test, operator strategies for recovering natural circulation flows, these calculations were restarted at 3835 s, and the following actions wre simulated:
1.
venting steam from the candy canes, 2.
bumping the reactor coolant pumps, and 3.
injecting a portion of the HPIS flow into the candy canes. -
a a w swAu eetAx Loca (s c riu.. cool. Acc. in)
TSAT A;e.TLloulD IN top LIVEL of core es.
t (SOLID) TsAT IN 10P LEYCL Of CO.E (DASH) TLloulD IN TOP LIvtL or ca.C i
I p
/
Q la.
i g
e.
-~
~
.a.
E I
I es.-
=
a
- 4 Fig.
5.
Liquid temp
- res decrease - K during 40 min cooldown.
The results of the imul ons are discussed individually in the remainder of this section.
~
(1)
Yenting Steam From leg Candy Canes Ya1 he hot-leg ndy canes are normally used to ~ expel air from the ary an acilitate refilling after a refueling outage.
Since info.
on on th valve dimensions was not provided, a diameter of 1 cm t
(0.394 i s as ', ed for the orifice.
The valves in both candy canes wre opened ful 5 s and remained open throughout the transient.
Critical flow through ese valves was computed using the model incorporated in TRAC-PF1/ Mod. O.
When the valves wre first opened, the pressure in the hot legs as 3.2 MPa, and the flow was 0.4 kg/s saturated vapor.
The pressure decreased steadily reaching 2 MPa (Fig.
36),
dien the calculation was terminated at 5400 s.
'From the time accumulator injection began (3290 s) until the calculation was terminated, inflow (Fig. 37) from both accumulators totaled about 20 m3 (20000 kg). The average mass flow supplied by the accumulators was 9.5 kg/s,
a a w suAu.-entAx 1.ocA (se riu coot. Acc. vrms) enizte entssuet see' (souo) msmaizza mssuet see e
m.
mes E
b 3
h&
b h
{
e.e w.
..a
/,
e e
e ae
. moe m asas aan mee
.e sees use TihC (e)
. 36.
Primary pressure decreases accumulators inject and secondary is cooled.
e a w suAu-entax.ocA (
riu, coot.'Acc. vrxis)
ACCUWutAToR EcfloN y
.e j
i 5
_/
5 g
O s-g M<
s M
a a
l e-1 g
l
..e a
B 8
3 3
..e
.a nao a u.e
.e aae m e
e see eso TlhC (.)
Fig. 37.
Liquid volume discharged from accumulators.
cccpared with the flow of 56 kg/s provided by the HPIS.
Of the 43000 kg that had b,een lost from the primary when filling and cooling of the steam generators began, about 33000 kg was recovered by the end of the transient, as shown in Fig. 38.
Accumulator injection produced a sharp dec ease in the vapor. fraction from 0.96 to 0.65 in level 6 of the upper ple as shown in Fig. 39.
Howver, the vapor fractions in the upper plen not. changed signi ficantly by opening the high-point vents.
The up er 1 of the downcomer (level 6) remained filled with vapor, and the vapor fr - fon in level 5 decreased from about 0.2 to 0.1, elle th igh-point vents e open (Fig. 40).
From the time natural circulation cease i
loop ntti the end of this transient, the vessel vent valves remained open g fluid (largely steam) to flow from the upper plenum (level 6) into the omer; the average vapor velocity of this. flow was approximately 1 m/s.
The imary cooldown rate, dich began at 2414s and continu ntil the transient was terminated at 5400 s, averaged 76 k/hr (137'F/hr) ds were cooled throughout the transient by liquid injected from the H
nd accumulators), and their temperatures closely followed the. If u d (saturation) temperatures in the aa EAx LocA (s su. coot. Acc. vtwis)
^
ss Loss rnow entwAny t
y-
_7 m.
g N-7 e-O g
Q
- 1 e
/l]
p 5
5 g
5
' '" 5 Au*A J
Tid (.)
Fig. 38.
Primary regains most of mass lost through break with accumulators injecting. -.,
s a w suAu-satAx Loca (s c riu, coot. Acc vtwis)
YotD IN THC UPPER PLEMUW u
i
~
~
i.
2
~ ff%y ]
p= :,~ l
{
%$g\\htrM N
E j
j if souxzvrt s sa w s e
Ij l
ll 2
u-i h
u-f I
s.
N. _.
/
F
~.
- a s see. -e -e see.
Time (s)
Yapor fractions in upper pl...
r ffected by opening candy-cane vents at 3835 s.
u a a w = *n -ancAx LocA s c ria, coot. Acc. vtwis) u [ g ]
volo IN T we Y
'~
(1
- twnm. lrri I
l-u-
il
\\
8 l
]
}
l saixrvet s j
l I
i u.
1 >I MS ese s 6 ados ai.e a noe 44 eine sees Time (s)
Fig. 40.
Vapor fractions in downcomer are not significantly affected by opening candy-cane vents.
30
(2)
Bumping Reactor Coolant Pumps
- One of the strategies that reactor operators can employ in attempting to recover natural circulation involves starting up one or more reactor coolant pumps then shutting them down.after about one minute.
