Mercury Target Research Articles

Luteinizing hormone provides a causal mechanism for mercury associated disease

Previous studies have demonstrated that the pituitary is a main target for inorganic mercury (I-Hg) deposition and accumulation within the brain. My recent study of the US population (1999-2006) has uncovered a significant, inverse relationship between chronic mercury exposure and levels of luteinizing hormone (LH). This association with LH signifies more than its presumed role as bioindicator for pituitary neurosecretion and function. LH is the only hormone with a rare and well characterized, high affinity binding site for mercury. On its catalytic beta subunit, LH has the structure to preferentially bind inorganic mercury almost irreversibly, and, by that manner, accumulate the neurotoxic element. Thus, it is likely that LH is an early and significant target of chronic mercury exposure. Moreover, due to the role of LH in immune-modulation and neurogenesis, I present LH as a central candidate to elucidate a causal mechanism for chronic mercury exposure and associated disease.

Defect structures in nickel and SUS304SS formed by the collapse of cavitation bubbles

A mercury target in high-power spallation neutron sources is subjected to pressure waves induced by a proton beam. The subsequent formation and collapse of cavitation bubbles lead to cavitation damage on the target vessel, especially the beam window. The cavitation damage in Ni and austenitic stainless steel SUS304SS were studied by using an electro-Magnetic IMpact Testing Machine (MIMTM) developed to simulate the damage. The existence of dislocations, stacking fault tetrahedra and vacancies was detected by positron annihilation lifetime measurements in Ni, and non-cellular dislocation structures were observed by transmission electron microscopy in Ni and SUS304SS. In addition, a high density of twins was observed in SUS304SS. These results were compared with those of high-speed compression tests using a high-speed projectile, proving that the cavitation damage caused by MIMTM corresponded to high-speed deformation.

3 MW solid rotating target design

A rotating solid target design concept is being developed for potential use at the second SNS target station (STS). A long pulse beam (∼1 ms) at 1.3 GeV and 20 Hz is planned with power levels at or above 1 MW. Since the long pulse may give future opportunities for higher power, this study is looking at 3 MW to compare the performance of a solid rotating target to a mercury target. Unlike the case for stationary solid targets at such powers this study indicates that a rotating solid target, when used with large coupled hydrogen moderators, has neutronic performance equal to or better than that with a mercury target, and the solid target has a greatly increased lifetime. Design studies have investigated water cooled tungsten targets with tantalum cladding approximately 1.2 m in diameter, and 70 mm thick. Operating temperatures are low (<150 °C) with mid-plane, top and bottom surface cooling. In case of cooling system failure, the diameter gives enough surface area to remove the decay heat by radiation to the surrounding reflector assemblies while keeping the peak temperatures below approximately 700 °C. This temperature should mitigate potential loss of coolant accidents and subsequent steam, tungsten interaction which has a threshold of approximately 800 °C. Design layouts for the sealing systems and potential target station concepts have been developed.

Solubility of helium in mercury for bubbling technology of the spallation neutron mercury target

The pitting damage of mercury target container that originates in the pressure wave excited by the proton beam incidence becomes a large problem to reach the high-power neutron source in JSNS and SNS. The lifetime of mercury container is decreased remarkably by the pitting damage. As one of solutions, the pressure wave is mitigated by injecting the helium micro bubbles in mercury. In order to inject the helium micro bubbles into mercury, it is important to understand the characteristic of micro bubbles in mercury. The solubility of mercury–helium system is a key factor to decide bubbling conditions, because the disappearance behavior, i.e. the lifetime of micro bubbles, depends on the solubility. In addition, the bubble generation method is affected by it. Moreover, the experimental data related to the solubility of helium in mercury hardly exist. In this work, the solubility was obtained experimentally by measuring precisely the pressure drop of the gas that is facing to mercury surface. The pressure drop was attributed to the helium dissolution into mercury. Based on the measured solubility, the lifetime of micro bubbles and the method of the bubble generation is estimated using the solubility data.

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.

