1

EXPERIMENTAL RESEARCH OF CAVITATION PERFORMANCE OF A LEAD

COOLANT AND MODELS OF MAIN CIRCULATION PUMP IN HEAVY LIQUID-

METAL COOLANT REACTOR PLANTS

A.V. Beznosov, T.A. Bokova, A.V. Lvov, P.A. Bokov (NNSTU n.a. R.E. Alekseyev, Nizhny

Novgorod, Russia)

Introduction

The conventional concept of cavitation in vane-type pumps combines the

characteristics of a pumped medium and the design features of a particular vane-type pump.

Different specialists give slightly different definitions to the term cavitation". Almost

in all definitions, a sign of cavitation is recognized as a discontinuity, or a break in the flow of

liquid drops with the formation of bubbles, voids, etc. moving with the flow of liquid and then

collapsing. As applied to vane-type pumps in calculations related to their operating cavitation

modes, a condition of cavitation occurrence is recognized when a critical pressure is reached

in the intervane space. In this regard, a critical pressure at which cavitation occurs means a

pressure of saturated liquid vapors at a given temperature.

Analysis of the references shows that during examining cavitation in pumps occurring

implicitly or explicitly, all the authors in any cases take water or liquids close to it in physical

properties (salt solutions, organic compounds, etc.) as pumped liquids. In rare cases of

calculating the pump flow part with organic liquids different from water: oil, kerosene,

gasoline, etc., the differences of their properties from water are taken into account using

empiric coefficients. The densities of the pumped medium are taken for the actual pumped

liquids. Cavitation and other characteristics are calculated using empiric formulas obtained in

water tests, although the same authors admit that the cavitation processes depend on the

thermodynamic properties of the liquid, the pressure of saturated vapors, the surface tension,

etc.

As evidenced by the opinion of all experts, it is obvious that in order to create a new

pump, it is necessary to know, first of all, the cavitation characteristic of a pumped medium,

by which all the authors mean saturated vapor pressure of such medium. Other properties

(thermodynamic, physical, etc.) of the pumped medium may be taken into account by means

of introduced empiric coefficients or ignored.

One of the factors determining the cavitation performance of each particular vane-type

pump includes physical properties of the pumped liquid. The incipient cavitation condition is

accepted to be a condition where an underpressure equal to the pressure at which the

continuity of the pumped medium is violated, and a new phase having a much smaller density

than the pumped medium is formed in the local areas of the transported pump flow. This

condition may be considered the most general formulation of cavitation, but it can be

determined only as a result of appropriate cavitation tests. The most obvious and simple

incipient cavitation condition, in explicit or implicit form, which is adopted by all the authors,

includes a condition of reducing the flow pressure to boiling, namely, vapor bubbling. This

value, saturated vapor pressure, is easily determined from relevant references and liquid state

tables.

The cavitation performance of a lead coolant significantly differs from that of other

coolants in nuclear reactors, such as water and sodium, and is probably close to such

characteristics of a lead-bismuth coolant.

Specific properties of a lead coolant, which determine the cavitation process in vane-

type pumps, include:

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low pressure of saturated vapor 1.44×10-17

Pa at t = 127 °С, 5.38×10-5

Pa at t =

527 °С;

high boiling temperature tboil = 1,750 °С under atmospheric pressure, and is

greater within the reactor loop;

surface tension σ 400 n/m at t = 350 °С, which is greater than that of water;

much greater density ρ = 10.5×10-3

kg/m³ at t = 450 °С;

thermal conductivity λ = 16.58 W/mK at t = 400 °С, which is greater than that

of water but is lower than that of sodium;

kinematic viscosity ν 20.99×10-8

m²/s at t = 400 °С.

Lead does not wet surfaces of steels and cast irons with protective coatings applied on

them; the contact angle of wetting of oxidized steels (in dry argon) is Θ = 110 – 120°.

A lead flow in the actual circuit always includes unsolved solid particles of impurities,

such as lead oxides or oxides and other compounds of construction material components,

which form cavitation cores and determine their number. In the cracks of such particles that

are hydrophobic with respect to lead, there are gases (vapor-gas mixtures) forming

discontinuities in lead, or gas bubbles, which then agglomerate.

