Michael Wohlmuther

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).

Production of ultracold neutrons from cryogenic 2H2, O2, and C2H4 converters

Ultracold neutrons (UCN) have been produced using the cold neutron (CN) beam FUNSPIN at SINQ on cryogenic oxygen (O2), tetradeuteromethane (C2H4), and deuterium (2H2) targets. The target cell (40 mm long, fiducial volume about 45 cm3) was operated between room temperature and 8 K and UCN were produced from gaseous, liquid and solid targets. UCN rates have been measured as a convolution of UCN production and transport out of the target and to the detector. At least within the accessible temperature range of this experiment, deuterium outperforms the other materials.

Neutron production and thermal moderation at the PSI UCN source

We report on gold foil activation measurements performed along a vertical channel along the tank of the ultracold neutron source at the Paul Scherrer Institute. The activities obtained at various distances from the spallation target are in very good agreement with MCNPX simulations which take into account the detailed description of the source as built.

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.

ACTIVATION OF TRACE ELEMENTS AND IMPURITIES - A NEW ANSATZ FOR MONTE CARLO CALCULATIONS

Abstract In the framework of activation calculations of accelerator components with Monte Carlo methods, an unsolved problem is to take into account the spallation products of trace elements and impurities in a bulk material. Because of the low probability of spallation reactions with these elements, a large number of primary particles are necessary to obtain some information about their spallation products. A new algorithm for treating high-energy reactions has been implemented into MCNPX 2.5.0 to overcome these deficiencies. With this algorithm, spallation reactions of all constituents of a material will be performed at each high-energy interaction. This leads to the production of spallation products from all elements in a material. We will present examples of how this new methodology influences the outcome of activation calculations.

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.

Implementation of Neutron Mirror Modeling Capability into MCNPX and Its Demonstration in First Applications

Abstract Reflection of thermal and cold neutrons by polished surfaces and so-called supermirrors effect radiation fields in and around neutron beamlines. To allow the prediction of these radiation fields with MCNPX 2.5.0, two new input cards were implemented for defining mirror properties of surfaces. Mirror properties can be linked to any type of surface, in contrast to other neutron optics codes, where the mirror properties are part of component descriptions, allowing the simulation of very complex neutron optical devices. First calculations are under way to verify the new capability against combinations of MCNPX and MCSTAS (neutron optics code) simulations. Also, simulations are under way to compare the predicted neutron beam characteristics against measurements conducted at Paul Scherrer Institut.

Comparison of Selected Codes for Calculating Induced Radioactivity at Accelerator Facilities

Abstract Component radioactivation is an important problem in accelerator facilities, impacting operations, maintenance, decommissioning, and disposal. Radionuclide inventories are calculated for an 8-cm-diam, 30.9-cm-long lead target irradiated by 660-MeV protons using the particle transport code MCNPX and the transmutation codes CINDER’90, ORIHET-3, and SP-FISPACT. The results using the various codes and data libraries are compared with experimental measurements. Comparisons are also made between the outputs of the three codes for nuclides not represented in the measurements. For more than half the nuclides studied, the codes agree with the measurements within a factor of 2, and nearly all agree within a factor of 10. The present set of codes and nuclear data files are largely adequate for calculating radioactivation in accelerator facilities, but there is room for substantial improvement for selected radionuclides.

Proton induced activity in graphite - comparison between measurement and simulation

The Paul Scherrer Institut (PSI) operates the Meson production target stations E and M with 590 MeV protons at currents of up to 2.4 mA. Both targets consist of polycrystalline graphite and rotate with 1 Hz due to the high power deposition (40 kW at 2 mA) in Target E. The graphite wheel is regularly exchanged and disposed as radioactive waste after a maximum of 3 to 4 years in operation, which corresponds to about 30 to 40 Ah of proton fluence. For disposal, the nuclide inventory of the long-lived isotopes (T(1/2) > 60 d) has to be calculated and reported to the authorities. Measurements of gamma emitters, as well as 3H, 10Be and 14C, were carried out using different techniques. The measured specific activities are compared to Monte Carlo particle transport simulations performed with MCNPX2.7.0 using the BERTINI-DRESNER-RAL (default model in MCNPX2.7.0) and INCL4.6/ABLA07 as nuclear reaction models.

Numerical flow simulation of the neutron source SINQ of PSI

The Swiss spallation source, SINQ, is in operation since 1996. Since its start-up a constant target development program has been pursued, which culminated in the operation of the liquid metal target MEGAPIE. However, the work-horse target consists of a bundle of Zircaloy-II rods filled with Lead, the so-called Cannelloni target. As the high intensity proton accelerator, HIPA, at the PSI constantly delivers higher beam currents to SINQ it is important to perform numerical simulations to understand the response of the target to the power deposition and to ensure the proper D2O cooling of the Cannelloni rod bundle. Steady-state 3D fluid simulations of the heavy water coolant flow are coupled with the heat source terms in the Cannellonis. The source terms are described as UDFs (User Defined Function) for all Cannellonis with its Zircaloy-II tube and Lead filling. The energy deposition functions for the heat source terms are calculated using MPCNX 2.7.0. Major results from the CFD simulations are the flow structure with the resulting pressure field and the temperature distribution in the target. The results show good agreement with respect to the measured values during operation of SINQ. Additionally the local convective heat transfer coefficients (HTC) are calculated.