Development of JSNS target vessel diagnosis system using laser Doppler method

When an intense pulsed proton beam with a power of 1 MW is irradiated to a mercury target, a pressure wave caused by the proton beam gives a vibration on the target vessel. Pitting damage also occurs on the target vessel, especially incident beam area, resulting in shortening of a life-time. It is very important to monitor the vibration of the target vessel from the view point of the life-time estimation. We developed the target vessel diagnosis system using laser Doppler method and successfully installed it in an actual pulsed spallation source. The diagnosis system consists of retro-reflecting corner-cube mirror (reflective mirror) on the target, mirror assembly in a reflector plug and laser source-detector. The newly developed reflective mirror, made by nickel, was installed by vacuum silver brazing on the target vessel to detect the target vibration. In order to pass the laser beam to the target vessel, a mirror assembly was installed inside the reflector plug. It is replaceable using a remote handling machine during a maintenance period. Nd-YAG laser beam (wave length: 533 nm) with the power of 50 mW was adopted to detect the target vibration. The first proton beam to the target in the spallation neutron source (JSNS) was provided on 30 May 2008. The first signal related to the target vibration was also detected by using this target vessel diagnosis system.

Post-irradiation examination of the Spallation Neutron Source target module

The Spallation Neutron Source (SNS) at Oak Ridge National Laboratory is an accelerator-based pulsed neutron source that produces high-energy spallation neutrons by bombarding liquid mercury flowing through a stainless steel target vessel. During operation the proton beam and spallation neutrons produce radiation damage in the AISI 316L austenitic stainless steel target vessel and water-cooled shroud. The beam pulses also cause rapid heating of the liquid mercury, which may produce cavitation erosion damage on the inner surface of the target vessel. The cavitation erosion rate is thought to be highly sensitive to beam power and predicted to be the primary life-limiting factor of the target module. Though cavitation erosion and radiation damage to the target vessel are expected to dictate its lifetime, the effects of radiation damage and cavitation erosion to target vessels in liquid metal spallation systems are not well known. Therefore preparations are being undertaken to perform post-irradiation examination (PIE) of the liquid mercury target vessel and water-cooled shroud after end-of-life occurs. An overview of the planned PIE for the SNS target vessel is presented here, including proposed techniques for specimen acquisition and subsequent material properties characterization.

Structural-hydraulic test of the liquid metal EURISOL target mock-up

Structural-hydraulic tests of the European Isotope Separation On-Line (EURISOL) neutron converter target mock-up, named MErcury Target EXperiment 1 (METEX 1), have been conducted by Paul Scherrer Institut (PSI, Switzerland) in cooperation with Institute of Physics of the University of Latvia (IPUL, Latvia). PSI proceeded with extensive thermal-hydraulic and structural computational studies, followed by the target mock-up tests carried out on the mercury loop at IPUL. One of the main goals of the METEX 1 test is to investigate the hydraulic and structural behaviour of the EURISOL target mock-up for various inlet flow conditions (i.e. mass flow rates) and, in particular, for nominal operating flow rate and pressure in the system. The experimental results were analysed by advanced time–frequency methods such as Short-Time Fourier Transform in order to check the vibration characteristics of the mock-up and the resonance risk. The experimental results (obtained in METEX 1), which include inlet flow rate, pressure of the cover gas, total pressure loss, structural acceleration, sound and strain data, were jointly analysed together with numerical data obtained from Computational Fluid Dynamics (CFD).

Operational Experiences of the Spallation Neutron Source Superconducting Linac and Power Ramp Up

The spallation neutron source (SNS) is a second generation pulsed neutron source and designed to provide a 1-GeV, 1.44-MW proton beam to a mercury target for neutron production. Since the commissioning of the accelerator complex in 2006, the SNS has started its operation for neutron production and beam power ramp-up has been in progress toward the design goal. All subsystems of the SNS were designed and developed for substantial improvements compared to existing accelerators because the design beam power is almost an order of magnitude higher compared to existing neutron facilities and the achievable neutron scattering performance will exceed present sources by more than a factor of 20 to 100. In this paper, the operational experiences with the SNS Superconducting Linac (SCL), Power Ramp-up Plan to reach the design goal and the Power Upgrade Plan (PUP) will be presented including machine, subsystem and beam related issues.