The small-break transient was restarted (at 3835 s) during the cooldown pert with the accumulators injecting to test this strategy.
We chose to the loop B (combined cold-leg) pump; this was equivalent to starting e pumps in loop B.
Based on information from engineers at B&W, he assumed t
. the pump could attain its rated speed 20 s after starting.
The pump was a
_d to operate for 30 s and coast to a stop in 70 s.
he loop flows fo this accident and recovery sequence are shown Fig. 41.
Since natural circulation was not recovered on the first att m at s, a second try was made at 4400 s.
Howver, this also failed to e +
sh natural circulation.
During the times een the pumps operated, the loo flows were not greatly affected.
Bumping the pumps produced
-convection flow of the tw-phase mixture in the upper plenum and the y r
loop-B candy cane through the loop-B steam generator.
Condensatio coo 'ng in the steam generator l'
produced the rapid decreases in pr. ry pressure observed in Fig. 42, l
coinciding with bumpin pumps.
Dur operation of the loop-B pumps, the temperature differe.
bet n the hot-and cold-legs, shown in Fig. 43, becomes very small.
mpi the loop-B pumps also established a sufficient pressure gradient betwe the ewngaer and the upper plenum to close the vent valves and to keep them c d until the pumps had stopped.
The rapid drops in prim stimulate large injections of ifquid by the accumulators, e
as in Fig.
However, after the pumps stopped the primary pressure remain table for i about 6 min causing the check valves in the accumulator-injection ing close, temporarily terminating flow into the downcomer.
As seen in
, the large injection flow (10000 kg) that occurred the second time the pumps wre started nearly restored the primary coolant mass to its pre-accident level.
l While the pumps are operating, the vent valves are closed and vapor from i
the upper plenum and vessel head is sept out of the hot legs.
The vapor l
fractions in the upper plenum and downcomer decreased markedly efle the' pumps j
wre operating, as shown in Figs. 46 and 47.
About 6 min after the pumps stopped however, the vapor fractions returned to the levels that preceded bumping the pumps.
This is also observed in Fig. 48, which shows the vapor --
3 & W SMALL-GREAK LOCA (S C FILL. COOL ACC PUNPS)
Loop FLOW RATES ameo esse-(00T) LOOP-8 MASS FLDw AATE l
(DASH) LOOP =Al WASS FLOW RATE l
W
. (SOLID) LOC #-A2 MASS FLOW AATE
, sene- !
4 g
I'
?
3
- 1 eene-d 8-N I
l!
[I mes-I t
use.
'I
!;j
'~............,
.3 u
-een use e
see noe mee seen asee use une aea Tiut(a)
~
41.
Loop flow rates show effe pumps.
umping reactor coolant e & W sWALL-SRfAK LOCA 3 FlLL, COQL, ACC PtwS)
LOOP-3 STCA NCRATOR we'
^
($OLID) LOOP STGCN PetWARY l
Y (oAss) Looe-a sfacu srcDear 5-h O
P.
^k
\\
.,l,
% - - ~,
..~
e see mio min aise ase use aies ese ease sees Tiut (e)
Fig. 42.
Primary pressure drops rapidly den pumps start at 3835 and 4400s..
l
( **
~,
t.
1 8 & W sWALL-99[AK LOCA (s G FILL. Cool. ACC. POWPs)
Looe-B Hof AND COLD LEG LloVID TEMPERATLRES m
(SOLID) LOOP-4 NOT LIG e.
(DAW) LOOP-3 COLD L{G m
5 m-
-. ~..,
m-1 F.
f.
h F:
!k
!}3 5.
~ ~
'N ! ).!k
- ki!.:kII'
- lt 8
ti 3
g!.
i!
i ase-g j.
'.I i
s m-y a
2&as 3ece ateo sees esos e
ese see see anse Taw (e) 43.
Hot-and Cold-leg tempera rence is small during pump operation.
8 a w sWAu-aacAa: LoCA ( G RILL. Cobt. ACC. rinses)
ACCuadVLAT INJECTloN
. p^)
y.
1 m-c L_.
E 7
g e.
\\
8
/
f l
4) i 3
e e
see m'as abe see nm seee noe soon ssee seen Ti d (e)
Fig. 44.
Abrupt drop in primary pressure produces large accumulator injection. - - -
g.
B & W sWALL-8REAK LocA (s G FILL. cool. Acc. PUwPS)
WAss Loss FROW PRlWARY as a.eee-n y sa.ee-m 5
se.
N
<3 g --
. -oose.
.e m
45.
Accu:nulator injections ne r ver mass lost during transient.
e a w suALL-entAr Loc c FILL. coot. Acc. Puurs)
VotD IN T PIR PLINuw u f
).
y I
.W L.j^.
7
.we. -J: ;:
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i Tr1 5
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't
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ees
. a.e is.a oo u.
. no.
Tim. (e)
Fig. 46.
Vapor fractions in upper plenum decrease during pump operation then return to previous levels.
