Large Break Loss of Coolant Accident

8/12/2019 Large Break Loss of Coolant Accident

1/11

~ PergamonAnn. NucL Eneryy,Vol.21. No. 3, pp. 17%187, 1994

Copyright 1994ElsevierScienceLtdPrinted n Gre at Britain.All rights eserved

0306-4549/94 6.00+ 0.00

L A R G E B R E A K L O S S O F C O O L A N T A C C I D E N T

A N A L Y S I S O F A D I R E C T C Y C L E S U P E R C R I T I C A L

P R E S S U R E L I G H T W A T E R R E A C T O R

S. KOSHIZUKA, K. SHIMAMURA an d Y. OKA

Nuclear Engineer ing Research Labora tory, Facu l ty o f Engineer ing , Univers i ty o f Tokyo ,2-22 Shi rane , Sh i raka ta , Toka i -mura , Naka-gun , Ibarak i 319-11 , Japan

R e c e i v e d 8 J u n e 1 9 9 3 )

A b s t r a c t - - L a rg e - b r e a k l o s s -o f - c o o la n t a c c id e n t ( L O C A ) w a s a n a l y z e d i n t h e c o u r s e o f t h e d e s i gn s t u d yconcern ing d i rec t -cyc le superc r i t i ca l -p ressure l igh t wate r reac tor (SCLWR). The advantages o f SCLWR

are a h ighe r therma l e ff ic iency a nd s impler reac tor sys tem than the cur ren t l igh t wate r reac tors (LW Rs) .A computer code was prepared for the ana lys i s o f the b lowdown phase f rom the superc r i t i ca l p ressure .T h e c a l c u l a t io n w a s c o n n e c t e d to t h e R E F L A - T R A C c o d e w h e n t h e s y s te m p r e ss u r e d ec r e a se d t o a r o u n da tmo spher ic p ressure . The ana lyze d acc iden ts a re 100 , 75 , 50 and 25 % co ld- leg and 100% ho t - leg breaks .F i r s t, b l o w d o w n a n d h e a t u p p h a s e s w i t h o u t a n e m e rg e n c y co r e c o o l in g s y s t em ( E C C S ) w e r e e v a lu a t e d . Alow-pressure coo lan t in jec t ion sys tem (LPC I) was des igned to f il l the core wi th wate r before the c ladd ing(s ta in less - stee l ) t emp era ture reached a l imi t o f 1260C. The LPC I cons i s t s o f four un i t s , each of which h asthe capac i ty 805 kg /s . An au tomat ic depressur iza t ion sys tem (ADS) was des igned to re lease the s teamg e n e r a t e d i n t h e c o re i n t h e c a s e o f c o ld - le g b r e a k s a n d t o p e r m i t o p e r a t i o n o f L P C I i n t h e c as e o f L O C A sof l ess than 100% break . F or a l l cases ana lyzed , the peak c ladd ing tem pera tures were lower than the l imi twhe n the designed ECC S i s implemen ted .

l . INTRODUCTION

We h a v e b e e n s t u d y i n g t h e d i r e c t -c y c l e s u p e r c ri t ic a l -p r e s su r e li g h t w a t e r r e a c t o r ( S C LW R ) ( O k ae t a l . ,1 99 :2 ; O k a a n d K o s h i z u k a , 1 9 92 ) . T h e t h e r m a le f fi c ie n c y w a s h i g h e r a n d t h e r e a c t o r s y s t e m w a s s i m -p l e r t h a n t h e c u r r e n t l i g h t w a t e r r e a c t o r s ( L W R s ) .P r i n c i p a l c h a r a c t e r i s ti c s o f S C LW R a r e l is t e d i n Ta b l e1. T h e s y s t e m p r e s s u r e w a s 2 5 0 b a r . T h e i n l e t a n do u t l e t c o o l a n t t e m p e r a t u r e s w e r e 3 1 0 a n d 4 1 6 C ,r e s p ec t iv e l y. T h e c o o l a n t t e m p e r a t u r e i n c r e a s e d c o n -t i n u o u s l y i n t h e c o r e . T h e s u p e r h e a t e d s t e a m w a sd i r e c t l y f e d t o t h e t u r b i n e a n d r e c i r c u l a t i o n l i n e s ,s t e a m s e p a r a t o r s a n d d r y e r s w e r e n o t n e c es s a r y. S t a in -l e ss - st e el w a s e m p l o y e d a s t h e c l a d d i n g m a t e r i a l t ow i t h s t a n d t h e h i g h p r e s s u r e a n d t e m p e r a t u r e . S i n c et h e e n t h a l p y d i f fe r e n c e w a s l a r g e b e t w e e n t h e c o r ei n l e t a n d o u t l e t, t h e c o r e m a s s f l o w r a t e w a s v e r y s m a l l.T h e r e f o r e t h e p i p e d i a m e t e r o f th e m a i n s t e a m l i n e sw a s m u c h s m a l l er t h a n t h e P W R s a s w el l a s th e n u m -b e r o f l in e s b e i n g r e d u c e d t o 2 . I n o r d e r t o a v o i d t h eh e a t t r a n s f e r d e t e r i o r a t i o n o f s u p e r c r i ti c a l w a t e r , t h eh i g h c o r e w a s a d o p t e d . T h e r e a c t o r v e s s e l i s i l l u s t ra t e din F ig . 1 .

