Passive core injection system with steam driven jet pump for next generation nuclear reactors

Passive core injection system with steam driven

jet pump for next generation nuclear reactors

Hikmet S. Aybar*, Nabil BeithouDepartment of Mechanical Engineering, Eastern Mediterranean University, G.Magosa, KKTC, Mersin 10,

Turkey

Received 12 August 1998; accepted 17 October 1998

Abstract

The steam driven jet pump (SDJP) is a device without moving parts, in which steam is used

as an energy source to pump cold water from a pressure much lower than the steam pressure

to a pressure higher than the steam pressure. In this study, a design of a passive core injection

system (PCIS) with SDJP is given for application to Boiling Water Reactor (BWR). The

operation range of PCIS depends on the operation characteristics of the SDJP. Thus, the

operation characteristics of the SDJP has been investigated in terms of independent para-

meters, such as water temperature, pressure, and steam inlet pressure which is the driving force

of the SDJP to obtain a wide operation range. The water tank pressure is chosen as 320 kPa to

increase the maximum operation pressure of the SDJP. The supply water temperature is chosen

as 15�Cwhich is about room temperature, and the steamwhich comes from the reactor pressure

vessel is assumed as saturated. The operation range of the SDJP in terms of the steam inlet

pressure is obtained from 2 to 10.5MPa. The study shows that when the discharge pressure

increases, the discharge mass ¯ow rate and the temperature increases as well. The probabilistic

reliability of the PCIS with the SDJP is assessed as well. The reliability assessment of the PCIS

with SDJP is investigated using a fault tree, and compared with the core injection system with

turbine-driven pump. It is found that the PCIS with SDJP is more reliable than the core injec-

tion system with turbine-driven pump.# 1999 Elsevier Science Ltd. All rights reserved.

1. Introduction

In the past years, several advanced light water reactor (ALWR) designs which

utilize passive safety systems have been proposed as a result of public concern about

Annals of Nuclear Energy 26 (1999) 769±781

0306-4549/99/$Ðsee front matter # 1999 Elsevier Science Ltd. All rights reserved

PII: S0306-4549(98)00091-7

*Corresponding author. Tel.: +90 392 366-6588; fax: +90 392 366-1217; e-mail: [email protected]

du.tr

nuclear power plant safety. The system simplicity, maintability, and the utilization

of passive features are the main design and safety objectives of the advanced nuclear

power plants. The steam driven jet pump (SDJP) or injector is an engineering device

in which steam is used as an energy source to pump cold water from a pressure

much lower than the steam pressure to a pressure higher than the steam pressure. The

SDJP has been used as an emergency feedwater supply device in ships and locomo-

tives for the last 40 years, and as a steam-jet air ejector to remove non-condensable

gases from the condenser in modern steam power plants today. The steam driven jet

pump (or injector, or ejector) is manufactured by a few companies for applications in

food, paper and power industries. However, commercially available SDJPs operate at

low pressure (< 2MPa). The SDJP has no moving parts and requires no external

energy supply. Because of these passive features of the SDJP, it is very attractive in the

design of next generation nuclear power plants. The utilization of the high pressure

steam driven jet pump (SDJP) as an emergency core cooling system in the advanced

nuclear reactor have been proposed by many reactor engineers. Suurman (1986) has

developed a high pressure SDJP for application to Canada Deuterium Uranium

(CANDU) reactors but the maximum discharge pressure was limited to about 4MPa.

Another high pressure SDJP has been developed by Soplenkov (1994) for application

to the secondary side of a VVER-440, however, the water inlet pressure was at the

same pressure as the steam generator so the pressure gain requested by the component

was very limited. Narabayashi et al. (1992) has proposed a two-stage SDJP system for

the passive core injection system (PCIS) for application to BWR. Recently, Cattadori

et al. (1995) have developed a high pressure steam driven jet pump and investigated

the operating parameters of this test SDJP experimentally.

The mathematical modeling of the SDJP has been done by Beithou and Aybar

(1998a) and the parametric analyses of the SDJP, in terms of steam pressure and

temperature, water pressure and temperature, have been done to obtain the operat-

ing parameter ranges of the high pressure SDJP (Beithou and Aybar, 1998b). In this

study, the pump characteristics of SDJP and the feasibility of a passive core injec-

tion system (PCIS) with the SDJP are investigated for application to BWR in terms

of the operation characteristics of the SDJP. The probabilistic reliability of the PCIS

with the SDJP has been assessed as well.

