ML20085A478

Forwards Draft Repts on Small Break Scenarios to Be Used in Small Break Cooldown Procedure.Comments Requested. Descriptions of RCS Leaks W/Auxiliary Feedwater & W/O Feedwater & Small Breaks in Pressurizer Encl

ML20085A478
Person / Time
Site:	Davis Besse, Crane
Issue date:	05/02/1979
From:	Rosalyn Jones BABCOCK & WILCOX CO.
To:	Kane E BABCOCK & WILCOX CO.
References
TASK-03, TASK-07, TASK-3, TASK-7, TASK-GB GPU-0397, GPU-397, NUDOCS 8307060521
Download: ML20085A478 (13)

Text

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i THE SAS'C0CK & alLCOX COMPANY

((6[Emli377f0310tN).

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PO'aER GENERATION GROUP g/ 7/p/ ig, g, hgji

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E.R. K.AKT., LICENSING

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From

• • • • • • 8 R.C. JN. ECCS.GA1.YSIS (2066) d3(

File no.

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P Date Subj.

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?'.AT 2.1979 SMMJ. mAK SCENARIOS gn.

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Attached are draft writeups on small break scenarios to be used in the l

small break cooldown procedure. You are requested to cocument on these writeups and place them in a form suitable for the procedure. The en-N closures are in three parts:

2 1.

RCS leaks (non-FZR) with auxiliary feedvater.

2.

P.CS leaks (non-PZR) without auxiliary feedvar er.

3.

Smil breaka in pressuriser.

A If you have any questions on these writeups, call either R.C. Jones 1

(X-2066). R.J. Sala (X-2600), or R.m Dunn (X-2138).

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RCJ/lc cc E.A. Vomac's 4

i B.M. Dunn R.J. Salm D.F. Hallman N.S. Elliott l

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i i i DISCRIFTION OF Sv.U L EZI.U: TRANSIr;T BEHAVIOR

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The response of the prisary syste= to a s all breat v1.ll di".~ar great.ly y

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depending on the break size, its location in the systen, operation of the

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reactor coolant pumps, su=her of ECCS syste=s functioning, and availability of

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secondary side cooling. Figures 1 ar.d 2, plots of RCS pressura and pressurizar y*

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level histories for various combination of parameters, indicate the vide range "gr r.

of behavior.

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CASES VITH AIDCTI.IART TEEDUATER a

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Curves 1 and 2 of Figure 1 show the behsvior.of F.CS pressure to breaks that are large enough in combination with the ECCS to depressurize 3,

the systen to a stable low pressure. ECCS injection easily exceeds core boil-Curves 1 and 2 ef Figure 2 show the pressuriser level transier.t. Rapidly off.

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falling pressure causes the het legs to quickly saturate. C 'd les temperature reaches saturation sonevhat later as RC punps coastdown or the RCS depressurizes f.

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1 belov the secondary side saturation pressure. Since these breaks are capable h,

t of depressurizing t!.e RCS without aid of the stean generators, they are largely insensitive to the availebility of auxiliary feedvster. Operation of the re-

c. t actor coolant pumps also plays little role in the course of events. Other than T*

verifying that all ESTAS actions have been completed. *he entrator needw to do nothing to bring the systen to a safe stable coadition.

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Curve 3 of Figure 1 shows the pressure transient for a break which is too s=all in combination with the operating HFI to depressurite the RCS. The I

steam generators are being relied upon to remove a portion of core decay heat.

If the reactor coolant pu=ps are not operating, and the pressure has stabilized f.re*-=~'MMq*

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I near the seco dary side pressure, RCS pressure cay evesta. ally bes;in falling j

again as the decay heat level decreases. If the RC pu=ps are operating, pressure 4j nay, or may not decrease, it could eve =n 21 ty increase to sc=e stable level as h-1 s

the E?I refill and repress =rize the 2CS. The assu=ption for this case is that i

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~j secondary cooling is ~ainemined.

