Thermo-hydraulics during start-up in natural circulation boiling water reactors

ELSEVIER

Nuclear Engineering and Design 146 (1994) 241-252

Nuclear Engin .ng D lgn

Thermo-hydraulics during start-up in natural circulation boiling water reactors Jing-Hsien Chiang a, Masanori Aritomi a, Ryuichi Inoue a, Michitsugu Mori h a Research Laboratory for Nuclear Reactors, Tokyo Institute of Technology, 2-12-10hokayama, Meguro-ku, Tokyo, 152, Japan b Tokyo Electric Power Company, 1-1-3 Uchisaiwai-cho, Chiyada-ku, Tokyo, 100, Japan

Abstract

The transient behavior of natural circulation for boiling two-phase flow was investigated by simulating normal and abnormal start-up conditions to research the feasibility of natural circulation BWRs such as the SBWR. It was found that the instabilities, which are out-of-phase geysering, in-phase natural circulation oscillation and out-of-phase density wave instability, may occur during the start-up when the vapor generation rate is insufficient. In this paper, the mechanism of in-phase natural circulation oscillation induced by hydrostatic head fluctuation in steam separators, which has never been understood well enough, is experimentally clarified. Next, the effect of system pressure on the occurrences of the geysering and the natural circulation oscillation are investigated. Finally, from the results, a recommendation is provided to establish the rational start-up procedure and reactor configuration for natural circulation BWRs.

1. Introduction

A concept of the natural circulation BWR, e.g. the SBWR [1], has been proposed to cope the fact that the safety of current LWRs is overly dependent on active engineered safety features. One of the most distinctive features of natural circulation BWRs is the elimination of recirculation pumps. From this approach, the natural circulation BWR gains an advantage in being a simplified primary cooling system and thus seems to be promising for the next generation of LWRs. However, the elimination of recirculation pumps brings some disadvantages simultaneously. Aritomi et al. provided a discussion concerning advantages and disadvantages on eliminating recirculation pumps [2]. From the discussion, it was clear that establishment of the rational start-up

procedure is one of the most important subjects to achieve natural circulation BWRs because they have to be heated by fission energy from start-up under low temperature and pressure conditions. If thermo-hydraulic instabilities were to occur during the plant start-up, they would induce fluctuation in reactivity and hamper the plant operations for raising power output. Furthermore, the thermo-hydraulic instability was experienced during start-up in thermal natural circulation boilers. Although there are no published papers about this instability, it is estimated that the instability might be a kind of geysering. On the other hand, geysering has never been experienced in the Dodewaard reactor, which is the only natural circu, lation BWR operating as a power reactor and operates start-up with the initial pressure being raised up to 0.5 MPa. The different behavior

0029-5493/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0029-5493(93)E0232-9

242

J.-H. Chiang et al. /Nuclear Engineering and Design 146 (1994) 241-252

during start-up between the Dodewaard reactor and thermal natural circulation boilers has never been clarified because geysering has not been investigated extensively. Griffith [3] investigated geysering in a vertical liquid column with a closed end which was heated near the bottom. Nakanishi et al. [4] investigated geysering in a single channel under forced circulation conditions. To study the flow dynamics during start-up of natural circulation BWR, some experiments, simulating both normal and abnormal start-up conditions of natural circulation BWRs, were performed with twin parallel boiling channels in pervious work of present study [2]. It was made clear that the following thermo-hydraulic instabilities might occur during start-up of natural circulation BWR according to its start-up procedure and geometric configuration: Upon increasing the heat flux, out-of-phase geysering is induced by condensation at first, thereafter in-phase natural circulation oscillation is caused by hydrostatic head fluctuation in steam separators, and finally out-ofphase density wave instability occurs. Although Nakanishi et al. [4] found that geysering was induced only at velocities lower than 0.02 m/s, our study indicated that geysering appeared in velocities up to about 0.2 m/s. Since geysering occurring in parallel channels had never been understood enough, the driving mechanism was investigated experimentally under both natural and forced circulation conditions [5]. The following were clarified from the result: The mechanism of geysering occurring in parallel channels under forced circulation conditions is identical to that under natural circulation ones. The formation of a large bubble covering the whole flow cross section and subcooled condition in the outlet plenum are necessary for geysering to occur. So long as both conditions are satisfied and condensation rate of a large bubble is superior to the whole circulation rate, flow reversal occurs and geysering is induced. Oosterkamp et al. pointed out that a thermal stratification, which occurred in the lower plenum, was possibly encountered during the natural circulation mode(at start-up of BWR) [6]. It was planned for the start-up procedure in the SBWR that coolant would be initially heated up using a

