2
Lead Eutectics
Lead-Bismuth-Eutectic (LBE) is one of the possible materials suitable as target material for a
liquid metal high power target because of its physical properties (mainly the low melting
point of 125 °C), although there are some disadvantages which have to be considered
carefully. One of these drawbacks is a considerable production of radiotoxic poloniumisotopes (208-210Po; α-emitter, half-lives from 138 d to 102 y) that have a fairly high vapour
pressure.
2.1. Investigation of volatile radio-elements produced in LBE
Detailed and careful radiochemical studies were necessary concerning the release behaviour
of Po and its compounds as well as other volatiles (e.g. Hg, Tl, I) before a licensing of the test
experiment MEGAPIE was possible. Fig. 4 shows examples for experimental release studies
relevant for the MEGAPIE project. As can be seen from these results, the release of the most
hazardous radionuclides of Po is slow for temperatures up to 700 °C, which is far above the
operation temperature of MEGAPIE. Hg is released in considerable amounts at the operating
temperature of MEGAPIE under inert conditions. However, in the presence of air its release is
retarded so that safety can be guaranteed in case of a leakage.
1.0
0.9
Ar/7%-H2
0.8
water-saturated Ar
0.8
0.7
0.6
fractional release
Fractional release of Po
1.0
0.5
0.4
0.3
0.2
0.1
-13
0.6
x(Hg)=10 , Ar/7%H2
-5
x(Hg)=6.8*10 , Ar/7%H2
-5
x(Hg)=6.8*10 , N2
0.4
-5
x(Hg)=6.8*10 , N2/5%O2
-5
x(Hg)=6.8*10 , N2/20%O2
0.2
0.0
0.0
400
600
800
1000
Temperature [K]
1200
1400
400
600
800
1000
1200
1400
Temperature [K]
Fig. 1: Release studies of the volatile elements
Po (right) and Hg (left) from irradiated LBE samples in dependence on the temperature under different cover gas
atmospheres
Related Literature
J. Neuhausen, U. Köster, B. Eichler
Investigation of evaporation characteristics of polonium and its lighter homologues selenium
and tellurium from liquid Pb-Bi-eutecticum
Radiochim. Acta 92, 917-923 (2004).
J. Neuhausen, B. Eichler
Study of the thermal release behaviour of mercury and thallium from liquid eutectic leadbismuth alloy
Radiochim. Acta 93, 155-158 (2005).
J. Neuhausen, B. Eichler
Investigations on the thermal release of iodine from liquid eutectic Lead-Bismuth alloy
Radiochim. Acta 94, 239-242 (2006).
J. Neuhausen
Investigations on the release of mercury from liquid eutectic lead bismuth alloy under
different gas atmospheres
Nucl. Instr. and Methods. A, 562, 702-705 (2006).
J. Neuhausen
Reassessment of the rate of evaporation of polonium from liquid eutectic lead bismuth alloy
TM-18-05-01, Paul Scherrer Institute, Villigen, Switzerland (2005)
J. Neuhausen
Gas phase concentrations of volatile nuclear reaction products in the MEGAPIE expansion
tank
TM-18-05-02, Paul Scherrer Institute, Villigen, Switzerland (2005)
F. Groeschel, J. Neuhausen, A. Fuchs, A. Janett
Beurteilung des all-einschliessenden Referenzstörfalls, Abschätzung der freigesetzten
Aktivität zur Berechnung der maximalen Personendosis
MEGAPIE Report MPR-3-GF34-01/0, Paul Scherrer Institute, Villigen, Switzerland (2006)
2.2 Distribution of radioelements in LBE
Since the liquid LBE in the MEGAPIE test experiment was continuously pumped through the
loop, one would expect a more or less homogeneous distribution of the dissolved
radioelements in the matrix. On the other hand, from chemical reasoning one can expect the
formation of insoluble compounds of the radioelements with the target and construction
materials, other impurities and among each other. These compounds may separate at exposed
positions like the heat exchanger or the surface of the expansion tank. Fig. 5 shows the LBE
surface of a proton irradiated liquid target from CERN-ISOLDE. A layer of dark material that
has separated on the surface is clearly visible. Fig. 6 shows a section of two γ-spectra taken
from the surface and bulk part of this target. A tremendous surface enrichment of luthetium is
observed.
