MEGAPIE Target Research Articles

GEANT4 characterization of the neutronic behavior of the active zone of the MEGAPIE spallation target

The increasing interest that GEANT4 is gaining nowadays, because of its special capabilities, prompted us to address its reliability in neutronic calculation for the realistic and complex spallation target MEGAPIE of the Paul Scherrer Institute of Switzerland. In this paper we have specifically addressed the neutronic characterization of the active zone of this target. Three physical quantities are evaluated: neutron flux spectra and total neutron fluxes on target's z-axis, and the neutron yield as a function of the target's altitude and radius. Comparison of the obtained results with those of the MCNPX reference code and some experimental measurements have confirmed the impact of the geometrical and proton beam models on the neutron fluxes. It has also allowed to reveal the intrinsic influence of the code type. The resulting differences reach a factor of ~2 for the beam model and 4–18% for the other parameters cumulated. The analysis of the neutron yield has led us to conclude that: 1) Increasing the productivity of the MEGAPIE target cannot be achieved simply by increasing the thickness of the target, if the irradiation parameters are not modified. 2) The size of the spallation area needs to be redefined more precisely.

Production threshold impact on a GEANT4 calculation of the power deposition in a fast domain: MEGAPIE spallation target

The calculation time in the Monte Carlo simulations consistently represents an essential issue. It is often very long, and its decrease constitutes a challenge for the simulator. Generally, an MC simulation is qualified as quality or not according to two main criteria: the calculation time and the accuracy of the results. However, in most cases, the optimization of one criterion affects negatively the other. Therefore, a compromise between both of them is always required in this kind of simulation. The present work aims at studying the impact of the production threshold (or cut) of the GEANT4 toolkit on the calculation of the power deposition in the MEGAPIE spallation target. The production threshold of secondaries is a GEANT4 intrinsic parameter. It indicates the limit of energy we can reach in the production of secondary particles. This study has allowed us to make the following conclusions. First, the influence of the cut on the calculation of the deposited power depends on the volume size, its arrangement and the importance of the electromagnetic processes occurring within. Second, the accuracy of the calculations can be acceptable only below a given value of the cut energy. Third, this accuracy remains almost unchangeable from a certain value of the cut. The study has also made it possible to explore the prevalence of certain interactions in the zone of spallation in the MEGAPIE target.

Nuclide production in spallation reactions: How useful are the simulations?

INCL (Liège IntraNuclear Cascade model) combined with a deexcitation code has been used a lot for numerous simulations of spallation reactions during the last two decades. We go back over some of those simulations to address the capabilities of such codes and to show some improvements. The four examples of simultation are: the EURISOL project, the MEGAPIE target, the ESS facility, and the cosmogenic nuclide production. The goal is to discuss respectively designing and optimisation, predictive power and reliability, feasibility and uncertainty estimate, and the use of modeling to mitigate lack of experimental data.

Distribution and surface enrichment of radionuclides in lead-bismuth eutectic from spallation targets

With the development of new high-power neutron spallation sources --both for scientific application and as neutron production tool for accelerator-driven systems-- the demand for experimentally obtained nuclear data on the residue nuclei production in the target is constantly increasing. In the present work, we examined two lead-bismuth-eutectic targets, irradiated with high-energy protons, concerning their radionuclide content and the spatial distribution of selected isotopes. The first one was the so-called ISOLDE target, being irradiated with 1-1.4GeV protons at CERN-ISOLDE, the second one was the MEGAPIE target, irradiated at PSI with 590MeV protons. In particular, we investigated the phenomenon of radionuclide enrichment on free surfaces in both targets. It turned out that considerable accumulation can be found especially in the case of lanthanides. The depletion process is enhanced at increased temperatures. The results are compared with theoretical predictions; some possible consequences of the findings are illustrated.