drain line, an auxiliary preheater and a pump to prevent thermal stratification and thermo-hydraulic instabilities from occurring. It seems that the start-up procedure is not simple and thereby abandons the advantage gained by eliminating recirculation pumps. The objective of this paper is to suggest a rational start-up procedure and a reactor configuration of natural circulation BWRs based on experimental results. At first, in-phase natural circulation oscillation is taken up, which cannot be found in Boures' [7] and Aritomi's [8] classifications of thermo-hydraulic instabilities in boiling two-phase flow. Its driving mechanism is investigated experimentally under various conditions (heat input, circulation fluid subcooling and nonheated riser length from the u p p e r e n d of the heated section to the outlet plenum.) Next, it is estimated from our proposed driving mechanisms of geysering and in-phase natural circulation oscillation that an increase in system pressure might stabilize these instabilities. Thus, the effect of system pressure on their occurrences are experimentally investigated. Finally, from these results and the experiences in the Dodewaard reactor and thermal natural circulation boilers, a rational start-up procedure was proposed.

2. Experimental apparatus Figure 1 shows a schematic diagram of the loop which carries boiling fluid in both natural and forced circulations by switching a pneumatic ball valve. Water is used as the test fluid. The total flow rate is measured by an orifice fiowraeter installed in the upstream of the inlet plenum. The steam generated in a test section is liquefied through a condenser installed in the separator. In the separator tank, the exit of a channel connecting with the outlet plenum is higher than the liquid level to simulate the separator of the SBWR. Figure 2 shows the geometry and dimension of the twin parallel boiling channels between the inlet and the outlet pler!um~. Each channel has a heater, a pair of electrodes and an orifice flowmeter for measuring flow oscillation. The

Z-H.

Chiang et al. /Nuclear Engineeringand Design 146 (1994) 241-252 . . . . . . . . . . .

243

'3' . . . . I i

-~ I |

Cmldmme¢ Eieclrm

Cooler Przhontz¢

Outlet

plenum

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F Te61

.-

DC source

_., --, i-~i-

.-

_

=

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,, -0--~ , Orifk:

II I

p<

~><1----

',',',

111---

----,,. . . .

¢ k ~

plenum

(

F - ~ l ~ h I ~

Inlet

'--I L__I Ine

Electre

Fig. 1. Schematic diagram of the loop. O--

i

D C m

O" . . . . . . . . . . .

~

;

....

, J

Fig. 2. Test section.

heater rod was made of thin stainless steel (6 mm O.D. and 1 m long), installed concentrically in an outer tube made of Pyrex glass (15.6 ram I.D.) and heated by a DC current. All tests were run under the symmetrical conditions in both channels. Experiments were performed to clarify the effect of system pressure on the driving mechanism of geysering in parallel channels for both natural and forced circulations. For realizing the mechanism of in-phase natural circulation oscillation, the experiments were also carried out with various inlet subcooling, non-heated riser length and system pressure under natural circulation condition. In these experiments, circulating fluid temperature was regulated to desirable value by using precooler and preheaters installed in a separator tank. System pressure was regulated with a

pressure control system composed of a pressure regulating valve, a relief valve and a compressor. The experimental conditions are listed in Table 1.

Table 1 Experimental conditions Test fluid System pressure Heat flux Circulation fluid temperature Non-heated riser length (From the top of heated section to the outlet plenum) Inlet velocity (only forced convection)

Water 0.1-0.2 MPa 0-800 k W / m 2 80-90°C 250-750 mm

0-1 m / s

244

J.-H. Chiang et al. /Nuclear Engineering and Design 146 (1994) 241-252 '

3. Natural circulation oscillation due to hydrostatic head fluctuation As above-mentioned, three kinds of flow instabilities were observed under conditions simulating start-up in natural circulation BWRs. As heat input increases, geysering occurs at first. Thereafter, another flow instability appears together with geysering. As heat input increases furthermore, geysering is suppressed only to have yet another instability appear. Typical records of this instability are shown in Fig. 3. It can be seen from the results that the oscillation of the circulation rate is synchronous with flow oscillations in both heated channels but is shifted by 180 ° against the pressure drop oscillation between the outlet plenum and separator. This instability is called "Natural Circulation Oscillation" in this study. Except for the phase of oscillation, natural circulation oscillation presents a more moderate oscillation than density wave's. Density wave instability is relatively high frequency oscillation in which the period is approximately one to two Tin : 85o0 q" "230kW/m 2

9000 175kW/m 2

9500 120kW/m 2

A

30 s

Fig. 3. Typicalmeasured results of natural circulation.

i

Natural circulation &

40

V

L.R. = 250mm

O O

P = 0.10MPa

A o 00

V O

v "10 .O m L-

ATsub

O

[] &

V

o_ 20

A

t~

v 5K o 10K : A 15K [] 2 0 K

O

V

OOo O A

A

[]

V v

v v v ~ oo ooGoo~,~.