21010 液体金属を内包した構造体の固液連成振動特性(原子力と材料・構造・組織(2),OS.4 原子力と材料・構造・組織)

The liquid mercury target system for the MW-class pulse spallation neutron source was installed at the material and life science experimental facility (MLF) in J-PARC. The laser Doppler vibrometer system was installed as diagnostic system on the structural integrity of the mercury target. Vibration measurements on the liquid mercury target vessel were carried out using this system. The amplitude of the first peak increase with an increase of the proton beam power. The delay time until the vibration starts in experimental results is longer than in analytical results. The slope of the regression line indicating the relationship between the delay time and the injection position of the proton beam is higher than that estimated by the sound velocity of the mercury. That is, the sound velocity of the mercury in the mercury vessel is predicted to be lower than that calculated using material constants.

21004 MIMTMを用いた水銀キャビテーションによる金属の壊食損傷構造(原子力と材料・構造・組織(1),OS.4 原子力と材料・構造・組織)

A mercury target in high-power spallation neutron sources is subjected to the pressure wave caused by proton beam. Subsequent formation and collapse of cavitation bubbles lead to cavitation damage on the target vessel, especially the beam window. The cavitation damage in Ni and austenitic stainless steel SUS304SS were studied by using MIMTM (electro-Magnetic IMpact Testing Machine) developed to simulate the damage. The existence of dislocations, stacking fault tetrahedra and vacancies was detected by positron annihilation lifetime measurement in Ni and non-cellar dislocation structures were observed by transmission electron microscopy in Ni and SUS316SS. High density of twins was also observed in SUS304SS. These results were compared with those of high speed compression test by using high speed projectile and proved that the cavitation damage by MIMTM corresponded to high speed deformation.

Mockup Experiments to Investigate the Leak Rate Correlation between Mercury and Helium for the Mercury Target System of J-PARC

Mockup Experiments to Investigate the Leak Rate Correlation between Mercury and Helium for the Mercury Target System of J-PARC

Effects of pitting damage and repeated stresses on lifetime of mercury target

Pitting damage influences the lifetime of Japan Spallation Neutron Source (JSNS) mercury target. The lifetime estimation was carried out taking into account the fatigue crack growth from the bottom of pits. Material crack propagation coefficients in the equation of Paris law were obtained based on the results of 3-point bending fatigue tests on the specimens with various pitting damages. Based on the estimated crack growth rate and the relationship between pitting damage and the number of impact pulses, the relationship between the total crack length and the number of pulses was estimated. The crack length increased rapidly over the crack length of 430 μm and the fatigue failure occurred at 8.4×10 7 pulses. The lifetime of the JSNS mercury target that suffers the pitting damage and repeated stress simultaneously with failure probability of 10% was 270 h at 1 MW operation.

Development of the Hg target in the J-PARC neutron source

A liquid mercury target system for the MW-class pulsed spallation neutron source is installed at the Materials and Life science experimental Facility (MLF) of J-PARC. MW-class proton bombardment of the mercury brings about intensive pressure waves that impact on mercury target vessel repeatedly at 25 Hz and can remarkably degrade the structural integrity and the lifetime of the vessel. In order to realize the high-power mercury target, R&D on the pressure wave mitigation technology is carried out experimentally and theoretically. One of the promising methods to mitigate the pressure waves is to inject microbubbles into the mercury. The microbubbles exhibit three effects for the mitigation: absorption, attenuation and suppression. An in-situ diagnostic system for measuring the dynamic responses induced by the pressure waves is installed in the Japan Spallation Neutron Source (JSNS). In this paper, we briefly describe the strategy to realize the high-power mercury target in the JSNS.

