2.1. Investigation of volatile radio-elements produced in LBE

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1 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 ( Po; α-emitter, half-lives from 138 d to 12 y) that have a fairly high vapour pressure 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 7 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. Fractional release of Po Ar/7%-H 2 water-saturated Ar fractional release x(hg)=1-13, Ar/7%H 2 x(hg)=6.8*1-5, Ar/7%H 2 x(hg)=6.8*1-5, N 2 x(hg)=6.8*1-5, N 2 /5%O 2 x(hg)=6.8*1-5, N 2 /2%O Temperature [K] 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, (24). J. Neuhausen, B. Eichler Study of the thermal release behaviour of mercury and thallium from liquid eutectic leadbismuth alloy Radiochim. Acta 93, (25). J. Neuhausen, B. Eichler Investigations on the thermal release of iodine from liquid eutectic Lead-Bismuth alloy Radiochim. Acta 94, (26).

2 J. Neuhausen Investigations on the release of mercury from liquid eutectic lead bismuth alloy under different gas atmospheres Nucl. Instr. and Methods. A, 562, (26). J. Neuhausen Reassessment of the rate of evaporation of polonium from liquid eutectic lead bismuth alloy TM , Paul Scherrer Institute, Villigen, Switzerland (25) J. Neuhausen Gas phase concentrations of volatile nuclear reaction products in the MEGAPIE expansion tank TM , Paul Scherrer Institute, Villigen, Switzerland (25) 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-GF34-1/, Paul Scherrer Institute, Villigen, Switzerland (26) 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

3 2 15 surface bulk 172 Lu counts 5 27 Bi Energy [kev] Fig. 3: Comparison of a section of the γ-spectra of the surface and bulk part of the ISOLDE LBE target 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. 8 7 specific Activity Bq/g residual sample mass [mg] Fig. 4: Etching of an LBE grain (photo on right side) with HNO 3. The curve (left) shows, the the Po activity in the first surface layer of around 5mg is a factor of ~1 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 8, Paul Scherrer Institute, Villigen, 28 F. v. Rohr, J. Neuhausen, D. Schumann, R. Eichler, R. Dressler Über das Verhalten von 21 Po in Blei-Wismut TM , Paul Scherrer Institut, Villigen 27

4 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 (T m = 175 C) and placed into the extraction device shown in Fig. 5. oxidizing potential 8 Po extracted from LBE [%] 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 25 C, 3 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 3 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 N 2 and He, experiments under air and O 2 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.

5 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. hydroxide mass 8 H 2 gas blanket weight loss [%] extraction ratio [%] N 2 gas blanket extraction ratio [%] N 2 with water removal N 2 without water remova (Na,K)OH to LBE mass ratio µ [/] temperature T ext [ C] Fig. 7: Dependence of polonium extraction from hydroxide to LBE mass ratio for hydrogen and nitrogen gas blankets; temperature of extraction 25 C; extraction time 3 min 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 H 2 and N 2, the hydroxide to LBE mass ratio µ was varied in the range of 1-1 to 1 1 under a constant temperature of 25 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 1 times lower amount of hydroxide mass is needed for H 2 compared to N 2, what confirms the observation made earlier. In Fig. 8, results of the extraction behavior on water content and temperature are plotted for N 2 as cover gas. Water removal is complete at temperatures above 35 C. The hydroxide mixture loses about 1% 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 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 5 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

6 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-DY34-1-V2, 28