Mercury Target System Research Articles

Quantifying the reduction in cavitation-induced erosion damage in the Spallation Neutron Source mercury target by means of small-bubble gas injection

A model developed to represent the progress of erosion damage in liquid-metal spallation target vessels was modified to incorporate the effect of gas injection on the erosion rate. The liquid mercury target system for the Spallation Neutron Source (SNS) at Oak Ridge National Laboratory now operates with helium gas injection to reduce target vessel fatigue stress and cavitation-induced erosion damage. Erosion damage is a primary degradation phenomenon affecting the service life of SNS target vessels, and cavitation mitigation techniques, such as small-bubble gas injection, have been implemented to reduce damage and extend target lifetimes. Erosion depths in samples removed from SNS targets after operation were measured using laser line scanning. These measurements confirmed that gas injection reduced erosion damage. However, quantifying the damage reduction due to gas injection was complicated by variations in lifetime, power, and gas injection rates between different targets. In this study, the operating power and gas injection rate of targets were incorporated into an erosion damage prediction model to quantify their effects on erosion damage reduction. Values of a power scaling factor, β, were calculated by comparing modeled with measured erosion damage. These values indicate that the use of gas injection at the SNS reduced damage to a level equivalent to operating targets without gas injection at 35–47% of the actual beam power. To account for the gas injection effect on the cavitation damage, a simple exponential form based on analysis of the scaling factor β was developed to incorporate the gas rate history with a scaling factor γ in the erosion damage modeling.

Water leakage due to the welding defect and improvement to reach 1-MW beam operation in the mercury target of J-PARC

ABSTRACTNeutron flux per pulse reached world record at neutron source in Japan Proton Accelerator Research Complex (J-PARC). In the J-PARC, mercury target system is used as a spallation neutron source. A target vessel has a multi-walled protection system that comprises a mercury vessel enclosed with a double-walled water shroud. This is to prevent the leakage of the mercury outside the target vessel. The multi-walled structure needed to be complicated with a lot of welding lines. However, during the operation, we faced an unscheduled shutdown due to water leakage to the intermediate layer between the mercury vessel and water shroud. An investigation on the cause of the leakage was carried out. It is deduced that the leakage path was formed due to the crack propagation from welding defects that are caused by the complicated multi-walled structure. The crack propagation is attributed to the repeated stress by pressure waves generated in the mercury target. Based on the investigation results, the design was improved to remove the welding line on the complicated structure and to realize the stable operation with 1 MW proton beam, which is the final design value of the neutron source in J-PARC.

Mitigation technologies for damage induced by pressure waves in high-power mercury spallation neutron sources (IV) – measurement of pressure wave response and microbubble effect on mitigation in mercury target at J-PARC –

ABSTRACTA mercury target system to produce neutron beams has been operated at the spallation neutron source in the Japan Proton Accelerator Research Complex (J-PARC). Pressure waves are generated in mercury by rapid heat generation due to bombardment by high-intensity short-pulse proton beams. The pressure waves not only cause cyclic stress but also induce the cavitation damage on the target vessel. Reduction of these pressure waves is important from the viewpoint of extending the lifetime of the target vessel in future power-up operations. The injection of microbubbles into mercury is effective for reducing pressure waves. Accordingly, a microbubble generator was installed in the mercury target vessel and an in situ diagnostic system that measures the displacement velocity of the target vessel induced by the pressure waves was also set up in J-PARC to investigate the effect of proton beam condition and the effect of the microbubbles. Consequently, we found that the peak displacement velocity of the target vessel decreased owing to microbubble injection. The ratios of the peaks obtained with bubble injection to that without bubble injection were 1/3 and 2/3 when the injected gas fractions were 0.4% and 0.1%, respectively.

