Jörg Neuhausen

Investigation of evaporation characteristics of polonium and its lighter homologues selenium and tellurium from liquid Pb-Bi-eutecticum

Summary The evaporation behaviour of polonium and its lighter homologues selenium and tellurium dissolved in liquid Pb-Bi-eutecticum (LBE) has been studied at various temperatures in the range from 482 K up to 1330 K under Ar/H2 and Ar/H2O-atmospheres using γ-ray spectroscopy. Polonium release in the temperature range of interest for technical applications is slow. Within short term (1 h) experiments measurable amounts of polonium are evaporated only at temperatures above 973 K. Long term experiments reveal that a slow evaporation of polonium occurs at temperatures around 873 K resulting in a fractional polonium loss of the melt around 1% per day. Evaporation rates of selenium and tellurium are smaller than those of polonium. The presence of H2O does not enhance the evaporation within the error limits of our experiments. The thermodynamics and possible reaction pathways involved in polonium release from LBE are discussed.

Accelerator waste as a source for exotic radionuclides

With the development of advanced large-scale accelerator facilities, considerable quantities of activated material are now being produced. From both a scientific and a technological viewpoint, many of these nuclides are of great interest for future research, and in fact cannot be obtained through other means. The Paul Scherrer Institute—operating one of the most powerful proton accelerators in Europe—is able to provide considerable amounts of several exotic long-lived radionuclides. The present report is focused on the separation and application of radionuclides with a special interest on nuclear astrophysics topics.

Radiochemical analysis of a copper beam dump irradiated with high-energetic protons

Abstract The radionuclide inventory of a copper beam dump from the 590 MeV proton accelerator of the Paul Scherrer Institute in Switzerland was determined, focusing on radioisotopes with half-lives of more than 60 d, and in particular, of long-lived isotopes with T1/2=104−107 years, which are important regarding radioactive waste management. The measurements were carried out using high resolution γ-measurement without sample destruction, as well as liquid scintillation counting (LSC) and accelerator mass spectrometry (AMS) after chemical separation. For the first time, a beam dump from a high power accelerator facility was completely characterized concerning the depth and radial distribution profile of the most hazardous and/or long-lived radionuclides. Moreover, it turned out that some of the investigated radionuclides, like for instance 26Al, 44Ti or 60Fe represent valuable material for application in several scientific fields like nuclear astrophysics, basic nuclear physics research, radiopharmacy and many others. Therefore, based on the analytical results, a special research and development program has been started at PSI objecting on specific preparative extraction of long-lived radioisotopes (ERAWAST — exotic radionuclides from accelerator waste for science and technology).

Study of the thermal release behaviour of mercury and thallium from liquid eutectic lead-bismuth alloy

Summary The release of mercury and thallium from liquid eutectic lead-bismuth alloy (LBE) under a flowing Ar/7%-H2 atmosphere has been studied in the temperature range from 408 to 1292 K using γ-ray spectroscopy. For technical applications such as liquid metal spallation targets or accelerator driven systems, where liquid LBE is planned to be used as target material, the release of radioactive mercury isotopes produced by spallation is expected to be one of the major safety issues. During short-term experiments significant amounts of mercury begin to evaporate from liquid LBE at temperatures starting from about 475 K. 80% of the mercury present in the sample is released from samples of approximately 1.5–3 g within one hour at temperatures higher than 625 K. Thallium release in the temperature range investigated is below experimental error. Long-term experiments reveal that even at temperatures as low as 476 K about 25% of the mercury present in the samples is released per day under a flowing Ar/7%-H2 atmosphere.

Investigations on the thermal release of iodine from liquid eutectic lead-bismuth alloy

Summary The release of iodine from liquid eutectic lead-bismuth alloy (LBE) under a flowing Ar/7%H2 atmosphere has been studied in a temperature range from 428 to 1223 K using γ-ray spectroscopy. During short-term experiments, significant amounts of iodine evaporate from liquid LBE at temperatures higher than 800 K. Long-term experiments reveal that the release of iodine at temperatures of relevance to technical applications, such as liquid metal spallation targets or Accelerator Driven Systems (ADS), is below our detection limits for periods up to 7 days. The release rate is determined by the desorption/evaporation process rather than by diffusion within the liquid alloy.