34 a
S 0 U sWALL-9 CAK LoCA (s C FILL. COOL ACC, PUWPs) volD IN THC DoWNCowCR M
t-
. * ~ ~ -
-..s..,
~ ~.
- i
- Ii
- A-I f
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Fig. 47.
Vapor fractions in downc then return to previous lev 1 crease during pump operation 1
e a w swAu.-e=CAx LoCA (s FILL, Cool, ACC, PuwPs)
WPoR FRACTIONS l P or CANDI CANES u
(SOLID) LOC TOP Of CANDY CA8C l
(DASH) Lo0P-8 TOP or CAACY Cast t-
//
j -
F "I. iiIi
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Fig. 48.
Vapor fractions in Loop-B Candy Canes decrease during pump operation then return to previous levels.
s.
stopped however, the vapor fractions returned to the levels that preceded bumping the pumps.
This is also observed in Fig. 48 Witch shows the vapor fractions in the candy canes.
The average vapor content of the tw-phase
~
mixture circulated by bumping the loop-B pumps was less than 20% of the total primary volume, however, natural circulation flows wre not m ained after being stimulated by pump operation.
The calculation was teminated at 4800 s after tw e.
to restore natural circulation by bumping the loop-B pumps had failed.
At t time the pressure was 2.1 MPa, and the cooldown continued.
(3)
Liquid Injection Into Hot-leg Candy Canes The presence of valves at the top of the
-leg andy canes affords the l
possibility of injecting cold liquid into e r ns.
To evaluate the effect of subcooled liquid introduced into the
-filled candy canes, the calculations were restarted at 3835 s with a FILL onent connected to the high-point venti valves.
Ten percent of the HPIS flow
.7 kg/s) was injected into the candy canes of each loop the cold-leg injectisn flows were reduced to 90% of their original deli y
te.
L Dividing the HPIS cold legs b tVe the and hot legs of each loop g
enhances the accumulator injection flo and hence refilling of the primary.
Figure 49 shows that ecumulators a e discharged about 19 m of liquid 3
at 4800 s into th ansi t with hot-eg injection.
From Fig. 30, only 3
q 12 m have been dis ge ft this time vith all the HPIS flow supplied to the cold legs. - When e *"nsierz was teminated (at 5300 s), most of the mass lost through the br had been recovered by the combined injection of accumula+
PIS; the
.cumulators injected 26 m3 (26000' kg) in 2000 s (Fig, giving average injection rate of 13 kg/s.
Thus, over the last 2000 the t'r ient, flow from the accumulators was equivalent to an increase 3% in e HPIS flow supplied to the plant.
The p pressure, shown in Fig. 50, continued to decrease as the plant cooled, eaching a value of 1.64 MPa (240 psia) when the calculation ended.
Injecting cold liquid into the candy canes caused flow surges in the hot legs similiar to those observed earlier in the transient before accumulator injection began.
These flow surges became more frequent and larger in amplitude, as the primary refilled.
This behavior can be observed from variations in the candy cane vapor fractions shown in Fig. 51.
During the 1500 s in W11ch liquid was being injected into the hot-leg candy canes, the vapor fractions in the upper plenum (Fig. 52) and downcomer (Fig. 53) wre -
o.
)*
B & W sMALL-SRIAX LoCA (5 C FILL. Cool. ACC, SPRAYS)
ACCUwuLAToR INJECTaoN w
3.
c E
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g 2o g
4-a 3
E s
k 1
- '. eee e.s
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Fig Accumulator infection en ed by hot-leg sprays.
s
-aREAK LOCA (
Flu COOL. ACC. SPRAYS)
LOOP-A STE GCMRATOR we (SOLID) LOOP-A STGDe PetWARY (om) Loor-A STctm stconcAav ue'.
E h
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ii.
A ase zwo adeo ase.se. 4i. sin u I
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l Primary pressure for transient involving
- Cooling, accumulator injection and hot-leg injection.
l !
i t
t*
s i.
I i
3 & W s4ALL-SRCAK LoCA (s o FILL, C00L ACC, SMtAY5)
WPoA FRACTIONS IN TOP or CAPOY Cads u
(5OLID) LOOP-A TCP 0F CA807 CA8C (DASH) LOOP-e TCP OF CMY Ca#C r
-Frm t
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s Yapor fractions in candy ca show effects of sporatic flow surges hot-leg i
. tion.
-antAK LOCA ( c FILL Coot. ACC, SMtAYS) 1 VotD IN THE UPPER PLENUni j
u l
J
=
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Time.
Vapor fractions in upper plenum reflect flow surges produced by hot-leg injection. -
l t
8 4 W sWALL-BREAK LoCA (s C FILL. Cool, ACC, SPRAYS)
VotD IN T>C DoWNCoWER u
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Vapor fractions in downcomer crease significantly with
[
hot-leg injection.
6
)
decreasing.
The reasi gl liquid fractions in the upper plenum and downcomer, accompanie e more frequent flow surges through the loops, caused the radially out i.v. Udough the vent valves to decrease markedly, so that ttese valves bega losing.
