Oxygen control systems and impurity purification in LBE: learning from
DEMETRA project
L. Brissonneau1*,
1
: CEA/DEN, Cadarache, DTN/STPA/LIPC, F-13108 Saint-Paul-lez-Durance, France
*corresponding author, laurent.brissonneau@cea.fr
Abstract
Operating a system using LBE (Lead Bismuth Eutectic) requires a control of the dissolved
oxygen concentration to avoid corrosion of structural materials and oxide build-up in the
coolant. Reliable devices are therefore needed to monitor and adjust the oxygen
concentration and to remove impurities during operation. In this article, we describe the
learning gained from experiments run in the framework of the DEMETRA project (IPEUROTRANS 6th FP contract) on the oxygen supply in LBE and on impurity filtration and
management in different European facilities.
An oxygen control device should supply oxygen in LBE at sufficient rate to compensate loss
by surface oxidation, otherwise local dissolution of oxide layers might lead to the loss of steel
protection against dissolution. Oxygen can be supplied by gas phase H2O or O2, or by solid
phase, PbO dissolution. Each of these systems has substantial advantages and drawbacks.
Considerations are given on devices for large scale facilities.
The management of impurities (lead oxides and corrosion products) is also a crucial issue as
their presence in the liquid phase or in the aerosols is likely to impair the facility,
instrumentation and mechanical devices. To avoid impurity build-up on the long term,
purification of LBE is required to keep the impurity inventory low by trapping oxide and
metallic impurities in specific filter units.
On the basis of impurities characterization and experimental results gained through filtration
tests in different loops, this paper gives a description of the state-of-art knowledge of LBE
purification with different filter media. It is now understood that the nature and behaviour of
impurities formed in LBE will change according to the operating modes as well as the method
to propose to remove impurities. This experience can be used to validate the basis filtration
process, define the operating procedures and evaluate perspectives for the design of
purification units for long-term application in lead-alloys liquid metal coolant systems.
Keywords : Lead Bismuth Eutectic ; oxygen control, purification, filtration
1. Introduction
The development of the chemistry control and monitoring is one of the issues that are critical
for nuclear systems using lead alloys either as a spallation target or as a coolant [1]. The aim
is on the one hand to avoid oxide formation, lead oxide or corrosion products, that might
induce fouling (deposits that eventually reduce the heat transfer capacity) or even plugging of
the smaller ducts. On the other hand, it is to ensure the formation of protective oxide layers
to prevent (or slow down) metallic element release. It is then necessary to maintain the
oxygen concentration within a range between the dissolution limit of the protective oxide film
to avoid fast dissolution of the metal and the solubility limit of oxygen to avoid formation of
lead oxide [2, 3]. It should be noted that the anisothermal nature of system might induce
mass transfer in particular from the hot point (higher dissolution rate or oxide removal, higher
solubility) to the cold point (higher deposition rate, lower solubility…). As oxygen is
consumed by oxidation of the surface, a constant oxygen supply is required. Oxygen can be
brought from gas or solid phase with respective advantages and drawbacks for both
methods.
One other important scope of development or validation is the management of impurities
(lead oxide and corrosion products) which presence in the liquid phase or in the aerosols is
likely to impair the facility, instrumentation and mechanical devices. Main contamination
sources (apart oxygen) are identified as the corrosion products (Fe mainly, Ni, Cr, etc.)
expected to be generated continuously at a rate depending on the operating temperature,
liquid metal flow rate, material composition…The contamination by impurities will occur
during start-up operation (first filling) or after maintenance or repair phases, and under
normal operation of the system at a continuous rate. Some impurities such as iron potentially
present long-term and cumulative effects. One of the processes considered for purifying LBE
relies on filtration, the other on sedimentation.
2. Definition of an oxygen concentration operating domain.
The oxygen is the most important chemical element for lead management, because of its
potential contamination of the liquid by forming solid oxides and its main influence on the
corrosion rate of structure. Excess of oxygen might come from the start-up operations
(dissolved oxygen in liquid metal), from the maintenance phase as well as possibly from bad
tightness (by air ingress) during normal operating mode. Oxygen consumption comes from
the corrosion (oxidation) of the structures.
The oxygen concentration must be adjusted to a specified value for corrosion control and to
avoid production of lead oxides (mainly PbO) during service lifetime. A relatively limited
working range for oxygen is therefore defined, in a range of temperature going from the cold
stop operation (typically 350°) to the hot spot on the fuel pin (typically 50°C above the normal
hot leg temperature). For the upper limit, this corresponds to the solubility limit of oxygen in
lead to avoid forming lead oxide. For the lower limit, this corresponds to the limit below which
the magnetite layer (Fe3O4) is dissolved and therefore no longer protects the metal walls from
corrosion [4].
We report only here the limits for LBE, but similar and close results might be obtained for
lead.
 Upper limit [5]:
Several authors propose a solubility limit for the formation of PbO from oxygen in LBE. From
comparison, it was decided to work with the following expression for the limit oxygen
concentration CO* from Gromov [4] and confirmed by other authors (see [5-8]):
5000
for 623K <T < 773 K
(1)
T
With CO* in wt% and temperature in Kelvin
This in good agreement with the solubility law proposed by Ganesan (most recent paper)
5100
for 812K <T < 1012 K
log( C O* )  4.32 
T
With CO* in at%
log( C O* )  3.2 
The solubility limit is reported in Figure 1.
 Lower limit:
Following [5], it might be considered that oxygen activity in Pb has a chemical potential close
to the one of PbO (though PbO might not be stable at low oxygen content). The free enthalpy
of the reaction (2a):
3Fedis + 4PbO ↔ 4PbL + Fe3O4
(2a)
where subscript dis holds for the element dissolved in liquid, is decomposed in
3Fes + 2O2 (g) ↔ Fe3O4 = 2 GO2(Fe3O4)
(2b)
4PbOs ↔ 2Pbs + 2O2
= -2 GO2 (PbO)
(2c)
3Fedis ↔ 3 Fes
= - 3RT ln aFe(dis)
(2d)
4Pbs ↔ 4 Pbdis
= 4 RT ln aPb(dis)
(2e)
4ºdis (= 4 PbOdis) ↔ 4 PbOS = -4 RT ln aO(dis)
(2f)
GO2 are the partial molar free enthalpies of oxygen as classically tabulated in Ellingham
diagram, the free enthalpies of formation of the pure species Fe, Pb at solid states are taken
to be 0 and therefore their activities are 1.