2. Steam driven jet pump

The SDJP is a kind of pump which does not have any moving parts. The steam

driven jet pump can be considered as an equivalent to a turbine-driven pump. All

thermodynamics processes in the SDJP rely on direct contact transport phenomena

between water and steam. Fig. 1 (modi®ed from Narabayashi et al., 1992) shows the

comparison of a turbine-driven pump and SDJP. Both systems utilize steam nozzles

to obtain a supersonic steam jet. In the turbine-driven pump, steam enthalpy is

converted to kinetic energy by the turbine blades, and the pump converts this kinetic

energy to pumping energy. In the SDJP, however, the steam enthalpy is converted

directly into kinetic energy of water and pumping energy.

770 H.S. Aybar, N. Beithou/Annals of Nuclear Energy 26 (1999) 769±781

2.1. Principle of SDJP

Fig. 2 shows the axial section of a steam driven jet pump, its dimensions, and

expected axial pressure distribution in the SDJP. This test SDJP is one which is used

in the experiment by Cattadori et al. (1995). Steam driven jet pump consists of three

sections: steam nozzle, mixing nozzle and di�user. The steam nozzle is producing a

nearly isentropic expansion and partially converting steam enthalpy into kinetic

energy. It can be a sonic nozzle or, if a stronger expansion is required, a supersonic

nozzle with the typical converging±diverging shape. The low pressure is obtained at

the exit of the steam nozzle, that is, at the inlet of the mixing nozzle. After the steam

Fig. 1. Turbine-driven pump and SDJP.

Fig. 2. Steam driven jet pump analysis model (all dimensions are in mm).

H.S. Aybar, N. Beithou/Annals of Nuclear Energy 26 (1999) 769±781 771

nozzle, the supersonic steam is blown into the cold water in the mixing nozzle. The

mixing nozzle is where water and steam come into contact. Steam transfers to water

heat (because of the temperature di�erence), mass (because of the related con-

densation) and momentum (because of the velocity di�erence). The steam con-

densation at the inlet of the mixing nozzle creates more pressure drop. Then, the

water is sucked from the water nozzle (gap between steam nozzle exit and mixing

nozzle inlet) by the near vacuum pressure. The water nozzle produces a moderate

acceleration and distributes the liquid all around the steam nozzle. The supersonic

steam jet accelerates the water in the mixing nozzle. The di�user is the section where

the liquid kinetic energy at the mixing section outlet is partially recovered, produ-

cing a further pressure rise. Because of the high pressure at the exit of the di�user,

water is injected to the reactor pressure vessel. The discharge pressure from the

SDJP should be higher than the steam inlet pressure. For a smooth startup and for

the discharge parameter control, two over¯ow exit lines are located in the mixing

nozzle of the SDJP (see Fig. 2). The ®rst one is the startup over¯ow line (OF1). It

removes the excess water in the mixing nozzle during the startup of the SDJP only.

After the startup, OF1 line will be always closed. The second over¯ow line (OF2) is

used for the ®ne control of the discharge mass ¯ow rate and pressure, and it dis-

charges the excess water from the last part of the mixing nozzle during the SDJP

operation.

Fig. 3 shows the processes of the SDJP on a Mollier diagram. As it is seen, the

process in the steam nozzle is nearly isentropic. As the steam expands in the nozzle,

its pressure and temperature drop, and it is expected that the steam starts conden-

sing. However, due to the high velocity, the residence time of the steam in the nozzle

is small, and there may not be su�cient time for necessary heat transfer and for-

mation of liquid droplets. As a result of this, the condensation of the steam is

Fig. 3. Mollier diagram representation of the SDJP processes.


delayed for a little while (supersaturated steam state). In the mixing nozzle, since the

pressure increases a little bit, the process is shown as a constant pressure process.

2.2. Mathematical model

The mathematical model of the SDJP and its numerical solution have been done

and reported before (Beithou and Aybar, 1998a). Here, the description of the

mathematical model is given shortly. The steam driven jet pump consists of four

sections: steam nozzle, water nozzle, mixing nozzle, and di�user. The one-dimen-

sional control volume method is used to develop the mathematical model for the

SDJP.

2.2.1. Steam nozzle

In the steam nozzle, the goal is to evaluate the pressure (P), density (�), velocity

(V), and internal energy (u) all over the converging±diverging nozzle (i.e. the sub-

sonic±supersonic nozzle). Since there exists single phase steam in the steam nozzle,

the governing equations (i.e. mass, momentum, and energy equations) for the

steady-state subsonic or supersonic single-phase steam ¯ow can be written as fol-

lowing:

d

dx��VA� � 0 �1�

VdV

dx�1

�

dp

dx� 0 �2�

du

dx�P

�

1

V

dV

dx�1

A

dA

dx

� �� 0 �3�

In addition to the governing equations, the state equation � � ��P; u� is used.