Non-destructive testing of the MEGAPIE target

Non-destructive testing (NDT) is one important part of the post-irradiation examination (PIE) of the MEGAPIE target. It includes visual inspection and ultrasonic measurement of the beam window of the T91 LBE container and gamma mapping of the beam window of the AlMg3 safety-container. The visual inspection showed no visible failure in the proton beam window area of the T91 LBE container. The ultrasonic measurement demonstrated no detectable change in the wall thickness of the T91 beam window, which implies no severe corrosion effect induced by flowing LBE during the four-month irradiation period. The gamma mapping provided the distribution of 22Na, a spallation product, in the proton beam window area of the AlMg3 safety-container. The result was used to evaluate the accumulated proton fluence distribution profile, the input data for determining irradiation parameters. A maximum proton fluence of 1.9 × 1025 p/m2 was deduced. The corresponding displacement damage degree in the T91 beam window was 7.1 dpa.

Radiochemical Determination of Polonium in Liquid Metal Spallation Targets

The MEGAPIE target, consisting of 82 litres of lead-bismuth eutectic (LBE), was irradiated close to the megawatt range (0.8 MW) from August to December 2006 in the SINQ facility at PSI. After a cooling period of 5 years, a post-irradiation examination (PIE) program was started and samples were taken from different positions in the target. In this paper we focus on the measurement of α-emitting 208-210Po in the MEGAPIE target. The experimental results are compared with theoretical predictions obtained by FLUKA and MCNPX calculations.

Analysis of the 207Bi, 194Hg/Au and 173Lu distribution in the irradiated MEGAPIE target

Samples obtained from different locations within the prototype liquid metal spallation target MEGAPIE irradiated in 2006 at PSI were analysed using γ-spectrometry. A variety of radionuclides formed by reaction of the target material, lead–bismuth eutectic (LBE), with the proton beam and secondary particles were identified. While nuclides representing the target material itself (207Bi) and nuclides of noble metals were found in LBE samples throughout the target, nuclides of electropositive metals were found to be quantitatively deposited on free surfaces and material interfaces within the target system. This behaviour is analysed in more detail based on results obtained for three nuclides representing groups of elements with distinct chemical behaviour, namely 207Bi, 194Hg/Au and 173Lu. Quantitative analysis results are given and compared with predictions obtained using nuclear physics calculations for those nuclides showing rather homogeneous distribution within the target. Possible reasons for the separation of radionuclides from the liquid metal and their deposition on surfaces are given, and consequences arising for nuclear facilities utilizing liquid metals are discussed.

PIE preparation of the MEGAPIE target

The MEGAPIE target, after successfully operating for 4months at a beam power of 0.77MW, is now being prepared for post irradiation examination PIE. The lead–bismuth eutectic (LBE) target was irradiated from August until December 2006, and in this period received a beam charge of 2.8Ah of 575MeV protons. After that, the target was stored in the target storage facility of PSI, waiting for its post irradiation examination. In the meantime several campaigns of tests have been conducted by PSI and ZWILAG, the interim storage facility of Swiss nuclear power plants. In these tests the feasibility of the conditioning of the target and the extraction of sample material for the PIE has been proven. After transport to the hot cell facility at ZWILAG in June 2009, the dismantling of the MEGAPIE target started. It finally was cut into 21 pieces. Ten of these pieces will be shipped to the Hot Laboratory of PSI (‘PSI hotlab’) to extract samples from the structural materials as well as from the LBE. Currently it is foreseen that the sample extraction will start in the first half of 2011. The remaining parts of the MEGAPIE target were conditioned as radioactive waste. The present paper will mainly focus on the dismantling and first visual inspection of the MEGAPIE target. In addition an outlook on the PIE phase of MEGAPIE is given.

Recent Developments on Micrometric Fission Chambers Dedicated for High Neutron Fluxes

With the development of innovative nuclear systems and new generation neutron sources, the nuclear instrumentation should be adapted. Since several years, we developed microscopic fission chambers to study the transmutation of minor actinides in high thermal-neutron fluxes. The recent developments done to fulfill the drastic conditions of irradiations are described in this paper together with the feedback from the measurements. Two installations were used: the HFR of the ILL for its high thermal neutron flux and the MEGAPIE target which was the first liquid Pb-Bi spallation target in the MW range.

