Spallation Target 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.

Imaging and reconstruction of high-energy proton beam spot before target based on secondary gamma rays.

The distribution and stability of the incident proton beam spot are critical for the stable operation of a high-power spallation target. This study proposes a method to capture images of the incident proton beam spot based on secondary gamma rays. The distribution of the backward secondary gamma rays irradiated by the primary proton beam on the incident surface of the target is close to the distribution of the protons and can be measured at a low-radiation position far from the target area. A relation of distributions between the incident protons and the secondary gammas is constructed by using the point response function of this pinhole imaging system. The proposed method of imaging is suitable for monitoring the distribution of the proton beam on the target in facilities that use a beam power of several megawatts or tens of megawatts, such as spallation neutron sources or accelerator-driven subcritical systems.

Research on proton beam spot imaging based on pixelated gamma detector

The secondary particles from the spallation target of the China Spallation Neutron Source are mainly gammas and neutrons, which are related to the incident proton. The reconstruction of the proton beam spot could be implemented based on the distribution of the positions of secondary gammas or neutrons. The methods of pinhole imaging and Compton imaging are developed by measuring the position distribution of gammas based on the pixelated detector. The secondary gammas could be detected by the pixelated gamma detector directly. The neutron can be identified by detecting the characteristic (478 keV) γ-rays from the 10B(n, α) reactions. In order to detect secondary neutrons, a layer of 10B converter is added before the pixelated gamma detector. The pixelated gamma detector is sensitive to the characteristic (478 keV) γ-rays and then the neutron imaging could be achieved based on measuring the position distribution of the characteristic (478 keV) γ-rays.

Beam-target configurations and robustness performance of the tungsten granular flow spallation target for an Accelerator-Driven Sub-critical system

The dense granular flow spallation target is a new target concept proposed for an Accelerator-Driven Sub-critical (ADS) system. In this paper, the beam-target configurations of a tungsten granular flow target for the ADS with a thermal power of 1 GW is explored. The beam profile options using different scanning methods are discussed. The critical geometry parameters are adjusted to investigate the performance of the granular target from the aspects of neutron efficiency, stability and temperature distribution in target medium. To figure out how the target under accident conditions would behave, different clogging conditions are induced in the simulation. The dynamic processes are analyzed and some important parameters such as abnormal temperature rise and beam cutoff time window are obtained. The response of the sub-critical reactor to a clogging accident is also investigated. It is indicated that the monitoring of the granular flow by the neutron detectors in the sub-critical core will be effective.

Experimental demonstration of the short bunch extraction by bunch rotation in a high-intensity rapid cycling proton synchrotron

Short bunch proton beams are of great significance for the applications of white neutron beams and muon beams. The accelerator complex of China Spallation Neutron Source (CSNS) was designed to support the applications mainly based on neutron scattering techniques where the proton pulse length is not very sensitive. Some theoretical and experimental studies have been performed to see if one can extract a short-bunch proton beam by bunch rotation from the rapid cycling synchrotron (RCS) at CSNS. The experimental results at RCS have evidently displayed the bunch lengthening and rotation process, which demonstrates the effectiveness of this method even with a very short available time for the RF gymnastic processes and a high-intensity beam. With a beam power of 50 kW and normal longitudinal emittance at the injection, the proton beam with a bunch length of about 53% with respect to the one in the normal operation mode was obtained and transported to the spallation target. With a reduced longitudinal emittance at injection and the beam power of 30 kW, the shortest extraction bunch length obtained is about 26% of the one in the normal operation mode. Different machine settings have also been tested to show the impact of the desynchronization between the RF and magnetic fields, the influence of the non-adiabatic risetime and the adiabatic decay time of the RF voltage on the extraction bunch length. The experimental results are well consistent with the theoretical and simulated ones. It is interesting to observe that space charge has a beneficial effect on the bunch lengthening which will result in a shorter bunch at the extraction with the later bunch rotation. The controlled desynchronization method between the RF and magnetic fields in an RCS was also proven successful.