Cavitation cores, which are weak points in a lead flow in the circuit, may also be the

finest (in the order of micrometers) bubbles of unsolved gases (vapor-gas mixtures).

An optimal pump design requires the following:

Specifying the pump head and feed determined from hydraulic characteristics

and adopted temperature differences and power of the reactor loop;

The rotating speed of the main circulation pump, which is conventionally

determined based on the specific speed coefficient ns and characterizes the

type of a pump. This coefficient is determined on the basis of cavitation

characteristics of the pumped medium, which were unknown for the lead and

lead-bismuth eutectics. In Russia and other countries, the liquid lead cavitation

is calculated and studied theoretically by examining the stability boundaries of

a metastable liquid state under negative pressures. Liquid lead is modeled

using the molecular dynamics method and applying the numerous interatomic

interaction potential. As a result of these studies, we propose analytical

temperature dependences of the nucleation rate, which are used for

extrapolation of the calculation results into a domain of practical importance.

The outcome of these conceptually important results for engineering design of

pumps for pumping lead and lead-bismuth coolants cannot be utilized at

present time due to the assumptions applied in calculations and theoretical

research.

Comprehensive experimental studies of the cavitation performance of lead and lead-

bismuth coolants are carried out at the Nizhny Novgorod State Technical University

(NNSTU). These studies include the following:

Determining the presence of gas and conditions for breaking the liquid lead

column and lead-bismuth eutectics;

Determining the cavitation performance of a lead coolant at Т=5000С in two

different centrifugal pumps on two different benches;

Determining cavitation of a lead and lead-bismuth in an ejector (Venturi

nozzle);

Determining the cavitation performance of a pump as part of an axial wheel

complete with a hydrostatic bearing at Т=420-5500С;

Determining the cavitation performance of the main circulation pump model

(М 1:3) of the BREST-OD-300 reactor plant. The pump feed is 2,000 t/h (200

3

m3/h), the pump head is 2.0 m of lead column, and the lead temperature is up

to 5500С.

Experimental Validation of Previously Unknown Property of Gas Cavitation of a Lead

Coolant and Its Characteristics Using Three Independent Methods on the NNSTU FT-3

Bench

The objective of this work was to confirm the absence of vapor cavitation and the

presence of gas cavitation in the lead coolant flow as well as to determine the conditions of

gas cavitation occurrence and explore its characteristics in the HLMC flow.

Description of FT-3 Test Bench

The FT-3 KI TsN test bench is designed to carry out research tests for identifying

cavitation characteristics of a lead coolant.

The lead coolant circuit (Figure 12) includes the following elements:

- A centrifugal electric pump (delivered by NNSTU); pump feed – 6...20 m³/h, pump

head – 6...1 m of liquid column, operating medium – heavy liquid-metal coolants, temperature

of pumped HLMC – up to 550 °С. Pump drive – an induction electric motor, current – three-

phase, (rated) power – 7.5 kW. The electric pump has a frequency control of shaft speed

within the range of 800 – 1,500 rpm (complete with a frequency converter). The electric pump

has a gland sealing of the shaft for gas.

Figure 1 Diagram of the FT-3 KI TsN Test Bench with an Impeller Pump

1 – melting tank; 2 – liquid-metal pump; 3 – dump tank; 4 – head tank; 5 – filter; 6 – suction pipe; 7 – pump pressure

tank; 8 – ejector; 9 – ejector inlet pressure tank; 10, 11 – ejector pressure tanks; 12 – ejector outlet pressure tank; 13 –

flow-metering tank; 14 – gas mass-exchanger pressure tank; 15 – gas mass-exchanger; 16 – hydrogen cylinder; 17 –

argon cylinder; 18 – high-pressure header; 19 – low-pressure header; 20 – ejector pressure tank high-pressure header;

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21 – ejector pressure tank low-pressure header; 22 – gas header of gas mass-exchanger; 23 – vacuum pump; 24 –

compressor; 25 – gas accumulating tank; 26 – gas flow meter; 27 – gas flow meter of gas mass-exchanger; 28 –

magnetic flow meter; 29 – refrigerator; 30 – trap; 31 – damper; 32 – express-freezing section

Fig. 2. FT-3 Test Bench Appearance

With a view to improving the representativeness of the research results, conditions of

occurrence and characteristics of gaseous cavitation in the lead coolant flow were determined

using three independent methods.