Curre 3 of Figure 2 shows pressurizer level t

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ture quickly falls to the saturated te=perature of the secondary side and controls j_

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primary systen pressure at s.aturation. The cold leg f:emperature remains slightly 1.

subcooled. If the EPI refill and repressurize the RCS, c' s hot legs can once

= ore become subcooled. The operator needs only to verify ESyAS actions and, if

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RC punps have not failed, lexve one ru= sing in each loop.

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Curve 4 of Figure 1 sbcvs the be'.avior of a small break in which the te 2

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actor coolant purrps have been lost and the break is too s=all in combination

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with the EP1 available to depressuri=e the pri=sry syste z.

Although auxiliary feedwater is available, loss of primary sys.te= coolant eventually leads to in-T

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terruption of circulation around the loop. This is followed by a gradual re-pressurization of the primary systen. Although not likely, it is conceivable 2

2 that the pri=ary systen could repressurize as high as the pressurizer safety be valvss before the pressure stabi1*ses. This is shown by the dashed line in Figure 4 1 -

Once enough inventory has been lost from the primary system to allow direct stvas condensation 1.s the r6gion of the secas generators adjacent to the secondary side coolant, t':e steam condensation forces a depressurizarfon of E

the pri=ary systers down to the saturation pressure of the secoc.iary side.

l'l Since the cooling capabilities of the secondary side are =eeded to remove l

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decay heat, pressure vill not fall below that on the secondary side. HPI is sufficient to exceed boil-off in the core, andcondensation in the steam generators renoves decay heat energy frc-the systes. The reactor. coolant system is in a

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action. The pressurizer level transient is characterized by Curve 3 of Figure l

q 2 during the depressurf r.atiei. sed Curve 4 of Figure 2 dttri :y, the re~cra.y repre.ssurization phase. The dashed line indicates the icvel behavior if pressure is forced up to the pressurizer safety valve setpoint. During this transient, w

bot leg temperature vill rapidly approach saturation with the initial system m

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9 depressurization and it vil.1 remain saturated during the vbole transient. Cold

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les temperature vill approach saturation as circulation is lost, but may remain slightly subcooled during the repressurization phase of the transient. Once i

direct steam condensation develops in the steam generator, ROS depressurization k

vill cause the cold les ta=peratures to quickly reach saturation. Cold les a

f temperatures may once more subcool slightly once the primary systen has stabilized at secondary side pressure. Subsequent filling of the pri=ary system by the EFI may cause temporary icterruption of steam condensation in the stean generator as the primary side level rises above the secondary side level. If the de-

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pressurizatiou capability of the break and the 3FI is insufficient to offset decay heat, the prir.ary systen vill once more repressurize until enough RCS i

coolant is lost to allow direct steam condensation in the steam generator. At this point the primary system will once again dep tessurize to secondary side 1

preasure. This cyclic behavior will stop once the HF; and break can balance 1

decay heat or the operator takes some a. tion.

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Curve 5 of Figure 1 shows the behavior of RCS pressure to a break in which RC ptraps are lost, but v'-ich don not lesd to intarruption of natural circula-tion. The high pressure injection is available and exceeds the leak flow before

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the pressurizer has e=ptied.

The primary systen es=ains subcooled and natural circulation to the steam generctor removes core decay heat. As Curve 5 of Tigure 2 shows, the pressurizer never er.pties and continues to control primary system pressure.

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There are three basic c. lasses of break respense for small breaks without 9

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auxiliary feedvater. These are:

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Those breaks capable of relieving all decay heat via the break.

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Breaks that relieve decay heat.rith bech the EPI injection and via the break.

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Breaks which do not automatically act= ate t',e HPI and result in i

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14 The system pressure transients for these breaks are depicted in Tigure 3.

