111th International Conference on Nuclear Engineering
Tokyo, Japan, April 20-23, 2003
“ICONE11- 36408“
VALIDATION OF THE LEAD COOLANT TECHNOLOGY
FOR BREST REACTORS
Z.I. Yemelyantseva
FSUE RDIPE, Moscow,
Russia
Fax: (095) 9752019
Y.I. Orlov
RF SRC IPPE, Obninsk,
Russia
V.N. Leonov
FSUE RDIPE, Moscow,
Russia
e-mail: [email protected]
P.N . Martynov
RF SRC IPPE, Obninsk,
Russia
1. INTRODUCTION
Higher requirements to operational safety
and reliability of nuclear reactors stimulate the search
of new coolants having advantages over traditional
ones (water, sodium, etc.).
One of such coolants is liquid lead. In terms
of its physical and chemical properties, liquid lead is
close to the eutectic alloy of lead and bismuth. A
large amount of data on physical, chemical, thermal
and other lead properties has been accumulated. A
methodological and experimental base is available to
be used for validation of the lead application as a
coolant of the BREST-OD-300 power reactors.
Lead dissolves many chemical elements and
compounds including components of structural materials. The potential consequence is damage to materials and loss of the circuit’s leaktightness. The efficient reduction of the material dissolution (corrosion)
rate occurs in the presence of iron and chromium oxide based protective films on the steel surfaces. Liquid lead interacts noticeably with oxygen. The result
of such interaction may be slag (phases containing
oxides of the coolant as such, structural steel components, etc.) that can deposit on the circuit surfaces and
deteriorate its thermohydraulic characteristics.
In general, the amount of oxygen in the
coolant and in the circuit as a whole is an extremely
important factor responsible for the normal operation
of the circuit. Excessive oxygen causes slagging of
circuits. Oxygen deficiency leads to dissociation of
protective oxide films on structural materials and development of corrosion processes. So successful operation of the BREST-OD-300 reactor facility (RF)
requires to control the coolant quality, i.e. to maintain
of the optimal amount of impurities (oxygen, oxide
compositions based on structural materials, etc.).
Also, measures should be provided to prevent contamination of the gas volume equipment and
purify the gas circulating through the reactor gas volume against the lead evaporation and material corrosion products and other impurities.
All these tasks are solved through implementation of the lead coolant technology. The term of
“coolant technology” means a complex of organizational and technical measures, processes and systems
(devices) for their implementation taken and used to
A.D.Yefanov
RF SRC IPPE, Obninsk,
Russia
V.A. Gulevsky
RF SRC IPPE, Obninsk,
Russia
ensure the specified (required) purity of the circuit
and the corrosion resistance of its structural materials
in the design, startup, repair and operation of an experimental rig or reactor facility.
2. COOLANT IMPURITIES
Impurities, their quantities, forms of existence and composition may and should have an impact on the processes of mass transfer in the liquid
lead circuits, liquid lead properties and serviceability
of the RF as a whole.
Main associated elements of lead ores (Cu,
Te, Bi, Ag, Au, Sn, As, Sb, Zn) are referred to genetic
(initial) impurities.
It is currently suggested that lead of the Pb00 and Pb-000 grades will be used to fill the primary
circuit of the BREST-OD-300 RF. The amount of
impurities in it is very small and they cannot have any
noticeable impact on the physical and chemical processes running further in the circulation circuit.
The composition, physical state and amount
of operational impurities depend much on the stage
and conditions of the circuit operation.
During operation of the circuit and following
its filling with the coolant, most of the impurities are
formed through the interaction of the coolant and
structural materials with the air oxygen usually
caused by loss of the circuit leaktightness.
2.1. INGRESS OF IMPURITIES OF STRUCTURAL MATERIALS
An additional amount of impurities results
from the dissolution of the structural material components. This dissolution may occur at an immediate
coolant contact with the circuit surfaces. If there are
oxide films on the circuit surfaces, components diffusing to the coolant from the structural materials
through these films are dissolved.
Practically any impurities (they are largely
Fe as well as Ni, Cr, Mn, Si, etc.) released into the
coolant from the structural material through dissolution or diffusion through the films start to interact
with the oxygen dissolved in the coolant or oxygen
ontaining compounds that results in the formation of
oxide phases with different compositions.