Method 1 of Cavitation Research

The essence of the first method of cavitation research consists in the following (Figure

3).

Using pump 1, the lead having the temperature of 400 – 500 °С with a variable flow of

5 – 30 m³/h is pumped out from tanks 2 and 3 connected with each other to head tank 5 at

valve V1 being closed. At this time, the elevation point of the lead empty level changes from

+200 mm off the pump impeller axis (initial position upon the bench filling-up) to approx. -

1,000 mm (depending on the pump speed and pressure р0 over the lead empty level), after

which the pumping stops, probably, due to gaseous cavitation. In any case, unless the

pumping stops earlier, pump starvation occurs when the lead empty level in tanks 2 and 3

reaches the lower edge of the pump suction tube due to gas intake into the pump. Figure 11

shows diagrams of the geometric suction head i

sH in the process of level lowering in tanks 2

and 3, as well as a behavior of the velocity head g

v

2

2

decreasing with reduction of the lead

flow according to the pump head and rate; the figure also shows a behavior of losses in the

suction pipeline (with inlet) – lossh and losses in the pump flow part – pump.fl.p.h , and the

minimum static level of the pump deepening – hst.

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Fig. 3. Pressure Change in Different Elements of Impeller Pump Suction Pipeline During Tests According to

Method 1

The physical significance of рcs consists in the fact that at pressure relief in the lead

flow to this value, there occurs an expansion of gas bubbles located directly in the lead, in the

crevices of solid particles of impurities in the flow and in the cracks of construction member

surfaces where a two-component flow is formed: lead – gas (vapor-gas mixture). Growing in

size, the gas bubbles agglomerate forming gaseous cavities and bugholes – all this results in

an increase of the channel hydraulic resistance, and in case of the pump flow part – in a

decrease of the pump head and feed.

Method 2 of Cavitation Research

The essence of the second method, regardless of the first method for determining

conditions of occurrence and gaseous cavitation characteristics (Figure 3), consists in the

following. Lead circulation on the following route was set: pump 1 – head tank 5 – valve V1

– tanks 2 and 3 – pump 1. By readjusting the position of valve V1, the level in tanks 1 and 2

was set so that the absolute pressure at the pump inlet was about 1.0 atm. Then the gas

pressure in the gas system was relieved to atmospheric, and the gas system was evacuated as

far as the moment of a decrease in the pump output due to pressure drop in the pump flow part

and occurrence of gaseous cavitation.

Method 3 of Cavitation Research Using an Ejection Device

The essence of the third testing method consisted in the following. Circulation on the

following route was arranged: pump 1 – pipeline with valve В4 supplying the lead coolant to

the ejector – ejector – tanks 2 and 3 – pump 1. Increasing the rate of flow through the ejector

contributed to the ejector hydraulic resistance surge along the lead coolant without ejection of

protective gas and with gas ejection by the lead flow in the ejector narrowed part. The surge

in the ejector hydraulic resistance should be indicative of gaseous cavitation occurrence in the

ejector narrowed part. The bench parameters before and after gaseous cavitation occurrence

were registered. The measured absolute pressure in the lead flow in the ejector narrowed part

was compared with the values рcs obtained during testing according to the first and second

methods.

Determination of Cavitation Performance of a Pump

To ejector

From ejector Absolute pressure line

(without P0 constant)

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The occurrence of cavitation processes is monitored according to the pump starvation

characterized by an abrupt change in the electric motor load, a reduction of the flow rate down

to its termination during testing using Method 1 as well as by changes in its vibrational and

acoustic properties.

The cavitation performance is determined using two methods.