At equilibrium G (eq.2a)= 0 and then using Eq.(2b) to Eq.(2f) yields:
2 GO2(Fe3O4)-2GO2(PbO)+4RTlnaPb(dis)-3RTlnaFe(dis)-4RTlnaO(dis) = 0
(3)
GO2(PbO) and GO2(Fe3O4) are given by classical thermodynamic data (reported in [5])
The lead activity is taken as 1.
According to Henry’s law (justified by the very few amount of O and Fe atoms dissolved in
liquid):
aFe= CFe/C*Fe
(5)
and
aO= CO/C*O
(6)
where C* holds for the solubility limit for precipitating Fe and PbO respectively.
The iron solubility limit in Pb is given in [4]:
logC*Fe (wt%)= 0.34-3450/T
(7)
With C*Fe in wt% and 823K <T<1053 K
By using equations (4) to (7) in Eq.(3), it comes easily:
*
log( CO min )   3 log C Fe
 2.355  10600
4
T
(8)
With COmin in wt% and temperature in Kelvin, and is better reliable with T ≥ 450°C.
Considering an iron activity of one in the liquid, it yields the lower limit curve as reported in
Figure 1. Obviously, it is more likely that iron concentration would not reach the limit for iron
precipitation (at maximum it should correspond to the equilibrium with magnetite Fe3O4), and
hence the above equation should be considered only as indicative of the lower limit, as
magnetite decomposition could occur at higher oxygen concentrations, depending on iron
concentration in LBE; obviously, it varies as aO4.aFe3. It should be noticed that, due to the
anisothermal nature of a nuclear system, iron concentration should vary in the circuit, some
precipitations might occur in the cold parts, allowing for some release in the hot parts until
the concentration comes into equilibrium with magnetite.
The lower limit can be also be derived by writing that the activity of oxygen in the magnetite
is the one of oxygen dissolved in lead :
aO(Fe3O4)=aO(Pb)=O.xO,
by using the relationship between oxygen activity coefficient O as given by Ganesan [7]:
 O2 ,1at %  exp  16.906 * T   121349 .T  


PO2 
xO
GFe3O4 
aO Fe3O4 
T
xO Pbeq _ Fe3O4  

 O Pb) 

exp  16.906 * T   121349

.
T


(9)
(10)
It can be shown in Figure 1 that very few difference arise from the different methods used for
calculating the lower limit. between equations (8) and (10). In expression (10), the influence
of iron activity is not directly expressed as the dependence of the limit with the iron
concentration but the solubility limit should always behave as the solubility product aO4.aFe3.
A large domain of oxygen concentration vs. temperature (simple hatched, Figure 1) can be
defined, between the lower temperature/higher oxygen content that permits the lead oxide
formation and the higher temperature/lower oxygen content that permits magnetite formation.
The operating domain is about 3 orders of magnitude large in oxygen concentration between
10-3 to 5.10-1 ppm. However, a narrower domain should be considered:


At cold stop at 340°C, no lead oxide should be allowed to form, and as oxygen
content increase the oxidation rate of the structures, it should be limited to a
minimum: then the upper limit should be one or two order of magnitude lower. To
enlarge the operating domain to the high oxygen content, it has to be questioned if
PbO precipitation might be acceptable at cold stop. Considering 4000 metric tons of
Pb in the vessel and a cold stop at 350°C, PbO would precipitate if oxygen content is
higher than 10-1 ppm. If operation are conducted at oxygen level of 5.10-1 ppm (higher
limit of the large operating domain described above), up to 22.31 kg of PbO can
precipitate. So the highest reasonable limit for the oxygen content is 8.10-2 ppm.
As the lower limit is defined with the assumption of iron saturated lead alloy, which is
not conservative, and as the temperature on the hot spot of the claddings can be up
to 40°C higher than the lead at core exit, and also as some temperature transients
can be experienced, the highest temperature to be considered should be 50°C higher
than in the hot leg. As a consequence, the lower limit is about one order of magnitude
higher.
As a consequence, the normal operating domain should be about two orders of magnitude
for oxygen content between 10-3 and 10-1 ppm, which is quite narrow (see Figure 1, double
hatched). However, some short transients should be allowed in the larger domain.
1,E+02
PbO (Gromov)
1,E+01
PbO (Ganesan)
1,E+00
Large operating domain
[O] (ppm)
1,E-01
1,E-02
Narrow operating domain
1,E-03
1,E-04
operating extension
if protected claddings
1,E-05
minimal concentration to form Fe3O4
___ at Fe saturation in Pb (Gromov)
___ idem (Ganesan)
----- at 10% Fe saturation in Pb
1,E-06
1,E-07
340
360
380
400
420
440
460
480
500
520
540
560
Temperature (°C)
The operating domain could be enlarged if GESA coatings is applied only on the claddings.
Still, the hot legs remain to be protected. It means that the lower temperature to be
considered in the system is the temperature of the hot leg: 480°C. According to the lower
limit draws on the figure above, it would enlarge the domain down to about 2.10-4 ppm of
oxygen.
If a coating is applied on the hot leg, the oxygen content could be as low as 5.10-5 ppm, in
order to avoid steel dissolution in the cold plenum at 400°C. As the kinetics of corrosion are
very low at this temperature, some excursion at lower values could be accepted on restricted
periods.