When the calculation was terminated l
recover ci.rculation flows appeared iminent.
6 III.
USIONS A RECOMMENDATIONS In bse of operator action to mitigate the plant response to a l
small-break L the void volume in the primary will increase until the outflow through the break equals the inflow from the HPIS.
This occurred at about 6000 s in these calculations, for dich the critical flow out the break corresponded to that for a break with an equivalent diameter of 0.039 m (1.54 in).
,0perator actions to cool the plant are effective in depressurizing to the setpoint of the LPI pumps even though the accumulators are isolated, and natural circulation flow is not reestablished.
Cooling of the core is by HPIS flows sich mix in the downcomer with steam flowing out through the vessel %
6
v.
[
ve.nt valves.
A vary wak themal coupling exists betwen the saturated fluid in the vessel and hot legs and the subcooled ifquid in the steam generators, loop seals, pumps and cold legs..This coupling is characterized by sporadic
~
fluw surges from the hot legs into the steam generators.. As the liquid in the steam generators is cooled, it becomes heavier and intemittentl ttles into the cold legs. This draws saturated fluid from the hot legs ugh the candy canes and into the steam generators.
Betwen one and tw hours into the small-break transien perator actions (opening vents and injecting sprays into the hot-leg candy c and bumping the coolant pumps) wre found to be inef ual in restoring natural circulation flows.
The inability to restore a aint natural circulation is attributed to the steam volume present in r
y and the behavior of the vent valves in the vessel.
We concluded tha y or volumes 5 20% of the primary volume. (excluding the pressurizer) wuld necessary to support natural circulation flows.
Although the mass lost thr gh the break can be replaced by inflow from the HPIS
'r ooldown, the void volume remains relatively unchanged as the specific 1
liquid decreases.
After te hours, the plant had depressurize a condition such that recovery of natural circulation flow was not require o, continue cooling the core.
b) M ditional e o(of plant c oldown beginning imediately after natural circulation w ce qd and including accumulator injectiondroduced similar results.
The ajator injection flows added 14 - 23% to the HPIS flow, thus increasing a n 4hich the primary regained the mass lost through the break.
Howver is was offset to a large extent.by more rapid cooling surization, so that natural-circulation flow was not reest shed befor he plant had cooled nearly to the LPI setpoint.
the e down transient with accumulator injection, both the loop-B p.
re arted and allowd to run for 30 s before being tripped.
This was twice in attempting to artificially stimulate natural-circulat on flows.
Although this technique proved ineffective, our evaluation of the plant behavior leads to the conclusion that starting one l
pump in each loop with operation continuing for 60 s may be more effective in l
establishing natural circulation.
l
' Yenting steam from the hot-leg candy canes should be effective in recovering natural circulation because the venting reduces pressure in the upper plenum, thereby pemitting the vent valves to close. This did not occur,
v.
f e in'our calculations probably because the high-point vents (1-cm diam) wre too sr$all to relieve all the steam vaporizing from the surface of the fluid in the vertical section of the hot leg.
The use of a larger valve could be effective in establishing a
closed and liquid-filled flow path.
- mver, the possibility of the valves failing open and causing or aggra ng an accident 6
wuld raise additional safety concerns.
The injection of 10% of the HPIS flow into the candy ca
. ems to provide the most promising means for recovering natural cfr tion.
Condensation of vapor in the hot-leg candy ca reduces the pressure and stimulates flow from the upper plenum towar e ste generators, thereby closing the vessel vent valves.
The observ eh r makes this strategy promising for establishing natural convectio Obviously, the
- valves, control s, and piping necessary to accommodate the sprays wuld introduce additi'onal safety concerns and increase the probabili of small-break LOCAs from the hot-legs.
We recommend that similar ca c la os performed to evaluate the
~
observed behavior for a range of brea es.
dditional calculations should use lowr rates of cooldown to allow e time for evaluating strategies for recovery of natural c
'fon before plant is depressurized to the LPI setpoint.
The siz hig - oint vents, the hot-leg injection flow, and the strategy for startin el pumps should be treated as parameters in further calculations...The goal
' tM** Mialyses should be to define the conditions necessary for reestablishi nd maintaining natural circulation flows during SB-LOCA l
i 1
l l
l 1 l
,.).
REFERENCES 1.
NRC Memorandum from R. J., Mattson, NRP/DSI, to 0. E. Bassett. RES/DAE,
" Request for TRAC Calculational ~ Assistance,"-dated October 23, 1981.
2.
" TRAC-PF1, An Advanced Best-Estimate Computer Progra Pressurized Water Reactor Analy si s,"
Los Al amos National Labor report (in preparation).
3.
" Final Safety Analysis Report, Oconee Nuclear Station, Units 1, and 3,"
Duke Powr Co. (May 1969).
4.
G. J. E. Willeutt, " TRAC-PD2 Calculation of Small Cold-Leg Break in a Babcock and Wilcox Lowred Loop Plant,"
Al.
National Laboratory informal report LA-SBTA-TN-81-3 (October
).
f b?