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As the result, some of the oxygen is spent for building
up oxide films on these steels and the rest of the oxygen is spent for oxidizing metal impurities that have
released from the steel to the coolant. So the process
of gradual coolant deoxidation is observed during
operation of any leaktight circuit. Different external
and internal (relative to the circuit) sources of oxygen
are used for its compensation.
As has been noted above, normal operation
of circuits is possible only if a strictly specified optimal range of the oxygen concentration (activity) values in the coolant is maintained. If this condition is
observed, the intensity of the mass transfer processes
is minimal.
The region of the allowable (optimal) dissolved oxygen concentration is limited here in the
range of 3⋅10-7 to 2.5⋅10-5 mass.%. The lower boundary is determined by the condition of the iron oxide
stability at the temperature of 650oC and the upper
concentration - by the lead saturation with oxygen
(prior to crystallization) at the minimum coolant temperature of 380oC. Sustaining the activity in this optimal region [7] excludes both crystallization of the
oxygen impurities in the cold sections of the circuit
and their clogging and increased corrosion of the circuit’s structural materials in the conditions of ao<aomin
when there is no oxygen in the coolant for forming
iron oxides and “healing” defective oxide films on
steels.
3. PROCESSES AND METHODS OF THE
COOLANT TECHNOLOGY
The result of calculations and experiments
has been the selection of the following lead coolant
technology methods accepted for further development:
- hydrogen purification of the coolant and the circuit;
- control of the coolant quality in terms of the oxygen content using a solid-phase oxidizer;
- coolant purification of solid-phase impurities;
- gas purification of suspended particles and aerosols.
3.1. HYDROGEN PURIFICATION OF THE
COOLANT AND THE CIRCUIT
The method is based on the coolant and circuit interaction with the hydrogen, water vapor and
hydrogen and water vapor mixtures.
If the selection of the reducing gas mixture
composition is correct, the coolant is purified of the
coolant oxides with the return of pure Pb to the coolant. Conditions for the formation of the Fe3O4 phase,
the basis of protective oxide coatings, are simultaneously created.
The use of the hydrogen and water vapor
mixtures during purification and control of the oxygen
content is most effective in the event of these mix-
tures being introduced immediately to the circulating
coolant flow. The gas phase is dispersed in the coolant using a special device (injector (ejector), etc.) and
a two-component “coolant-gas” flow is formed. As it
circulates over the circuit, the oxidizing and reducing
gas phase is delivered to the local sections of the circuit surfaces including also to the impurity concentration and deposition areas. As the lead oxide based
deposition interacts with the reducing gas phase, partial or complete hydrogen reduction of the lead from
these depositions takes place, respectively with partial
or complete disintegration of depositions.
The BREST-OD-300 RF circuit is characterized with small values of the average lead rates contributing to the gas phase separation and agglomeration, the existence of three free levels with the possibility of the “light” phase separation on them, a small
length of the channels with a horizontal flow and determining (in terms of the length) vertical downcomer
and riser sections, the absence of auxiliary pumps and
an insignificant head of the main circulation pumps.
A considerable amount of the lead coolant in the circuit automatically limits the allowable value of the
volumetric gas content in the purification mode due to
an inadmissible “swelling” of the level during gas
introduction to the lead volume. All this and the tank
design of the circuit impose considerable restrictions
on the design approaches and circuit treatment process modes.
1. Calculations have shown that organization of a
two-component “coolant-gas” flow circulating
over all areas of the reactor facility’s primary circuit in the BREST-OD-300 RF conditions if the
gas component during the two-component flow
formation consists of the bubbles with the size of
100 µm and less.
2. Tests on hydrodynamic rigs and water models of
ejector and injector devices with the gas delivery
thereto both forcedly and naturally from the reactor’s gas cavity using the rarefaction created in
the devices by the coolant flow and mechanical
gas dispersers have shown that each of the considered methods and devices has drawbacks and
merits of its own and can be used in the BRESTOD-300 RF in principle.
3. A theoretical possibility has been shown to
model processes of transportation and separation
of a finely disposed gas phase in liquid lead at
rigs and on models with water. There has been
confirmed the possibility of the finely dispersed
gas phase transportation to long distances (>10
m) in the conditions of the above processes, including in descending low-rate (V<1.3 m/s)
flows.
4. The possibility of the gas phase transportation by
low-rate (even at V≤0.15 m/s) coolant flows was
confirmed at the liquid metal circuit.
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5.
There has been experimentally proved the possibility of the hydrogen reduction of the lead oxide
even at unfavorable conditions of the process
running (very low rate (0.15 m/s) of the descending coolant flow, temperature low for the lead
circuit (370oC)).