Cavitation damage prediction for the JSNS mercury target vessel

The liquid mercury target system for the Japan Spallation Neutron Source (JSNS) at the Materials and Life science experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC) is designed to produce pulsed neutrons. The mercury target vessel in this system, which is made of type 316L stainless steel, is damaged by pressure wave-induced cavitation due to proton beam bombardment. Currently, cavitation damage is considered to be the dominant factor influencing the service life of the target vessel rather than radiation damage. In this study, cavitation damage to the interior surface of the target vessel was predicted on the basis of accumulated damage data from off-beam and on-beam experiments. The predicted damage was compared with the damage observed in a used target vessel. Furthermore, the effect of injecting gas microbubbles on cavitation damage was predicted through the measurement of the acoustic vibration of the target vessel. It was shown that the predicted depth of cavitation damage is reasonably coincident with the observed results. Moreover, it was confirmed that the injection of gas microbubbles had an effect on cavitation damage.

ChemInform Abstract: Proton Bombardment in Mercury Target for Neutron Production ‐ Impact Dynamics on Interface Between Liquid and Solid Metals

AbstractReview: 36 refs.

Material issues relating to high power spallation neutron sources

Innovative researches using neutrons are being performed at the Materials and Life Science Experimental Facility (MLF) at the Japan Proton Accelerator Research Complex (J-PARC), in which a mercury target system is installed for MW-class pulse spallation neutron sources. In order to produce neutrons by the spallation reaction, proton beams are injected into the mercury target. At the moment, when the intense proton beam hits the target, pressure waves are generated in mercury because of the abrupt heat deposition. The pressure waves interact with the target vessel, leading to negative pressure that may cause cavitation along the vessel wall, i.e. on the interface between liquid and solid metals. On the other hand, the structural materials are subjected to irradiation damage due to protons and neutrons, very high cycle fatigue damages and so-called "liquid metal embrittlement". That is, the structural materials must be said to be exposed to the extremely severe environments. In the paper, research and development relating to the material issues in the high power spallation neutron sources that has been performed so far at J-PARC is summarized.

Cavitation erosion induced by proton beam bombarding mercury target for high-power spallation neutron sources

A liquid mercury target system for a megawatt-class spallation neutron source is being developed in the world. Proton beam is injected to the mercury target to induce spallation reaction. The moment the proton beams bombard the target, pressure waves are generated in the mercury by the thermally shocked heat deposition. The pressure waves excite the mercury target vessel and negative pressure that may cause cavitation along the vessel wall. Gas-bubbles will be injected into the flowing mercury to mitigate the pressure waves and suppress the cavitation inception. The injected gas-bubbles conditions were examined and the effects were predicted experimentally and theoretically from the viewpoints of macroscopic time-scale and microscopic time-scale, i.e. in the former is dominant the interaction between the structural vibration and the pressure in mercury, and in the later is essential the pressure wave propagation process.

Proton Bombardment in Mercury Target for Neutron Production - Impact Dynamics on Interface between Liquid and Solid Metals

Innovative researches using neutrons are being performed at the Materials & Life Science Experimental Facility (MLF) in the Japan Proton Accelerator Research Complex (J-PARC), in which a mercury target system is installed as MW-class pulse spallation neutron sources. In order to produce neutrons by the spallation reaction, proton beams are injected into the mercury target. At the moment when the intense proton beam hits the target, pressure waves are generated in mercury because of abrupt heat deposition. The pressure waves interact with the target vessel leading to negative pressure that may cause cavitation along the vessel wall, i.e. the interface between liquid and solid metals. Localized impacts by microjets and/or shock waves that are caused by cavitation bubble collapse impose pitting damage on the vessel wall. The pitting damage that degrades the structural integrity of the target vessel is a crucial issue for the high power mercury targets. Therefore, the mitigation techniques for the pitting damages and cavitation are needed to reach the MW-class pulsed spallation neutron sources.

Damage inspection of the first mercury target vessel of JSNS

The liquid mercury target system for the Japan Spallation Neutron Source (JSNS) began operating in May 2008. A mercury target vessel composed of type 316L stainless steel suffers radiation damage in the proton and neutron environment. In addition to this damage, the inner wall of the target vessel in contact with mercury is damaged by cavitation-induced erosion, which is caused by rarefied pressure waves. The target vessel was replaced with a new target in November 2011, because the pneumatic bellow type flange was damaged during the Great East Japan Earthquake, that occurred on March 11, 2011. Before replacing the target, 50mm diameter disk specimens were cut from the beam window of the target vessel in order to investigate the cavitation damage inside the target vessel and to evaluate the change in the mechanical properties due to radiation damage. A video camera was used to inspect the inside of the target vessel and no flow induced erosion damage was detected on the flow guide by visual inspection. Cavitation damage from the proton pulse was concentrated at the center and approximately 15mm from the center of the beam window. In addition, the depth of the cavitation damage was quantitatively measured using a laser profilometer and a metallurgical surface replica. Based on the detailed measurements, it was concluded that the eroded damage depth of the beam window was 250μm after 475MWh of operation.

