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HPPA5 - SCKCEN, Mol, Belgium, May 6-9, 2007 Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio- and Environmental Chemistry Paul Scherrer Institute, CH-5232 Villigen PSI, Switzerland

Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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Page 1: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 2: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 3: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 4: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 5: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 6: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 7: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 8: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 9: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 10: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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]

Page 11: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 12: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 13: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

HPPA5 - SCK⋅CEN, Mol, Belgium, May 6-9, 2007

Driving force: Po bound in oxide phase?

Page 14: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 15: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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

Page 16: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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)

Page 17: Polonium Behavior in eutectic Lead Bismuth Alloy...Polonium Behavior in eutectic Lead Bismuth Alloy J. Neuhausen, F. v. Rohr, S. Horn, S. Lüthi, D. Schumann Laboratory for Radio-

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