Two-phase flow model for energetic proton beam induced pressure waves in mercury target systems in the planned European Spallation Source

Two-phase flow calculations are presented to investigate the thermo-hydraulical effects of the interaction between 300 kJ proton pulses (2 ms long, 1.3 GeV) with a closed mercury loop which can be considered as a model system of the target of the planned European Spallation Source(ESS) facility. The two-fluid model consists of six first-order partial differential equations that present one dimensional mass momentum and energy balances for mercury vapour and liquid phases are capable to describe quick transients like cavitation effects or shock waves. The absorption of the proton beam is represented as instantaneous heat source in the energy balance equations. Densities and internal energies of the mercury liquid-vapour system are calculated from van der Waals equation, and the general method how to obtain such properties is also presented. A second order accurate high-resolution shock-capturing numerical scheme is applied with different kinds of limiters in the numerical calculations. %Our two-fluid model is very similar to the RELAP5 thermo-hydraulical code %which is used in the nuclear industry to simulate nuclear power plant accidents. Our analysis show that even 75 degree temperature heat shocks cannot cause considerable cavitation effects in mercury in 2ms long pulses

Design strategy for devices under high radiation field in J-PARC muon facility

The intensity of proton accelerator has attained the order of mega-watt, and several MW-class proton accelerators have started operating in the world. In comparison with the other MW-class accelerators, Japan Proton Accelerator Research Complex (J-PARC) is unique due to the muon facility. Materials and Life Science Facility, J-PARC, has not only the mercury target but also the graphite target to provide spallation neutrons and muons, respectively. The muon target is inserted upstream of the neutron target in the proton beam line. Full use of several mutually linking Monte-Carlo codes enables the design work of devices in the muon facility, which has to be operated under high radiation field from the muon target.

Bubble flow simulations in target vessel

Target vessel wall being damaged by pitting caused by pressure waves generated when pulsed high-power beams are injected is a crucial issue in the realization of a pulsed high-power mercury target for spallation neutron sources. The injection of micro-bubbles into mercury is a prospective technology for use in mitigating the pressure wave and the resulting pitting. The bubble flow field in an actual target vessel was numerically examined and the optimal conditions for bubble injection were investigated. The results indicated the possibility of the bubbles being distributed over the core regions when they are injected in the appropriate position according to their diameter.

Beam commissioning for neutron and muon facility at J-PARC

At J-PARC, an intensive pulsed 3-GeV proton beam (1 MW and 25 Hz) extracted from a rapid cycling synchrotron (RCS) is introduced to a mercury target for a neutron source and to a carbon graphite target for a muon source. In order to obtain the beam characteristics before irradiating the targets, beam commissioning was carried out using a beam dump during the accelerator study. Along the beam line to the dump, several beam monitors were located for beam diagnostics. It was found that all the beam monitors functioned as expected.

Development on mercury pump for JSNS

A permanent magnet rotating type induction pump (PM pump) was developed to provide mercury to a liquid mercury target system in Japan Spallation Neutron Source (JSNS). Mechanical pumps, such as a gear pump and a centrifugal pump, have risk of leakage of mercury from the seal parts. Induction pumps can avoid mercury leakage because they have no seal parts. The PM pump could be compact compared with a conventional induction pump; however, power loss must be reduced to avoid overheating of the system. Then optimizations for the thickness of mercury duct wall and width of the duct were carried out to reduce heat loss due to the eddy current in the duct generated by the induction, and to reduce flow-induced loss. As for the flow-induced loss, backflow would occur at the outside of the duct because of the difference in the Lorentz force between inside and outside of the duct. Consequently, the developed PM pump could have sufficient performance for the target system in JSNS and operate with low vibration.

J-PARC status update

The J-PARC (Japan Proton Accelerator Research Complex) project is close to its completion in construction and the initial user operation is scheduled to start by the end of 2008 (day-one). Following the linac test, the 3 GeV synchrotron successfully ejected 3 GeV protons in October 2007. In the materials and life science experimental facility, all components, e.g., the mercury target, cryogenic moderators, beam windows, remote handling devises, passed functional tests and being ready to receive beams in May 2008. Along with the neutron target system completion, neutron instrument construction has significantly accelerated. Currently, seven instruments are expected to be completed by the time of day-one in December. Installations of the muon target station and a secondary decay/surface muon channel are in progress for the first beam acceptance in September 2008. In addition to the project status, J-PARC user access policy and user program of neutron and muon are overviewed.