1
(223/16)*4.10-7*4.109 = 22300 g
Another strategy to be considered is tantalum coatings. As tantalum does not dissolve in Pb,
the tantalum oxide formation is to be avoided
If equilibrium with tantalum oxide Ta2O5 is considered, the oxygen content at equilibrium in
Pb is between 10-18 at 350°C to 10-12 at 550°C. If Tantalum is used as coatings for the SGU
tubes and the hot leg, the oxygen content in the lead should be about less than 10-15 ppm.
If large surfaces of tantalum are present in the vessel, oxygen initially present in the lead will
be trapped at the hotter surfaces. Then colder surfaces will oxidize and hotter will be
reduced. Any extra oxygen, from the gas phase… must be trapped in order to prevent further
oxidation of the tantalum (which would eventually lead to large release of tantalum oxide in
the coolant).
Accidental conditions
1) Air ingress
In case of a large air ingress in the primary vessel (e.g. the air leak in the argon flow of
Superphénix that lead to the introduction of 52 kg of oxygen in the sodium), it is likely that
oxides would form first at the surface, due to the low solubility of oxygen in the lead compare
to the one in sodium (about 3 orders of magnitude at 480°C). The dissolution of lead oxide
might be mainly driven by the presence of vortices at the surface.
Such leak could be detected in the cover gas (N2 or O2 detection E..G by MS or gas
chromatography) of by the slow increase of the oxygen content in the lead by the oxygen
probes.
2) Water ingress
In case of leak at SG, H2O decomposition could lead to oxygen increase in the lead, as well
as H2. However, as H2O is very stable compare to O2 in lead, most of H2O should be
eventually found in the cover gas. This could be detected by O-meter or H-meter in the gas
plenum or by MS or IR spectroscopy.
In both cases it seems that the kinetics of lead pollution should be rather slow and detection
should arise before considerable amount of oxygen are dissolved in lead.
However, after detection, the coolant has to be cleaned.
See further: H2 reduction ?, filtration ?
4.3. Recommendations for oxygen control in large scale facilities
In large scale facilities, oxygen control in liquid lead or lead-bismuth is a real challenge: the
targeted oxygen content must be maintained at any time of operation in every part of the
coolant. Oxygen control of liquid lead alloys in industrial-scale reactors involves conditioning
of liquid metal amounts in the order of 1000 metric tons rather than 0.1–1 metric tons as in
today’s experimental facilities. Many oxygen sources and wells may exist in the facility. The
working reactor is by nature anisothermal, with complex hydrodynamic features that might
induce particular local flow rates and the oxidation process depends on time, temperature
and oxygen content.
It is believed that due to large surface areas susceptible to oxidise in the reactor, the more
difficult task is to supply enough oxygen to the coolant to prevent oxygen depletion due to the
oxidation of the (hot) steel surfaces.
Too weak oxygen content may locally lead to the dissolution of the oxides on the hot parts of
the steel elements and eventually to their direct dissolution in the coolant. It should be
underlined that even when the targeted oxygen content is reached, or at even lower
concentrations, some lead oxides may still exist in some parts of the circuits (calm, cold
parts) which might play the role of local sources for oxygen and might lead to incorrect
understanding of the real oxygen content in the coolant (especially if oxygen release is
located near an oxygen sensor). In the general case, it should be assumed that the oxygen
content might be heterogeneous in all parts of the circuit. This has been clearly illustrated by
the STELLA tests. It is then recommended that some mass transfer model should be used to
describe at least roughly the oxygen and iron behaviour in the circuit.
It can probably be assumed that the oxygen content of the liquid metal initially exceeds the
range of oxygen control and that the additional oxygen makes up for the comparatively high
initial oxidation rates of the reactor walls and internals, especially if no appropriate preoxidation treatment has been applied to the metallic construction materials. In this case,
mass-exchangers for oxygen transfer have to be designed chiefly for replacing oxygen once
the actual content falls below the target value. The amount of oxygen that has to be replaced
depends on the specific materials (steels) used for the reactor components, the surface area
in contact with the liquid metal and local temperatures, and also on the targeted oxygen
content, which should be as low as possible, observing a sufficient safety margin from the
transition to substantial dissolution of the materials.
In the case of the currently studied pool-type reactors, the general strategy of oxygen
transfer consists in extracting a certain amount of liquid metal for conditioning in the massexchanger(s) and re-feeding into the pool, preferentially where oxygen is predominantly
consumed in the designed steady state of operation (core, steam generators). The massexchanger may be either an internal component of the reactor or part of an external loop,
irrespective of whether solid/liquid or gas/liquid oxygen-transfer is chosen. The basic
requirements are that it permits to deliver oxygen at a sufficient rate that compensates
oxygen consumption by oxidation of steels, without leading locally to the formation of lead
oxide and ideally:
-
can be easily controlled by the oxygen sensors,
-
need the less maintenance or intervention by personal,
-
not lead to risk of contamination or induce a risk for the safety, with most simple
designs and auxiliary systems.
Considering the designed steady state of operation of the reactor—involving oxidation of the
reactor components at a constant level of oxygen dissolved in the liquid metal—, the mass of
dissolved oxygen would change at an instantaneous rate rmO , if oxygen control is not
applied ( rmO  0 for oxygen consumption). Dividing by the mass of liquid metal in the reactor,
mLM, yields rcO , the corresponding rate at which the average oxygen concentration would
change without oxygen control. rcO , along with the minimum temperature Tmin in the massexchanger, is of critical importance for the minimum percentage of the liquid-metal inventory
that must be recycled. It will drive the pumping capacity needed for an efficient oxygen
control [11]. The corresponding maximum residence time of the liquid metal, max, calculates
as
 max 
cO* (Tmin )  cO
 rcO
(22)
where c O denotes the average oxygen concentration in the reactor pool (target
concentration) and cO* is the saturation concentration of oxygen in the regarded liquid metal,
and can be calculated according to equation (1).
 LM ;min , to be supplied in the oxygen-transfer
The minimum mass flow of liquid metal, m
device(s) follows as:
m LM ;min 
mLM
(23)
 max
The minimum mass flow in the oxygen transfer device is obtained when saturated LBE exits
the device.