J l
O h V -
.N ENCLOSURE 2 FRELIMINARY EVALUATION OF LANL CALCULATIONS 0F B&W SMALL BREAK BEHAVIOR Although we have not completed our detailed review of the LANL calculations in this matter, we have reviewed their draft report. We have determined that the LANL results do not indicate an imediate safety problem for the reasons stated below.
The LANL calculational results are not inconsistent with staff testimony at the TMI-1 restart hearing.
Licensee and NRC staff witnesses testified that natural circulation could exist at B&W Plants in two basic modes.
- 1) Single phase natural circulation in which water would flow through the coolant loops driven by density differences and 2) Boiler-condenser in the steam generators.
It was further stated that natural circulation in mode 1 could be blocked by steam formation at the 3
top of the hot legs. This volume of steam was stated to be about 360ft per loop or approximately 7% of the primary system volume.
It was further stated that for natural circulation to occur in the boiler-condenser mode, a steam condensing l
surface must exist.
Post THI-2 accident emergency operating procedures require that the secondary system steam generator water level be raised to 95% of the operating range following a small break LOCA.
If the primary system water level were above the 95% water level t-the secondary side, the condensation surface would be lost and natural circulation would not occur. The core would be protected however, since the 95% water level is well above the elevation of the top of the core and continued water loss from the break would establish natural circulation l
l in the boiler-condenser mode before the core could be uncovered. The 95% level Therefore, corresponds to a primary system void fraction of approximately 20%.
for void fractions of between approximately 7 and 20%, single phase natural circu-The 20% void fraction lation is not expected to occur in B&W designed-reactors.
value is consistent with that report by LANL.
. No core uncovery occured in the LANL calculations.
The LANL calculations indicate that although very little natural circulation occurred, the core remained covered
~
and cooled. The calculations indicate that the reactor system lost mass for
-\\
approximately 75 minutes and then began to fill. At the end of the calculations (2.5 hours5.787037e-5 days <br />0.00139 hours <br />8.267196e-6 weeks <br />1.9025e-6 months <br />) the reactor system was calculated to be gaining coolant inventory.
The calculations were not continued far enough in time to determine if natural circulation would be established. As previously stated, the reactor system was still filling at the end of the LANL calculations.
In a previously submitted TRAC calculation for which the break was isolated, the analysis was carried out suffi-ciently long that the reactor system completely filled and natural circulation was calculated to be reestablished in the single phase mode.
The NRC staff is currently evaluating the validity of the TRAC code regarding these results.
I f
The break size assumed by LANL was too large for natural circulation heat removal to l
l be required.
Restoration of single phase natural circulation is not required at B&W Plants for break sizes large enough to remove decay heat. Analysis performed i
by B&W* and presented to the TNI hearing board indicated that no secondary cooling 2
was required for brehks of 0.02 ft and larger.
For break sizes of.01 't2 and smaller natural circulation and steam generator decay heat removal was shown to be required by B&W if only one ECCS train was operational. The LANL calculations assumled two ECCS trains were operational.
The additional ECCS flow would condense more decay heat steam than in the B&W calculation so that a break size of less than 2
the.01 ft would remove decay heat. This is because the break flow would be increased as steam flow was decreased.
The draft LANL report indicated that the
- " Evaluation of Transient Behavior and Small Reactor Coolant System Breaks in the 177-FA-Plant" May 7, 1979
l break size of their analysis was equivalent to an effective diameter of 1.54 inches or 0.0129 ft. The draft report indicated that the break was sufficient to remove
~
all decay heat.
This result is consistent with the B&W calculations.
Plant operating procedures do not require that natural circulation be restored for the reactor system conditions calculated by LANL. The draft LANL report incorrectly indicated that the operator would attempt to restore single phase natural circulation by bumping reactor coolant pumps and would be unsuccessful.
In fact restoration of natural circulation would not be required by the procedures since the core is expected to be cooled and the primary system depressurized.
The available ICC instrumentation (core exit thermocouples) would be used by the operator to confirm adequate core cooling.
The operator is required to raise the secondary system water level to 95% of the operating range to establish a condensing surface well above the top of the core and to cool the primary system by controlling secondary system steam flow.
The TRAC analyses in'dicated that the operator would be successful in performing these operations required by the procedures.
A limited amount of two-phase natural circulation occurred in the LANL calculation.
This is evident from comparing the calculations of primary system depressurization response when operator action was assumed to cool the secondary system and the base case when no operator action was assumed.
For the depressurization case the primary system tended to follow the secondary system pressure decrease indicating that the boiler-condenser mode of natural circulation was removing some heat from the primary system.
The TRAC Code which was used in these calculations is still in the development stage i
and neither the code nor the input for these calculations has been subject to detailed evaluation by NRR, and there has been no QA by the contractor as yet, i
l We have not yet had time for a detailed review.
However, even if the analyses show unexpected SBLOCA recovery behavior, it could be due to modelling considerations i
l 4-rather than real phenomena. We have requested RES to expeditiously examine these
~
analyses and confirm their validity.