2.2.2. Water nozzle

With the knowledge of steam exit pressure from the steam nozzle and the supply

water pressure and temperature, the water nozzle velocity can be determined by

writing the Bernoulli equation between the tank and the exit of the water nozzle

which gives:

Vwc �PT

�Tÿ

Pc

�wc

� �� 1=2�4�

where the subscript ``T'' de®nes the water properties in the water supply tank, and

Pc is the pressure at the section ``c'', and �wc is the liquid density at the pressure Pc

(see Fig. 2). To ®nd the water mass ¯ow rate, the continuity equation can be used


_mw � �wcVwcAwc �5�

2.2.3. Mixing nozzle

Vapor exits for the steam nozzle enters the mixing nozzle with high kinetic energy

and relatively low pressure, and vapor mixes with the sub-cooled liquid creating a

lower pressure which sucks the liquid from the water tank. The subcooled water

continues to condense the vapor under two phase ¯ow until complete condensation,

at which the ¯ow becomes a single phase ¯ow at the rest of the mixing nozzle. Thus,

the mixing nozzle has been modeled by dividing into two sections: two-phase ¯ow

mixing section, and single-phase ¯ow section. The mixing section has been modeled

as a nozzle that contains a vapor cone. This cone describes the vapor plume (see

Fig. 2). The governing equations for two-phase ¯ow (three governing equations for

steam and three governing equations for water) have been taken from RELAP5/

MOD2 Code manual (Ransom et al., 1985), and modi®ed for this problem.

2.2.4. Di�user

The di�user is a subsonic diverging nozzle. The ¯ow area changes and the prop-

erties from the mixing nozzle at the di�user inlet (i.e., the section ``d'') are known.

To ®nd the pressure, velocity and temperature pro®les all over the di�user, the

mass, momentum and energy equations given in Eqs. (1±3) are used for single-

phase water.

In the numerical solution of the mathematical model, the iterative ®nite di�erence

procedure with under-relaxation has been used to approximate the derivatives in the

governing equations which are in terms of the mesh points. For the state equation,

the steam table subroutines have been used in the developed computer program.

Fig. 4 shows the axial pressure pro®le obtained from the mathematical model in

the SDJP and the Cattadori's experimental pressure pro®le for comparison. The

steady-state operation parameters of the SDJP have been investigated using the

mathematical model in terms of the four independent parameters, which are steam

supply pressure, steam temperature or superheat, supply water pressure (i.e. water

tank pressure), and supply water temperature (Beithou and Aybar, 1998b). The

main dependent parameters are discharge pressure, discharge water temperature,

and discharge water mass ¯ow rate, which are important parameters for the appli-

cation of the SDJP to the BWR as a PCIS. The OF2 mass ¯ow rate setting is

another important dependent parameter in addition to these three main dependent

parameters, because the OF2 mass ¯ow rate a�ects the discharge mass ¯ow rate.

The discharge pressure of the SDJP is obtained 10±20% higher than the steam inlet

pressure.

3. Application to BWR

The concept of the passive core injection system (PCIS) with the high pressure

SDJP for boiling water reactors is shown in Fig. 5. In case of emergency, the


motor-driven water supply valve (WSV) will open, ®rst and SDJP will be ®lled up

with water which comes from the water tank. Then, the motor-driven steam supply

valve (SSV) will open and steam goes to the SDJP from main steam line. After the

steam nozzle, the supersonic steam is blown into the cold water in the mixing nozzle.

In the mean time, the ®rst over-¯ow line valve (OF1 check valve) must be open to

drain the excess water which was initially in the SDJP. While steam is being con-

densed, the supersonic kinetic energy of steam accelerates the water in the mixing

nozzle into the main nozzle which is located just before the di�user throat. Then, the

OF1 check valve is closed due to the drop in static pressure inside the mixing nozzle

resulting from the acceleration of ¯ow in the mixing nozzle itself. The water is

sucked from the water nozzle because of the pressure di�erence between water tank

and mixing nozzle. The secondary over¯ow line has a motor-driven valve (OF2

valve), and it is controlled by steam pressure. The optimum secondary over¯ow rate

depends on steam inlet pressure. The OF2 valve will tend to open at low steam inlet

pressure allowing a suitable liquid over¯ow rate, however, the OF2 valve will be

almost close at high steam inlet pressure. In addition to that, the supply water ¯ow

rate may need to be controlled by means of the WSV controlled by the steam inlet

pressure. The full condensation is obtained just before the throat between the mixing

nozzle and di�user. While the liquid velocity decelerates, the pressure in the di�user

Fig. 4. Pressure pro®le in the SDJP.


increases. Because of the high pressure in the di�user, the discharge check valve will

open to inject water to the reactor pressure vessel.