Fig. 2: The LBE surface of a target irradiated with 1 GeV protons at CERN-ISOLDE
200000
surface
bulk
counts
150000
172
Lu
100000
50000
Fig. 3: Comparison of a section of the γ-spectra
of the surface and bulk part of the ISOLDE
LBE target
207Bi
0
1040 1050 1060 1070 1080 1090 1100 1110 1120
Energy [keV]
As a second example, first laboratory model studies showed that Po migrates to the surface in
solidified LBE sample, where it is accumulated. Evidence of this behaviour can be seen in
Fig. 7, which shows the specific activity of the LBE that was successively etched from a
polonium containing sample. α-measurements show that this process proceeds with a
considerable velocity. For example, a five-fold increase of the α-counts within one hour was
observed from a freshly homogenized sample of polonium-containing LBE. The driving force
of this process is not yet clear, but the results already now strongly suggest that a
homogeneous distribution of Po in the MEGAPIE target can not be expected after a cooling
time of more than 2 years. The behaviour of other safety-relevant radionuclides in the
solidified target material has not been studied so far, and the phenomena itself requires further
extended and detailed basic research.
800
specific Activity Bq/g
700
600
500
400
300
200
100
0
100 0
8 00
600
4 00
200
0
residual sam ple m ass [m g]
Fig. 4: Etching of an LBE grain (photo on right side) with HNO3. The curve (left) shows, the the Po activity in
the first surface layer of around 50mg is a factor of ~10 higher as in the remaining sample..
Related Literature
J. Neuhausen, S. Heinitz, F. v. Rohr, S. Lüthi, S. Horn, D. Schumann
Nuclear Reaction Product Behavior in Liquid Metal Targets
ARIA 08, Paul Scherrer Institute, Villigen, 2008
F. v. Rohr, J. Neuhausen, D. Schumann, R. Eichler, R. Dressler
Über das Verhalten von 210Po in Blei-Wismut
TM-18-08-01, Paul Scherrer Institut, Villigen 2007
2.3. Extraction of Po from LBE as a purification method
Handling and managing the waste of lead bismuth eutectic (LBE) reactor coolant requires
severe safety precautions. Due to neutron capture of Bi during operating time, the hazardous
element Polonium is formed in considerable large amounts. To avoid Po release by
evaporation or sputtering effects from LBE and minimize contamination in case of reactor
leakage, a liquid-liquid extraction technique using molten alkaline hydroxides was proposed
[1]. The aim of this study was to determine polonium removal efficiency as a function of the
main process variables.
Experimental
LBE samples were irradiated at the SINQ spallation source and diluted, homogenized and cut
in 1 g samples to be used for experimentation. NaOH and KOH were crushed and mixed to
give a eutectic mixture (Tm = 175 °C) and placed into the extraction device shown in Fig. 5.
100
oxidizing potential
Po extracted from LBE [%]
80
60
40
20
0
hydrogen
nitrogen
helium
air
oxygen
gas atmosphere
Fig. 5: Extraction device setup used for
experimentation
Fig 6: Extraction ratio of Po using different gas
blankets performed at 250 °C, 30 min of extraction
time and µ = 2
After heating up and water removal from the hydroxide melt, a certain temperature was
adjusted. LBE samples were added and the extraction started in a desired gas blanket and
hydroxide to LBE ratio µ. First, different gases were used to determine the extraction
behavior under different oxidation potentials. A second run of experiments were done under
different extraction temperatures. The extraction was stopped by solidification of both liquids
after 30 min of extraction time. The initial and final Po concentrations in LBE and in the
hydroxide phase were determined via liquid scintillation counting with diluting each sample
in acidic conditions.