0

1O0 200 Heat flux (kW/m 2)

300

Fig. 4. Periods of natural circulationoscillationin reference to heat flux and inlet subeooling. times the time required for a fluid particle to travel through the heated channel [9]. Figure 4 shows typical periods of natural circulation oscillation in reference to inlet subcooling. If density wave instability occurred in present conditions, as shown in Fig. 4, its period should be 1-8 seconds. It is made clear from the figure that the period of natural circulation oscillation is much longer than that of density wave instability and that it becomes shorter with an increase in heat input and with a decrease in inlet subcooling. Figure 5 shows the effect of subcooling on natural circulation characteristic curves (i.e. time-average natural circulation rate vs. modified heat flux.) A solid line in the figure means saturated condition at the exit of the heated section. The figure indicates that a characteristic curve exists even under different inlet subcooling conditions because all exit conditions are almost saturated. Under these conditions, natural circulation rate is governed by the vaporization rate in the heated section. Therefore, we tried to arrange the periods shown in Fig. 4 with natural circulation rate. The results are presented in Fig. 6. It can be seen from the figure that the period of natural circulation oscillation is well correlated with the natural circulation rate even under different subcooling conditions.

J.-H. Chiang et aL ~Nuclear Engineering and Design 146 (1994) 241-252 i

.~ 0.5 ..-.~>'

i

i

i

i

i

Natural circulation L.R. = 250ram P = 0.10 MPa v 5K ^ T ~ .O 10 K •-=-suv . . 15 K [] 20K

i

i

i

I

i

'

.A~ ~f/q~

/ . z " O/°°°

i

i

[

i

i

~

Z~Tsub = 10 K P -- 0.10 MPa

0 . 5

i

/

/

Natural circulation

^^o~2 ~°°°°

o 250mm ~o2 L.R. : A 500ram ,-g,e,/" [] 750mm

o >

.'~ 0.25

.¢- 0.25 (9

I

0

I

I

I

I

I

I

i

2 4 6 8 10 q" / (Cp, ATsub/h fg) (iW/m 2)

12

Fig. 5. Natural circulation characteristic curve in reference to inlet subeooling.

Figure 7 shows the effect of non-heated riser length on the natural circulation characteristic curve. Heat input is selected for the abscissa in the figure because distance between the inlet and outlet plenums is equal even for changing the non-heated riser length, that is, the sum of the

'

i

•

i

Natural circulation A

40 []n V

%~, L,.

~

L.R. = 250mm P= 0.10 MPa v 5K ATsub: o 10K A15K n 20K

v~oo o

i

~/~"

_#7

ID a. 20

i

245

'o12' ' '0'.4' ' Average inlet velocity (m/s)

'o.6

Fig. 6. Relationship between period of natural circulation oscillation and time-averaged natural circulation rate in referenee to inlet subcooling.

0

I

I

I

I

I

I

I

2.5 Heat input (kW)

I

I

5

Fig. 7. Natural circulation characteristic curve in reference to non-heated riser length.

heated section length and non-heated riser one. Although somewhat of a deviation is shown in the figure, it is very small. This deviation may be induced by longer risers which can induce a greater amount of vapor in accordance with the same vapor generation rate, but the volume ratio of riser to the whole section is small in the present experimental apparatus. The relationship between periods of natural circulation oscillation and natural circulation rate is also shown in Fig. 8. It is seen from the figure that the period can be arranged with natural circulation rate though data seem to scatter slightly. From the results shown in Figs. 5 to 8, it is supposed that the vaporization rate dominates over the period of natural circulation oscillation. In the present experimental apparatus, the flow cross section of the connection channel is wider than that of the heated section, so the friction loss can be negligibly small as compared with total pressure drop therein. Furthermore, acceleration loss is also negligibly small because the connection channel is not heated. Hence, the hydrostatic head is predominant over the pressure drop in the connection channel. According to the above results and considerations, the driving mechanism of in-phase natural