As mentioned previously, in the SDJP there are four independent parameters:

steam pressure, steam temperature (saturated or superheated), water pressure, and

water temperature. The water pressure and temperature are de®ned by the design of

the water tank. Since the water tank is inside the reactor containment, the water

temperature is assumed to be around room temperature. The steam is taken from

main steam line directly, and therefore it is assumed that it is saturated steam. The

operation range of the SDJP for the water tank pressure of 200±260 kPa has been

investigated previously (Beithou and Aybar, 1998b). The maximum operation steam

pressure was obtained as 8.7MPa. In this study, the higher water tank pressure

(320 kPa) is used in order to increase the maximum operation pressure. When the

steam inlet pressure increases, the steam nozzle exit pressure increases as well.

However, the steam nozzle exit pressure must be less than the water tank pressure to

get suck water. Therefore, the water tank pressure must be increased. The water

tank can be designed as an elevated and pressurized (with a gas) closed tank to get

the pressure of 320 kPa at the inlet of water to the SDJP. For example, the 20m

elevated open tank from the level of the SDJP can give approximately 300 kPa at the

water inlet to the SDJP. Then, the water pressure can be assumed to be a ®xed

parameter. Now, only the steam pressure will be independent parameters. The

Fig. 5. Passive core injection system (PCIS) with SDJP.


computer program which was developed as a result of the mathematical model has

been run for the ®xed water temperature of 15�C, and pressure of 320 kPa, and the

steam is saturated. The steam inlet pressure is the only variable. The steam pressure

and dependent parameters for the operation of the SDJP with a 320 kPa water tank

pressure are show in Table 1. Since the pressure gradient between water tank and

mixing nozzle inlet would be very high for the low steam inlet pressure, the supply

water mass ¯ow rate is needed to adjust by WSV to obtain 10±20% higher discharge

pressure. The OF2 valve is fully open to hot suppression pool which is located inside

the containment, and it is at the pressure of containment. The area of the OF2 valve

for fully open is 1.131�10ÿ4m2. The over¯ow mass ¯ow rate is de®ned by the pres-

sure gradient between the pressure in the mixing nozzle and the pressure of con-

tainment. In the simulation from which the values of Table 1 are obtained, the

containment pressure is assumed at the atmospheric pressure. However, the con-

tainment pressure can be higher than the atmospheric pressure under accidental

conditions. In such a situation, the over¯ow mass ¯ow rate will decrease because the

pressure gradient between the mixing nozzle and containment pressure decreases,

and the water ¯ow rate must be adjusted using WSV. This detail analysis is left for

future study. As is seen in Table 1, the operation steam inlet pressure range of the

SDJP is between 2 and 10.5MPa.

The discharge mass ¯ow rate and temperature are very important parameters

because they will de®ne what amount of energy can be removed from the reactor

core. Fig. 6 shows the pump characteristics in terms of the discharge ¯ow rate; the

discharge pressure and the discharge temperature versus the discharge mass ¯ow

rate. The discharge pressure is higher than steam inlet pressure by 10±20% at every

point. As is seen in that ®gure, when the discharge pressure decreases, the discharge

mass ¯ow rate and the discharge temperature decrease as well. If a pipe break is

considered such as given by Cattadori et al. (1995), after the break, the reactor sys-

tem pressure decreases and the water leakage from the reactor pressure vessel lessens

as well. That is, the required makeup water supply ¯ow rate to the reactor decreases

Table 1

The operation parameter ranges of SDJP

Independent parameters

Supply water pressure (kPa) 320

Supply water temperature (�C) 15

Steam quality 1.0

Steam pressure (MPa) 2.0±10.5

Dependent parameters

Supply water ¯ow rate (kg/s) 3.5±13.5

OF2 Mass ¯ow rate (kg/s) 1.5±6.3

Discharge pressure (MPa) 2.2±12.7

Discharge temperature (�C) 147±192

Discharge mass ¯ow rate (kg/s) 3.1±12.7


while the reactor system pressure decreases after the break. As we mentioned before,

the reactor pressure is the driving force for the SDJP, namely, it is the steam inlet

pressure. After a pipe break, the reactor system pressure decreases, and the dis-

charge pressure of the SDJP decreases too. As a result of this, the discharge mass

¯ow rate lessens, however, the required makeup water ¯ow rate decreases as well. As

is seen, the accident conditions of the system and the operation characteristics of

SDJP have the same trend.

