Spallation Neutron Source Mercury Target Research Articles

Strain prediction and benchmark with measurement in the Spallation Neutron Source mercury target without gas injection

The 1.4-MW Spallation Neutron Source (SNS) currently operates as the world's highest time-average power spallation source and uses mercury as the spallation target material. The mercury circulates through a stainless-steel target vessel and heat exchanger loop that removes the heat deposited from the incident proton beam in the target. The mercury vessel is part of the fixed but replaceable target module. The target module is not safety credited in the SNS design, but reliable long life is vital for meeting operational goals and minimizing cost and waste. SNS is a short-pulse ($\ensuremath{\sim}0.7\text{ }\text{ }\ensuremath{\mu}\mathrm{s}$) neutron source with a repetition rate of 60 Hz with 1.0 GeV protons onto the target. The short-pulse operation introduces two mechanisms that threaten the longevity of the mercury vessel. The pulsed target heating is essentially isochoric, which results in a distributed pressure field in the mercury with each pulse. After each pulse, the pressure propagates, interacts with, and strains the stainless-steel vessel. For intermittent times, this pressure field leads to mercury cavitation. The first mechanism of concern for the target lifetime is high-cycle fatigue to the stainless-steel vessel and the second is cavitation erosion. At the onset of the SNS project, the R program sought to address the need for an accurate and practical method to estimate the fatigue life of the mercury vessel. Around 2000, the R team conducted a series of in-beam mercury target experiments at the Weapons Neutron Research Facility at Los Alamos Neutron Science Center. These experiments successfully produced a small database of pulse response strain data. The data were used to develop and benchmark an explicit dynamic finite element analysis technique for estimating target fatigue life. SNS is now a mature facility with about 15 years of operation and 29 SNS target modules consumed. Since 2015, strain measurements have been collected from actual targets, and abundant target vessel strain data are now available in greater detail than was possible in 2000. Variances in pulse response data have been characterized using tolerance interval analysis to benchmark the simulation technique more thoroughly. The simulated results were found to match measurements generally well, but the method is always conservative. A potential adjustment to the cavitation threshold parameter value was suggested. Variances in the pulse response data are moderate, which is largely explained by the fact that data were acquired in 60-Hz burst mode and by the lasting vibration of the vessel between pulses.

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.

Strain measurement in the recent SNS mercury target with gas injection

High-radiation-tolerant fiber-optic strain sensors were recently developed to measure the transient proton-beam-induced strain profiles on the mercury target vessel at the Spallation Neutron Source (SNS). Here we report the strain measurement results and radiation-resistance performance on the latest SNS mercury target vessel equipped with helium gas injection. The results have demonstrated the efficacy of gas injection to reduce the cyclic stress on the target module.

Initial implementation of helium Gas into the SNS mercury target for mitigation of fatigue and cavitation damage

The short-pulse, 60 Hz beam used at the Spallation Neutron Source (SNS) initiates pressure waves in the mercury target with each pulse that 1) lead to huge numbers of fatigue stress cycles in the target vessel, and 2) pitting / erosion damage to the target vessel caused by mercury cavitation. Both phenomena can lead to target leaks and interruption of neutron production.The number of beam pulses expected over a successful target life is on the order of 109; the number of response stress cycles at a vessel position can exceed 1 per pulse. This number is even beyond the regime typical of high-cycle fatigue. It is important to be aware that contemporary knowledge fatigue damage includes probabilistic behavior for many materials, and that for many materials a true endurance limit does not exist. Obtaining data in the giga-cycle regime is difficult, particularly repeatable data. In this regime, the effects of very small material defects, inclusions and inhomogeneity become relevant as their size approaches that of the material microstructure. Detecting and controlling such defect sizes is problematic in the SNS mercury target module weld assembly.While considerable efforts are being made to improve the robustness of the SNS target, one thing that can be assuredly stated is that reducing high-cycle fatigue stress magnitude will add margin to its fatigue life. Injecting gas into the target mercury is one way to accomplish this. The degree of pulse stress reduction to be expected from a given gas condition is difficult to predict. There are SNS experimental data and JPARC target gas injection experience to expect stress reduction to ½ ~ ¼ of that without gas throughout much of the target mercury vessel.As a first step towards reduction of cyclic stresses and mitigation of cavitation damage, SNS will implement gas injection in its mercury target module beginning in the fall of 2018. The project is known as Gas Injection Initial Implementation, or GI3. GI3 will comprise of a once-through system. That is, the injected helium will be processed through the Mercury Off-gas Treatment System (MOTS) rather than recycled for re-injection. GI3 will employ relatively low flow rate of up to 1 slpm of helium gas through a total of sixty orifices, each with a diameter of 8 microns. The upstream supply pressure will be approximately 6.9 atm.GI3 in envisioned as a first step towards a more complete and complex gas injection system that would include much higher flow rates, gas re-circulation, and the potential combination of small gas bubble and gas wall supplies. Despite this reduced scope, there are significant design, installation, operational, and safety related challenges associated with the project. This paper and presentation will describe these challenges and the strategies employed to overcome them.