Development and Validation of Analysis Code for Spallation Products Behavior in Lbe Coolant System of Ads Comparing with the Distribution Data in Megapie Spallation Target

Development and Validation of Analysis Code for Spallation Products Behavior in Lbe Coolant System of Ads Comparing with the Distribution Data in Megapie Spallation Target

Microstructural investigation and identification of intermetallic σ-phase in solution annealed 316L-type austenitic stainless steel

MYRRHA (Multi-purpose hYbrid Research Reactor for High-tech Applications) is an accelerator driven system, currently under development at SCK CEN in Mol, Belgium. This nuclear system will use liquid Lead-Bismuth eutectic alloy as a spallation target for fast neutron production and as coolant. The ideal structural material for a liquid metal cooled reactor should be unsusceptible to both liquid metal embrittlement and liquid metal corrosion, while possessing high toughness. Nuclear grade austenitic stainless steels similar to AISI 316L have therefore been chosen as the main candidate structural materials for MYRRHA. In the framework of the qualification of those candidates, a specific batch of this material has shown unexpectedly poor mechanical properties, which triggered the need of in-depth microstructural analysis. The behaviour was attributed to the unexpected and undesired presence of intermetallic σ-phase. The σ-phase was identified with a high confidence by combining the data for chemical composition from energy-dispersive X-ray spectroscopy and crystallographic information from electron backscatter diffraction by comparing simulated Kikuchi diffraction patterns with experimentally recorded ones. At first sight the optical appearance of σ-phase resembles δ-ferrite islands, which results in the risk of overlooking this when only classical material qualification methods are used. When left undetected, testing this material including the brittle σ-phase in a liquid metal environment, in combination with miniature mechanical test specimens, could lead to misinterpretation of embrittlement of the austenitic matrix.

Power control of CiADS core with the intensity of the proton beam

This paper reports the control method for the core power of the China initiative Accelerator Driven System (CiADS) facility. In the CiADS facility, an intense external neutron source provided by a proton accelerator coupled to a spallation target is used to drive a sub-critical reactor. Without any control rod inside the sub-critical reactor, the core power is controlled by adjusting the proton beam intensity. In order to continuously change the beam intensity, an adjustable aperture is considered to be used at the Low Energy Beam Transport (LEBT) line of the accelerator. The aperture size is adjusted based on the Proportional Integral Derivative (PID) controllers, by comparing either the setting beam intensity or the setting core power with the measured value. To evaluate the proposed control method, a CiADS core model is built based on the point reactor kinetics model with six delayed neutron groups. The simulations based on the CiADS core model have indicated that the core power can be controlled stably by adjusting the aperture size. The response time in the adjustment of the core power depends mainly on the adjustment time of the beam intensity.

A Thermo-Hydraulic Investigation of the Unprecedented TMRS Upper Neutron Spallation Target

Abstract The next-generation neutron spallation target station, the Target–Moderator-Reflector System (TMRS) Mk. IV, will be installed in 2021. This iteration features an unprecedented, water-cooled, third internal target aptly named the upper target. With the upper target designed completely by analysis, a complementary empirical investigation was undertaken to ascertain target conformance to those computational results which deemed the cooling efficacious. Three facets of the target were designated for verification: displacement under hydraulic load, critical fluid velocities, and the characteristic heat transfer coefficient (HTC). With the potential for flow maldistribution under excessive displacements, static pressure testing was performed. Discrepancies of an order of magnitude became evident between empirical and simulated displacements, 1.499 mm versus 0.203 mm, respectively. A closed-water flow loop reproducing the flow parameters intrinsic to the TMRS Mk. IV was constructed. Utilizing particle image velocimetry, global fluid dynamics were observed to be analogous to computer simulation. Furthermore, crucial velocities such as those at the point of beam impingement were met or exceeded, thus satisfying cooling requirements by a preponderance. A graphite susceptor mirroring nominal beam geometry was coupled to a solenoid coil to replicate a prodigious peak heat flux of 169 W/cm2 via induction heating. Matching peak heat flux within 3% engendered a HTC of 80% that of simulation. Consistent with analysis, the local HTC sufficiently mitigated nucleate/flow boiling. In summary, the analytically derived upper target design empirically demonstrated sufficient cooling despite quixotic beam conditions and unforeseen displacements.

Laser Heating Study of the High-Temperature Interactions in Nanograined Uranium Carbides.

Nanograined nuclear materials are expected to have a better performance as spallation targets and nuclear fuels than conventional materials, but many basic properties of these materials are still unknown. The present work aims to contribute to their better understanding by studying the effect of grain size on the melting and solid–solid transitions of nanograined UC2−y. We laser-heated 4 nm–10 nm grain size samples with UC2−y as the main phase (but containing graphite and UO2 as impurities) under inert gas to temperatures above 3000 K, and their behavior was studied by thermal radiance spectroscopy. The UC2−y solidification point (2713(30) K) and α-UC2 to β-UC2 solid–solid transition temperature (2038(10) K) were observed to remain unchanged when compared to bulk crystalline materials with micrometer grain sizes. After melting, the composite grain size persisted at the nanoscale, from around 10 nm to 20 nm, pointing to an effective role of carbon in preventing the rapid diffusion of uranium and grain growth.