The water leakage rate during the pipe break (a 2 in. break) considered by Catta-

dori et al. (1995) in a real BWR is 58.0 kg/s for the system pressure of 8.7MPa, and

the water leakage rate decreases linearly with the system pressure. For the system

pressure of 2MPa, the water leakage rate is given as 19.4 kg/s. However, the SDJP

gives the discharge ¯ow rates of 9.4 and 3.2 kg/s for the system pressure of 8.7MPa

(i.e. the discharge pressure & 9.57MPa) and 2MPa (i.e. the discharge pressure &

2.2MPa), respectively. The comparison shows that the di�erence between the

required water ¯ow rate is six times higher than the discharge ¯ow rate of the SDJP.

Cattadori et al. (1995) has also mentioned that the SDJP used in the experiment is a

scaled-down model in terms of ¯ow rate scale of about 1:6. The PCIS with SDJP

considered in this study for application to BWR is not enough to mitigate the acci-

dent because of the small amount of the discharge ¯ow rate. Six of these small SDJP

in the design of PCIS may be used to satisfy the requirement of supply water ¯ow

rate in a real BWR. Alternatively, the dimensional analysis of the SDJP is needed to

see how to increase the discharge mass ¯ow rate. This will be considered as future

study.

Fig. 6. Pump characteristics of SDJP.


4. Reliability assesment

The probabilistic reliability assessment of the high pressure passive core injection

system (PCIS) utilizing a SDJP has been investigated. The SDJP is the main com-

ponent of the PCIS (Fig. 5), and there is no any reason for failure of the SDJP itself

since the SDJP does not have any moving parts and does not utilize any power other

than steam heat energy. The PCIS system with SDJP can fail due to the failure of

the valves and pipe breaks only. According to the operation procedure mentioned

above, the SDJP fails to start operating:

a. if the water supply valve (WSV) fails to open,

b. if the steam supply valve (SSV) fails to open,

c. if the over-¯ow check valve (OF1 valve) fails to open,

d. if the discharge check valve fails to open,

e. if the over-¯ow check valve (OF1 valve) fails to close,

f. if the water supply pipe fails,

g. if the steam pressure (i.e. system pressure) is out of range of the SDJP design

operation pressure.

Events (a)±(e) are caused by primary fault events, and event (g) leads to failure to

operate. A fault tree analysis of the system (Fig. 7) has shown that the probability of

Fig. 7. Fault tree for the SDJP.


a SDJP for failure to start on demand is 6.3E-3 per demand. The failure data have

been taken from McCormick (1983) and Modarres (1993). Since the operation range

of the SDJP is very wide in term of the system pressure, the failure rate of event (g)

is assumed negligible. If the SDJP is replaced with a turbine-driven pump in the high

pressure core injection system with turbine-driven pump (Fig. 5), there are no over-

¯ow valves, but there is still a need for water and steam supply valves. In this case,

the system can fail, not only because of the failure of the valves or pipe breaks but

due to the failure of the turbine or pump. Only the probability of a turbine-driven

pump failing start on demand is 3E-2 (Modarres, 1993) which is greater than the

failure probability of the whole system of the PCIS with SDJP.

5. Conclusion

In this study, the application of a steam driven jet pump (SDJP) as a high pressure

passive core injection system (PCIS) to a BWR is investigated, and the concept of

PCIS with SDJP is given. The SDJP is analyzed in terms of the operation param-

eters. The water tank pressure is chosen as 320 kPa to increase the maximum

operation pressure of the system to apply to BWR. The water supply tank must be

elevated and pressurized such that it gives the pressure of 320 kPa at the inlet of

water to the SDJP. The supply water temperature is chosen 15�C, and the steam

which comes from the main steam line is assumed to be saturated steam. The

operation range of the SDJP in terms of the steam inlet pressure is from 2 to

10.5MPa. The study shows that when the discharge pressure (which is 10±20%

higher than steam inlet pressure) increases, the discharge mass ¯ow rate and tem-

perature increases as well.

The reliability assessment of the PCIS with SDJP is investigated using a simple

fault tree, and compared with the core injection system with a turbine-driven pump.

The supplied water pressure and temperature are not included as fault events in the

fault tree, since water is always drawn from a cold suppression pool and the tem-

perature of the water is the temperature of the containment building (i.e. room

temperature). It is found that the PCIS with SDJP is more reliable than the core

injection system with turbine-driven pump.

References

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of Engineering for Gas Turbines & Power, Transaction of ASME, submitted for publication

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