Liquid Metal Spallation Target Research Articles

Cavitation-induced erosion in liquid metal spallation targets – A comparison between a predictive method and measured erosion damage to stainless steel target modules at the Spallation Neutron Source

Neutrons are produced for neutron scattering instruments at the Spallation Neutron Source by bombarding liquid mercury with a pulsed high-energy proton beam. The liquid mercury target material flows through a replaceable stainless-steel target vessel and cavitates during operation due to the extremely high heating rate caused by each proton pulse. Cavitation-induced erosion damage to the mercury-facing surfaces of target vessels is a key concern for target lifetime and has been the subject of considerable research. While numerous experiments have studied and modeled the progression of cavitation erosion damage to liquid metal target containers, direct quantitative measurements of actual erosion of a spallation target vessel were previously not available. Recently, the erosion damage to multiple target vessels operated at the Spallation Neutron Source was measured using a high-resolution laser scanning system, which provided an opportunity to study the efficacy of a cavitation damage progression model. In this paper, a cavitation progression modeling procedure developed using empirical observations is compared to measurements of samples from target vessels after operation. The strengths and limitations of the technique are discussed and methods for improving future modeling and predictive accuracy are proposed.

Assessment of the power deposition on the MEGAPIE spallation target using the GEANT4 toolkit

This work aims at evaluating the reliability of the GEANT4 (GEometry ANd Tracking 4) Monte Carlo (MC) toolkit in calculating the power deposition on the Megawatt Pilot Experiment (MEGAPIE), the first liquid–metal spallation target worldwide. A new choice of codes to study and optimize this target is provided. The evaluation of the GEANT4 toolkit is carried out in comparison with the MCNPX and FLUKA MC codes. The MEGAPIE is an international project led by the Paul Scherrer Institute in Switzerland. It aims to demonstrate the safe operation of an intense neutron source to power the next generation of nuclear reactors, accelerator-driven systems (ADSs). In this study, we used the GEANT4 MC toolkit to calculate the power deposited by fast protons on the MEGAPIE target. The calculation focuses on several structures and regions. The predictions of our calculations were compared and discussed with that of the MCNPX and FLUKA codes, adopted by the MEGAPIE project. The comparison shows that there is a very good agreement between our results and those of the reference codes.

Three-dimensional CFD simulations to study the effect of impeller geometry on internal flow field in ADS upward spiral flow target

ABSTRACT The relationship between flow field characteristics and geometrical structures is an important issue in designing liquid metal spallation target for accelerator-driven sub-critical system (ADS). Upward spiral flow target (USFT) is a new type ADS windowless target in which an impeller is employed to generate upward spiral flow and regulate internal flow field. The impeller comprises a central shaft and several identical blades, and its geometry is very crucial for designing the target. In this paper, series models with different geometric parameters of the shaft and the blades are simulated by ANSYS CFX to analyse the distributions of internal flow field and the dimensions of stagnation zone in USFT. The results show that the shaft plays a primary role in guiding the upward flow while the blades are significant for generating the spiral flow. Finally, an optimized model with preferred geometric parameters is obtained by comparing the overall simulation results. The whole research has foundational signification for further studying USFT.

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.

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.

Radiochemical aspects of liquid mercury spallation targets

Liquid metal spallation targets using mercury as target material are used in state-of-the-art high power pulsed neutron sources that have been constructed in the USA and Japan within the last decade. Similar target concepts were also proposed for next generation ISOL, β-beam and neutrino facilities. A large amount of radioactivity will be induced in the liquid metal during operation caused by the interaction of the target material with the intense proton beam. This radioactivity – carried by a wide range of radioisotopes of all the elements of the periodic table from hydrogen up to thallium – must be considered for the assessment of safe operation and maintenance procedures as well as for a final disposal of the used target material and components. This report presents an overview on chemical investigations performed in our laboratory that deal with the behavior of radionuclides in proton irradiated mercury samples. The solubility of elements in mercury was calculated using thermodynamical data obtained by a semi-empirical method. It is found that several important nuclear reaction products will reach their solubility limit in a high power mercury spallation target. It is also experimentally proven that the radionuclides produced by proton irradiation are generally not homogeneously distributed in the liquid metal but tend to accumulate on vessel walls and free surfaces. The consequences resulting from this behavior for the operation of mercury spallation targets are discussed.