Vapour phase concentrations of volatile nuclear reaction products in the MEGAPIE cover gas

The licensing of the prototype liquid lead bismuth spallation target MEGAPIE required the assessment of the amount of radioactivity that can be released from the liquid metal to the gas phase under various scenarios. To estimate the radiological consequences of evaporation processes under normal operation conditions, the concentrations of hazardous volatile radionuclides in the gas phase of the MEGAPIE expansion volume were estimated using a simplified model based on an equilibrium state. In this report, we focus on those volatile impurities that do not show a strong retention in the liquid metal caused by chemical interactions, comprising mercury, cadmium and thallium. For mercury, temperature functions for the effective vapour pressure of mercury over liquid eutectic lead bismuth alloy were deduced from experimental data and compared to literature data available for the binary systems Hg–Pb and Hg–Bi. Conservative functions were selected from the data on hand for the evaluation of the maximum possible amounts of mercury, cadmium and thallium radioactivity in the gas phase of the MEGAPIE target. Substantial amounts of radioactive mercury are predicted to be released to the cover gas phase of MEGAPIE and liquid lead-alloy based spallation targets in general. The radioactivity resulting from evaporation of cadmium and thallium nuclides is expected to be low. Consequences for liquid metal spallation target systems are discussed.

Experience from the post-test analysis of MEGAPIE

The (MEGAWatt Pilot Experiment) MEGAPIE target was successfully irradiated in 2006 at the SINQ facility of the Paul Scherrer Institut. During the irradiation a series of measurements to monitor the operation of the target, the thermal hydraulics behavior and the neutronic and nuclear aspects, has been performed. In the post-test analysis phase of the project, the data were analyzed and important information relevant to accelerator-driven systems (ADS) was gained, in particular: (i) from the operation of the target several recommendations concern the simplification of the system and the improved reliability; (ii) data from the thermal hydraulic measurements have offered the opportunity to validate the codes used in the design phase; (iii) the neutronic analysis confirm the high performance of a liquid metal target and the importance of the delayed neutron measurements in an ADS target; (iv) the nuclear measurements of the gas released gave the opportunity to validate the codes used during the design phase and provided indications for the operation. From the results in these different domains recommendations to further development of ADS and heavy liquid metal targets are discussed.

Observed changes in electrical materials properties of ZrO 2 in Megapie leak detectors during irradiation

Reliable leak detection is of high importance for liquid metal targets. For the PSI Megapie target two novel approaches had been pursued: a thermocouple-based leak detector and one employing capacity sensors with impedance read out. Whereas the temperature based leak detector performed according to expectations, severe degradation of the impedance probes occurred during the irradiation period of Megapie. The electrical characteristics of the chosen insulator material ZrO 2 stabilized by 1% Y 2O 3 showed massive changes over time and they exhibited strong dependence on the prevailing environment conditions. Three different contributions to effective electrical conductivity in the supposed insulator could be identified: a prompt radiation induced, a temperature induced, and most probably a gas induced change in conductivity. These components, both increases and decreases in bulk conductivity of the insulating slabs in the impedance probes, demonstrated some cross-dependence on each other. The alterations in conductivity and the mutual influence of the identified factors changed with the irradiation time and the probable exposure to gaseous radiolysis products at the location close to the beam entrance window of the liquid metal target. A phenomenological and qualitative description of the observed effects as they evolved during Megapie operation is presented.

Analysis of nuclide production in the MEGAPIE target

MEGAPIE, the first liquid metal target irradiated by a proton beam at the MW power level, was successfully operated in 2006. A continuous beam of 575 MeV protons with a current up to 1.35 mA irradiated the liquid lead–bismuth target placed in the SINQ target location at PSI (Switzerland) for a period of 4 months. The activation of the lead–bismuth irradiated in MEGAPIE has been investigated. Experimental cross-sections and evaluated data available for neutron- and proton-induced reaction cross-sections at incident energies from 10 −5 eV to 600 MeV, and results of nuclear model calculations have been used to obtain nuclear reaction rates. Calculated nuclide and gas production rates are compared with calculations using the MCNPX and FLUKA Monte Carlo codes. The total activation of the LBE agrees well with the other codes. Discrepancies with FLUKA and MCNPX are mainly in two mass regions, where experimental data are scarce: the region 30< A<50, and the region 140< A<170. The results obtained can be used for the further study of the safe operation of liquid heavy metal targets of Accelerator-Driven Systems and spallation neutron sources and for the definition of the priorities in the development of evaluated nuclear data libraries at intermediate nucleon energies.

