Metal Spallation Research Articles

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.

Spallation in laser shock-loaded tin below and just above melting on release

Spall damage in solid materials has been one of the most widely studied shock-induced phenomena for several decades, for both applied and basic scientific motivations. Comparatively, very little data can be found yet about spallation in liquid metals. In a recent paper, we have reported an exploratory investigation of liquid spall in tin samples melted upon laser shocks of very high intensities. Here, we present further experimental results obtained over a lower pressure range, where we focus on the transition from the ductile fracture behavior of solid tin to the cavitating spall expected above melting. This transition is clearly evidenced from both time-resolved free surface velocity measurements and post-test examination of the recovered targets. The drop in tensile strength associated with melting is evaluated from the velocity profiles. Detailed views of the fracture surfaces in the spall craters provide an insight into the cavitation process. Experimental data are compared to preliminary computations to determine the loading pressures and to assess the overall consistency of our interpretation of the results.

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.

Requirements for a high-power target test facility

Future advances in several research areas are being based around accelerator facilities that employ targets for generating neutrons, neutrinos or rare isotopes. The demand for higher neutron intensities and particle or isotope production is driving facilities to higher accelerator powers and more intense beams on targets. The challenges to target development are significant for accepting higher power levels while maintaining reasonable lifetimes. In the case of short-pulse liquid metal spallation targets the most significant issue is cavitation damage erosion. Although research and development is underway, long-term progress is hampered by lack of a test facility that provides prototypic beam in a flexible testing and analysis environment. The requirements of such a facility are significant, but it was recognized by some in the spallation neutron source community that many of its features would be useful for the development of targets in other research areas. This paper outlines the requirements of a desired facility that serves not only the spallation source community, but also those of other research areas. The scope of requirements includes beam parameters, test cell infrastructure, remote handling, post-irradiation examination and waste handling. Discussion and consensus between the potential users is hoped to lead to collaboration towards making the facility a reality.

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.

The MEGAPIE 1 MW target in support to ADS development: status of R&D and design

The MEGAPIE project is aimed at designing, building and operating a liquid metal spallation neutron target as a key experiment on the road to an experimental accelerator driven system and to improve the neutron flux at the PSI spallation source. The design of the target system has been completed. The target configuration and the operating conditions have been defined and the expected performance assessed. A preliminary safety analysis has been performed considering normal, off-normal and accident conditions and a corresponding report has been submitted to the authorities for licensing. The experience gained up to now shows that MEGAPIE may well be the first liquid metal target to be irradiated under high power beam conditions.

Molecular dynamics simulations of spallation in metals and alloys

We report on the results of large-scale molecular dynamics simulations of the mechanical behavior of two-dimensional metallic systems. The specific impact phenomenon studied is that in which a flyer of mass M moving with x-velocity v impacts a target of mass 2M moving with x-velocity −v/2. Simulations of such a spallation experiment have been performed for a generic metal, modelled with an embedded atom potential and also for a Cu-Ni alloy system, modelled with truncated Lennard-Jones potentials.Our simulations indicate cold-welding upon impact, and shock wave generation, followed by rebound from the boundaries. The alloy was less ductile than the generic metal and consequently the system came apart due to the cooperative effect of the reflected shock waves.

Dynamic fracture by spallation in metals

Material responses and fracture criteria relevant to the description of spallation phenomena are reviewed. The state of the material at the instant of fracture is an important part of a fracture criterion. The stress state associated with plane strain is described, and the effect of the initial conditions and the time dependent conditions of the material on spallation are discussed. The important quantities in any fracture criterion are the applied stress state, the size of the critically stressed region, the critical value of stress at which fracture begins, and the critical crack velocity. Stress gradient and cumulative damage spall-criteria are discussed in relation to these more fundamental quantities. Finally, some experiments are suggested that would isolate important parameters of the spallation process.