E211 低濃度気泡流解析コードの開発(OS8 軽水路・新型炉・原子力安全(5))

Abstract In a sodium-cooled fast reactor, inert gas exists in the primary coolant system as bubbles or dissolved gas Such inert gas may cause disturbance in reactivity and/or degradation of IHX performance, and therefore, the inert gas behaviors have to be investigated to ensure the stable operation of a fast reactor Similarly, small bubbles exist also in the mercury target loop in J-PARC to suppress cavitation erosion. Those small bubbles should be removed immediately at the downstream of a target region because the small bubbles may accumulate at the upper plenum of a heat exchanger and degrade the heat exchange efficiency To simulate these inert gas behaviors in liquid metal flows, the Japan Atomic Energy Agency (JAEA) has developed a plant dynamics code VIBUL. In this study, new models, 1 e the bubble release and bubble carry under models, are introduced to simulate the bubble behaviors in the fast reactor and mercury target system Then, the small bubble behavior in the mercury target system is simulated to check the validity of the new models.

Study on Simulation Code for Dilute Bubbly Flow

In a sodium-cooled fast reactor, inert gas exists in the primary coolant system as bubbles or dissolved gas. Such inert gas may cause disturbance in reactivity and/or degradation of IHX performance, and therefore, the inert gas behaviors have to be investigated to ensure the stable operation of a fast reactor. Similarly, small bubbles exist also in the mercury target loop in J-PARC to suppress cavitation erosion. Those small bubbles should be removed immediately at the downstream of a target region because the small bubbles may accumulate at the upper plenum of a heat exchanger and degrade the heat exchange efficiency. To simulate these inert gas behaviors in liquid metal flows, the Japan Atomic Energy Agency (JAEA) has developed a plant dynamics code VIBUL. In this study, new models, i.e. the bubble release and bubble carry under models, are introduced to simulate the bubble behaviors in the fast reactor and mercury target system. Then, the small bubble behavior in the mercury target system is simulated to check the validity of the new models.

Gamma dose measurements and spectroscopy analysis for spallation products in JSNS mercury circulation system

Mercury used as a spallation neutron target at JSNS (J-PARC Spallation Neutron Source)� is highly activated by proton beam operation. In order to evaluate the feasibility of hands-on maintenance after nominal beam operation with 1 MW in the where a mercury circulation system was installed, gamma dose measurements and gamma spectroscopy analysis were carried out after low power operation during the commissioning period. By analyzing the data, it was found that radioactive spallation products adhered to the inner surface of piping wall of the circulation systemand those significantly raised the dose rates in the cell. In order to discuss about measures for future hot-cell entry, the gamma dose rates after the nominal operation were evaluated based on the measured data obtained in this work. 1) is a multipurpose research facility with muon and neutron generation target. The operation of the J-PARC spallation neutron source (JSNS) in MLF started in May 2008, and an effort is being made now to increase the proton beam intensity up to 1 MW together with many kinds of researches on materials and life science. 2. Mercury Target and Circulation System. In JSNS, mercury was selected as a material of neutron generation target in consideration of the neutron generation efficiency and the cooling. A drawing of the mercury target vessel and a mercury circulation system on a trolley is shown in Fig 1. Neutrons are generated by bombarding mercury in the target vessel with a proton beam, and the mercury heated by incident protons and accompanying nuclear reactions is circulated in the loop between the target vessel and the mercury circulation system for cooling. The target vessel is inserted to the core of the neutron station consisted of moderators, reflector, neutron beam shutters and shields. For the maintenance, the trolley is moved to the hot cell prepared for handling of activated devices. Mercury in the circulation system is highly activated due to radioactive isotopes produced via spallation reactions. Therefore most of the maintenance works for the mercury target and the circulation system were designed to be done by using remote handling system equipped in the cell. However a few works, which is expected to happen once or twice during the facility lifetime of about 30 years, will need to be done by hands-on; this will necessitate entrance of workers to the cell. Mercury in the circulation system can be transferred to the drain tanks under the circulation system on the bottom of the