3.2. SOLID OXIDIZER APPLICATION
Solid oxidizers are used to control the oxygen content, namely, to increase oxygen concentration
in the coolant. The oxide material (PbO) is confined
in the circulation circuit section (reaction volume)
with a limited capacity through which the coolant is
pumped. At that, the solid-phase oxidizing agent contacting with the coolant is dissolved with oxygen evolution, that is further transported over the circuit with
the coolant flow. By changing the coolant temperature
and/or flow rate in the reaction volume, the oxide
dissolution rate can be controlled in accordance with
the circuit demands for dissolved oxygen.
casing; 9 – lower perforated grid of the reaction volume; 10 – coolant inlet
Complex of rig tests in the mass exchange
apparatuses (MEA) of different designs implementing
the solid-phase method of the oxygen thermodynamic
activity (TDA) control in the lead coolant (Figs. 1 and
2) were conducted during the corrosion tests of structural materials planned for the use as part of the facility under design. The tests have helped to ensure the
modeling of the specified oxygen TDA levels with a
high accuracy on a considerable time base.
Figure 2
Diagram of the
MA-3 mass exchange apparatus
of
immersion
type:
Figure 1
Diagram of the
MA-2
mass
exchange
apparatus
of
immersion
type:
1 – breathing tank vessel; 2 – coolant outlet; 3 - heating portion of the internal heater; 4- charge of PbO
spheroids; 5 – deflector baffle for returning a part of
the oxidized coolant flow to the reaction volume inlet;
6 – upper perforated grid of the reaction volume; 7 –
mass exchange apparatus casing; 8 - reaction volume
1 – tank vessel; 2 – coolant inlet; 3 - heating portion
of the internal heater; 4 - charge of PbO spheroids; 5
– deflector baffle for returning a part of the oxidized
coolant flow to the reaction volume inlet; 6 – spacer
grid; 7 – mass exchange apparatus casing; 8 - reaction
volume casing; 9 – upper perforated grid of the reaction volume; 10 – lower perforated grid; 11- coolant
inlet
The direct examination of the MEA reaction
volume after long testing time has determined that the
use of a mass exchange apparatus did not lead to the
solid oxide phase release beyond the MEA boundaries
and formation of slag in the circuit.
It is known that the process of the iron diffusion release to the lead and the required output of the
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mass exchange apparatus with PbO in the “coolantstructural material” system are interconnected.
If there is no dissolved oxygen source
(MEA) in the said system, the iron should come to the
RF circuit in the “hot” section up to the level limited
by the solubility and conditions are created in the circuit’s “cold” section for crystallization of excessive
iron determined by the difference in the solubility of
this element in the lead at two temperatures of 550oC
and 420oC. The RF “cold” area is slagged and coolant
is deoxidized, which leads to corrosion degradation of
the structural materials in the “hot” area.
To prevent these negative phenomena, it is
necessary to stipulate continuous oxidation of the iron
coming to the circuit and maintaining of the oxygen
content in the coolant in the range optimal for the
steel state using the MEA.
4. COOLANT FILTRATION METHODS AND
AGENTS
The most efficient method of the coolant purification of solid impurities is its filtration. It is especially efficient for removal of large particles (≥10 µm)
from the melt.
High-temperature conditions of the filter operation (t≥450oC) and the aggressive effect of the lead
coolant with slag make the selection and validation of
the serviceability of filtering materials the most important stage of the studies.
Fibrous, granulated and grainy materials
have been studied for thermal stability of the strength
properties and chemical compatibility with the lead
melt and slag and candidate materials have been determined for continuous coolant treatment filters:
1. Silica glass fabric.
2. Carbon fabric and combinations of carbon
fabrics with glass fabrics of silica and mullite-silica compositions.
3. Mullite-silica kaolin fabric.
4. Card wire punched fabric of metal fibers
(metal felt).
5. Grainy granulated material - α-Al2O3 based
spheres and Al2O3 polished grain.
The above materials have been tested in the
lead melt under static and dynamic conditions and on
a circulation rig at the temperatures of 450÷600oC
based on the specified time of 360, 500 and
1000 hours.
The criterion of the satisfactory compatibility
of filtering materials with the lead melt is the absence
of chemical interaction and failure as well as functional deterioration of the material properties and reduction in the material strength.