Comparison between the direct TRAC results and those by B&W will be difficult to make however, because of differences in break size, assumed number of ECCS pumps and the inability of the TRAC code to calculate finite coolant levels within the reactor system.
l 6
ae e 4
e
~
.n....Y.
.. 2
- 2 Dr. :-:e..ry liyers
'.Sub:o.itti.e en Energy and the Envirordant Co mittee en Interior a. d Insular Affairs United States Hcuse of F.;presentatives
'elashin; ton, D.C.
20515
Dear Dr. !!yers:
'In early April you requested the NRC staff to coment on a statement in
~
. paragraph 619 of the TV.I Restart pa'rtial. Initial Decision.
The statement read as fo11cus:
"If, ho :ever, the voids are steam, as would be expected in a small-break LOCA, the bubble in the hot leg should.be compressed and con-dansed as the primary system pressure is increased by operation of t!.e RFI system."
The centext of this statement in paragraph 619 is a discussion of the recovery feca a small-break LOCA,when ECCS injection flow exceeds break flow so that the reactor system is fill.ing.
The discussion. addressed a Union of Concerned Scientists (UCS) concern that for this condition a steam bubble in the top of the hot. leg U-bends might prevent the reestablis.5qent of single-phase natural circulation.
l'a. Karrnerer of NRC replied to your request in a latter of April 8,1932.
The response included the following coments:
"If a ster.:ri bubble exists and primary syste i pressure is raised, the
. bubble will be. compressed and there will be condensation.. The con-densation o: curs because as you raise systr. pressure sys' tem tecoera-ture cross below.satur.atioE.
Condensation must occur to reach satura-tion conditions again.
The bubble'will not necessarily be completely condensed.
This depends on bubble size and pressure change."
(Sentence was' underlined in your April 19, 1932 l etter.)
' ~
In a letter. of April 19, 1982, you asked if the quote underlined above from our April 8,1952, response was correct.
Specifically, you questioned."... whether, in the case cf a small break LOCA, the heat transfer would be sufficient to keep the steam space from expanding into the core." The underlined quote is not correct because there cust be heat transfer to a heat sink to absorb the heat of vaporization associated with the condensation of stea.i, and the heat transfer' rate is important.
Also, the system temperature does not actually drop; rather, the saturation teperature increases with increasing pressure.
- -:owever, while the.cond'ensation rate of the hot leg bubble will affe:t the everall recovery behavier', it does not, by itself, g:vern the everall adecuacy of decay heat rr.cval during a small break 1.0CA in a plant desigr.ed by 59!. To be certain that there is no similar nisunderstanding reprding this matter, we are providing a copy of this letter to the Licensing-
- n.' ud_sti.11 <-- h
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l d
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...------s-.
--e
- he det i,; U-hsnds ur. der off-r.creal c:ndiciens is cc plex.
The cetiilsd 13havi r of the reactor undar thsse c:nditions is still being actively studied by 5.vi, tt,a ED: plant cuners, and the staff.
Although uncertainties remain, i.:hdir.; heat trar:sfer rates for stsam c:r.densaticn, vie ht'a reache'd s:ce e er.sr.al cen: lesions about the effect of'these un:ertainties and the potential
- ior red.;cina unacceptable'ecre heatup.
These ccnclusions are based en
-calculati:ns'by E&W and/or by a staff centractor of a mn.ter cf postulated Ua have concluded that the un:ertainties can be physically 5c.zil 5:-eak LOCAs..ndad r.nd that these bounding assumptions do not prc. duce unacc e-heatup.
n the enciesure, the staff addressas y ur cuesticas ra-;arding the ir.pict of steam conder.sati:n rates and supporting analyses and describes h:x the use of bounding assumu-ions ::ald not result in unacceptable core heatup.
Calculations related to these' conclusions tre identified.
Thtre are still uncertainties in the therc.al-hydraulic respense of t'he E&W design under sete of these conditicas. We are pursuing with B&W and BEM plant cuners the details of the ther r.al and hydraulic phenomena involved, including the possible need for additional sensitivity analyses and a small scale, inte-gral systems test. 0.r objective is to confirm the ana.lytical results described in.the 2nclosure and to aid in our further analysis of t. ore cecplex, cultiple failure events being studied in the context of the new symptcn-criented, cergency procedure guidelines.
SinCeral 0re:En ud t-f
- H. R. D:..ico Harold R. Denton, Director Office of Nuclear Reactor Regulation Enclesure:
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Enclosure J
DYllAMIC RESPONSE OF B&W REACTORS TO SMALL BREAK LOCAS Detailed analyses have been perfomed either by B&W'or by staff contractors for the two classes of small break LOCAs:
(1) those that can be subsequently isolated (e.g., letdown lines, PORVs), and (2) those that are not isolatable.
~
They are discussed in that order, below.
Small' Breaks Which Are Subsecuently Isolated We have had our contractor, the Los Alamos National Laboratory (LANL) perfom an analysis of a small break in the cold leg of,the B&% reactor coolant system that is subsequently isolated.