Bending Fatigue Strength of Austenitic Stainless Steel SUS316 in Mercury

Liquid mercury target system for spallation neutron source is installed at Materials and Life Science Experimental Facility in Japan Proton Accelerator Research Complex (J-PARC), which will promote innovative science. Austenitic stainless steel SUS316 for a structural material of the target vessel is contact with mercury, and imposed by cyclic pressure that is induced by pulsed proton beam injection at 25Hz. Therefore, it is important from the viewpoint of the structural integrity to investigate the effect of mercury on fatigue behavior of the structural material. Bending fatigue tests in mercury and air were performed to evaluate the effect of mercury on fatigue behavior. FRActure Surface Topography Analyses (FRASTA) were carried out to evaluate the change in fracture morphology with mercury. It was confirmed that the fatigue strength was decreased by mercury immersion in low cycle region less than 105 cycles and intergranular fracture surface was observed, while in high cycle region the mercury immersion effect was hardly recognized.

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

Checking the seal performance of the mercury piping network is very important for the mercury target system operation of J-PARC, and the test method for leaks using the pressure change measurement is preferable for this purpose because it can be carried out easily and precisely by measuring the pressure change, and it is free from the risk of mercury contamination. The piping network is pressurized by helium gas. Thus, the correlation between the helium leak rate and mercury leak flow rate was investigated experimentally by carrying out leak tests for helium and mercury with an identical mockup flange model. The results showed that the mercury leak flow rates of the experimental data were lower than those of the estimated value by 64% on average. It was also found that the threshold of the helium leak rate at which good seal performance for mercury can be obtained exists between 2.18 × 10−4 and 1.01 × 10−2 Pa.m3/s. This fact confirmed the sufficient safety margin of the mercury target system against the mercury leak, where 1 × 10−6 Pa.m3/s was adopted as the seal performance criterion.

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

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.

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.

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.

Development of a gas layer to mitigate cavitation damage in liquid mercury spallation targets

Establishment of a gas layer between the flowing liquid and container wall is proposed for mitigating the effects of cavitation in mercury spallation targets. Previous work has shown an order of magnitude decrease in damage for a gas layer developed in a stagnant mercury target for an in-beam experiment. This work is aimed at extending these results to the more complex conditions introduced by a flowing mercury target system. A water-loop has been fabricated to provide initial insights on potential gas injection methods into a flowing liquid. An existing full-scale flow loop designed to simulate the Spallation Neutron Source target system will be used to extend these studies to mercury. A parallel analytical effort is being conducted using computational fluid dynamics (CFD) modeling to provide direction to the experimental effort. Some preliminary simulations of gas injection through a single hole have been completed and show behavior of the models that is qualitatively meaningful.

1518 粉末焼結法によるバブリングメゾノズルの開発(S13 新機能多孔質材料の創製と評価,21世紀地球環境革命の機械工学:人・マイクロナノ・エネルギー・環境)

Liquid mercury target system for high power spallation neutron sources is being developed. When high intensity proton beams are injected into the target, pressure waves are generated by thermal shock in mercury and cavitation damage will be imposed inside the target vessel. Injection of micro bubbles into the mercury is effective to mitigate the pressure waves. In the previous report, we proposed a method of fabricating a micro nozzle using powder metallurgy method. SUS316L and molybdenum powders were used as the nozzle matrix material. In a combination of molybdenum and quartz fiber, we had produced a straight hole with a circular cross-section, in which the diameter is approximately 100μm. In this work, experiment was carried out to observe the shape change of glass fiber against temperature. Quartz's phase changed from liquid to gas when heating between 1030℃ and 1875℃. This result shows that change of glass phase is an important factor for fabricating a hole.