T h e p u r p o s e o f t h e p r e s e n t s t u d y w a s t o a n a l y z el a r g e - b r e a k l o s s - o f - c o o l a n t a c c i d e n t s ( L O C A s ) o f t h eS C L W R . T h e a x i a l c o o l a n t d e n s i ty c h a n g e w a s l a rg e r

t h a n t h a t o f b o il i n g w a t e r r e a c t o rs ( B W R s ) . T h e l o w e r

c o r e, t h e d o w n c o m e r a n d t h e c o l d p l e n u m w e r e f il le dw i t h h i g h - d e n s i t y w a t e r , w h i l e t h e u p p e r c o r e a n dt h e u p p e r p l e n u m w e r e f il le d w i t h l o w - d e n s i t y w a t e r ,s u p e r h e a t e d s t e a m . T h e r e w e r e n o r e c i r c u l a t i o n l i n e sl ik e t h e B W R . T h e p r e s s u r e w a s s u p e r c r it i c al . F r o mt h e s e c h a r a c t e r is t i c s, L O C A , p a r t i c u l a r l y t h e b l o w -d o w n p h a s e , w a s d i f fe r en t f r o m L W R s .

W a t e r i s a s i n g l e p h a s e f l u i d a t s u p e r c r i t i c a lp r e s s u r e , b u t s t e a m a n d w a t e r a r e s e p a r a t e d b e l o wt h e cr i ti c a l p o i n t , 2 21 b a r . N o c o n v e n t i o n a l L O C Aa n a l y s i s c o d e c a n h a n d l e i t. A c o m p u t e r c o d e w a sd e v e l o p e d t o a n a l y s e t h e b l o w d o w n f r o m t h e s u p e r -c r i t i c a l p r e s s u r e . W h e n t h e s y s t e m p r e s s u r e f a l ls

Table 1. Principal characteristics of SCLW R

Core dia/heightCoolant inlet/outlet temperatureCoolant inlet/outlet densityFuel rod/pellet diameterCladding/thicknessMaximum power densityPressureFlowrateCoolant average velocity, core inlet/outletPressure vessel height/inner diaCoolant loop numberInlet/outlet pipe diaThermal powerEfficiencyElectrical power

226/570 cm310/416C0.725/0.137 g/cm30.8/0.694 cmSS/0.046 cm363.6 M W/m 3250 bar2,032kg/s1.27/6.77 m15.7/3.26 m234/53 cm2.780 MW0.4121,145 MW

177


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178 S. KOSH1ZUKAet al.

Cold- leg Hot- leg

N I I I . ~ U p p e r p le nu m

I ~ ~ ~ ' ' ~ - D w n cm er

~ Lower p lenum

Fig. 1. Cross-sectional view of the SCL W R pressure vessel.

a r o u n d a t m o s p h e r i c p r e s s u r e , th e c a l c u l a t i o n i s c o n -n e c t e d t o t h e R E F L A - T R A C c o d e ( A k i m o t o a n dM u r a o , 1 9 9 2; A k i m o t oe t a l . 1992) , which wasd e v e l o p e d i n t h e J a p a n A t o m i c E n e rg y R e s e a r c h I n s t i -t u te b a s e d o n T R A C - P F I . T h e h e a t u p a n d r e f lo o d i n gp h a s e s a r e al s o an a l y z e d u s i n g R E F L A - T R A C .

F i r s t , t h e b l o w d o w n a n d h e a t u p p h a s e s w e r e c a l -c u l a t e d w i t h o u t a n e m e rg e n c y c o r e c o o l i n g s y s t e m( E C C S ) . N e x t , a l o w - p r e s s u r e c o o l a n t i n j e c t i o n s y s -t e m a n d a n a u t o m a t i c d e p r e s s u r i z a t i o n s y s t e m w e r ed e s i g n e d f r o m t h e h e a t u p a n a l y s i s . T h e n t h e b l o w -d o w n , h e a t u p a n d r e f l o o d i n g p h a s e s w e r e a n a l y z e di n c l u d i n g t h e d e s i g n e d E C C S .

2 M O D E L I N G

2 . 1. S u p e r c r i t ic a l p r e s s u r e b l o w d o w n m o d e l

A b o v e t h e c r i t i c a l p r e s s u re , t h e b o u n d a r y b e t w e e nl i q u i d a n d g a s d i s a p p e a r s . T h e w a t e r d e n s i t y c o n -t i n u o u s l y c h a n g e s w i t h t e m p e r a t u r e , b u t t h e s p e ci fi ch e a t s h o w s a m a x i m u m a t a p s e u d o - c r i t ic a l t e m -p e r a t u r e Tin. T h e d e n s i t y c h a n g e i s l a rg e a r o u n d t h eTin. F o r a n a l y z i n g th e b l o w d o w n p h a s e , t h e fo l l o w i n ga s s u m p t i o n s w e r e m a d e :

(1 ) The r e ac to r ves se l was f i l l ed wi th tw o typ es o ff l u i d c o r r e s p o n d i n g t o t h e i n l e t a n d o u t l e t d e n -

s i t i e s : h igh -dens i ty f l u id , wa te r (310C) and

l o w - d e n s i t y f lu i d , s t e a m ( 4 16 C ). T h e w a t e re x i s t e d i n t h e d o w n c o m e r, t h e l o w e r p l e n u m a n dt h e l o w e r c o r e . T h e u p p e r c o r e a n d t h e u p p e rp l e n u m w e r e o c c u p i e d b y t h e s t e a m . T h e fl u i dp o s s e s si n g i n t e r m e d i a t e p r o p e r t i e s i n t h e c o r e w a sn e g l e ct e d . T h e r e s u l t s o f c a l c u l a t io n w o u l d h a v eb e e n l i t t le a ff e c te d b y t h i s a s s u m p t i o n b e c a u s e o fa r e l a t i ve ly sma l l i nven to ry o f wa te r i n t he co re .