Design of the third-generation lead-based neutron spallation target for the neutron time-of-flight facility at CERN

The neutron time-of-flight (n_TOF) facility at the European Laboratory for Particle Physics (CERN) is a pulsed white-spectrum neutron spallation source producing neutrons for two experimental areas: the Experimental Area 1 (EAR1), located 185 m horizontally from the target, and the Experimental Area 2 (EAR2), located 20 m above the target. The target, based on pure lead, is impacted by a high-intensity 20-GeV/c pulsed proton beam. The facility was conceived to study neutron-nucleus interactions for neutron kinetic energies between a few meV to several GeV, with applications of interest for nuclear astrophysics, nuclear technology, and medical research. After the second-generation target reached the end of its lifetime, the facility underwent a major upgrade during CERN's Long Shutdown 2 (LS2, 2019-2021), which included the installation of the new third-generation neutron target. The first and second-generation targets were based on water-cooled massive lead blocks and were designed focusing on EAR1, since EAR2 was built later. The new target is cooled by nitrogen gas to avoid erosion-corrosion and contamination of cooling water with radioactive lead spallation products. Moreover, the new design is optimized also for the vertical flight path and EAR2. This paper presents an overview of the target design focused on both physics and thermo-mechanical performance, and includes a description of the nitrogen cooling circuit and radiation protection studies.

The High Intensity Proton Accelerator Facility

The High Intensity Proton Accelerator Facility at PSI routinely produces a proton beam with up to 1.4 MW power at a kinetic energy of 590 MeV. The beam is used to generate neutrons in spallation targets, and pions in meson production targets. The pions decay into muons and neutrinos. Pions and muons are used for condensed matter and particle physics research at the intensity frontier. This section presents the main physics and technology concepts utilized in the facility. It includes beam dynamics and the control of beam losses and activation, power conversion, efficiency aspects, and performance figures, including the availability of the facility.

Study on collective friction and wear behavior of W-Ni-Fe alloy balls

Tungsten alloy balls are considered as one of the important candidate spallation targets for China initiative Accelerator Driven Subcritical System. The wear of these balls in the spallation target loop greatly affects their service life. In this work, the collective friction and wear of the balls at 250 °C were studied by using our self-developed equipment. The results reveal that the mass loss ratio of balls first rises and then falls, experiencing different stages, and its maximum is about 0.32 wt%. For each stage balls have their own unique wear characteristics. In the end a new layer with a structure of amorphous surrounding nanocrystallines is formed on the ball surface. The role of wear debris in the all process is discussed.

Third-Generation CERN n_TOF Spallation Target: Final Design and Examinations of Irradiated Prototype

Third-Generation CERN n_TOF Spallation Target: Final Design and Examinations of Irradiated Prototype

Monte-Carlo calculations of the energy resolution function with Geant4 for analyzing the neutron capture cross section of 232Th measured at CSNS Back-n

The Back-n facility at the China Spallation Neutron Source (CSNS Back-n facility) is a newly-built time-of-flight (TOF) spectrometer for neutron nuclear data measurements. The Energy Resolution Function (ERF) of a neutron spectrometer is essential for the analysis of the data measured on the facility, especially in the resolved energy region, as the ERF represents the intrinsic broadening effect of the measured resonance peaks. For the constructing of the ERF, the most crucial component need to be investigated is the neutron production and transport in the spallation target, which could only be inferred from detailed Monte-Carlo simulations. In the present work, the simulation is performed with the Monte-Carlo toolkit Geant4 on account of its flexible features of particle tracking and parameter recording. The production and transport of neutrons in the target–moderator–reflector (TMR) assembly of the Back-n facility are simulated, and the simulation data are analyzed to deduce the distributions of the time spent of neutrons’ transport in the TMR assembly. Based on the distributions of neutrons with different energies from eV to MeV, the ERF of the Back-n facility is constructed by adapting the RPI resolution function-form which is extensively used in neutron nuclear data evaluation. The energy dependence of the ERF is also parameterized to ensure its applicability for the nuclear data evaluation in the resolved energy region. Furthermore, the standard neutron capture cross section of 197Au is taken to verify the accuracy of the ERF by comparing the measured data and the theoretical ones from the ENDF/B-VIII.0 library.