246

Z-H. Chianget aL ~Nuclear Engineering and Design 146 (1994) 241-252 I

Natural circulation 40 ATsub = 10 K P = 0.10 M P a

[] 250mm L.R.: A 500mm a 750mm

0 A0

"10 O .m

O /`

t-

°

n 20

13 d

A

Oo

00.2

'

0'.3

ooo

'

o

0'.4

o

o.s

Average inlet velocity (m/s) Fig. 8. Relationship between period o f natural circulation oscillation and time-averaged natural circulation rate in reference to non-heated riser length.

circulation oscillation is reasonably considered as follows: Under conditions where there is an insufficient vaporization rate, while vapor accumulates in a channel connecting the outlet plenum through the separator tank, the hydrostatic head

decreases, and the circulation rate increases. On the other hand, while the accumulated vapor flows out and water is fdled with therein, the hydrostatic head increases and thus the circulating rate decreased. These phenomena repeat periodically. In the present experimental apparatus, density wave instability occurs for heat fluxes higher than that at the maximum circulation rate (for example, at the turning point of natural circulation characteristic curve shown in Figs 5 and 7.) So long as the vaporization rate becomes high with increasing heat input, mixture flow in the connection channel becomes stable, so that in-phase natural circulation oscillation is suppressed. As indicated already, the oscillation of the pressure drop between the outlet plenum and the separator plays an important role in natural circulation oscillation. To comprehend this instability, the relationship between the pressure drop oscillation and the natural circulation rate one was examined. Figure 9 shows the relationship between the amplitude of flow oscillation and average natural circulating rate. The dimensionless amplitude of natural circulation oscillation is defined by (Utmax -- Utmin)//(Utmax q- Utmin),

(1)

/-~tmax a n d Utmin are maximum and minimum circulation velocities respectively. Furthermore, we introduced dimensionless amplitude of the pressure drop oscillation in the connection channel defined by where

1.5

i

i

i

i

i

i

i

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

l

Natural circulation

g +

P = 0.10MPa

( Aelmax -- A P l m i n ) / p , gh ,

(2)

1

V

0 0 V

V

~ 0.5 I

L.R. :

, 02 •

500 mm

•

750 mm

. . . . . . .

.,.

0

,, •

3

A

O O O VV

L.R. = 250 mm v 5K --uATsu=:° 10K ,~ 1 5 K

ATsub = 10 K •

A

[] 2 0 K

, , , .

0

. . . . . . .

°

4

0.5

A v e r a g e i n l e t v e l o c i t y (m/s)

Fig. 9. Typical amplitudes of natural circulation oscillation.

where APlm~x and APlmin are maximum and minimum pressure drop oscillation, p, saturated liquid density, g acceleration due to gravity and h vertical height of the connection channel. This dimensionless amplitude of the pressure drop means the amplitude of void fraction oscillation in the connection channel. Figures 10(a) and (b) represent relationships of the dimensionless amplitude of pressure drop oscillation with the dimensionless amplitude of natural circulation oscillation and with average natural circulation rate. The arrangements shown

Z.H. Chiang et al. ~Nuclear Engineering and Design 146 (1994) 241-252 in Figs. 10 are useful for only one parameter but hardly interpret their physical phenomena universally. Then, examining thoroughly these data, we tried to arrange the dimensionless amplitude of the pressure drop oscillation in the following way; pt(Utmax -- Utmin)(Utmax"l- Utmin) 2

p,( U2tmax-- U2mfm) 2

=

(3)

The right side of Eq. (3) means an amplitude of a kinetic energy for natural circulation rate calculated in terms of flow cross section in the heated section. The results are illustrated together with the effect of pressure on natural circulation oscillation in which is discussed later, as shown in Fig. 15. The dimensionless amplitude of pressure drop oscillation in the connection channel is well correlated with the one of the kinetic energy for natural circulation rate. This arrangement indicates that more violent to pressure drop oscillation in the connection channel caused by void fluctuation under insufficient vapor generation may induce more strong oscillation in natural circulation rate. These results, then, support our proposed model of the driving mechanism in regard to natural circulation oscillation.