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For Class 1. it is seen that the RC system 7tess.:re decraases rather smoothly o

,3 throughout the transient. For the larger breaks in this class. CFT actuation and 1.PI injection will probably occur. For the s= aller breaks of this class I'

only. CFT actuation vill occur. Auxiliary feedwater injection is not necessary s

for the shert tern stabilization of these breaks.

j Tor Class 2 breaka, the MC pressure vill rapidly reach the ESTAS trip f

1 signal 62 to 3 min). With e.he RPIs on, a slev systen depressurization vill f

be established coincident with the decrease in cere decay heat. No CFT actua-T tion is expectsd. Auxiliary feedwater is not necessary for the short tera

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stabilir ation of these breaks.

t Autotatic ESTAS actuation vill not occ*ar fer Class 3 *oreaks. Once the SC second:.ry side inventory is boiled off, systa=s repressurization vill occur V

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as the break is not capsble of removing all the f.ecay heat being generated in the core. Systes repressurization to the POEr er the pressurizer safety L

valves will occur for snaller breaks in this cla.ss.

For the "zero" break t

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d case, repressurization to the POK7 vill occur i= the first 5 sinutes. Operator J

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action is required within the first 20 =inutes to ensure core coverage through-out the transient. For the 177-TA lowered-loop plants, this action can be either manual actuation of the auxiliary feedwater cystem or the EPI systes.

The establishment of auxiliary feedvater vill rapidly depressurize the RCS to the T. STAS actuation pressure, and system pressure vill stabilize at a

2 eitber the secondary side SG pressure or at a pressure where the EFI equa.ls W

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Tor the Davis-Besse plants (raised loop design) operator action is J

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necessary at some time greater than 20 rtinutes (probably 40 ninutes) due to the increased inventory in the loops that is available to drain into the re-L actor vessel. However, due to the low shutoff head on the HPI system, the a

operator nist establish auxiliary feedwater in order to depressurize the RCS.

t Pressurizer level response for the first two classes of breaks will be similar to that depicted for Case 1 and 2 breaks discussed in the previous

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M section. For the Class 3 breaks, pressuriz.er level response vill be as shown I

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r in Figure 4 The minious refill time for the pressuriser 1, that for the "zero"

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l break and is shovu on Tigure 4.

Af ter initially drawing inventory from the

.c pressurizer, the system pressurization vill cause the pressurizer level to 1

increase, pessibly to full pressurizer level. Once the operator action to re-i store auxiliary feedwater has been taken, the system depressurizatica vill result in outsurge from the pressurizar, probably resulting in a couplete loss

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of pressurizer level. For the smaller breaks of Class 3 which results in a system repressurization following the actuation of the !IPI systen, pressuriser level vill increase and then stabilize.

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t'ithout auxiliary feedvater, both the hot les and cold icg te=peratures vill saturate early in the transient a d, for the Class 1 and 2 breaks, vill remain saturated. For the Class 3 breaks, once auxiliary feedwater is established.

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g temperature corresponding to the SG secondary side pressure and vill re=aia b

there throughout the re=ainder of the transient. Het leg te=peratures will I

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The systen pressure transient for a < mil brea in the pressurizer vill 9-be.have in a sinf1=r ranner to that previously discussed. The in.icM d= pres-

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surization, bovever, vill be core rapid due to scez= relief out the break.

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The pressurizer level response for these accidents will initially behave a

4 like a verf small break without auxiliary feedvater. As shout on Yigure 5. an i

2 initial rise in pressurizer level vill occur doe to the pressure reduction in y

3 the pressurizer and the subsequent insurge into the pressurizer from the RCS.

i once the reactor tripw. system contraction results in a decreasing level in 1

the pressurizer. Tlashing vill ultimately occur in the hot leg piping causing e

For an insurge into the pressurizer and vill rapidly fill the pressurizer.

the rer.ainder of the transient, the pressurizer will renzin solid. Towards the later stages of the transient.' the pressurizer vill be filled by a two-phase nizture. However, the indicated level vill show that the pressuriser is only partially full.

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