The absolute amount of oxygen that has to be supplied is | rmO | , irrespective of the actually
 LM  m LM ;min . However, the choice of m
 LM significantly influences the
recycled mass flow m
driving force available for oxygen transfer, resulting from the difference between cO - the
oxygen concentration to be achieved in the mass-exchanger- and cO* at the temperature of
oxygen transfer. For isothermal conditions in the oxygen-transfer sub-system,
 m LM ;min 
 cO  cO*
(24)
cO  cO*  1 

m
LM


 LM  10 m LM ;min can provide 90% of the theoretical maximum driving
showing that, e.g., m


 LM   ).
force (calculated with for m
Proper dimensioning of the oxygen-transfer system for the steady state of operation critically
depends on correct estimation of the need for oxygen supply in the reactor. Quantitative
data, from long-term experiments on the performance of ferritic/martensitic steel T91 in
flowing LBE at 550°C, cO ≈ 1.6×10-6 mass% and a flow velocity v = 2 m/s, indicate rates of
material consumption (in terms of metal recession) in the order of 0.01 and 0.001 µm/h for
the first 4,000 h of exposure and the following period from 4,000 to 15,000 h, respectively
[13]. Assuming that the consumed steel is quantitatively converted into oxides—either
immediately at the specific site of consumption or, after convective transport in the form of
dissolved metals, somewhere else within the reactor—the estimated decrease (without
oxygen control) of the dissolved mass of oxygen relative to the effective surface area is
approximately 0.003–0.03 g/(m² h). In this estimation, the steel has been identified with pure
iron (Fe), converted quantitatively into magnetite (Fe3O4). Multiplying by the effective surface
area gives | rmO | .
The European demonstration plants studied with respect to the application of lead alloys in
industrial-scale nuclear reactors are the XT-ADS and EFIT. The reactor of XT-ADS contains
2000 metric tons LBE, its total internal surface area of 1300 m², the surface with
temperatures ranging from 385°–>400°C that are regarded as decisive for oxygen
consumption from the liquid metal correspond to an effective area of 150 m². The larger
reactor EFIT contains 6000 metric tons liquid Pb at a total surface area of 7500 m², from
which 3000 m² at 400°–480°C are considered responsible for the main part of oxygen
consumption.
Still using data for T91 oxidation rates obtained at 550°C, the estimated need for oxygen
supply at constant cO = 10-6 mass% varies
 For XT-ADS from 0.45 to 4.5 g/h. Assuming 0.45 g/h oxygen consumption and
 LM ;min = 0.1 kg/s, as follows
Tmin = 400°C, the minimum mass flow to be recycled is m
from the respective rcO = -6.25×10-12 mass%/s. Decreasing Tmin by 50°C
 LM ;min = 0.4 kg/s.
(Tmin = 350°C) results in m

For EFIT, from 9–90 g/h, which corresponds to rcO = -(4.17×10-11–4.17×10-
 LM ;min
) mass%/s. The minimum mass flow through the mass-exchanger m
10
calculates as 5.96–59.6 kg/s at Tmin = 400°C, and 1.75–17.5 kg/s at Tmin = 450°C.
 LM ;min actually required for keeping up the
In both cases, but particularly for XT-ADS, m
targeted oxygen concentration may be somewhat smaller than estimated, as the
temperatures in the reactor are lower than the temperature at which the oxidation data used
for approximating oxygen consumption was determined (with T91 steel at 550°C).
The main outcome of the analysis is, that the pumping power required for oxygen control in
the steady operating state of the investigated reactors is probably marginal in comparison to
the capacity of pumps installed for supporting heat removal from the liquid-metal pool, even if
provision is made for temporarily transferring higher amounts of oxygen after accidental loss
of oxygen.
The volume flow of pure gaseous oxygen (referred to 25°C) that has to be supplied
continuously to a gas/liquid mass-exchanger follows from the estimated need of oxygen and
the ideal gas law as VO2 ; 25C = 5.8 cm³/min and 116–1160 cm³/min for steady-state operation
of XT-ADS and EFIT, respectively. In order to prevent significant formation of solid PbO in
the mass-exchanger or to meet safety regulations, dilution with Ar or another inert gas (with
negligible solubility in the liquid metal) may be required. Estimating the oxygen flux through
the gas/liquid interface on the basis of the experience from operating the CORRIDA loop,
i.e., jO ≈ 2×10-8 kg/(m² h), results in a free liquid-metal surface inside the oxygen-transfer
device in the order of 6 m² and 120–1200 m² for XT-ADS and EFIT, respectively. This
means, that jO must be at least two orders of magnitude higher, so as to achieve a
reasonable size or number of mass-exchangers for gas/liquid oxygen-transfer on the
industrial-scale. For judging the technical feasibility for industrial purposes, it is therefore
essential to prove the limit of jO imposed by the transfer kinetics, which can only be done by
specifically designed experiments.
Considering transfer by solid phase, it should be considered that a typical mass exchanger
contains 50 kg of PbO pellets, corresponding to 3.6 kg of oxygen. With an oxygen supply
rate of 0.45-4.5 g.h-1, it leads to a total consumption of the PbO in the device in such short
periods as 33-330 days. Even if one such mass exchanger supplies oxygen per steam
generator and one for the core, the higher oxygen supply rates would require frequent filling
operations that might be detrimental to personal exposure and reactor availability. These
operations should then be performed automatically, without stopping the auxiliary loop and
the need of personal near the exchanger (radiation exposure, see later).
It has been shown by STELLA experiments that oxygen control by solid phase is not trivial:
there is no indication on the state of the PbO pellets, they might be dissolved in LBE/Pb or
covered by contamination oxides (iron, chromium oxides). Indeed, Russians specialists
recommend much more complex mass exchanger designs that the one used in STELLA [14].
A precise control of the temperature and LBE/Pb mass flow must be performed and pellets
contamination by oxides must be avoided by using a LBE/Pb feedback in the device. Doing
so, dissolved iron and chromium precipitates as oxides before they reach the pellets.
