HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Polonium Behavior in eutectic Lead Bismuth Alloy
J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. SchumannLaboratory for Radio- and Environmental Chemistry
Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Outline
1. Importance of Po for spallation targets
2. A short look on MEGAPIE licensing
3. New results on polonium behavior in LBE
4. Things remaining to be done
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Spallation Products
Nuclides formed by nuclear reactions
Spallation, Fission, Neutron capture
All elements of periodic table with Z ≤ Ztarget + 1
For LBE: Significant amounts of highly radiotoxic Po-isotopes are formed (MEGAPIE: gram amounts, future ADS systems: some orders of magnitudes more)
Evaluation of consequences for the behavior of the system
Normal operation conditions
Accident scenarios
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
MEGAPIE Licensing: Accident ScenariosCompared to a solid target: faster release of volatiles
3 Source terms in case of break:
Expansion volume
Spilled LBE
LBE adhering to the target walls
Dominant for accident case:
LBE adhering to the target walls,
Effected by slow cooling of target
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Expansion Volume: Determination of vapour pressure
Po data:
only unreliable vapor pressure data due to experimental problems arising from α-decay
Licensing authorities demanded a reevaluation of data
Transpiration method
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Time and temperature dependent Po-release
0 5 10 15 20 25 30
0.00.10.20.30.40.50.60.70.80.91.0
646 K 721 K 867 K 916 K 968 K
Frac
tiona
l rel
ease
of P
o
Heating time [d]400 600 800 1000 1200 1400
0.00.10.20.30.40.50.60.70.80.91.0
Ar/7%-H2
water-saturated Ar
Frac
tiona
l rel
ease
of P
o
Temperature [K]
Reasonable aggreement with earlierstudiesOnly a few Bq Po are expected to bereleased from expansion tank basedon equilibrium vapor pressure
1.00E-18
1.00E-16
1.00E-14
1.00E-12
1.00E-10
1.00E-08
1.00E-06
1.00E-04
1.00E-02
0.7 0.9 1.1 1.3 1.5 1.7 1.9 2.1 2.3 2.5
1000/Temperature [K]
log
p(Po
/Tor
r) e
ff at
xPo
=3.3
*10-6
Joy/Brooks_Po in BiBuongiorno_Po in LBENeuhausen_Po in LBE
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Spilled and adhering LBE: Evaporation rateBAG demanded a reevaluation of evaporation rate stated in PSAR
500 600 700 800 900 1000 1100 12001E-11
1E-10
1E-9
1E-8
1E-7
1E-6
1E-5
1E-4
1E-3
JN_short (Ar-7Rel-Rate_vsT_w/o_vacuum2) JN_long (Ar-7Rel-Rate_vsT_w/o_vacuum2) Tupper (Ar)
Eva
pora
tion
rate
/mol
e fra
ctio
n [g
cm-2
s-1]
Temperature [K]
0.0008 0.0010 0.0012 0.0014 0.0016 0.0018 0.0020-11
-10
-9
-8
-7
-6
-5
-4
-3 [20.06.2005 14:32 "/rate/Graph1" (2453541)]Linear Regression for RelRatekula_logRm:Y = A + B * X
Parameter Value Error------------------------------------------------------------A 0.97126 0.21556B -6049.14964 199.71861------------------------------------------------------------
R SD N P-------------------------------------------------------------0.99352 0.18984 14 <0.0001------------------------------------------------------------
log(
Eva
pora
tion
rate
/mol
e fra
ctio
n [g
cm-2
s-1])
1/T [K]
Cumulative 210Po activity released from a 4m2 surface of liquid LBE cooled from 305 to 125° C in 11 min
0.00E+00
2.00E+04
4.00E+04
6.00E+04
8.00E+04
1.00E+05
1.20E+05
1.40E+05
1.60E+05
1.80E+05
0.25 2.25 4.25 6.25 8.25 10.25
cooling time [min]
cum
ulat
ed a
ctiv
ity r
elea
sed
[Bq]
0
50
100
150
200
250
300
350
Tem
pera
ture
[°C]
equation (4)equation (8)Temperature
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Cooling of inner surfaces and radionuclide release
0.0
50.0
100.0
150.0
200.0
250.0
300.0
350.0
400.0
0 200 400 600 800 1000 1200 1400 1600 1800 2000
Time [min]
Tem
pera
ture
[C]
Targetkopf geschlossen,Abkühlung durch radiale Wärmeleitung, Strahlung und Konvektion
Central RodEMPTHXOil ShieldUTECentral TubeHe-I Enclosure
Cooling curves for many parts of the target inner surface have been calculated for different accident scenarios
For each of these surfaces the evaporation of Po and Hg were calculated using temperature dependent evaporation rates
Acceptable Doses are obtained by ESS41 calculations in case of filtering of the gas can be assured.Additional filters including a mobile filter unit was installed
Evaporation of Polonium
0.00E+00
5.00E+07
1.00E+08
1.50E+08
2.00E+08
2.50E+08
3.00E+08
3.50E+08
0 500 1000 1500 2000 2500
Time [min]
Act
ivity
[Bq]
Targetkopf geschlossen
Central
EMPS
TH
Lower Target
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Recent investigations
Investigation of older SINQ-irradiated LBE samples containing 210Po revealed that polonium is enriched at the surface of the solid samples.