The essence of determining the cavitation performance using the first method consists

in a stepwise variation (reduction) of the lead column pressure relative to the horizontal plane

of entry into the pump impeller if the pump parameters are properly retained. This pressure is

changed by repositioning (lowering) the lead empty level in the pump tank (suction tube) in

the course of pumping over the coolant to the head tank at closed valve VS7 and retention of

the bench parameters prior to and at the moment of cavitation processes occurrence in the

pump flow part. Repositioning and retaining the position of the lead empty level H in the

pump tank relative to entry into the pump impeller are carried out in a sequential order at the

following H values: +100 mm; 0 mm; -200 mm; -400 mm; -600 mm; -800 mm; -1,000 mm; -

1,200 mm (tentatively).

The essence of determining the cavitation performance using the second method

consists in reducing pressure at the entry into the pump impeller by evacuating gas from the

protective gas system (vacuum degassing).

Determination of Cavitation Performance of a Lead Coolant Using an Ejection Device

The moment of gaseous cavitation occurrence in the lead flow narrowed part in the

ejector will be characterized by an abrupt increase in the ejector hydraulic resistance. During

testing, variation of the lead flow rates occurs through the ejector with measuring pressure at

characteristic points of the ejector route.

Dependence of Maximum Suction Head on Operating Parameters (Method 1)

The analysis of the results at a variable pump speed and the gas system pressure of 500

00,1 atm (1,000 – 1,500 mm of lead column) at t = 450 – 500 °С shows that pump

starvation might occur due to gaseous cavitation occurrence in the pump impeller and due to

lowering of the lead empty level to the lower edge of the pump suction tube, gas intake and jet

break. According to the experiment, since the pump starvation occurred at one and the same

suction head, regardless of the pump speed, then the first assumption is more likely. In any

case, the head HSmax

exceeded 1,050 mm at the lead density of 10.5 10³ kg/m³ at testing

temperature. If it is right, then there was the negative (bursting) stress Рcs in the lead flow in

the pump impeller.

Analyzing the diagrams of dependence of the suction head HS on the feed G (Figure 4)

shows that the coolant supply failure (G = 0) occurs at the same suction head at speeds of n =

1,000, 1,100 and 1,200 rpm.

G, m³/hour

G, m³/hour

a) b)

Hs,

mm

Hs,

mm

1,000 rpm

1,100 rpm

1,200 rpm

1,000 rpm

1,100 rpm

1,200 rpm

900 rpm

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G, m³/hour

G, m³/hour

c) d)

a) without any excess pressure; b) with the excess loop pressure of 0.1 atm; c) with the excess loop pressure of

0.3 atm; d) with the excess loop pressure of 0.5 atm

Fig. 4. Dependences of Suction Head Нs on Feed G at Various Pump Speeds

The tests, as it was expected, proved a material dependence of the values Рcs and HSmax

on gas pressure in the circuit (under other equal conditions). The pump starvation and

reduction of HS at 900 rpm and at lesser flow rates and heads quite clearly show that gaseous

cavitation accompanied by the pump starvation and reduction of the pump suction head

occurs at lesser pump speeds, which is probably determined by the cavitation performance of

the pump itself.

Dependence of Gaseous Cavitation Characteristics and Conditions of Its Occurrence on

Pressure in the Gas System (at Excess Pressure and Vacuum Degassing)

Figure 5 featuring dependence of the maximum suction head at which pump starvation

occurred on the pump speed at a variable pressure in the gas system shows that with an

increase in the gas system pressure, the form of the dependence Hmax = f(n) changes. This

result may be accounted for by the fact that with an increase in gas system pressure, the

density of the medium conveyed by the pump rises.

a) b)

c) d)

a) without any excess pressure; b) with the excess loop pressure of 0.1 atm; c) with the excess loop

pressure of 0.3 atm; d) with the excess loop pressure of 0.5 atm

Fig. 5. Dependence of Maximum Head on Electric Pump Speed at Variable Pressures in the Gas System

Dependence of Load on the Pump Electric Motor on Pressure in the Gas System (Method 2)

Hs,

mm

Hs,

mm

900 rpm

1,000 rpm

1,100 rpm

1,200 rpm

900 rpm

1,000 rpm

1,100 rpm

1,200 rpm H

ead

Hm

ax,

mm

Head

Hm

ax,

mm

Head

Hm

ax,

mm

Head

Hm

ax,

mm

Speed, rpm Speed, rpm

Speed, rpm

Speed, rpm

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With pressure reduction in the bench gas system, there was a gradual decline in load

on the electric motor. Under the pressure of 0.2 kgf/m2 (atm), there was a discontinuity of the

lead flow in the pump impeller, which corresponded to the results of Method 1 and the

calculation-theoretical conclusions presented in the technical publications.