(2 ) S ince the i n i ti a l c r i t i ca l f l owra t e a t t he b reak wasm u c h l a rg e r t h a n t h e c o r e f l o w r a t e a t n o r m a lo p e r a t i o n , z e r o v e l o c i t y o f t h e c o r e f l o w w a sa s s u m e d a s t h e i n i t ia l c o n d i t io n . T h i s i m p l i e dt h a t t h e i s o l a t i o n v a l v e s c l o s e d in s t a n t a n e o u s l y.

(3 ) The f lu id i ne r t i a was neg lec t ed . The o u t l e t f l ow a tt h e b r e a k a t t a i n e d c r i t i c a l v e l o c i ty a t t h e s a m et i m e a s t h e b r e a k o c c u r e d . P r a c t i c a l l y i t n e e d s

seve ra l s econd s fo r t he acce l e ra t ing f lu id t o a t t a inc r i t i ca l ve loc i ty. Th i s s imp l i f i ca t ion made theb l o w d o w n t i m e s h o r t e r , w h i c h l e d t o a n e a r l i e rh e a t u p o f t h e c la d d i n g .

( 4) T h e s t o r e d h e a t a t s t e a d y - s t a t e o p e r a t i o n w a sred i s t r i bu ted in t he fue l a t t ime ze ro . The in i t i a lc l a d d i n g s u r f a c e t e m p e r a t u r e b e c a m e s l ig h t l y h i g hw i t h t h i s a s s u m p t i o n .

( 5) T h e t e m p e r a t u r e d i s t r i b u t i o n i n t h e f ue l w a s s i m -p l i fi e d t o b e u n i f o r m d u r i n g t h e b l o w d o w n p h a s e .T h e c o r e w a s u n c o v e r e d j u s t a f t e r t h e b r e a k a n da l a rge nega t ive r eac t iv i ty was in se r t ed . The re fo re

t h e c o r e p o w e r d e c r e a s e d t o t h e d e c a y h e a t l e v e la n d t h e t e m p e r a t u r e d i f f e r e n c e i n t h e f u e l w a ssma l l .

( 6) T h e c r i ti c a l m a s s f l o w r a t e w a s c a l c u l a t e d u s i n gM o o d y ' s m o d e l w h e n t h e q u a l i ty w a s l a rg e r th a n2 % a n d l o w e r th a n 1 0 0 % a t t h e b r e a k . O th e r w i s e ,t h e a c o u s t i c s p e e d w a s u s e d . T h e c h o k e p r e s s u r e a tt h e b r e a k r e m a i n e d s u b c r i t i c a l t h o u g h t h e s y s t e mp r e s s u r e w a s s u p e r c r it i c a l. T h u s M o o d y ' s m o d e lw a s a p p l i c a b l e t o t h e a n a l y s is .

( 7 ) D i t t u s - B o e l t e r ' s c o r r e l a t i o n w a s e m p l o y e d f o re s t ima t ing the hea t t r ans fe r coe ff i c i en t f rom thec l a d t o s u p e r c r i ti c a l w a t e r. T h i s c o r r e l a t i o n p r o -v ides conse rva t ive va lues a roun d Tin .

( 8) T h e c o r e p o w e r w a s e v a l u a t e d f r o m p o i n t k in e t i ce q u a t i o n s i n c l u d i n g s i x g r o u p s o f d e l a l y e d n e u -t ro n s a n d a d ec a y h e a t o f A N S + 2 0 % . T h ei n s e r te d r e a c t iv i t y w a s c a l c u l a t e d f ro m t h e c o r ec h a r a c t e r is t i c s a n d t h e a v e r a g e c o o l a n t d e n s i t y a teach t ime s t ep .

( 9) H e a t c a p a c i t y o f th e r e c t o r v e s se l a n d o t h e r s t r u c -tu re s was neg lec t ed .

T h e n o d e d i a g r a m u s e d f o r t h e c o l d - l e g b r e a k a n a l y -s is is dep ic t ed in F ig . 2 . Wh o le r eac to r ves se l was

r e p r e s e n t e d b y t h r e e n o d e s . T h e d o w n c o m e r, t h e


3/11

Large -break loss -of -coolant acc ident ana lys is 179

S t e a m n o d e # I [I

IS t e a m 1

n o d e # 2 1I I [ o r e

LL i q u i d n o d e Cr i t i c a l f l ow

Fig . 2 . Rea c to r mo de l f o r b lowdow n ana ly s i s o f t he co ld - l egbreak .

l o w e r p l e n u m a n d t h e l o w e r c o r e w e r e s i m u l a t e d b yo n e li q u id n o d e o c c u p ie d b y u n i f o r m w a t e r . T h eu p p e r p l e n u m a n d t h e u p p e r c o r e w e r e re p r e s en t e d b ys t e a m n o d e s 1 a n d 2 , r e s p e c t i v e l y. I n i t i a l l y t h e f l u i dp r o p e r t i e s i n b o t h s t e a m n o d e s w e r e t h e s a m e . M a s sa n d e n t h a l p y b a l a n c e s w e r e c a l c u l a t e d i n e a c h n o d eb y c o n s i d e r i n g th e i n t e r a c t i o n w i t h o t h e r n o d e s . N o d ev o l u m e s w e r e v a r i a b l e s.