A safe concept of oxygen control must also include a loss-of-oxygen scenario, e.g., take into
account temporary failure of the oxygen-control system, after which raising the oxygen
content of the whole liquid-metal inventory by an exceptionally high amount, in appropriate
time is required. It should be also underlined that the grace period, define as the time which
leads to the total consumption of oxygen by the structures if no oxygen is provided, might be
quite short. For a 9 g.h-1 oxygen consumption in EFIT, it is about 2 hours, which may be very
uncomfortable for reactor management in case of the momentary loss of an auxiliary loop.
Though being out of range of operation, it can be considered that the dissolution rate of the
magnetite is rather slow, and that no real damages are caused to the materials immediately
after the end of the grace period: the surfaces at the hottest temperature where dissolution is
fast should have the thicker oxide layers. However, the dissolution of the magnetite itself
might induce a large mass transfer in the reactor and completely disturb its chemistry.
4.4. Advantages and drawbacks
Oxygen control by gas phase works: NRI experiments permit to maintain oxygen
concentration between 10-5 to 10-6 wt% with H2/H2O ratio of 1/11. In CORRIDA, the right
order of magnitude of oxygen quantities to reach is searched by injecting air without
bubbling. The difference between the two is the size of the loop (100 l of liquid for CORRIDA
and 1.7 l for COLONRI). The main point is the amount of oxygen to be injected compared
with the system. The bigger is the facility the more efficient must be the mass transfer device,
as more oxygen must be supply. It is more difficult to obtain the right amount of oxygen with
H2/H2O, as enough oxygen must be supply to offset consumption while remaining at the
equilibrium concentration.
However, for a nuclear system, using gas can cause contamination problems in case of
leakage.
The advantages of oxygen control by dissolution of PbO are the efficiency of mass exchange
between solid and liquid phases, the elimination of some gas lines and to avoid the possible
solid oxide formation and eventually excess of slagging within the system. This leads to
cleaner operation and less degradation of the thermal hydraulic performance. Moreover, an
easy control could be performed via the flow rate or the temperature regulation [14].
The disadvantages include having to insert a fixed reserve of solid oxides into the LBE/Pb
system that, if depleted, is harder to replenish [15]. It is a drawback for the maintenance of
this system. Nonetheless, it requires a complex design for the mass exchanger, especially in
order to avoid the risk of oxide (of iron…) precipitation on lead oxide solid, leading to sluggish
dissolution kinetics.
To synthesize, the main drawbacks and advantages of the two Oxygen Control System
(OCS), gas and solid phase types, are reported in Table 1:
Advantages
Drawbacks
Gas phase type
- Same device for O2 control and
purification by H2
- No operation on the device in
normal operation
- Regulation rely on dissolved
oxygen measurements in liquid
phase by oxygen sensors if non
equilibrium gases (Ar/H2…) is
needed
- Need for a (very) efficient bubbling
if equilibrium gases (H2/H2O) are
used
- Risks of oxide formation (Ar/O2…)
- Large gas flow rates at the
beginning of operation
- Need of a large dilution gas loop
(Ar) with purification system (Fission
Products, Activated Products,
Tritium, Polonium)
- Risk of contamination exposure for
operators (gas leakage)
Solid phase device
- No gas management
- No risks of plugging (oxide
formation)
- rather easy control by flow rate
and temperature
- More complex design for mass
exchangers
- More maintenance : pellets
filling
- Personal exposure (Activated
corrosion products as 54Mn or
60
Co deposited on the cold part of
the device)
- Risks of oxide precipitation on
pellets and sluggish kinetics for
dissolution
Table 1: Advantages and drawbacks of each method of oxygen control systems.
5 Purification
5.1. Liquid phase filtration
In principle, only the solid impurities, non dissolved elements or oxides particles should
be gathered in a line containing filtration capabilities. The filtration appears as one of the
most suitable process for the lead alloys coolant but its efficiency depends on numerous
parameters, constants or variables during the operations.
The type of impurities (nature, form and size, concentration…) influences the choice of the
filtration method to trap impurities (surface filtration or deep bed filtration), the type of filter
medium, the filter removal ratings and the required level of filtering as well. The Russian
experiments showed that the filtering media used in deep bed filtration with a texture
promoting coagulation-sedimentation and adhesion mechanisms were more promising to
retain fine dispersed particles (size ≤ 1μm) [16]. Liquid metal properties (viscosity, density,
corrosion properties) and operating parameters (flow conditions, flow rate and temperature
ranges, pressure, filter pressure drop monitoring) can affect the choice of the filter medium
and the design of the filter unit. The flow rate through a filter medium affects the retention of
colloidal particles and aggregates. Some manufacturers recommend not to exceed a filtration
rate (flow velocity divided by the filter area) of about 2 cm/s for liquid metals. Otherwise, the
filters housing must be properly implemented in the loop to avoid turbulence at the inlet of the
filters created by elbows or valves. In STELLA loop, a horizontal portion of the circuit was
selected for the filter housing location to avoid the return of impurities trapped by the filter to
the liquid metal flow by hydrodynamic separation. Moreover, the implementation of the filter
unit in the cold parts of the circuit must be preferred compared to hot zones to prevent
dissolution of metal from structural wall and mass transfer to other cold surfaces of the
system. The mode of operation (continuous or batch), the duration of the process and the
regeneration process are other parameters to consider for the design of the filtration system.
Different types of filters were also tested on STELLA loop (see Figure 5). The aim of the
STELLA filtration test programme was to assess the suitability of commercialized and
conventional filters applied to LBE purification. Analysis of all these design and operating
parameters in the comparison of the possible filtering media depends mainly on technical
characteristics data given by manufacturers. Thanks to the feedback from Russian LBE
filtration studies [3] and preliminary experiments on STELLA loop, three kinds of filters (see
Figure 5) were selected by favouring media with appropriate removal ratings (few to tens
µm), and specifications such as high holding capacity or high filter area, low clean pressure
drop, good mechanical strength for high operating pressures and temperatures.