Fig. 6. Variation in Load on the Pump Electric Motor Due to Pressure in the Bench Gas System

Experimental Determination of Cavitation Performance of a Nozzle Device (Method 3)

While arranging gas circulation through the gas inlet union from the gas volume in the

gas system to the lead flow in the nozzle device narrowed part, the hydraulic resistance

increases insignificantly. Gaseous cavitation in the nozzle device occurs at relatively low

speeds of the lead in the nozzle device narrowed part of 5.0 – 5.42 m/s, which to a

considerable extent is determined by the reverse pressure at the device outlet (Figure 1.40).

The underpressure in the nozzle narrowed part at which gas cavitation starts is 0.3 –

0.4 kgf/cm² (atm), which correlates with the testing results of the first and second methods.

Fig. 7. Dependence Diagram of Mean Pressure in Lead Coolant Flow According to Nozzle Length at G3 =

0.43 m³/s, G2 = 0.55 m³/s, G1 = 0.5 m³/s

Fig. 8. Dependence Diagram of Mean Flow Rate of Lead Coolant According to Nozzle Length at G3 = 0.43 m³/s,

G2 = 0.55 m³/s, G1 = 0.5 m³/s

Pu

mp

mo

tor

loa

d, A

Bench gas volume pressure, kgf/cm2

900 rpm

1,000 rpm

1,100 rpm

Pkg

f/cm

2 (

atm

)

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In the course of tests conducted on the FT-3 bench in cavitation modes, including the

last hours and in other modes, there were no changes detected in the pump feed and head on

the relevant routes. There was no strange knock or vibration in the pump. In some cavitation

modes, there was a minor change detected in the acoustic properties.

Fig. 9. Photo of Disk Outer Surface Fig. 10. Photo of Impeller Vane End

Based on these results, it is possible to come to the conclusion that during operation of

vane-type pump impellers made of steel 08Х18Н10Т at the temperature of lead coolant -

pumped medium of 470 – 500 ºС during several dozens of hours under gas cavitation

conditions, there occurs local destruction of oxide films (a few micrometers in thickness). No

sign of erosive-corrosive wear of the surface material of the pump flow parts was detected.

Based on the research performed, the following findings have been obtained:

During testing using the first method, the pump starvation might occur either due to

gas cavitation or due to lowering of the lead empty level to the lower edge of the

pump suction tube or combination of these factors. In any case, the maximum

suction head exceeded 1,050 mm at the lead density of 10.5 10³ kg/m³ at testing

temperature.

The critical pressure during testing using the first method is close to the value of

0.0 kgf/cm² (atm), and the pump flow part may have local tensile and bursting

stresses in the lead flow, and formation of gas bubbles – development of gas

cavitation. The pump does not pass into the regime of failure and impeller operation

in a steam-and-gas mixture; it operates in the lead-gas two-component flow

environment, but with lesser head and flow.

During testing using the second method, occurrence of gas cavitation is registered

at the lead flow pressure of 0.5 – 0.3 kgf/cm² (atm), and occurrence of developed

gas cavitation at 0.2 kgf/cm² (atm). At the time of occurrence of developed gas

cavitation, there were registered pump head and flow values accounting for 70 –

80% of the original values, and the electric motor power decreased ~ 2 times as

much.

While testing using the third method, there was no chance to specify conditions of

gas cavitation occurrence. It is reasonable to presume that under testing conditions,

gas cavitation in the nozzle device narrowed part was initiated at the average flow

pressure of 0.3 – 0.4 kgf/cm² (atm) and the flow rate of 5 m/s and more.

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The gas cavitation characteristics obtained using the three independent methods

satisfactorily correlate with each other taking into consideration additional factors

affecting the processes under control.