T h e c a l c u l a t i o n p r o c e d u r e i s s h o w n i n F i g . 3 . F i r s t ,

t h e e x i s te n c e o f l i q u id i s j u d g e d . W h e n l i q u i d r e m a i n s ,t h e c r i t i c a l f l o w i s e v a l u a t e d u s i n g t h e l i q u i d p r o p e r -t ie s . M a s s c o n s e r v a t i o n e q u a t i o n s a r e :

w~'~ l = w~'~ -A w '~, ~t -d t (1 )

w~g ' = w~g~--Aw~ g,~,2 dt , (2)

w~g ' = W~gz Aw ~g,~g2 d t , (3)

w h e r e w d e n o t e s t h e t o t a l m a s s i n t h e n o d e . S u b s c r i p t s1, g l a n d g 2 r e p r e s e n t t h e l i q u i d , s t e a m 1 a n d s t e a m 2n o d e s , r e s p e c t i v e l y. T h e s u p e r s c r i p t s r e p r e s e n t t h et i m e s t e p s . T h e q u a n t i t y Aw e , , d e n o t e s t h e c r i t i c a lm a s s f l o w r a t e a t t h e b r e a k a n d Aw gj ~ g2 r e p r e s e n t s t h em a s s t r a n s f e r f r o m s t e a m n o d e 1 t o 2. T h e s t e a mp r e s s u r e w a s c a l c u l a t e d f r o m t h e e n t h a l p y, t h e t o t a l

m a s s a n d v o l u m e o f t h e n o d e . T h e m a s s t r a n s f e rAw gl ~ g2 w a s i t e r a t i v e l y e v a l u a t e d t o b a l a n c e t h e p r e s -s u r e s o f t h e t w o s t e a m n o d e s .

A f t e r t h e l i q u i d m a s s w a s e n t i r e l y l o s t a t t h e b r e a k ,s t e a m i n n o d e 2 f lo w e d o u t . A t t h i s p h a s e t h e m a s sc o n s e r v a t i o n e q u a t i o n s a r e :

w ~ g~ - = w ~ g t - - A w ~ g , . ~ d t , ( 4 )

w ~g = w ~ 2 + ( A w ~ g , . g 2 - - A w ~ , t ) d t . ( 5 )

T h e e n t h a l p y c o n s e r v a t i o n e q u a t i o n s a r e :

~ s ~ ~

[ Cr i t ica l f low of l iqu id ]

[S t eam node #2 ex pands [

[ S team node #2 pressure decreases ]

Mass t ransfer f roms t eam node # l t o #2

F - -

i ea t t ransfer f rom coreto s team node #2

not ex is t s

[ Cr i t ica l f low of s team [

] S team nod e #2 inventory decreases ]

[ S team nod e #2 pressure decreases ]

I Ma ss t ransfer f rom I .~ . . .. . ._s team node #1 to #2

Heat t ransfer f rom coreto s team node #2

Fig . 3 . Calcula t ion f lowcha r t for b lowdow n analys is of the co ld- leg break .


4/11

180 S. KOSHIZUKA t al.

h~ +' = h~, (6)h k I kgl = hgl, (7)

h ~+ 1 = h ~2 + [ O + h ~ - h ~ 2 ) Aw g , ~ g 2]d t /w ~ g +1 , 8 )

where h denotes the enthalpy per unit mass. The quan-tity Q represents the heat transfer from the core tosteam node 2. This is calculated using Dittus-Boe lter'sheat tran sfe r coefficient. The flow velocity used in thecorrelation is evaluated from Aw g I - g 2 .The density ofliquid is kept constant, so that the volume of the liquidnodes is uniquely determined from the total mass. Thevolume of steam node 2 increases with a decreasingvolume of liquid node, while the volume of the steamnode 1 is constant. The density of steam was evaluatedfrom the total mass a nd volume of the node. The other

properties o f fluid were obt ained from the density andenthalpy using a steam table.The node diagram used for the hot-leg break is

depicted in Fig. 4. The whole pressure vessel is rep-resented by liquid and steam nodes. The calculationprocedure is shown in Fig. 5. Steam flows out at thebreak. When the pressure decreased to the saturatedpressure of the liquid, flashing occurred. The heattransfer from the core to the coolant was calculatedusing the steam velocitiy derived from this flashing.However the heat was transferred only to the liquid.This ass ump tion implies that the liquid level increasedup to the upper part of the core due to the flashing,and the heat was main ly consumed in the boiling oftwo-phase mixture.

The mass conservation equations are representedby :

w* = w~, (9)

W ~ : W ~g - m l ~ c r i t d t . 1 0 )

When the pressure was higher than the saturated oneof the liquid, the masses corresponding to k + 1 time

I LSteam node [---- Critical flow

ig. 4. Reactor model for blowdown analysis of the hot-leg

break.

step were equal to those asterisked. If not, the flashingmass was incorporated int o the calculatio n as :

W~I1-1 w~l --~ ), (11)

w ~ + = w * + w * z , (12)

where Z denotes the f raction of the flashed steam a ndis evaluated from the mixt ure of saturated liquid andsteam en thalpies :

h g ~ a t ( P * ) z + h , s a t ( P * ) ( 1 - Z )= hk, (13)

where the subscripts lsat and gsat denote saturatedliquid and steam, respectively. The saturated enthal-pies are the functions of pressure p*, which is evaluatedexplicitly from the steam node properties. Theenthalpy per unit mass is kept consta nt as equations

(6) an d (7) when the flashing does not occur. Wi th theflashing, the liquid and steam enthalpies are calculatedfrom :

h ~ + I = h ~ , t ( p * ) + O d t / w ~ j + ~ , (14)

h i + , . ~ + , k . , * . 1 5 )h g W g - [ - h g s a w I ~ .

The steam volume increases with a decreasing liquidvolume due to the flashing.

2 . 2. H e a t u p a n d r e f lo o d i n g m o d e l

REFLA-TRAC is a best-estimate code for pre-dicting LWR plant transients. The results of the blow-

down calculation are converted to the input data ofthe REFLA-TRAC code and successive heatup andreflooding phases are analyzed. Though the REF LA-TRAC code is available at a pressure lower than 170bar, the c alcul ation is proceeded to this code when thesystem pressure is low enough and the blowdown isalmost finished.