Initial observations of cavitation-induced erosion of liquid metal spallation target vessels at the Spallation Neutron Source

During operation of the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory the mechanical properties of the AISI 316L target module are altered by high-energy neutron and proton radiation. The interior surfaces of the target vessel are also damaged by cavitation-induced erosion, which results from repetitive rapid heating of the liquid mercury by high-energy proton beam pulses. Until recently no observations of cavitation-induced erosion were possible for conditions fully prototypic to the SNS. Post-Irradiation Examination (PIE) of the first and second operational SNS targets was performed to gain insight into the radiation-induced changes in mechanical properties of the 316L target material and the extent of cavitation-induced erosion to the mercury vessel inner surfaces. Observations of cavitation-induced erosion of the first and second operational SNS target modules are presented here, including images of the target vessel interiors and specimens removed from the target beam-entrance regions.

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.

Effect of tensile stress on cavitation damage formation in mercury

Cavitation erosion or so called pitting damage was investigated under tensile stress conditions in mercury. In MW-class liquid metal spallation targets, pitting damage is a critical issue to satisfy required power and/or lifetime of the target vessel. Cavitation occurs by negative pressure which is induced through pressure wave propagation due to proton beam injection. Pitting damage is formed by microjet and/or shock wave during cavitation bubble collapse. A mercury target vessel suffers tensile stress due to thermal stress or welding. In order to investigate the effect of tensile stress on pitting damage formation, cavitation erosion tests were performed using stress imposed specimens in mercury. An ultrasonic vibratory horn and electro-Magnetic IMpact Testing Machine (MIMTM) were used to vary the cavitation intensity. In the incubation period of pitting damage, damaged area was slightly increased with increasing imposed tensile stress. In the steady state period, a mean depth of erosion was increased by the tensile stress. Additionally, in order to quantitatively evaluate the effect of tensile stress, an indentation test with Vickers indenter was carried out to quasi-statically simulate the impact load. From the measurement of the diagonal length of the indent aspect ratio and hardness, it is recognized that the threshold of the deformation, i.e. pitting damage formation, was decreased by the tensile stress.

Electromagnetic pump for a liquid metal spallation target: calculation, diagnostics, reliability

Electromagnetic pump for a liquid metal spallation target: calculation, diagnostics, reliability

Adaptation of the law of the wall boundary treatment to include volumetric heating effects

Computational Fluid Dynamics (CFD) simulations of liquid–metal spallation targets, such as MEGAPIE and ESS, which utilize the High Reynolds number k – ε turbulence model, invariably incorporate an implicit law of the wall treatment in which a linear or logarithmic fit to the velocity and temperature profiles is made next to heated, non-slip surfaces. The law is well-established, but has been derived from the assumptions that the wall shear stress and the normal heat flux are constant through the viscous sub-layer and buffer zone, which lie beneath the turbulent boundary layer. However, in the case of the heat flux, this condition will be violated for applications in which there is intense volumetric heating in the near-wall layers. This is just the case for the spallation reactions taking place in liquid–metal targets as a result of proton bombardment. In this article, a modified law of the wall is derived to be used under such conditions. Use of the law is illustrated by means of flow in a flat channel and one application to a spallation target. From the applications considered, it is found that the effect of the modification is small, provided the local mesh resolution is chosen appropriately. Specific recommendations regarding optimum mesh size for liquid–metal heat transfer problems are given, which will be of general interest, with or without volumetric heating.

The MEGAPIE-TEST project: Supporting research and lessons learned in first-of-a-kind spallation target technology

The megawatt pilot experiment (MEGAPIE) has been launched by six European institutions (PSI, FZK, CEA, SCK-CEN, ENEA and CNRS), JAEA (Japan), DOE (US) and KAERI (Korea) with the aim to carry out an experiment, in the SINQ target location at PSI (Switzerland), to demonstrate the safe operation of a liquid metal (lead–bismuth eutectic, LBE) spallation target hit by a ∼1 MW proton beam. The European Commission has joined the MEGAPIE project through the 5-year (2001–2006) project named MEGAPIE-TEST. This project has been formally concluded with an International Workshop, where the results and the lessons learned during the project have been summarised. This work presents a review of the outcome of that Workshop.