The calculations were performed with the advanced TRAC computer code, and we have some preliminary results. We are
. not aware of any calculations that have been performed by B&W in which a small break LOCA was subsequently isolated.
In our analysis, the system was assumed to lose primary coolant from the break until the upper vessel head region, pressurizer, and hot leg U-bends were filled with steam.- At that time, the break was assumed to be isolated by the operator.
Approximately.1,000 seconds later, it was assumed the operator began a controlled secondary system depressu 'zation.
The analyses showed that the flow of cold water from the two high pressure injection pumps, coupled with controlled secondary. system depressurization, would condense the hot leg bubbles and restore natural circulation. "We are still evaluating the ability of the TPAC code to model the heat conduction in the liquid near the steam-liquid' interface which is required to accur'ately cal-culate the primary system refilling process. However, steam condensation rates are not expected to influence the overall conclusion that no unacceptable
~
. core heatup wocid occur,.as explained in the following paragraphs.
If the steam was condensed at 100% efficiency by the cold HPI water, the top of the hot leg U-bends would refill.with liquid and single phase natural,
circulation would be restored.
If, however, the steam condensation rate is very low, and, in fact, in the limit the steam is assumed not to con-dense at all, two possibilities would result.
s: urnon:qkn ceau n OW
l-L The first possibility is that the HPI pumps would repress'urize the system sufficiently to compress the steam. to a small enough volume,to allow liquid on the upstream side of the hot leg U-bend to spill over into the downstream side and resume natura,1 circulation.
If this did not' occur, the HPI would continue to inject ECC water and repressurize the reactor coolant. system until.the pressure re' aches either the PORV or the safety valve set pressure.
We would expect that core cooling would be maintained by a " feed and bleed"' -
process until the steam bubble at the top of the U-bend eventually condensed by heat transfer to the pipes and.across the liquid vapor interface. Once the bubble condenses, single phase natural circulation throughout the steam generators would be resumed.
The other possible behavior if the steam condensation rate is low is associated with a design difference in B&W reactors.
One plant designed by B&W, the Davis-Besse plant, does not have a "high head" HPI pump (one that can pump water into the primary system at or above the safety valv'e. set point).
The shuto.ff head of this pump is about 1700 psi.
In the event the cold water *from'the HPI pumps does not condense the steam in'the hot leg U-bend, the system.may repressurize to abovd the shutoff. head of the HPI pump, and eventually reach the PORV or the safety valve setpoint.
The system will begin to lose primary coolant through the PORV or the safety valves and drain down.
Once the. primary coolant level
~on the downstream side of the hot leg U-bend extends into the steam generator tube region below the condensing surface of the secondary coolant, steam in the primary system will begin to condense, lowering the primary system pressure and closing the PORV or the safety valve.
~
As the system pressure decreases below 1700 psi, the HPI will actuate and begin
' to fill the primary syste:h.
This would result in covering the condensing sur-face in the steam generator, and producing another repres'surization, which',.
in turn, could stop HPI flow and cause the PORV or the safety valve to open and release enough coolant to reestablish a condensing surface.
A number of
~
these cycles may occur before the charging system comcletely refills the system or before the steam bubble is condensed by heat transfer to pipes and across the liquid vapor interface.
Since the condensing surface in the steam genera-tors is above the elevation of the tcp of the core, natural circulation should be established before the hot leg steam bubble extends into the cor.e.
DESIGMS3 02 ICE-::[
lfrju
,R.-
e m,e 3,
Small Breaks _ Which Are Not_ Isolated Small break LOCAs which are not subsequently isolated have been The analysis performed by by both B&W and by the staff's contrac, tor, LANL.They used a, com 2
B&W was for a 0.01 f t cold leg break.
With the exception of the to the requirements of Appendix K to 10 CFR 50.
i d by assumed number of HpI pumps available,'the modeling assumptions requ re Appendix K do not affect the thermal-hydraulic models of interest Thus, the results of the hydraulic analysis should be realistic.
small break LOCA.
The 0.01 square foot break was selected since this size is insufficient for remove decay heat via break flow (thus requiring the steam generators It was also,predic.ted to result i.n t'he repressurization decay heat removal).
phenomenon for the reactor coolant system.
From these analyses, B&W concluded that a range of small break be postulated in which steam generated in the core would accum l circulation top of the hot leg U-bends and cause an interruption of natura The interruption of natural circulation flow would isolate the steam being produced in the reactor core from the steam generator he flow.
i net steam accumulation in the system was calculated to cause the pr m Th'is repressurization was calculated to continue system to repressurize.
i t to until the primary system coolant' loss through the break was suffic en It is expected uncover a steam cond'ensing surface in the steam generators.
i f
that some steam generated in the core would flow into,the upper el lder water in the downcomer annulus via the vent valves and condense However, cold water from one HPI pump was not calculated to that region.
be sufficient to condense all,of the steam generated in the c~ ore.,
Similar to the isolated break case previously discussed, the rep l
i of the reactor coolant systeh caused by interruption of natural circu i
l tion, viould lead to a boiler-condenser mode of two-phase natural
~
and subsequently reduce system pressure.
i and exceed the break flow, the system cooiant inven begin to increase.
h ctor pressure, liquid level in the hot leg piping, and liquid level in t e vessel are shown for this case as calculated by B&W.
i to increase.
to. the. staff teminate at about the time system inventory beg ns However, the continued recovery of the event is considered relevan concern, and is described further below.