The com pon ent diagram is depicted in Fig. 6. Com-ponent 7 represents the core. The core component isdivided into 14 nodes, eight of which are connectedto heat conductors representing the fuel rods. Onlythe hottest channel is modeled and cosine power dis-tributi on is assumed in the axial direction. Comp onen t5 represents the lower plenum. It is connected to pipecompon ent 8 representing the downcomer. This com-ponen t is divided into I0 nodes. Valve compo nent 2was attached to the downcomer. The upper plenumrepresented by c ompone nt 6 was connected to valvecomponent 4. The ECCS was connected directly tothe lower plenum using one valve and one fill com-ponen t in the present model.

In the case of cold-leg break, valve 9 was openedand valves 2 and 4 were closed. Valve 2 was openedwhen LPCI was actuated, and valve 4 was openedwhen ADS was actuated. On the other hand, valve 4

was opened and the others are closed when simula ting


5/11

Large-break loss-of-coolant accident analysis 181

+At

~ . . . _ _ S TA RT

[ Crit ical f low at break [

[ Steam inventor~d e c r e a s e s

[ [ Stea m pressure decreases [

Core m ass f low rate I

= Flashing mass [ [ Core f low = 0 [

Iladd ing temperature [

Fig. 5. Calcula t ion flowchart for blowd own a nalysis of the hot-leg break.

5 .P lenum

6.Plenum

7.Core

Fig . 6. Nodal iza t ion for the RE FL A-T RA C code .

t h e h o t - le g b r e a k . L P C I i s s i m u l a t e d i n th e s a m e w a ya s t h e c o l d - l e g b r e a k , w h i l e t h e e f f e c t o f A D S w a sneg l ec t ed .

3 B L O W D O W N A N D H E A T U P A N A L Y S E S

3.1 . 100 cold- leg break

T h e i n i ti a l b l o w d o w n p h a s e w a s a n a l y z e d w i t h t h e

a s s u m p t i o n s e x p l a i n e d i n t h e p r e v i o u s s e c t i o n s . T h e

c a l cu l a ti o n w a s c o nn e c t e d to t h e R E F L A - T R A C c o dea t 2 0 s. F i g u r e 7 a s h o w s t h e p r e s s u r e t r a n s i e n t a t t h ec o r e . A p o i n t o f i n f le c t i o n a p p e a r s a t 6 s w h e n w h o l e

w a t e r w a s l o s t a t t h e b r e a k a n d s t e a m r e le a s es t a rt s . T h e d e p r e s s u r i z a t i o n s p e e d w a s r e l a t iv e l y s m a l ld u r i n g t h e w a t e r r e le a s e . T h i s i n f l e c t io n p o i n tw o u l d n o t b e s o c l e a r if t h e c o n t i n u o u s d i s t r i b u t i o no f c o o l a n t p r o p e r t i e s i n t h e c o r e i s c o n s i d e r e d . T h e

p r e s s u r e d e c r e a s e d a r o u n d a t m o s p h e r i c p r e s s u r e a t


6/11

82

a )

Cg. a

=

2Q .

250

200

150

100

50

00 2O 4 0 6 0

T i m e [ s ]

S . K O S H I Z U K Ae t a l

(a) 250

2 0 0

P .u

1 5 0

-,1

100PQ .

50

080 100 120 0 50 100

T i m e [ s ]150 200

b) 2000

Oo=

D

._=OO

_mt,3

1500

1000

5 0 0

, , . / I ip f

/ I/

f

1 I

t

Icore in le t

c o r e m i d d l e

core ou t l e t0 I =

0 20 40 60 80 100 120

T i m e [s]

F i g . 7 . 1 0 0 c o l d - le g b r e a k w i t h o u t E C C S : a ) c o r e p r e s-s u r e ; a n d b ) c l a d d i n g s u r f a c e t e m p e r a t u r e .

2 0 s . S i n c e t h e b l o w d o w n m a s s f l o w r a te i s a l m o s t z e r oa n d t h e r e a c t o r v e s s e l w a s f i l l e d w i t h a l m o s t u n i f o r ms t e a m , t h e c a l c u l a t i o n i s s m o o t h l y p r o c e e d e d t o t h eR E F L A - T R A C c od e.

T h e c l a d d i n g s u r f a c e t e m p e r a t u r e s a r e s h o w n i nFig . 7b . S ince the red i s t r ibu t io n o f the s to red hea t int h e f u e l i s a s s u m e d t o b e c o m p l e t e d a t t i m e z e r o , t h ei n i t ia l c l a d d i n g s u r f a c e t e m p e r a t u r e i n t h e c o r e m i d d l ew a s v e r y h i g h . T h i s h i g h t e m p e r a t u r e w a s s o o nd e c r e a s e d b y t h e r e v e r s e d c o r e f l o w d u e t o t h e b l o w -d o w n . T h e t e m p e r a t u r e s w e r e k e p t b e l o w 7 0 0 C f o r 1 0s a n d b e g a n t o i n c r e a s e l i n ea r l y. T h e c l a d d i n g s u r fa c et e m p e r a t u r e i n t h e c o r e m i d d l e r e a c h e d 1 5 0 0 C f o r100 s af ter the break.