Radiochemical Determination of Rare Earth Elements in Proton-Irradiated Lead–Bismuth Eutectic

Various types of proton-irradiated lead-bismuth eutectic (LBE) samples from the MEGAPIE prototype spallation target were analyzed concerning their content of (148)Gd, (173)Lu, and (146)Pm by use of α- and γ-spectrometry. A radiochemical separation procedure was developed to isolate the lanthanide fraction and to prepare thin samples for α-ray measurement. The results prove a substantial depletion of these three elements in bulk samples, whereas accumulation on the LBE/steel-interfaces was observed. The amount of material accumulated on surfaces was roughly estimated by relating the values measured on the sample surfaces to the total surface of the inner target walls. The amount of (148)Gd, (173)Lu, and (146)Pm was then quantified by summing up the contributions from every sample type. The results show a reasonable agreement with theoretical predictions. The obtained results are of utmost importance for the evaluation of the performance of high-power spallation targets, especially concerning the residual nuclide production, the physicochemical behavior of the produced radionuclides during operation, and in terms of an intermediate or final disposal.

Excitation functions for the production of long-lived residue nuclides in the reaction natBi(p;xn,yp)Z

Experimental and theoretical studies were performed to complete the data sets on the production of residue nuclides in natural Bi irradiated with protons. For the long-lived and difficult to measure isotopes 108mAg, 10Be, 26Al as well as 129I and 36Cl, chemical systems had been developed aimed to isolate them from the bismuth matrix. The separation methods are based on distillation, precipitation and ion exchange. From the content of the radionuclides determined by accelerator mass spectrometry and γ-spectrometry, the production cross sections in the proton energy range from 100 to 2600 MeV were calculated. The experimental results were compared with theoretical predictions. Significant deviations between experiment and theory over the whole energy range were observed with increasing discrepancies for the production of the lighter residue nuclides.

Radiochemical separation and analytical determination of 10Be from proton-irradiated graphite targets

High-concentration samples of carrier-free 10Be (up to 1 μg 10Be per gram graphite) were separated from proton-irradiated graphite targets used at the meson production unit of the Paul Scherrer Institute (PSI). 10Be concentrations were determined both with accelerator mass spectrometry (AMS) and inductively coupled plasma mass spectrometry (ICP-MS). The analytical results from both methods are in good agreement. The highly enriched 10Be-samples will be used in scientific experiments relating to basic nuclear research and nuclear astrophysics in the framework of an international collaboration aimed to exploit accelerator waste material for scientific applications.

Radiochemical determination of 129I and 36Cl in MEGAPIE, a proton irradiated lead-bismuth eutectic spallation target

Abstract The concentrations of the long-lived nuclear reaction products 129I and 36Cl have been measured in samples from the MEGAPIE liquid metal spallation target. Samples from the bulk target material (lead-bismuth eutectic, LBE), from the interface of the metal free surface with the cover gas, from LBE/steel interfaces and from noble metal absorber foils installed in the cover gas system were analysed using Accelerator Mass Spectrometry at the Laboratory of Ion beam Physics at ETH Zürich. The major part of 129I and 36Cl was found accumulated on the interfaces, particularly at the interface of LBE and the steel walls of the target container, while bulk LBE samples contain only a minor fraction of these nuclides. Both nuclides were also detected on the absorber foils to a certain extent (≪ 1% of the total amount). The latter number is negligible concerning the radio-hazard of the irradiated target material; however it indicates a certain affinity of the absorber foils for halogens, thus proving the principle of using noble metal foils for catching these volatile radionuclides. The total amounts of 129I and 36Cl in the target were estimated from the analytical data by averaging within the different groups of samples and summing up these averages over the total target. This estimation could account for about half of the amount of 129I and 36Cl predicted to be produced using nuclear physics modelling codes for both nuclides. The significance of the results and the associated uncertainties are discussed.

Adsorption of volatile polonium and bismuth species on metals in various gas atmospheres: Part I – Adsorption of volatile polonium and bismuth on gold

Abstract Polonium isotopes are considered the most hazardous radionuclides produced during the operation of accelerator driven systems (ADS) when lead–bismuth eutectic (LBE) is used as the reactor coolant and as the spallation target material. In this work the use of gold surfaces for capturing polonium from the cover gas of the ADS reactor was studied by thermochromatography. The results show that gaseous monoatomic polonium, formed in dry hydrogen, is adsorbed on gold at 1058 K. Its adsorption enthalpy was calculated as –250±7 kJ mol−1, using a Monte Carlo simulation code. Highly volatile polonium species that were observed in similar experiments in fused silica columns in the presence of moisture in both inert and reducing gas were not detected in the experiments studying adsorption on gold surfaces. PoO2 is formed in both dry and moist oxygen, and its interaction with gold is characterized by transport reactions. The interaction of bismuth, present in large amounts in the atmosphere of the ADS, with gold was also evaluated. It was found that bismuth has a higher affinity for gold, compared to polonium, in an inert, reducing, and oxidizing atmosphere. This fact must be considered when using gold as a material for filtering polonium in the cover gas of ADS.