Development of microbubble generator for suppression of pressure waves in mercury target of spallation source

A MW-class mercury target for the spallation neutron source is subjected to the pressure waves and cavitation erosion induced by high-intense pulsed-proton beam bombardment. Helium-gas microbubbles injection into mercury is one of the effective techniques to suppress the pressure waves. The microbubble injection technique was developed. The selection test of bubble generators indicated that the bubble generator utilizing swirl flow of liquid (swirl-type bubble-generator) will be suitable from the viewpoint of the produced bubble size. However, when single swirl-type bubble-generator was used in flowing mercury, swirl flow of mercury remains at downstream of the generator. The remaining swirl flow causes the coalescence of bubbles which results in ineffective suppression of pressure waves. To solve this concern, a multi-swirl type bubble-generator, which consists of several single swirl-type bubble-generators arraying in the plane perpendicular to mercury flow direction, was invented. The multi-swirl type bubble-generator was tested in mercury and the geometry was optimized to generate small bubble with low flow resistance based on the test results. It is estimated to generate the microbubbles of 65 μm in radius under the operational condition of the Japanese Spallation Neutron Source mercury target, which is the sufficient size to suppress the pressure waves.

Status of R&D on mitigating the effects of pressure waves for the Spallation Neutron Source mercury target

The Spallation Neutron Source (SNS) at the Oak Ridge National Laboratory has been conducting R&D on mitigating the effects of pressure waves in mercury spallation targets since 2001. More precisely, cavitation damage of the target vessel caused by the short beam pulse threatens to limit its lifetime more severely than radiation damage as well as limit its ultimate power capacity – and hence its neutron intensity performance. The R&D program has moved from verification of the beam-induced damage phenomena to study of material and surface treatments for damage resistance to the current emphasis on gas injection techniques for damage mitigation. Two techniques are being worked on: injection of small dispersed gas bubbles that mitigate the pressure waves volumetrically; and protective gas walls that isolate the vessel from the damaging effects of collapsing cavitation bubbles. The latter has demonstrated good damage mitigation during in-beam testing with limited pulses, and adequate gas wall coverage at the beam entrance window has been demonstrated with the SNS mercury target flow configuration using a full scale mercury test loop. A question on the required area coverage remains which depends on results from SNS target post irradiation examination. The small gas bubble technique has been less effective during past in-beam tests but those results were with un-optimized and un-verified bubble populations. Another round of in-beam tests with small gas bubbles is planned for 2011.The first SNS target was removed from service in mid 2009 and samples were cut from two locations at the target’s beam entrance window. Through-wall damage was observed at the innermost mercury vessel wall (not a containment wall). The damage pattern suggested correlation with the local mercury flow condition which is nearly stagnant at the peak damage location. Detailed post irradiation examination of the samples is under way that will assess the erosion and measure irradiation-induced changes in mechanical properties. Similar samples were cut from the second SNS target after it was removed from service in mid 2010. More extensive damage was observed on the target inner wall but damage to the containment wall was minimal.

Summary of cavitation erosion investigations for the SNS mercury target

Intense proton beam-induced heating of the spallation neutron source mercury target will cause pressure spikes that lead to the formation of cavitation bubbles in the mercury. Erosion of the mercury container walls caused by violent collapse of bubbles could potentially limit its service lifetime. In-beam tests for a limited number of pulses (<1000 pulses for each test target) have demonstrated that pitting damage appears to be especially sensitive to beam intensity, surface treatment, and gas injection. Using results of off-line pressure pulse tests conducted for a million cycles or more to scale the results from limited in-beam tests, it is concluded that the mercury target will last at least two weeks at a time-averaged proton beam power level of 1 MW. However, because of remaining uncertainties, it is concluded that further research and development and target design efforts are needed to verify these conclusions and extend the target to higher operating powers and longer lifetimes.

04/00211 Materials research and development for the spallation neutron source mercury target: Mansur, L. K. Journal of Nuclear Materials, 2003, 318, 14–25

04/00211 Materials research and development for the spallation neutron source mercury target: Mansur, L. K. Journal of Nuclear Materials, 2003, 318, 14–25

Materials research and development for the spallation neutron source mercury target

In the Spallation Neutron Source target, the structural material will be exposed to intense pulsed fluxes of high-energy protons and neutrons, which produce radiation damage. These pulsed fluxes also lead to pressure pulses created by beam heating. In turn, the pressure pulses give rise to fluctuating stresses in the 316 LN austenitic stainless steel target vessel, and to cavitation in the liquid mercury spallation target. Corrosion reactions and related changes in mechanical properties also may occur through contact with flowing mercury. We describe the materials research and development program for the spallation target. The program covers the areas of cavitation erosion, radiation effects, and compatibility. Cavitation erosion work includes pressure wave tests at the LANSCE proton accelerator, as well as laboratory tests that simulate aspects of the actual in-beam exposures. Materials irradiations are being carried out in spallation environments at high-energy and high-power proton accelerators. Other experiments are conducted at irradiation facilities that simulate aspects of spallation conditions. Extensive radiation damage and transmutation calculations supplement these experiments. Compatibility work includes both thermal convection and pumped flow loop tests to examine temperature gradient mass transfer, as well as fatigue and tensile tests in contact with Hg. Based on the information developed for radiation effects and compatibility with mercury, our analysis indicates that the target will meet its intended service requirements. In the past year and one half the new issue of cavitation erosion has been included in the program. Both in-beam and laboratory experiments indicate that cavitation erosion may occur in the target. The highest priority activity is now to determine whether cavitation erosion will limit target lifetime to a level below the lifetime limit set by radiation effects.

Pressure and stress waves in a spallation neutron source mercury target generated by high-power proton pulses

The international ASTE collaboration has performed a first series of measurements on a spallation neutron source target at the Alternating Gradient Synchrotron (AGS) in Brookhaven. The dynamic response of a liquid mercury target hit by high-power proton pulses of about 40 ns duration has been measured by a laser Doppler technique and compared with finite elements calculations using the ABAQUS code. It is shown that the calculation can describe the experimental results for at least the time interval up to 100 μs after the pulse injection. Furthermore, it has been observed that piezoelectric pressure transducers cannot be applied in the high γ-radiation field of a spallation target.