3 .2 . 1 0 0 h o t - l e g b r e a k

T h e p r e s s u r e t r a n s ie n t a t t h e 1 0 0 % h o t - l e g b r e a k i s

d e p i c t e d i n F i g . 8 a . A p o i n t o f i n f l e c t i o n i s a l so

b ) l O O O

~ 8OO. o

ff 600,~E

m 400c

~5_m0 200

core in le t

c o r e m i d d l e

core ou t l e t

//

//

//

//

/i

// / . . . .I

/ / j ' -

~ ,,

0 50 100 150 200

T i m e [ s ]

F i g . 8 . 1 0 0 h o t - le g b r e a k w i t h o u t E C C S: ( a ) c o r e p r e s s u r e;a n d b ) c l a d d i n g s u r f a c e t e m p e r a t u r e .

observed a t 0 .7 s . In the in i t i a l s t age , s t eam wasr e l e a s e d f r o m t h e h o t - l e g a n d r a p i d d e p r e s s u r i z a t i o noccur red . When the p ressure decreased to the sa tu -r a t ed o n e a t 0 . 7 s , w a t e r b e g a n t o f l a sh a n d t h ed e p r e s s u r i z a t i o n s p e e d b e c a m e s l o w. T h e c a l c u l a t i o ni s p r o c e ed e d t o t h e R E F L A - T R A C c o d e a t 30 s.

F i g u r e 8 b s h o w s t h e c l a d d i n g s u r f a c e t e m p e r a t u re s .S i n c e t h e c o r e m a s s f l o w w a s m a i n t a i n e d , d i s -c o n t i n u i t i e s o f t e m p e r a t u r e a r e o b s e r v ed a t 3 0 s , th et i m e o f c o d e s w i t c h i n g . T h i s i s a t t r i b u te d t o t h e d i f fe r -e n c e s o f h e a t t r a n s fe r c o e f fi c ie n t , n o d a l i z a t i o n , t w o -p h a s e f l o w m o d e l , e tc . b e t w e e n t h e t w o c o d e s . T h eR E F L A - T R A C c o d e g av e lo w e r t e m p e ra t u re s t h a nt h e i n i t i a l b l o w d o w n c o d e . A c t u a l l y t h e t e m p e r a t u r ed i s c on t i n u i ti e s w e r e s m a l l a n d n o t s o m u c h i m p o r t a n tb e c a u s e t h e c l a d d i n g s u r f a ce s w e r e s u f f i ci e n t ly c o o l e d

b y s a t u r a t e d s t e a m d u r i n g t h e b l o w d o w n p h a s e u p


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Large-break loss-of-coolant accident analysis 183

( a )

.=

in=

n

2 5 0

2 0 0

1 5 0

1 0 0

5 0

0

0 2 0

I I

2 5

. . . . 5 0

7 5

4 0 6 0 8 0 1 0 0

T i m e [ s ]

b) 1 2 0 0

1 1 0 0

o

l O O O

a . 9 0 0 , ,

c 8 0 0

, Y7 0 0

6 0 0

0 2 0 4 0 6 0

T i m e I s ]

- - 2 5

. . . . 5 0 - -- - - - - ~

8 0 1 0 0

Fig. 9.75, 50 and 25% cold-leg breaks without ECCS: (a)core pressure; and (b) cladding surface temperature a t

hottest point.

to 100 s. The heatup of the cladding was prolongedcompared with the cold-leg break.

3 .3 . 75 , 50 an d2 5 cold- leg break s

In general 100% break LOCA is not always theseverest case, thus the smaller size breaks have to beanalyzed. As shown in the prev ious analyses, the cold-leg break provided a severer result than the hot-legbreak. The refore 75, 50 and 25% cold-leg breaks wereanalyzed. The pressure and the cladding surface tem-peratures in the initial blowdown phase are shown inFigs 9a and b, respectively. The blowdown periodwas extended due to the smaller critical flowrate. Thetemper ature reached the first peak when wate r wascompletely lost. This is attributed to the enhanced

core flow when stea m release starts. The claddin g

surface temperature began to increase again as thepressure and the core flow decreased. The claddingtemperature is higher for smaller break size when thepressure decreased around the atmospheric pressure.Therefore a depressurization system or a high-pres-sure coolant i njecti on system to cope with LOCA s lessthan 100% break is necessary.

In the 25% break L OCA analysis, the initial outletflowrate at the break is almost similar to the steadystate main flowrate. One intact loop of two can feedwater until the isolation valves are closed. Theassum ption tha t the i nitial core flow is set to zero maynot be appropriate. In order to analyze a smaller breakLOCA, more refined calcul ation is necessary.

4 ECCS DESIGN

Stainless-steel (SS) was employed as the claddingmaterial of SCLWR to withstand the high coolanttemperature. The peak temperature of SS claddingshould be lower than 1260C at LOCA (Coffman,1976). In the present study, LPCI was designed toreflood the whole core, the lower plenum and thelower downcomer until the cladding surface tem-l~rature reaches 1260C. The 100% cold-leg breakwas used for this evaluation. Time delay of ECCSinit iati on was assumed to be 30 s for starti ng up theemergency diesel generato rs.

The number of LPCI systems was four; two ofthem were connected to the cold-legs and the othertwo were connected directly to the downcomer. Twoof four systems were assumed to be unav ailab le : oneis the break line and the other is failed. The requiredmass flowrate of one system was 805 kg/s/uni t, whichwas 1.7 times larger capacity compared with BWR-5's, when assuming the downcomer width of 50 cm.The capacity can be reduced when a smaller down-comer width is used.