Poral filters are made of sintered Stainless Steel (316L) matrix and are generally more
appropriate for cake filtration. These filters are known to have very good resistance to high
temperatures and high flow rates. PALL Dynalloy mesh media filters are specifically used for
deep bed filtration. They are made of a uniform layer of woven stainless steel (316L) fibres.
This layer is compacted and sintered to ensure good resistance and integrity. The porosity of
this medium is high (~60%) which improves its hydraulic resistance and its retention
capacity. These Dynalloy mesh medium filters have good mechanical resistance at high
temperature - up to 477°C according to manufacturer’s data. A particular welding technique
(steel/mesh sandwich weld method) has been developed to improve the thermal and
hydraulic resistant of Dynalloy media filter prototypes. For PALL Standard PMM cartridges,
the medium is a thin matrix of stainless steel (316L) powder within the pore of SS wire mesh.
This results in a porous material with high mechanical properties (no wire mesh shift, pore
size integrity maintained). The maximum admissible temperature for use is 677°C.
Four filtration tests are presented with two Poral filters of different removal ratings, a Dynalloy
filter and a PMM cartridge. Main results and operating conditions of STELLA filtration tests
are summarized in Error! Reference source not found.:
Poral filter
CL20
Poral filter
CL10
Filter
Dynalloy
15CO
PMM 150 filter
Removal
ratings
35 μm
20 μm
10 μm
9 μm
Temperature
400 °C
400 °C
400 °C
400 °C
0.25 / 0.36
0.44 / 0.3
0.2 / 0.5
0.08
3.10-7 wt%
-
-
1.9.10-5 wt%
1.9.10-7 wt%
1.8 / 3.2
mbar/h
30
mbar/h
-
2.5
mbar/h
156 h
24 h
98 h
306 h
Type
Filter
characteristics
Operating
conditions
Filtration tests
parameters
Flow rate
range (m3/h)
Oxygen
concentration
Average
pressure
drop rise
Duration of
the test
Pb7Bi3-Bi with PbO-Bi micron-size agglomerated particles
Impurities characterisation
forming filaments containing iron, chromium oxides + In, Sn,
Pb, Si
Trapping method
Surface (inner layers into filters)
Table 2: Synthesis of results and operating conditions of LBE filtration tests
Monitoring of the pressure drop values (figure 6) showed a steadily increase over time for the
4 filters tested. It suggests that impurities were gradually trapped. It could be assumed that
the pressure drop continued to increase beyond the maximum value of the pressure
transmitter measurement range (max. 1040 mbar) and would have been rising up to a
plateau. Only Dynalloy filter pressure drop monitoring reached this plateau (about 620 mbar)
whereas other tests were stopped before reaching it. Many assumptions are possible to
explain this plateau: hydraulic limit of the line, no more impurities in LBE to be trapped by the
filter or local stretching of the meshes of the filter, leading to the release of some particles
previously trapped.
Metallographic observations by optical of scanning electronic microscopy showed no stretch
of the filter mesh nor filter degradation, when previous tests on vertical lines lead to large
cracks in filter welds. In general, the retention of the particles lead to the formation of an
inner (close to the mesh) grey compact deposit and an outer porous deposit of some
hundreds of microns. The microstructure of the deposits is complex. The main and larger
compounds are Bi and Pb7Bi3, together with PbO, which is usually more easily detected by
X- Ray diffraction than by SEM analyses, certainly due to the sub-micron size of the PbO
particles. Micron size particles or filaments of oxides are aggregated between grains of
heavy metal compounds. The oxides are generally iron rich but may contain chromium, lead,
and in some cases indium, tin, silicon…The atomic ratio between the elements is generally:
Cr:1, Fe:2, Pb 5-8, Bi 8-10. No impurities were trapped in the mesh of the filters: Deposits
observed at the inner surface did not seem to have entered into the medium filter. Globally,
impurities were trapped by surface filtration, not by in-depth retention
For PALL PMM 150 cartridge, very few deposits were observed on this filter and there were
fewer filaments retained than in the other media filters: a few on the surface of the mesh and
nothing on the surface of the perforated internal wall.
5.2. Sedimentation, cold trapping
Sedimentation or settling process based on precipitation of impurities could be investigated
as an alternative purification system. Considering the properties of impurities (micrometric
size, low density, tending to agglomerate), it should be possible to separate them in the liquid
phase in a (cold) quasi-stagnant auxiliary vessel.
Sedimentation principle is based on the difference of density between solid impurities and
liquid. The impurity comes to the surface because their density is lower.
The sedimentation velocity vs (m/s) is given from the Stokes law by the following relationship:
d 2 .(    ).g
p s
l
v 
s
18.
(25)
The time of sedimentation depends principally on: dp (m), the particle diameter; the other
parameters are: relative density of LBE, ρl (kg/m3) and lead oxide, ρs (kg/m3) and viscosity of
fluid, μ (kg/m/s).
This method of purification by sedimentation requires a specific tank in the liquid circuit. After
filling, the LBE into this tank, particles tend to float and settle to the interface. Thereafter, the
clean liquid metal is draining from the bottom while keeping in the tank the impurities
concentrating at the liquid/gas interface before starting the recirculation of the clean liquid
metal in the loop.
The study of this process was foreseen on STELLA; the loop was equipped with a
decantation tank but this process was not yet tested. No test has been carried out to rate the
feasibility and efficiency of this device. Nonetheless, some restriction could be addressed for
the purification of micrometric size particles which could be carried along by thermal
convection flow.
However, the sedimentation principle was studied by NRI in the COLONRI loop using
another device. The loop is equipped with an equalising tank used as a cold trap. This
system permits to precipitate and condensate impurities which float on the colder surface of
the liquid. Impurities can be separate from liquid phase and concentrated in the equalizing
tank using a metallic mesh filter implemented in the bottom of the equalising tank next to the
draining line.