I

I

I

247

4. Effect of s y s t e m pressure on t h e r m o - h y d r a u l i c instabilities

4.1. Effect o f system pressure on geysering As mentioned in the Section 1, the formation of a large bubble covering the whole flow cross section is necessary for geysering to occur in parallel boiling channels regardless of the difference of natural and forced circulation conditions. A large bubble is hardly ever formed with an increase in system pressure. Therefore, it is supposed that geysering will be suppressed under a high pressure condition. The effects of system pressure on geysering were investigated under both natural and forced circulation conditions. It has to be noted that the driving mechanism of geysering in parallel boiling channels occurs regardless of circulation conditions, as has been revealed in authors' previous work [5]. Moreover, in the case of natural circulation, the upper limitation of velocity for geysering to occur cannot be evaluated quantitatively because natural circulation oscillation appears together with geysering beyond a certain heat flux. Hence, the effect of system pressure on the occurrence of geysering was investigated under forced circulation conditions.

i

I

i

t

Natural circulation

Natural cimulation -r"

P = 0.10 MPa

P = 0.10 MPa

Q.

0

A

(3.

VV

0

vV V

V

0

V

V

E <3

O O

0.5

A A O

A•

•

250 mm v 5K O 10 K ATsu b : " 15K [] 20 K L.R. =

ATsub=10K L.R.: I

• 500 m m • 750 mm I

I

I

I

I

I

I

0.5 ( Utmax - Utmln ) / ( Umlax + Utmin )

O

A

A A £

I

O

[]

o

[]

n 0.5 <3

•

O

I

Q. <3

E

0 0

0

•

•

E

L.FI. = 250 mm

13. <3

ATsub = 10 K L.R.: I

I

0.2

AT

500 mm • 750 mm I

0.3

-

~-suo I

I

v 5 K o 10K A 15K o 20 K I

0.4

0.5

A v e r a g e i n l e t v e l o c i t y (m/s)

Fig. 10. Amplitudesof pressure drop oscillationin connectionchannel;(a) relationshipwith amplitudeof natural circulation rate oscillation;(b) relationshipwith averagenatural circulationrate.

J.-H. Chianget al. ~Nuclear Engineering and Design 146 (1994) 241-252

248

Figure 11 shows flow stability maps for geysering in reference to system pressure. Although outer tubes of the present test section was made of Pyrex glass, system pressure was limited up to 0.2 MPa, it is seen from the figure that an increase in system pressure reduces the Upper limitation of the circulation rate at which geysering occurs as well as narrows the region in which it

0.05

I

0.05

I

I

I

i

I

F o r c e d circulation - ..... ---

~ T s u b = 10 K L.R. = 2 5 0 m m v

P = 0.10MPaP = 0.15MPaP= 0.20MPa-

I

Stable region o"

E ,-z

0 [

.m z=.

Stable region

Unstable

region

"5

-0.~

I

I

I

I

0.1 0.2 Inlet velocity (m/s)

I

).3

Fig. 11. Effect of system pressure on flow stability map for geysering.

I

~ T s u b = 10 K L.R. = 2 5 0 m m

I

I

I

I

---

P = 0.10MPaP = 0.15MPa-

.....

P = 0.20MPa-

I

Large bu.,~! -,,~1~'~

Churn & turbulent flow

m -.,z o-

E

0

..._......~Bu bbly flow

OccUrs.

To study the reason why an increase in system pressure stabilizes the flow for geysering, the effect of system pressure on flow pattern maps was investigated under forced circulation conditions by means of observation. The test section was reformed to a single heated channel to suppress flow fluctuation. The results are shown in Fig. 12. It is clear that the lower limitations of the large bubbly flow region coincide under various pressure conditions. However, the large bubbly flow region becomes narrower and the upper limitation is shifted to lower quality with increased system pressure, As the system pressure increases, bubbles leaving the heated surface become smaller due t o a decrease in the surface tension and in the differential density between liquid a n d vapor phases. Consequently, at higher system pressure, a large bubble is hardly ever

I

..g_ "5

Liquidsingle phase flow Su~ice boiling

o" UJ

-0.05~

'

' ' 0.2 ~ ' 0.3 ' 0.1 Inlet velocity (m/s)

'

0.4

Fig. 12. Effect of system pressure on large bubbly flow region.

formed and churn and turbulent flow readily occurs. A comparison between the flow pattern maps shown in Fig. 12 and the unstable region for geysering shown m Fig. 11 indicates that the unstable region for geysedng is almost included in a large bubbly flow region except for both low velocity and quality. This fact supports our proposed model that the existence of a large bubble is necessary for geysering. In a single channel for forced circulation, stable subcooled bubbly flow is maintained though differential pressure between the inlet and outlet plenums fluctuates due to condensation of bubbles. In contrast with this, the fluctuation of the pressure drop induces the velocity oscillation in parallel channels. Then, bubbles can coalesce during low velocity. Once a large bubble arrives at the outlet plenum, it is mixed with subcooled water therein and is condensed. If the condensation rate is superior to the whole circulation rate, flow reversal is caused and thus geysering zs generated. That is our opinion on how geysering is induced under both low velocity and quality conditions where only subcooled bubbly flow appears. It was made clear in our previous paper [5] that the periods of geysering appearing under various conditions in both natural and forced