ADS is also designed for the two purposes. One isto accelerate the depressurization speed of LOCAsless than 100% break. Without ADS, LPCI had towait until the system pressure decreased to the LPCIpump head. The blowdown calculation results of 75,50 and 25% break LOCAs without ECCS showedthat the cladding surface temperature increased sig-nificantly before the system pressure decreasedenough. The other function is to release the steamgenerated in the core in the case of cold-leg break.Without the steam outlet at the hot-leg side, the corepressure increases and the ECC water cannot enterthe core but spill out from the cold-leg break. Thisphen omen on is similar to the steam bin ding in PWRs.The characteristics of the LPCI and ADS design

are summarized in Table 2. ADS should consist of


8/11

1 8 4 S . K O S H I Z U K A e l a l

Ta b l e 2 . E C C S f o r l a rg e b r e a k L O C A o f S C LW R

L o w p r e s s u r e c o o l a n t i n je c t io n s y s t e m ( L P C I ) 4 ~ 2 : C o l d - l e gN u m b e r o f u n i ts : [ 2 : D o w n c o m e r

C a p a c i t y : 8 0 5 k g / s u n i t

Ti m e d e l a y : 3 0 s

A u t o m a t i c d e p r e s s u r i z a t io n s y s t e m ( A D S ) L o c a t i o n : H o t - l e gTo t a l c r o s s s e c t i o n : 2 . 2 1 0 ~ ( m 2 )Ti m e d e l a y : 3 0 s

m u l t i p l e v a l v e s f o r e n h a n c i n g s a f e t y a n d r e l i a b i l i t y,bu t such d e t a i l ed des ign i s le f t f o r fu tu re s tudy

5 . R E F L O O D I N G A N A LY S E S

5.1 . 100 cold- leg break

T h e E C C S d e s i g n e d i s a d d e d t o t h e m o d e l . I n F i g .6 , t h e v a l v e c o m p o n e n t s 2 a n d 4 , w h i c h r e p r e se n t

L P C I a n d A D S , r e s p ec t i v el y, a r e o p e n e d a t 3 0 s . Tw oo f f o u r L P C I s y s t e m s a r e a ss u m e d t o b e a v a i l a b l e .C o n d e n s a t i o n i s n e g le c te d in t h e R E F L A - T R A C c a l-c u l a t i o n t o a v o i d n u m e r i c a l i n s t a b i l it y.

T h e p r e s s u r e , c l a d d i n g s u r f a c e t e m p e r a t u r e , m a s sf l o w r a t e a n d v o i d f r a c t i o n t r a n s i e n t s a r e d e p i c t e d i nF i g . 1 0 . W h e n E C C S i s a c t i v a t e d , th e v o i d f r a c t i o n o f

t h e l o w e r p l e n u m d e c r e a s e s i m m e d i a t e l y. T h e l o w e rp lenu m i s f i l led wi th t he E CC wa te r w i th in 10 s and

a ) 2 5 0

200

150

1

a .

50 ~,~.

00 50 100

T i m e [ s ]

150

c ) 4

3 0 0 0 - -

~ 2000,

1000

o~ o

- 1 0 0 0 1

-20000

200

_ 1 c o r e I

I I A I L_ /v /

line

50 100 150 200

T i m e [ s ]

(b) 1200

1000o

, 800 / \

ta. \\E 6 0 0 r ~ _

X '~c

4 0 0 X \ -

0 0

00 5 0

i I

100 150

- - co re inletcore middlecore outlet

d ) 1.0

0 .8

:: 0.60

0 . 4O>

0.2

0 .0 - ~200 0 100 150

l l , ,I l l r l lI I i ~ t .

I I ,

, II l JI I

I , I Ii l i ,I I

i I tI . I l

5 0

T i m e [ s ] T i m e I s ]

i

lower plenumc o r e

upper plenum

Fig. 10. 100% cold-leg break with EC CS: (a) core pressure; (b) cladding surface tem perature; (c) mass

flowrate ; and (d) vo id fraction.

200


9/11

Large-b reak loss-of-coo lant accident analysis 185

( a ) 2 5 0

2 0 0

L.. u

~ 1 5 0

- i

u~ 100

a

5 0

00 5 O 1 0 0

T i m e [ s ]

1 5 0 2 0 0

( c ) 1 0 0 0 0

5 0 0 0

_o~ 0

o

m - 5 0 0 0

1 0 0 0 0

0 5 0

I- - c o re

b r e a k l i n e

1 0 0 1 5 0

T i m e [ s ]

b ) l o o o

~ 8 0 0o?._,.=

Q .

E

t

~ 5

c o r e i n l e t

c o r e m i d d l e

c o r e o u t l e t

6 0 0

4 O 0

2

00 5 0 1 0 0 1 5 0

T i m e [ s ]

d )

c0o

0

2 0 0

1 . 0

0 . 8 - ~

I

o 5 / 1I

0 . 2 I

Io o \

0 5 0

I I

l o w e r p l e n u m

. . . . c o r e

- - - - - u p p e r p l e n u m

1 0 0 1 5 0

T i m e [ s ]

Fig. 11. 100 hot- leg break with EC CS : (a) core pressure; (b) cladding surface tem peratu re; (c) massflowrate ; and (d) void fract ion.

2 0 0

2 0 0

r e f l o o d i n g o f t h e c o r e t a k e s p l a c e. A s i l l u s t r a te d i nF i g . 1 0 b , t h e c l a d d i n g s u r f a c e t e m p e r a t u r e a t t h e c o r ei n l e t b e g i n s t o d e c r e a s e a t 3 5 s a n d q u e n c h i n g o c c u r sa t 6 0 s . B o t h c l a d d i n g s u r f a c e t e m p e r a t u r e s a t t h e c o r em i d d l e a n d o u t l e t r e a c h a m a x i m u m a t 5 0 s . T h ec l a d d i n g a t t h e c o r e o u t l e t i s q u e n c h e d a t 110 s.Q u e n c h i n g a t t h e c o r e m i d d l e i s n o t o b s e r v e d u n t i l2 0 0 s b u t i t w i l l s o o n b e a c h i e v e d . T h e h i g h e s tc l a d d i n g t e m p e r a t u r e o f 1 0 0 0 C i s o b s e r v e d a t t h ec o r e m i d d l e a t 5 0 s . T h i s i s s a t i s f a c t o r i l y l o w e r t h a nt h e l i m i t , 1 2 6 0 C .