The equalising tank, in the upper part of the loop (Figure 2), is the container where impurities
of liquid metal condensate and float on the surface of the liquid metal. In fact, this tank is
usually operating at about 350oC, which is the lowest temperature in the loop. However, the
main stream of LBE is flowing through the square structure and only partially bypassed into
the equalising tank. In this way, only about 50% of the flux is purified into the tank and the
rest continues flowing along the loop. At the end of each experiment, the equalizing tank is
opened and the impurities are removed by manual/mechanical way. The disadvantage of this
method is that it is not possible to quantify the amount of impurities collected because some
LBE is co-trapped with the impurities.
However, this method is valid for purification, since the analyses of the liquid metal flowing in
the loop confirmed its good composition after the several thousand hours. In particular
analyses of the LBE after 7000 hours in operating conditions were carried out. The bulk,
homogeneous material was a solid solution of the alloying elements (43.9% Pb and 56.1%
Bi) and a small percentage of Si (about 0.2%). Small, localized precipitates (<20 μm) were
observed. They were homogeneously distributed in the bulk alloy and their analyses showed
enrichment in elements such as Fe (about 80 wt%), Cr (about 10 wt%), Ni (about 3 wt%), Sn
(about 1.8 wt%).
For this device as well as for a conventional settling vessel, the tank must be opened during
maintenance period to remove impurities by manual/mechanical way. This problem remains
a main issue for an industrial use. Another solution to investigate could be the use of an
overflow line at the upper part of the vessel so that floating impurities would be entrained with
a fraction of the liquid and then could be trapped in a filter.
5.3. Gas phase
Appropriate filters are required for purification of gas circuits from aerosols. Paper, tissue or
‘Argon’ type filters were used to collect aerosols. Deposits trapped by the filters were a thin
black powder containing mainly particles of 2 µm sizes or less, with presence of lead and
bismuth (mainly in the form of Bi, Pb7Bi3, PbO) and traces of magnetite. Examinations of filter
located at long distance from the testing tank showed less presence of PbO.
5.4. Discussion on filtration
Tests of about hundreds of hours had shown a good mechanical resistance at 400°C: no
defects on the welds and nor stretch of the wires were observed -only little distended for
Dynalloy- and the pore integrity was maintained, and there was very few oxidation of the
stainless steel mesh.
Results of metallographic analyses showed that impurities were trapped mainly by surface
(cake) filtration for these conventional type filters. Analyses detected lead oxide (PbO) in
deposits but not in large amount and also presence of iron and chromium oxides. These
oxides were probably coming from peeling of the oxide coatings of structural materials. Also
the initial impurities of lead bismuth (tin, indium) tend to be trapped. Deposits formed uniform
layers on the inner surface of filters: sometimes thicker in the downstream part of the filters
with variable thickness, according to the radial height in the filter. Trapped particles were
varied in size and shape. They were micron-size aggregated particles of lead oxide and long
micron-size filaments of iron and chromium oxides contained in a heterogeneous mixture
composed of Pb7Bi3-Bi (and to some extent PbO). These differences in size, structure and
shape involve different mechanisms and rates of trapping according to the medium design
parameters (removal ratings and the holding capacity), but also depending on operating
conditions.
It must be considered that the amount of impurities present and the oxygen content in LBE
were probably not the same at the beginning of each test. The first test with Poral CL20 filter
was performed after a period of maintenance when the amount of impurities is assumed as
not negligible. However, the level of impurities in the loop was never known nor the rate of
their formation. It was therefore difficult to compare filters tests one with each other, and not
possible to provide a quantitative analysis of filters efficiency. The only way to evaluate filters
real-time performance was by following the variations in the pressure drop over time due to
the filter module, thus giving an indication of the increase in resistance linked to particle
retention in the filter.It was estimated that only few grams of Fe and Cr oxides were trapped
on these classical filters.
In order to provide efficient removal of impurities from the liquid and gas systems by filtration
devices, impurities have to circulate through this one for being trapped. The impurities that
remain stagnant or stuck to walls can not be trapped and could participate to mass transfer.
Moreover, it is known that impurities are not uniformly present in all cold and hot parts of a
circuit. Whatever the choice of the filter medium, these points must be solved when
designing the circuits and the purification unit as well as the issue of the filter replacement
after its lifetime expiration.
Finally, these filter media are promising. The real trapping of impurities for all filters tested
(Poral, Dynalloy and PMM cartridge filters) was confirmed by the pressure drop monitoring
as well as the metallographic post-analyses.. Qualification by more long-term tests (2000
hrs) using other removal ratings and under various operational conditions (high or low air
pollution level) with monitoring of the impurity content by regular samplings would better
assess their performance in terms of mechanical and thermal resistance, trapping
efficiency,…
5.5. Recommendations for basic processes definition
Some preliminary recommendations can be set out for the design of the filtration units.
Nature and behaviour of impurities formed in LBE change according to the operating mode
(initial start-up and filling, re-start or transient states, off-normal operating conditions, normal
operations and steady states). In normal operating conditions, fine dispersed particles of iron
and chromium oxides are coming from metallic impurities dissolved in the coolant by
diffusion through the oxide coating of structural materials.
The use of a reducing gas in the filter unit should make it possible to reduce the quantity of
impurity, the main impurities being the lead oxide that is reduced back to liquid lead. The
subsequent destruction of the oxide particles is only possible by using micrometric hydrogen
gas bubbles for a combined action: first the breaking of the crystallite or agglomerate
structure, and then the lead oxide reduction with hydrogen through the fresh lead oxide
surface generated by the cracking of the particles. The iron oxide reduction is kinetically very
slow so that these particles or aggregates containing iron oxides remain stable in solution
and should be trapped in a special filtration unit. The bubbling allows for the removal of the
sticking deposits on the structures, and to transport them to the purification unit. Then,
remaining slags are purified in the filtration medium not only by filtration, but by combined
mechanisms of coagulation/concentration and filtration. The trapping in filters is done by
adhesion strength between particles aggregates and the surface of the filter material. Under
these operating conditions, mainly non reducible iron and chromium oxides (Fe3O4, Fe2O3,
Cr2O3) should be trapped by the filtration process [3, 16].