DESIGbTIDOEpf3II, Certified 37 E
.pm
~
. in the steam generator As the system refills, the steam condensing surfacebubble will.be t will again be recovered by liquid, and a steamThe scenario is now the top of the hot leg U-bend.
ibed.' That is, if the similar to the isolated break case.previously descr
'i singl.e ~ phase natural steam is rapidly condensed during the refilling proces, pressure w circulation will be reestablished and primary systemIf the steam with no significant repressurization.
ould repressurize until hot leg U-bend is not rapidly condensed, the. system w face was reestablished, (1) the break flow exceeded the Hp,I flow and a condens fficiently (2) the' system repressurized and compressed the steam hot leg U-be'nd could h
small enough volume so that water upstream of t eleg U-bend and spill over into the downstream side of the hot tpoint was reached.
natural circulation, or (3) the p0RV/ safety valve se ld occur since the For the Dav'is-Besse plant, we believe only option 1 wou tem at or above the HpI pumps are not sufficient to pump water into the sys
~
~
safety valve setpoiot.
sponse of B&W-designed The staff has also been calculating and analyzing the rel circ
~
reactors to small break LOCAs in which natura hot. leg U-bends.
Our con-rupted by steam accumulation at the top of thed a few.smal,1~ b tractor at LANL has recently complete ither preposed by
' looked at four recovery enhancement actions presently e (1) high These four options are:
B&W or being considered by the staff.
(3) secondary side de-
~
t point vent operation, (2) momentary pump restar,f the hot leg All pressurizaition, and (4) ECC spray at the top o their ability to enhance i
of these options are being investigated to.detem ne
, the reestablishment of sing eInitial results of our contractor's calc l
k (i.~e., nominal decay heat, portion'of the accident.
show that for a realistically calculated small breawith a break l
t two HpI pumps available, etc.) in a B&W p an,
ld occur, the system did of that for which B&W predicts repressurization wou Although our filled with steam.
not repressurize once the hot leg U-be'nds the rea' son the LANL calcu l
evaluation is not yet complete, we believe that ated in the core vented did not show a repressurization is because steam generl s to the upper reaches of the vessel annu u ll of the steam produced in th from tvio HpI pumps was sufficient to condense a AL DESIC.MID OR b
lk Certified 37_
F
~
. In figures 4 The results of' the TRAC analyses are shown in figures 4 through 7.
We believe they and 5, the B&W results are overlayed to show the differences.
The results of the can be attributed to two versus one HPI pump being available.
analyses to investigate natural circulation recovery enhancement met T.hese ana-lyses only recently presented to the staff at a meeting with LANL.
have not been documented by LANL in a formal report, and we have not re-
~
However, based on information. received at the meet-viewed them in any detail.
ing with the contractor, the results show that the hot leg U-be following recovery from the.small break LOCA (1.75 inch diameter), an d were of the natural circulation recovery enhancement methods previously liste However, LANL reported that the core remained cover'ed and decay They attributed the heat removal effective.
. heat was continuously removed from the core.
l upper to inte'rnal recirculation (steam exiting the core is vented to the vesse This situa-annulus and condensed by the cold HPI water entering the downcomer lly removed tion physically could only persist until the decay heat was eventua t
l entirely by the break flow or the system eventualTy sas refilled and na circulation reestablished.
d Before widely disseminating the results of the LAt L calculations, our Office of Regulatory Research to carefully document and eva
~
We believe this careful approach is assist us in confirming their validity.
l justified because the analyses showed that core decay heat w removed and that no core uncovery or heatup was predicted.
In sumary, although we are continuing our evaluation of the rate unisolated reactor steam bubble condensation in.the recovery from both isolated and that coolant system sma.ll-break LOCAs,in reactors like TMI-1, we do n k LOCAs steam bubbles present in the reactor coolant. system resulting fr (either isolated or unisolated) in either the cold er hot legs will result in unacceptable heatup of the core.
E SIG D 0?
XAL
~
O D%s Certified 37_V I
e
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. Figure 4
(,
Co=parison of T2AC and MW break flows.
skw Lowrato-Leor c.01-n2 cold-LEC RREAK tu.c'
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Figure 5 Cc=parison of fEAC af.i! MV pri=ary pressures.
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=
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SkW LowtRED-Loof 0.01-rT2 COLD-LEG 81tt AK *
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=u 4m cJ-e.
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1000 2000
.m soon m
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Figure 6 WC-calculited core liqul'd vole =e fraction,.
u s
B&W LOWERED-LC*.' O.05-TT2 COLD-LEG BREAK tic O#
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h m.
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5 94 sn-w 340-O i
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{
m C-ca3:ulated ma=i=u= averste rod claddi g te=perature.
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