A l a r g e o s c i l l a t i o n o f t h e c o r e m a s s f l o w is o b s e r v e dd u r i n g t h e q u e n c h i n g p h a s e i n F i g . 1 0 c. T h i s c o r -r e s p o n d s t o v o i d f r a c t i o n o s c i l l a t i o n s i n t h e l o w e r

p l e n u m a n d t h e c o r e a s s h o w n i n F i g . 1 0 d. T h i sp h e n o m e n o n i s c o n s i d e r e d i n s ta n t a n e o u s v o i d g e n-e r a t i o n i n t h e c o r e. T h e c o r e i s o n l y d i v i d e d i n t o 1 0n o d e s , e a c h o f w h i c h s t o re s l a r g e a m o u n t o f h e a ta c c u m u l a t e d i n t h e h e a t u p p h a s e . A l a r g e a m o u n t o fs t e a m i s g e n e r a t e d w h e n o n e n o d e r e p r e s e n t i n g 1 / 10c o r e i s q u e n c h e d . T h e r e f o r e r e v e r s e c o r e f l o w o c c u r sa n d t h e c o r e v o i d f r a c t i o n i n c r e a s e s. S i n c e t h e s t e a mg e n e r a t e d i n t h e c o r e i s r e l e a s e d t h r o u g h A D S , t h ec o r e p re s s u re s o o n d e c r ea s e s a n d t h e E C C w a t e re n t e r s t h e c o r e a g a i n .

W h e n t h e u p p e r p l e n u m i s c o m p l e t e l y f il l ed w i t ht h e E C C w a t e r a t a r o u n d 1 30 s, t h e m a s s f l o w b a l a n c eb e c o m e s s t a b le . O n e t h i r d o f t h e E C C w a t e r f l o w s o u t


10/11

186 S. KOSHIZUKA t al.

of the break line, and the remai nder flows up throug h (a) 30othe core and goes out of ADS.

5 .2 . 100 ho t - l eg b r eak

The 100% hot-leg break is analyzed with thedesigned ECCS. ADS is neglected in the present case. ~The calcula tion results are shown in Fig. I 1. The -~-temperature at any location in the core shows almostthe same behav ior due to the core flow maint ain ed byflashing. Whe n LPCI is initia ted at 30 s, the claddin g a.temperatures decrease rapidly. This is attributed toswitching from the blowdown calculation to theRE FL A- TR AC as explained before. The temperaturedecrease continues until about l0 s after LPCIinitiation because the cold water injected into thelower ple num is carried to the core. This is verified bylarge positive mass flowrate in the core as shown inFig. 1 c. The positive core flow conti nues for l0 s and (b) 1200reverse flow occurs for several seconds. This reversedflow is derived from rap id v oid genera tion in the core.This corresponds to the spike of the void fraction in ~' othe lower ple num as shown in Fig. 11 d. After the shortreverse flow, the core flow stagnates for about l0 s.Durin g this period, the cladding surface temperatures eLgradually increase and the void fractions in the Ecore and the upper ple num are kept high. Here the en 8o0

r-

void fractio n in the uppe r plen um is main tain ed h5o

between 0.3 a nd 0.7, while that o f the core reches more

than 0.9. This implies that the initial ECC water flows ~ 4ooup to the upper plenum and it is sustained by thesteam in the core. At 55 s, positi ve core flow occurs 20cagain. The cladding surface temperatures an d the voidfractions in the core and the upper plenum decreaseagain. Th e core flow reaches steady state at 90 s.

Consequent ly the claddin g surface temperatures arekept low eno ugh d urin g the whole period of the 100,4hot-leg break.

2 5 0

2 0 0

1 0 0

\\1o ~

0 5 0 100

i m e s ]

I

2 5 %

. . . . 5 0 %

7 5I

1 5 2 0 0

1000 Z

oo

2 5

5 0

7 5

5 0 1 0 0 1 5 0 2 0 0

Time Is]

Fig. 12. 75, 50 and 25% cold-leg breaks with ECCS : (a) corepressure ; and (b) cladding surface temperature at the hottest

point.

5 .3. 7 5 , 5 0 a n d 2 5 co ld -l eg b r eaks

In the LOCA analyses of less than 100% break,

ADS and LPCI were activated at 30 and 35 s, respec-tively. The pressure trans ients are show n in Fig. 12a.The pressure drops to atmospheric pressure a veryshort time after ADS is activated. This behavior isequivalent to the initial blowdown of the 100% hot-leg break. The transients of hottest cladding surfacetemperatures are shown in Fig. 12b. The temperaturetransients during the heatup phase are almost thesame among three cases of break size, thoughiniti ation of heatup is earlier in the larger br eak size.The tem perat ure peak emerges at 55 s in any case. T hehighest claddin g surface tempe ratur e is 1070C in thecase of a 25% break, though the differences between

the break sizes are small. A 5 s delay of LPCI acti-vation leads to the same delay time of the peak clad-ding temperature compared with the 100% breakLOCA. The cladding temperature decreases smoothlyafter the peak and will soon be quenched after 200 s.

Smaller breaks show almost the same behavior asthe 100% break with the combinat ion of ADS andLPCI.

6 . C O N C L U S I O N

Large-b reak LOCA s, 100% hot-leg and 100, 75, 50and 25% cold-leg breaks of SCLWR are analyzed.


11/11

Large Break Loss of Coolant Accident

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