Z-H. Chiang et aL ~NuclearEngineering and Design 146 (1994) 241-252

circulations are well correlated with delay time for boiling defined by p,(i'-im)

,rB

q"

,

4.2. Effect o f system pressure on natural circulation oscillation

Figure 14 shows the characteristic curves of natural circulation, i.e. circulation rate vs. heat

•

I

•

I

I

•

I

'

/

Natural circulation

L.R. = 250 mm 20

10 K o P = 0.10 MPa ATsub =

A P = 0.15 MPa a P=0.20MI~

.-.~

19 n

10

Forced circulation

.¢dY&" /=ILB ~r" •

0

.

o.flr / .~A

/

I

i

• P = 0.10 MPa • P = 0.15 MPa • P = 0.20 MPa .

I

10

.

I

.

l

,

20

Delay time for boiling (s) Fig. 13. Periodsof geyseringin referenceto systempressure.

i

i

i

i

Natural circulation 10 K L.R. = 250mm

.~dHl~ A . n ~~ ' ~ ' ° °

ATsu b =

(4)

where, i' is saturated enthalpy of liquid, iin inlet liquid enthalpy, p, liquid density and q " calorific power per unit volume. Eq. (4) means period required for boiling of subcooled water flowing into the heated section. Therefore, the periods of geysering under various system pressure conditions in both natural and forced circulation were arranged with the delay time for boiling and the results are shown in Fig. 13. The flow reversal makes the fluid temperature in the inlet plenum higher than that of the circulation fluid. Hence, i~n is corrected by the temperature measured in the inlet plenum instead of the circulating fluid temperature shown in Fig. 13. As well as previous results, the periods of geysering are in agreement with the delay time for boiling even at various pressures.

•

.6

249

0.4

A

+ / go.21-

/I f

'~

o0.,0+a

p : A 0.15MPa a 0.20MPa

O I

0

I

I

100 200 Heat flux (kW/m2)

I

300

Fig. 14. Naturalcirculationcharacteristiccurvein referenceto systempressure.

flux, for various system pressures. In these runs, the conditions were the same as those stated in our previous paper [5] except for system pressure. The present experimental apparatus has the identical characteristic curve for different system pressures so long as the inlet subcooling and heat inputs are equal. This indicates that void fraction scarcely varies for the same vaporization rate within the pressure range operated in the present work. The effect of system pressure on the stability map for geysering under natural circulation oscillation is discussed in the next chapter, Next, trying to arrange the dimensionless amplitude of the pressure drop in the connection channel, defined by Eq. (2), with the amplitude of the kinetic energy for the natural circulation rate defined by Eq. (3), the results are shown in Fig. 15 with solid mark. It is seen from the figure that the amplitudes of the pressure drop is also well correlated with the amplitude of the kinetic energy for the natural circulation rate under the present experimental conditions. Moreover, the effect of system pressure on periods of natural circulation oscillation was investigated and the results are shown in Fig. 16. It is clear from the figure that the periods are influenced by system pressure. Although the

250

Z-H. Chianget aL /Nuclear Engineering and Design 146 (1994) 241-252 i

i

i

i

#

i

i

i

Natural circulation P = 0.10MPa

500

..... ~I ""

= 250

I

o 200

a,<( Ao,~ ~

"I" ~ /" I

I

I

I

I

I

I

I

I

0.5

0

(ZSPlmax - APimin ) / P , g H Fig. 15. Effect of system pressure on relationship between amplitude of pressure drop oscillation in connection channel and amplitude of kinetic energy for natural circulation rate.

characteristic curves of natural circulation are almost identical for different system pressures, the periods shorten with an increase in system pressure. It is estimated from the figure that the volume of a bubble decreases and the number of bubbles flowing in the connection channel in-

•

i

i

Natural circulation

40 L.R. = 250mm ATsu b --- 10 K o 0.10MPa P :A 0.15MPa

O

"10 o .¢(9

O

O

a. 2(]