Results and discussion
As it can be seen from Fig. 6, the extraction ratio of Po is highly dependent on the oxidizing
potential of the cover gas present at extraction. Compared to inert gases as N2 and He,
experiments under air and O2 show significantly lower Po extraction rates. A strong reduce of
Po removal efficiency at presence of oxygen was also reported by [1]. Either an oxygen
barrier between both liquids [2] or different extraction kinetics is believed to explain this
observation.
For hydrogen as a reducing agent, the effect on the extraction ratio is opposite. The reducing
ability is increasing the Po transfer rate to the hydroxide phase yielding higher extraction
efficiency. As the chemical extraction process still remains unclear, no proved explanation
could be given here. Hydrogen is the favourable gas blanket to be used in future Po extraction
devices that may be developed to a technically mature state.
weight
loss [%]
H2 gas blanket
95
90
150
200
250
300
350
400
100
60
80
N2 gas blanket
40
20
extraction ratio [%]
extraction ratio [%]
80
hydroxide mass
100
100
N2 with water removal
60
40
20
N2 without water remova
0
0
0.1
1
10
(Na,K)OH to LBE mass ratio µ [/]
Fig. 7: Dependence of polonium extraction from
hydroxide to LBE mass ratio for hydrogen and
nitrogen gas blankets; temperature of extraction 250
°C; extraction time 30 min
150
200
250
300
350
400
temperature Text [°C]
Fig. 8: Influence of water content on the extraction
coefficient as function of temperature. The upper
graph denotes the water release from hydroxide as
function of temperature.
If polonium should be extracted from targets according to the method described above, it is of
inherent importance to minimize the quantity of hydroxide melt to decrease waste disposal
efforts and costs. Moreover, the optimum phase ratio at a certain temperature should be
known to reach the best removal efficiency.
In order to determine the extraction performance for H2 and N2, the hydroxide to LBE mass
ratio µ was varied in the range of 10-1 to 101 under a constant temperature of 250°C. As it can
be seen from Fig. 7, polonium removal gets worse with lower µ since the volume of the
extractant decreases. Moreover, to extract an equivalent amount of Po from LBE, a 10 times
lower amount of hydroxide mass is needed for H2 compared to N2, what confirms the
observation made earlier.
In Fig. 8, results of the extraction behavior on water content and temperature are plotted for
N2 as cover gas. Water removal is complete at temperatures above 350 °C. The hydroxide
mixture loses about 10% of its weight by water evaporation. At lower temperatures a certain
amount of water still remains within the extraction system and negatively influences
polonium removal. This may be explained by the influence of water on the polonium uptake
thermodynamics.
2.4. MEGAPIE-PIE
With the irradiated LBE target from the MEGAPIE experiment, PSI is now equipped with a
unique archive of potential data concerning liquid metal target technology. Consequently, an
extended program for sample taking and following investigations has been established, which
covers also around 50 LBE samples from several exposed positions. With the help of these
samples, we would like to determine the total radionuclide inventory as well as reconstruct the
spatial distribution of safety-relevant isotopes within the entire target. Additional samples are
taken from exposed positions such as the target window, the former liquid metal-cover gas
interface, components of the cover gas system, heat exchangers, electromagnetic pumps and
others. From the analysis of these samples, essential new knowledge about chemical and
physico-chemical processes like migration, segregation, adsorption, diffusion, corrosion
embrittlement etc. within such a liquid metal target will be gained in the frame of extended
studies.
BFT
FGT
BW
Fig. 9: The cutting plan for the MEGAPIE target and some of the sections to be studied in the Post Irradiation
Examination Program
Related Literature
Y. Dai, J. Neuhausen, D. Schumann
Specimen extraction plan for MEGAPIE PIE
MEGAPIE-Report MPR-11-DY34-001-V2, 2008