Two filtration methods and systems could be proposed. Cake filtration with conventional
filters (Poral, Dynalloy and PMM cartridge filters) which are suitable to remove impurities
formed consequently to an excess of oxygen (after a large air ingress and during filling, startup and re-start operations) in order to prevent a large oxidation of the structure and the
formation of lead monoxide. These filters could be located in filtration units especially
designed for batch operation. For the whole service time and normal operating modes, active
oxygen control for corrosion protection, and impurity reduction and elimination processes are
required. Special prototype filters have to be designed promoting deep bed trapping for thin
particles of non-reducible oxides. The coagulation and adhesion mechanisms are mainly
depending on the filtration medium holding capacity and characteristics such as tortuosity,
porosity, specific surface area. These characteristics have to be taken into account in the
design of the filtration unit. For instance, a Dynalloy medium with multiple layer of different
ratings removal should improve the depth filtration. Other important parameters are
specifications of filtration rates which must not exceed a maximum value of about 2 to 10
cm/s.
Filter unit systems could be proposed for semi-continuous operations in auxiliary circuits or
by-pass lines of the facility. Moreover, filtration processes need to be coupled with other
systems of management of impurities: H2 gas bubbling (in a vessel or into the filter housing)
could be used for the reduction of lead oxides and the cracking of oxide particles, or
sedimentation to remove large agglomerates during start-up or restart.
The regeneration is an important point in order to increase the service life-time of these units,
and reduce the need for removal/replacement. Large quantity of oxides is likely to be in
circulation in the coolant and not only because in the case an air ingress. In worst estimation,
it could be assumed that the magnetite layer, which is half of the thickness of the duplex
oxide formed on the T91 steel, is removed by the LBE flow. As a consequence, with typical
values of oxidation rates considered above, (0.45 g of oxygen consumed per hour on 3000m²
for EFIT) it would lead to about 7 kg of oxide to be filtered in a year. It could be estimated
that about 8 grams of iron-chromium oxides (about 1/3 of total trapped mass) were trapped
on the 250 cm² filters tested in STELLA, which lead to a pressure drop increase after the
filter of about 0.5 bar. This means that, even if large surface area filters are used,
regeneration should be necessary to prevent their frequent replacement. The replacement
operation could be very detrimental, as it is probable that the filters would contain some
contamination products (mainly 54Mn from 55Fe activation, and to a lower degree 60Co from
austenitic stainless steels activation).
6. Conclusion
Numerous lessons are learned from experiences performed within the DEMETRA project on
practical technological problems related to LBE chemistry management.
Concerning oxygen control, gas/liquid oxygen-transfer by injection of H2/H2O was found to be
quite an efficient way to control the oxygen in small loop, but was quite ineffective on larger
facility. In that latter case, the use of humidified dissolved air was found to be appropriate for
maintaining a constant level of dissolved oxygen, as demonstrated by operating the
CORRIDA loop. The amount of oxygen actually supplied through the plane gas/liquid
interface inside the mass-exchanger of the CORRIDA loop was however small in comparison
to the need for oxygen estimated for industrial-scale reactors. In order to prevent a
prohibitively large size or number of gas/liquid mass-exchangers, industrial-scale application
requires an oxygen flux that is at least two orders of magnitude higher than was necessary
for operating the CORRIDA loop at the target oxygen concentration, so that further
experiments determining the kinetic limits are recommended.
Concerning oxygen control by the solid phase method, it was found that simple mass
exchanger does not allow for an satisfying oxygen supply and mass exchangers with more
refined designs are required. One of the main difficulties is to determine and control the
dissolution kinetic to adjust the oxygen concentration in a whole system. By regulating the
coolant flow rate (adjusting bypass valve opening), temperature and bypass opening
duration, the oxygen concentration in LBE could be controlled.
Estimations of the mass-flow of liquid metal that has to be conditioned in either solid/liquid or
gas/liquid mass-exchangers are encouraging, showing that the required pumping power is by
far lower than the capacity of pumps provided for ensuring sufficient heat removal from the
liquid metal Whether solid/liquid and gas/liquid oxygen-transfer is more appropriate for
industrial-scale oxygen-control is a question of the kinetics of the respective transfer
process—i.e., how compact the transfer device can be—and the peripheral equipment
required for supplying oxygen in the form of PbO or gaseous oxygen (O2). Both types of
oxygen transfer can, in principle, be performed in mass-exchangers that are either integrated
into the reactor or part of an external loop. In the case of the latter, oxygen transfer can be
optimized without restrictions imposed by the temperature distribution inside the reactor.
Nevertheless it has to be pointed out that, as demonstrated by the tests in the different
facilities, LBE chemistry is the result of numerous interactions between the impurities present
in the coolant in various chemical forms, leading to a complex chemical behaviour. The
reactor chemistry has to be understood and then controlled, if proper oxygen control wants to
be reached at all times and locations, especially if out of range conditions were attained in
some parts of the circuits.
The filtering tests performed in the STELLA loops brought large information on the nature of
the impurities that can be trapped by classical stainless steel mesh filters in a LBE system.
They can also serve to validate the basic design of the filtration processes, to define the
operating procedures and to assess the perspectives for the design of purification units for
long-term application in lead-alloy liquid metal coolant systems.
Nevertheless, kinetics of impurity formation remains still unknown, especially under nonisothermal conditions of dynamics loop and transient states. This is coupled with the issue of
on-line and quantitative measurements of impurity contents in liquid or gas. Thus, most of the
filtration experimental program does not lead to a parametric and qualitative definition of the
process. This point should be improved in the future for the next steps of performance
assessment, engineering study and design, and finally for the demonstration of an industrial
purification system.
Acknowledgments
This work was funded by the UE commission, in 6th Framework Programme for Research
and Technological Development (PCRD), in the DEMETRA domain (DEvelopment and
assessment of structural materials and heavy liquid MEtal technologies for TRAnsmutation
systems) of the EUROTRANS (European Research Programme for the Transmutation of
High-Level Nuclear Waste in Accelerator Driven Systems).
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