[] 0.20MPa °Ooo

41~...q) OOO O O u ~"8=I~,tAA A

(]

' 012 ' ' ' 01.4 ' ' Average inlet velocity (m/s)

I

Natural.'~

100

Tran

\" region

' 0.6

Fig. 16. Effect of system pressure on periods of natural circulation oscillation.

region 0

a~

•

ZSTsub = 10 K L.R. P • 250mm 0.15MPa • 250ram 0.20MPa

=

i

Z~Tsub = 10 K L.R. = 250ram

circ~..!a!i°_n & ~ a Stable region osc,,nat,on - " ~ region \

¢=

"~= (z

i

Natural circulation

/

L.R. ZSTsub v 250mm 5 K o 250mm 10 K A 250mm 15 K , [] 250mm 20 K 0 500mm 10K ® 750mm 10 K , o ~ "

m E

i

/

o e

I

\.

"'~•

\,

m ~

~

•

•

Zx

a Non_b(~iling,regi(tn

i

. ~.,..

~.

I

0.1 0.2 0.3 0.4 System pressure (MPa)

e

0.5

Fig. 17, Effect of system pressure on stability maps for geysering and natural circulation oscillation.

creases as system pressure increases. These effects may increase bubble velocity. Therefore, increasing system pressure shortens the period.

5. Consideration on start-up procedure The thermo-hydraulic instability, which appeared during start-up in the thermal natural circulation boilers and which seemed to be a kind of geysering, has never been experienced during start-up in the Dodewaard reactor. Clarifying this difference is very significant to establish a rational start-up procedure in natural circulation BWRs. Therefore, the effect of system pressure on the occurrence of geysering was also investigated experimentally under natural circulation conditions. Figure 17 shows a flow stability map for both geysering and natural circulation oscillation in reference to system pressure. It is clear that raising system pressure produces an effect of suppression to both instabilities of geysering and natural circulation oscillation. In this study, the system pressure has been raised up to only 0.2 MPa owing to the test section made of glass. However, it is supposed from the figure that both instabilities hardly occur when system pressure beyond 0.5 MPa. This supposition can be mutu-

J.-H. Chiang et al. / Nuclear Engineering and Design 146 (1994) 241-252

ally proved with the instance of the Dodewaard reactor which is started up from 0.5 MPa initial pressure. Eliminating recirculation pumps and associated equipment could not only avoid the trouble caused by their active functions but also might bring some disadvantages related to the start-up procedure. From our fundamental studies on natural circulation, thermo-hydraulic instabilities during the start-up should be cautiously reconsidered to validate the concept of natural circulation BWRs. The following procedures are worth considering for the start-up procedure: (1) After coolant is preheated up to about 80°C at atmospheric pressure in a method other than fission energy, it is decompressed down to the saturated pressure and is heated by fission energy. (2) Under subcooled condition at atmospheric pressure, coolant begins to be heated by fission energy. (3) In the same way as the start-up in the Dodewaard reactor, after coolant is pressurized up to a certain value, coolant begins to be heated by fission energy. It is clear from our results that a large bubble existing in the exit of the heated section and subcooled condition in outlet plenum are necessary conditions for geysering to be induced. Case (1): even though the water surface at the top of the chimney is saturated, coolant in the exit of the reactor core is still subcooled due to large hydrostatic head in the tall chimney. Furthermore, it is well known that a larger bubble is readily formed under decompressed conditions. Therefore, this case of start-up procedure is undesirable under considering the flow stability, though it is used for current BWRs using recirculation pumps to heat up the coolant. Case (2): from our results and the fact that geysering occurred in thermal natural circulation boilers, it is supposed that thermo-hydraulic instabilities may be induced in this procedure of start-up. To avoid recirculation in the chimney and to secure effective natural circulation, it is possible design to divide chimney, which is applied to the Dodewaard reactor. Since the flow area of the divided chimney is wider than that of

251

the reactor core, it is undeniable that void fraction fluctuation therein induces in-phase natural circulation oscillation while the vaporization rate is insufficient and stable two-phase flow is not yet established. On the other hand, it seems from the experience in the Dodewaard reactor and our results that a pressurized start-up procedure such as Case (3) can prevent thermo-hydraulic instabilities from occurring. Hence, the authors recommend the pressurized start-up procedure as being the best way. Since the heated section in the present experimental apparatus consisted of only one rod, our recommendation is still an inference for a fuel bundle. Namely, large scale proof testing is necessary to establish a rational start-up procedure in natural circulation BWRs.