The silica fiber glass fabric showed good filtering properties during rig tests. The melt purification efficiency is ∼50.0%.
The strength of the material after tests in the
lead melt under static conditions at the temperature of
550oC (depending on the time of the contact with the
lead) decreases significantly and the embrittlement
state is recorded.
The mullite-silica (kaolin) fabric after being
held in the lead melt at the temperature of 550oC during 360 hours fell by the factor of ∼3.5. The mullitesilica fabric samples are currently under the additional
tests as a promising material.
In terms of preserving strength characteristics, the carbon fabric showed a good result (the
strength decreased by the factor of 1.5) after thermal
tests in the air at the temperature of 450÷600oC during
500 hours. The effect of the environment on the fabric
properties was observed – the strength decreased by
the factor of ∼2.5 after holding in the lead melt at
these temperature and time conditions. Following
tests at the SM-2 rig, the carbon fabric layers were
pure, did no contain visible lead inclusions, preserved
their strength and elasticity and no fiber failures were
found.
The card wire punched fabric of metal fibers
(metal felt) preserved its strength properties after being hold in the lead melt at the temperature of 550oC.
Preliminary estimates show that the efficiency of the
lead melt purification of impurities for metal felt is
not less than 52.0%.
The research of a grainy material of corundum (spheres of Al2O3) after tests in the static conditions in the lead melt at the temperatures of 450, 550
and 550oC hase shown that the strength of the spheres
changed insignificantly after 500 hours of being held
in the lead; it subsequently stabilizes and does not
practically change. The strength properties of the polishing grains of Al2O3 after all tests in the lead melt
remain unchanged. The granules and spheres of Al2O3
are wetted with lead insignificantly and do not have
surface changes as pits and caverns.
Figs. 3 and 4 show photographs of the inlet
and outlet surfaces of metal felt and polishing grains
respectively. It is seen that the inlet surface of the
filtering materials is contaminated with impurities and
the outlet surface of the polishing gains and metal felt
is practically pure of accumulated slag. Rough estimates show that the efficiency of the lead melt purification of slag deposits is ∼45 and 52.0% for these
materials.
The tests of the filter’s mockup at the SM-2
circulation rig have shown that the serviceability of
filtering models of fibrous and grainy materials is
efficient enough.
The coolant flow rate via the filter has not
decreased during the tests. It demonstrates that the
filter was not completely clogged with impurities.
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Figure 3 Filtering module with the charge of Al2O3 polishing grains after tests as seen from the coolant inlet
side (upper picture) and outlet side (lower picture). Pb
temperature - 550oC
Figure 4
Filtering element of metal felt after tests as
seen from the coolant inlet side (upper picture) and outlet
side (lower picture). Pb temperature - 550oC; suspended
matter (Fe2O3) con centration at the inlet - 4×10-3 mass.%
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REFERENCES
1.
2.
3.
4.
M.P.Smirnov. Lead Refining and Treatment of
Semifinished Products. Moscow. Metallurgiya.
1977.
I.S.Kulikov, Thermodynamics of Oxides. Reference Book. Moscow. Metallurgiya. 1968. 342 p.
Structure, Nuclear Dynamics, Thermodynamics
and Impurity State of Lead and Bismuth Melts
(Modern State of the Problem). Review. FEI0290. Moscow. TsNIIatominform. 2000.
Problems of the Heavy Liquid Metal Coolant
(Lead-Bismuth, Lead) Technology. Proc of the
International Conference “TZhMT-98”. Obninsk.
V1. 1999. pp. 92-106.
5.
6.
7.
8.
J.R.Weeks, A.J.Romano. Liquids Curves and
Corrosion of Fe, Ti, Zr in Liquid Bi-Pb Alloys.
Corrosion. 1969. Vol.25. No. 3, pp. 131-635.
Experience of Design and Operation of SolidElectrolyte Activity Meters of Oxygen in the
Lead-Bismuth Coolant. Proc. of the International
Conference “TZhMT-98”. Obninsk. V2. 1999.
pp. 631-635.
V.I.Subbotin, M.N.Ivanovsky, M.N.Arnoldov.
Physical and Chemical Fundamentals of Using
Liquid Metal Coolants. Moscow. Atomizdat.
1970.
Use of Hydrogen and Water Vapor Mixtures in
the Heavy Coolant Technology. Proc. of the International Conference “TZhMT-98”. Obninsk.
Moscow. V.2. 1999. pp. 712-719.
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