Neutronic characterization of the MEGAPIE target

The MEGAPIE project aimed to design, build and operate a liquid metal spallation neutron target of about 1 MW beam power in the SINQ facility at the Paul Scherrer Institut (Villigen, Switzerland). This project is an important step in the roadmap towards the demonstration of the accelerator driven system (ADS) concept and high power liquid metal targets in general. Following the design phase, an experimental program was defined to provide a complete characterization of the facility by performing a “mapping” of the neutron flux at different points, from the center of the target to the beam lines. The neutronic performance of the target was studied using different experimental techniques with the goals of validating the Monte Carlo codes used in the design of the target; additionally, the performance was compared with the solid lead targets used before and after the MEGAPIE experiment.

Liquid metal leak detection for spallation neutron sources

Short response times and the harsh radiation environment in liquid metal targets of spallation neutron sources pose stringent requirements and demanding boundary conditions for reliable liquid metal leak detection. For the PSI Megawatt pilot experiment (Megapie) liquid metal target two novel approaches have been pursued: a thermocouple-based leak detector and one employing impedance read out. During the whole operational life of the Megapie target satisfactory performance has been achieved. How these concepts can be further improved and easily be adapted to other installations based on the PSI experience will be described here.

Stressed capsules of austenitic and martensitic steels irradiated in SINQ Target-4 in contact with liquid lead–bismuth eutectic

In the MEGAPIE target, the steels used for the proton beam entrance window and other components in the spallation reaction zone suffer not only from the irradiation damage produced by protons and neutrons but also from the corrosion and embrittlement induced by liquid lead–bismuth eutectic (LBE). Although these effects have been separately studied by a number of authors, the synergistic effects of irradiation, LBE corrosion and embrittlement are little understood. This work presents detailed analyses of two stressed capsules made of the austenitic steel EC316LN and the martensitic steel 9Cr2WVTa, which were irradiated in SINQ Target-4 in contact with LBE at calculated temperatures of 315 and 225 °C, respectively. The Electron Probe Microanalysis (EPMA) on the cross-sections of the capsules showed that the stagnant LBE induced only slight corrosion on both capsules and no cracks existed in the wall of the EC316LN capsule. Some cracks were observed in the electron beam weld (EBW) and its vicinity of the 9Cr2WVTa capsule, which can be attributed to the high stress inside the wall, the hardening of the material induced by either welding (without re-tempering) or irradiation, and the effects of LBE embrittlement.

Turbulent heat mixing of a heavy liquid metal flow in the MEGAPIE target geometry—The heated jet experiment