6. Conclusion

With an aim toward establishing a rational start-up procedure for natural circulation BWRs, the driving mechanisms of in-phase natural circulation oscillation, which may be induced during the start-up, were investigated in parallel boiling channels under both natural circulation conditions. In addition, the effect of system pressure on the occurrences of the geysering and the natural circulation oscillation were also investigated experimentally. The following are clarified: (1) Both instabilities occurred while the vapor generation rate was insufficient. (2) The amplitude of pressure drop oscillation in a non-heated vertical channel in the downstream to the heated section such as steam separators is well correlated with the amplitude of the kinetic energy for the natural circulation rate. Therefore, in-phase natural circulation oscillation is induced by hydrostatic head fluctuation in the non-heated vertical channel. (3) Condensation rate greater than the circulation rate induced flow reversal and thus geysering. An existence of a large bubble covering the whole channel section and subcooling in the outlet plenum are necessary to attain the condition inducing geysering.

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(4) Geysering would be suppressed by increasing system pressure because it becomes difficult for a large bubble to be formed. (5) As system pressure is increased, the region where natural circulation oscillation occurs becomes narrower because the void fraction fluctuation becomes weaker. It is recognized that scaling-cum-dimensional analyses would need to be conducted to establish a foundation for application of experimentallyderived stability maps and boundaries to full scale BWR prototypes. Nevertheless, the results of studies presented in this paper do indicate important trends and phenomenological considerations that will need to be accounted in the next generation of BWRs. The present work has been performed at the Research Laboratory for Nuclear Reactors at the Tokyo Institute of Technology in collaboration with the Tokyo Electric Power Company.

7. Nomenclature Cp t g h f8 i q" L.R.

T u ut

a& aPe ae3

A ub

Specific heat of liquid (kJ / k g K), acceleration due to gravity ( m / s 2 ) , latent heat of vaporization (El/kg), enthalpy (kJ/kg), calorific power per unit volume (kW/m3), length of non-heated riser in the downstream of the heated section (m), temperature (°C), inlet velocity (m/s), total circulation velocity calculated in term of a channel flow area in the heated section (m/s), differential pressure between an outlet plenum and a separator tank (kPa), pressure drop between an inlet and an outlet plenums (kPa), pressure drop between an inlet and a separator tank (kPa), inlet subcooling (K),

p ~'a

density ( k g / m 3 ) , delay time for boiling (s).

ZI. Superscripts (i) '

i-th channel, saturated liquid phase.

7.2. Subscripts in max rain ,

inlet, maximum, minimum, liquid.

8. References [1] D.R. Wilkins, J.K. Quirk and R.J. McCandless, Status of Advanced Boiling Water Reactors, Proc. 7th Pacific Basin Nuclear Conference, San Diego, March 1990, pp. 261. [2] M. Aritomi, T. Nakahashi, J.H. Chiang, M. Wataru and M. Mori, Transient behavior of natural circulation for boiling two-phase flow (experimental results), Proc. 6th Nuclear Thermal Hydraulic, ANS 1990 Winter Meeting, Washington DC, November 1990, pp. 313-320. [3] P. Griffith, Geysering in fiquid-filled lines, ASME Paper 62-HT-39 (1962). [4] S. Nakanishi, S. Ishigai, M. Ozawa, Y. Mizuta and H. Tarui, Flow instability in boiling channels (2rid Report, Geysering), Trans. JSME Vol. 44 (1978) 4252-4262 (in Japanese). [5] M. Aritomi, J.H. Chiang and M. Mori, Transient behavior of natural circulation for boiling two-phase flow (2rid Report, Mechanism of geyseringO,Proc. 1st JSME/ASME Joint Int. Conf. Nucl. Engrg., Tokyo, November 1991, pp. 87-94. [6] W.J. Oostekamp and G. Koopmans, Start-up of natural circulation BWRs, Proc. 1st JSME/ASME Joint Int. Conf. Nucl. Engrg., Tokyo, November 1991, pp. 357-363. [7] J.A. Boure, A.E. Bergles and S.L. Tong, Review of twophase flow instability, Nucl. Engrg. Des. 25 (1973) 165. [8] M. Aritomi, Handbook of Two-Phase Flow; 6. Flow stability, (corona CO., 1985) pp. 171-186 (in Japanese). [9] M. Aritomi, S. Aoki and A. Inone, Instabilities in parallel channel of forced-convection boiling up-flow system; (II) Experimental results, J. Nucl. Sei. Technol. 14 (1977) 88-96.