Polonium Behavior in eutectic Lead Bismuth Alloy

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1 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

2 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

Spallation Products Nuclides formed by nuclear reactions Spallation, Fission, Neutron capture All elements of periodic table with Z Z target + 1 For LBE: Significant amounts of highly radiotoxic

3 Spallation Products Nuclides formed by nuclear reactions Spallation, Fission, Neutron capture All elements of periodic table with Z Z target + 1 For LBE: Significant amounts of highly radiotoxic Poisotopes 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

4 MEGAPIE Licensing: Accident Scenarios Compared 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

experimental problems arising from α-decay Licensing

5 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

6 Time and temperature dependent Po-release log p(po/torr) eff at x Po =3.3*10-6 Fractional release of Po E E E E E E E E K 721 K 867 K 916 K 968 K Heating time [d] 1.00E /Temperature [K] Joy/Brooks_Po in Bi Buongiorno_Po in LBE Neuhausen_Po in LBE Fractional release of Po Ar/7%-H 2 water-saturated Ar Temperature [K] Reasonable aggreement with earlier studies Only a few Bq Po are expected to be released from expansion tank based on equilibrium vapor pressure

7 Spilled and adhering LBE: Evaporation rate cumulated activity released [Bq] BAG demanded a reevaluation of evaporation rate stated in PSAR 1.80E E E E E E E E E E+00 Cumulative 210 Po activity released from a 4m 2 surface of liquid LBE cooled from 305 to 125 C in 11 min equation (4) equation (8) Temperature cooling time [min] Temperature [ C] Evaporation rate/mole fraction [gcm -2 s -1 ] log(evaporation rate/mole fraction [gcm -2 s -1 ]) 1E-3 1E-4 1E-5 1E-6 1E-7 1E-8 1E-9 1E-10 JN_short (Ar-7Rel-Rate_vsT_w/o_vacuum 2 ) JN_long (Ar-7Rel-Rate_vsT_w/o_vacuum 2 ) Tupper (Ar) 1E Temperature [K] -3 [ :32 "/rate/graph1" ( )] Linear Regression for RelRatekula_logRm: Y = A + B * X -4 Parameter Value Error A B R SD N P < /T [K]

8 Cooling of inner surfaces and radionuclide release Cooling curves for many parts of the target inner surface have been calculated for different accident scenarios Temperature [C] Targetkopf geschlossen, Abkühlung durch radiale Wärmeleitung, Strahlung und Konvektion For each of these surfaces the evaporation of Po and Hg were calculated using temperature dependent evaporation rates Central Rod EMP THX Oil Shield UTE Central Tube He-I Enclosure Time [min] Evaporation of Polonium 3.50E E+08 EMPS 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 Activity [Bq] 2.50E E E+08 Central 1.00E+08 TH 5.00E+07 Targetkopf geschlossen Lower Target 0.00E Time [min]

9 Recent investigations Investigation of older SINQ-irradiated LBE samples containing 210 Po 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

10 Segregation of Polonium in LBE A study of the phenomenon of polonium segregation was started. LBE-samples containing 210 Po 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 specific Activity Bq/g mass of remaining sample after etching [mg]

11 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 th hour 100th hour 10th hour first hour A detailed study of the segregation process and its kinetics may be of importance with respect to target and core handling and decommissioning. counts Energy [MeV]

12 Time dependence total α-counts Parameter Value Error A B R SD N P < Sqrt(t)[h 1/2 ] Typical for a diffusion controlled Process

13 Driving force: Po bound in oxide phase?

14 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 HNO 3, evaporation to dryness, solution in diluted HNO 3 and subsequent liquid scintillation counting. Advantage of this method: It makes us independent from 206 Po supply of CERN Disadvantage: Analytics takes more time and effort than γ-spectrometry that can be used with 206 Po

15 First Results of Vacuum Release As expected, much faster release in vacuum ( mbar) is observed in 1h isochronous measurements, compared to Ar/7%-H 2 at ambient pressure Polonium Release [%] 100 Vacuum, 2x10-5 mbar Ar/7%-H 2, 60 ml/min Temperature [ C] Detailed studies at various pressures including time dependent studies at different temperatures are planned to develop a mathematical description of Po evaporation behavior

16 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 H 2 Po 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)

17 Development of Containment Strategies MEGAPIE: 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

2.1. Investigation of volatile radio-elements produced in LBE

2.1. Investigation of volatile radio-elements produced in LBE 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