Experimental Research of Cavitation Performance of Pump Axial Impeller and

Hydrostatic Bearing on the NNSTU FT-4A Bench

The objective of this work was to confirm the absence of conventional vapor

cavitation and erosive damages to the axial impeller at the shaft speed up to 1,200 rpm,

Т=420-5500С, pressure at the impeller inlet ~ 0.5 kgf/cm

2 (atm) and less.

Figure 11 shows a coolant circulation diagram in the test bench at the axial impeller

rotation.

1 – upper bearing unit; 2 – gas seal assembly; 3 – axial impeller; 4 – outlet straightener; 5 – MCP shaft

simulator; 6 – bearing housing; 7 – shaft; 8 – pull-out part housing; 9 – HLMC tank

Fig. 11. Coolant Circulation Diagram in Experimental Section

The lead coolant circulates through the bench channels as follows. During the shaft

rotation, the axial impeller supplies the lead coolant from the bottom upwards to the outlet

straightener. Coming out of the outlet straightener, the main flow of the coolant goes upwards

to the tank cover, then it turns round 180°С and goes down to the impeller inlet. The rated

parameters of the circulation high-temperature (400-550°С) flow of the lead coolant at

n=1,200 rpm – feed – 1,000 – 1,200 t/h, head – approx. 1.5 m of lead column, impeller inlet

pressure – 0.8 – 0.5 kgf/cm2 (atm). A portion of the flow with relatively small feed (approx.

0.5 – 0.8 t/h) is supplied to the hydrostatic bearing.

The axial impeller used to provide the coolant circulation through the hydrostatic

bearing was in the following state after the tests:

The axial impeller vanes were irregularly covered with black protective oxide

coatings. The vane periphery was wetted with lead. (The axial impeller vanes

are made of steel 3);

To pressure measuring

system

Coolant level

Working chamber pressure tapping line

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The vane edge deformation was registered. A possible reason for deformation

of the edges included the fact that the vane thickness in this section was less

than 1 mm, and low strength performance of steel 3 at 550 °С;

There was no sign of erosive wear on the impeller vanes and the outlet

straightener blades made of steel 12Х18Н10Т.

a) b)

a) axial impeller; b) axial impeller vane with a deformed edge

Fig. 12. Axial Impeller After Tests

Experimental Research of Cavitation Performance of Main Circulation Pump Model on

the NNSTU FT-4A Bench

The objective of work carried out on the NNSTU FT-4 bench includes performance of

cavitation, energy and resource tests of an axial main circulation pump (MCP) flow part

model of the BREST-OD-300 reactor plant, tests in potential emergency situations and

bearing unit variants of such pumps.

Description of Bench and Testing Procedure

The major bench equipment includes:

- a circulation electric pump with a replaceable induction motor with speed control

within the range of 600-3,000 rpm;

- a vapor generator;

- a flow meter;

- a gate valve (flow adjuster);

- a drain tank;

- a gas system;

- pressure tanks (for measuring pressure of the lead coolant).

The circuit basic characteristics: flow rate up to 2,000 t/h; electric pump head up to 2

m of lead column; temperature up to 5500С; lead weight – 10 t; protective gas – argon; circuit

core elements material – steel 08Х18Н10Т.

Cavitation tests of the MCP model flow part are carried out using the following

procedure. Circulation of a lead coolant is arranged across the circuit with a sequential

increase of the electric pump speed. At every fixed speed, the bench parameters are measured,

following which using a vacuum pump, the bench gas system is degassed retaining its

parameters. Upon occurrence of gaseous cavitation, the electric pump feed and head decrease,

which is registered by means of the bench instruments.

After completion of the tests, the electric pump flow part is checked for any cavitation

erosive damage.

Vane edge

deformation

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Conclusion

It has been experimentally justified, using circulation benches with a high-temperature

lead coolant and centrifugal, axial pumps and an ejector (Venturi nozzle) under the conditions

of reactor loops, that there is a previously unknown property of a lead coolant, i.e. gaseous

cavitation, and its characteristics have been analyzed using independent methods.

The performed tests contribute to reasonable calculation of the flow part of pumps

using heavy liquid-metal coolants and development of design and operational documentation

of MCP circuits with fast-neutron reactor plants cooled by lead and lead-bismuth coolants.