A holistic approach to the optimization of neutron beam transport at the LANSCE facility

An advanced neutron beam collimation system has been developed as part of an effort to optimize neutron beam transport at the Weapons Neutron Research (WNR) Facility [1], located within the Los Alamos Neutron Science Center (LANSCE). The goal of this work was to develop and demonstrate techniques to tailor neutron delivery in order to provide maximum available flux on sample with a specific beam profile, while simultaneously reducing unwanted background. A holistic approach was taken, upgrading the facility spallation target, facility metrology infrastructure, and flight path shutter insert to support the implementation of this advanced collimation system. Modern instruments and software were employed to conduct facility surveys, characterization of as-built geometry of critical components, 3D ray tracing and neutron transport calculations. Beam images and flux measurements were acquired after installation to evaluate the performance of the collimation system and to demonstrate consistent agreement with MCNP simulations which showed an 87% increase of flux on sample while providing suppression of background neutron impingement on the sample frame by 103.

Preliminary study on spallation target characteristics in an Accelerator-Driven system

This study aims to investigate the neutronic characteristics of spallation targets for Accelerator-Driven subcritical System (ADS) and find the optimal target design for reducing the strength of the required beam current. All the calculations were conducted using the MCNP6.1 with cross-section library ENDF/B-VII. In this study, the influence of several parameters on spallation targets is investigated, such as neutron production with various spallation target layouts, spallation neutron distribution with different proton beam energy levels, and spallation neutron spectrum.

GEANT4 characterization of the neutronic behavior of the active zone of the MEGAPIE spallation target

The increasing interest that GEANT4 is gaining nowadays, because of its special capabilities, prompted us to address its reliability in neutronic calculation for the realistic and complex spallation target MEGAPIE of the Paul Scherrer Institute of Switzerland. In this paper we have specifically addressed the neutronic characterization of the active zone of this target. Three physical quantities are evaluated: neutron flux spectra and total neutron fluxes on target's z-axis, and the neutron yield as a function of the target's altitude and radius. Comparison of the obtained results with those of the MCNPX reference code and some experimental measurements have confirmed the impact of the geometrical and proton beam models on the neutron fluxes. It has also allowed to reveal the intrinsic influence of the code type. The resulting differences reach a factor of ~2 for the beam model and 4–18% for the other parameters cumulated. The analysis of the neutron yield has led us to conclude that: 1) Increasing the productivity of the MEGAPIE target cannot be achieved simply by increasing the thickness of the target, if the irradiation parameters are not modified. 2) The size of the spallation area needs to be redefined more precisely.

Initial years’ neutron-induced cross-section measurements at the CSNS Back-n white neutron source * *Supported by the National Key Research and Development Plan (2016YFA0401600) and the National Natural Science Foundation of China (11675155, 11790321)

The Back-n white neutron source (known as Back-n) is based on back-streaming neutrons from the spallation target at the China Spallation Neutron Source (CSNS). With its excellent beam properties, e.g., a neutron flux of approximately 1.8×107 n/cm2/s at 55 m from the spallation target, energy range spanning from 0.5 eV to 200 MeV, and time-of-flight resolution of a few per thousand, along with the equipped physical spectrometers, Back-n is considered to be among the best facilities in the world for carrying out nuclear data measurements. Since its completion and commencement of operation in May 2018, five types of cross-section measurements concerning neutron capture cross-sections, fission cross-sections, total cross-sections, light charged particle emissions, in-beam gamma spectra, and more than forty nuclides have been measured. This article presents an overview of the experimental setup and result analysis on the neutron-induced cross-section measurements and gamma spectroscopy at Back-n in the initial years.

Feasibility of nuclide-identified imaging based on the back-streaming white neutron beam at the China Spallation Neutron Source.

Back-streaming neutrons through the incoming proton channel at the spallation target station of the China Spallation Neutron Source have been studied as a white neutron source (Back-n). We report a physical study on white neutron imaging based on the Back-n beamline. The wide neutron energy spectrum spanning from 1eV to 100MeV was very suitable for nuclide-identified imaging as well as measurements of nuclear data and other applications. We calculated the back-streaming white neutron energy spectrum using the Monte Carlo N-Particle code. A comparison of the results of calculation with those of the FLUktuierende KAskade code and experimental measurements were carried out. The energy resolution of the Back-n beam was close to 5% depending on the neutron energy and the modes of operation, and its spatial resolution could attain the order of tens of micrometers.