VIMOS, near-target beam diagnostics for MEGAPIE

A correct intensity distribution in the incident beam is of highest importance for the Megawatt Pilot Experiment (MEGAPIE) liquid–metal spallation target at PSI. VIMOS is a new optical system which monitors the glowing of a tungsten mesh heated by the proton beam by means of a fast radiation-resistant camera. The experience gained with the system during 3 years is very positive, further improvement in terms of easy maintenance is still planned.

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.

Investigations on the release of mercury from liquid eutectic lead–bismuth alloy under different gas atmospheres

Eutectic lead–bismuth alloy (LBE) samples with mercury concentrations similar to those expected for a liquid metal spallation target were prepared. The release of mercury from these samples under a continuous flow of gas was studied as a function of temperature, using γ-spectroscopy and 203Hg as a radioactive tracer. Various gas compositions ranging from a reducing atmosphere (Ar/7%H 2) to oxidising conditions (moist air) were applied. Under reducing conditions, substantial release of mercury from LBE started at temperatures around 473 K. At about 571 K, half of the mercury present in the samples was released within 1 h. Oxidising conditions led to a strong decrease in mercury release. In 1 h-experiments, no release of mercury was observed for temperatures up to 935 K within the limits of experimental error when humid air was used as a carrier gas. Apparently, the formation of an oxide layer on the surface of the melt inhibits the evaporation of Hg. This effect is of importance with respect to accident scenarios in liquid metal spallation targets, where liquid LBE could be exposed to air as well as cooling water. In such a case, a substantial decrease in the evaporation rate of mercury can be expected due to the formation of an oxide layer.

Improved cavitation resistance of structural materials in pulsed liquid metal targets by surface hardening

Cavitation damage of structural materials due to pressure waves is expected to be one of the major life-time limiting factors in high power liquid metal spallation targets under pulsed operation. Two methods are developed for the European Spallation Source (ESS) to mitigate this damage: Introduction of gas bubbles to suppress the pressure pulse, and surface hardening of structural materials to reduce their vulnerability. Surface hardening of four 8–13% Cr martensitic steels was performed by thermal treatment with pulsed or scanned electron- and laser beams as well as by nitriding in a r.f. plasma. Parametric dependences of these processes were investigated and partially optimised. Thermal treatment with electrons reached HV-hardness values up to about 600, laser treatment up to 710, and nitriding above 1200. After mechanical and metallographical characterisation, specimens of 12% Cr steel were tested in liquid mercury under pulsed proton irradiation, and under mechanical pulse-loading. Surface damage was analysed by optical, confocal laser, and scanning-electron microscopy, showing in all tests much better resistance of the hardened material compared to standard condition. Application of these techniques to a spallation target is outlined.

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.

CFD Design Support at PSI for the International MEGAPIE Liquid-Metal Spallation Target

The MEGAPIE (Megawatt Pilot Experiment) Project has been primarily initiated in response to interest in the Accelerator Driven System (ADS). The ADS is a nuclear reactor concept with a fuel cycle which can burn minor actinide waste products from conventional reactors, while also taking advantage of the safety characteristics of a sub-critical reaction process. By this means, the fission chain reaction is maintained by additional neutrons generated by protons in a liquid-metal spallation target. The goal of the international MEGAPIE project is to design and construct such a target and install and test it over a period of one year in the SINQ spallation neuron source facility at the Paul Scherrer Institut (PSI). To aid the design process, Computational Fluid Dynamics (CFD) is being used to optimise the thermal- hydraulic behaviour of the target. Results obtained so far indicate that it should be possible to remove 700 kW of heat deposited by the proton beam in the target, under steady-state conditions (40kg/s total Lead Bismuth Eutectic flow rate), without peak window temperature rising above about 385°C, when using a bypass flow of 2.5 kg/s and a slanted- end guide tube. A tentative peak window temperature of 400°C is currently considered allowable, on the basis of material strength after irradiation, and CFD simulation is currently being validated by suitable LBE experiments in similar geometry.