Consequences:
a) For release measurements, the samples have to be homogenized.
b) Suitable analytics have to be developed to determine the α-activity.
c) The process of segregation may be of importance for the handling of LBE after shutdown of the system and for the interim storage of decommissioned target and core material
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Segregation of Polonium in LBE
A study of the phenomenon of polonium segregation was started.
LBE-samples containing 210Po were etched and the etching solutions analyzed by liquid scintillation counting. As shown in the figure, polonium is highly enriched in the surface.
Surface layer appr. 20 μm
Can be homogenized by melting
1000 800 600 400 200 00
100
200
300
400
500
600
700
800
spec
ific
Act
ivity
Bq/
g
mass of remaining sample after etching [mg]
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Segregation of Polonium in LBE
α-spectra successively taken from a freshly irradiated and homogenized sample shows that substantial surface enrichment occurs within hours or days in a solid sample.
A detailed study of the segregation process and its kinetics may be of importance with respect to target and core handling and decommissioning.
4.6 4.8 5.0 5.2 5.40
200
400
600
800
1000
1200
coun
ts
Energy [MeV]
290th hour 100th hour 10th hour first hour
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Time dependence
0 2 4 6 8 10 12 14 16 18 2016000
18000
20000
22000
24000
26000
28000
30000
Parameter Value Error------------------------------------------------------------A 17034.35626 29.90417B 697.99463 2.36045------------------------------------------------------------
R SD N P------------------------------------------------------------0.99819 177.27283 320 <0.0001------------------------------------------------------------
tota
l α-c
ount
s
Sqrt(t)[h1/2]
Typical for a diffusion controlled Process
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Driving force: Po bound in oxide phase?
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Release studies in vacuum for windowless target design
A procedure was devised to homogenize the Po concentration in LBE samples by melting. Analysis of α-activity is the done by dissolving part of the sample in concentrated HNO3, evaporation to dryness, solution in diluted HNO3 and subsequent liquid scintillation counting.
Advantage of this method: It makes us independent from 206Po supply of CERN
Disadvantage: Analytics takes more time and effort than γ-spectrometry that can be used with 206Po
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
First Results of Vacuum Release
200 400 600 800 1000
0
20
40
60
80
100 Vacuum, 2x10-5 mbar Ar/7%-H2, 60 ml/min
Pol
oniu
m R
elea
se [%
]
Temperature [°C]
As expected, much faster release in vacuum (2×10-5 mbar) is observed in 1h isochronous measurements, compared to Ar/7%-H2 at ambient pressure
Detailed studies at various pressures including time dependent studies at different temperatures are planned to develop a mathematical description of Po evaporation behavior
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Further studies required: Other Mechanisms of Po transport
At the presence of hydrogen and/or water and additional agitation, a process has been observed that leads to increased vapor phase concentrations of Po
The process seems to be more or less temperature independent
Is it really simply formation of H2Po and its evaporation?
The mechanism of this process and its relevance for a spallation system needs to be clarified
Aerosol transport may play a role (has been observed in many loop systems, MEGAPIE experience will be valuable, not limited to LBE nor Po)
HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007
Development of Containment StrategiesMEGAPIE: evaporation based on the complete inventory is compatible with safe
operation and accident scenarios (probably for long-term irradiation (ADS), reduction of the inventory is desirable)
Bonding of volatiles before they leave the condensed phase
Metal absorbers, salt melts, CeBi,
Deliberate evaporation and bonding to catcher/filter systems in the gas phase
Megapie-experience: Ag/Pd-absorbers for Hg and possibly halogens
Efficiency has still to be evaluated in PIE