The MEGAPIE target installed at the Paul–Scherrer Institute is an example of a spallation target using eutectic liquid lead–bismuth (Pb 45Bi 55) both as coolant and neutron source. An adequate cooling of the target requires a conditioning of the flow, which is realized by a main flow transported in an annular gap downwards, u-turned at a hemispherical shell into a cylindrical riser tube. In order to avoid a stagnation point close to the lowest part of the shell a jet flow is superimposed to the main flow, which is directed towards to the stagnation point and flows tangentially along the shell. The heated jet experiment conducted in the THEADES loop of the KALLA laboratory is nearly 1:1 representation of the lower part of the MEGAPIE target. It is aimed to study the cooling capability of this specific geometry in dependence on the flow rate ratio ( Q main/ Q jet) of the main flow ( Q main) to the jet flow ( Q jet). Here, a heated jet is injected into a cold main flow at MEGAPIE relevant flow rate ratios. The liquid metal experiment is accompanied by a water experiment in almost the same geometry to study the momentum field as well as a three-dimensional turbulent numerical fluid dynamic simulation (CFD). Besides a detailed study of the envisaged nominal operation of the MEGAPIE target with Q main/ Q jet = 15 deviations from this mode are investigated in the range from 7.5 ≤ Q main/ Q jet ≤ 20 in order to give an estimate on the safe operational threshold of the target. The experiment shows that, the flow pattern establishing in this specific design and the turbulence intensity distribution essentially depends on the flow rate ratio ( Q main/ Q jet). All Q main/ Q jet-ratios investigated exhibit an unstable time dependent behavior. The MEGAPIE design is highly sensitive against changes of this ratio. Mainly three completely different flow patterns were identified. A sufficient cooling of the lower target shell, however, is only ensured if Q main/ Q jet ≤ 12.5. In this case the jet flow covers the whole lower shell. Although for Q main/ Q jet ≤ 12.5 the flow is more unstable compared to the other patterns most of the fluctuations close to the centerline are in the high frequency range (>1 Hz), so that they will not lead to severe temperature fluctuations in the lower shell material. In this case the thermal mixing occurs on large scales and is excellent. For flow rate ratios Q main/ Q jet > 12.5 complex flow patterns consisting of several fluid streaks and vortices were identified. Since in these cases the jet flow does not fully cover the lower shell an adequate cooling of the MEGAPIE target cannot be guaranteed and thus temperatures may appear exceeding material acceptable limits. All conducted experiments show a high sensitivity to asymmetries even far upstream. A comparison of the numerical simulation, which assumed a symmetric flow, with the experimental data was due to the experimentally found asymmetry only partially possible.

Temperature monitoring of the lead bismuth eutectic flow in the MEGAPIE target

Introduction. An Electromagnetic Pumps System for the MEGAPIE target has been developed, produced and tested at the Institute of Physics, University of Latvia, during the recent two years in the framework of the MEGAPIE-TEST Project (MEGAWatt Pilot Experiment – TESTing) funded within the Euratom 5 Framework Programme as an implementation of transmutation technology [1]. The EMPS operates being submerged in lead bismuth eutectic (LBE), whose temperature fluctuates depending on the proton beam trip in the range 220–380◦C with the temperature changing rate 5–10◦C/s. The electromagnetic pump system is responsible for the lead bismuth eutectic flow in the MEGAPIE target. Stop of the flow, e.g., as a result of the channel plugging with He bubbles, leads to local overheating of the proton beam inlet window and very likely to its disintegration. Therefore, monitoring of the flow during the target operation is a very important task. Results of original electrodynamics and thermo hydraulic calculations of the electromagnetic pump system are presented in the report. These results assume the procedure of lead bismuth eutectic flow monitoring through electrical regimes for electromagnetic pumps and lead bismuth eutectic temperatures measurements. The procedure is based on the strong correlation between the lead bismuth eutectic temperature at the EMPS inlet and outlet, the flow rate and the electrical regime for the pump. A special PC code, which allows controlling the intensity of the lead bismuth eutectic flow in the EMPS channel at steady and transient temperature regimes of the target operation, has been developed and proposed. There are recommendations on the PC code adoption for the MEGAPIE target control system at the end of the presentation.

Experience of calculation, design, fabrication and testing of the electromagnetic pump system for the MEGAPIE target

Experience of calculation, design, fabrication and testing of the electromagnetic pump system for the MEGAPIE target

Assessment of the lifetime of the beam window of MEGAPIE target liquid metal container

The lifetime of the beam window of the T91 liquid Pb–Bi container in the MEGAPIE target is discussed based on the present knowledge of lead bismuth eutectic (LBE) corrosion, embrittlement and radiation effects in relevant conditions. In the MEGAPIE target, since the high hydrogen production will likely reduce the oxygen content to a low level, LBE corrosion may reduce the wall thickness up to 2%. In addition, the corrosion induced grain boundary dissolution will promote LBE embrittlement on the T91 steel in the beam window. The DBTT data and fracture toughness values of T91 specimens tested in contact with LBE suggest a lower bound of the lifetime of the T91 beam window to be limited to a dose of 6 dpa, corresponding to 2.4 Ah proton charge to be received by the target in about 20 weeks in the normal operation condition.