Accelerator-driven Subcritical System Research Articles

Investigation on the Use of the Monte Carlo Iterative k-Source Scheme for the Study of Neutron Subcritical Multiplication

Monte Carlo neutron transport codes have traditionally used a fixed-source scheme to simulate a subcritical system with an external source. The efficiency of this scheme is known to depend on the subcriticality level: The lower the subcriticality is, the worse is the efficiency. We have investigated an alternate iterative scheme, namely, the Monte Carlo iterative k-source (IKS) scheme, for the study of neutron subcritical multiplication. Our results show that the iterative scheme not only is as accurate, effective, and computationally efficient as the fixed-source scheme but also has the additional advantage of being weakly dependent on the subcriticality level. Also, the efficiency of this scheme is unaffected by the change in the location of the external source, unlike the fixed-source scheme where the efficiency decreases as the source is moved away from the fissile core center. The algorithm of this scheme is very similar to the algorithm of the eigenmode iterative scheme and hence can be easily implemented in the existing Monte Carlo codes. Our work establishes the validity and accuracy of the Monte Carlo IKS scheme, and with its incorporation in the production-level codes, it can be used for the physics design and analysis of accelerator-driven subcritical systems.

Inherent Safety Characteristics of Lead Bismuth Eutectic-Cooled Accelerator Driven Subcritical Systems

An accelerator driven subcritical system (ADS) is a new nuclear energy system which could not only produce clean energy but also incinerate nuclear waste. In this paper, inherent safety analysis of an ADS is performed with neutronics and thermal-hydraulics coupled code named ARTAP. Five typical accidents are carried out, including the cases of proton beam interruption, transient overpower, reactivity insertion, loss of flow, and loss of heat sink. The transient simulations are performed in the average channel and the hottest channel of the fuel pin in the ADS core. The simulation results for beam interruption show that the highest temperature of the pellet is in the middle of the fuel element in the average channel, while the peak temperature of the cladding is in the top of the fuel element. After the beam is interrupted for 20s, the maximum temperature drops at the fuel center, the cladding inner surface, and the outlet coolant in the hottest channel are 644.46K, 162.27K, and 136.42K respectively. For transient overpower accidents with the increase of beam intensity, the maximum temperature of the fuel and the cladding are below the safety limit. Concerning the reactivity insertion accident, it is found that the ADS has good inherent safety and its margin of criticality safety is large. The calculation results for loss of flow show that the power drop is small due to low sensitivity of the subcritical core to negative reactivity feedback, and the maximum temperature of the cladding reaches 1726K, which means the fuel element would rupture. However, the power and the temperatures of fuel, cladding, and coolant could decrease quickly to the safety level after the proton accelerator is cut off under a loss of flow accident. The results also show that the peak temperature of the cladding is lower than the safety limit under a loss of heat sink accident. The present simulation results reveal that the ADS has a remarkable advantage against severe accidents. It also implies that its inherent safety characteristics could ensure reactor shutdown by cutting off the proton beam after accidents occur.

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.

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.

Status and outlook: Research and development on the neutron source for BNCT

<p indent=0mm>Boron neutron capture therapy (BNCT) is regarded as a revolutionary means for high-accuracy cancer therapy with cell level selectivity. It has unique therapeutic effects for some malignance, such as glioblastoma multiforme, melanoma, recurrent head and neck malignance. It can also be used for cancer treatment of deep organ, such as liver and lung. Even though its principle was proposed about <sc>80 years</sc> ago, it has never been utilized in routine clinic therapy in hospital until 2020. BNCT, as a binary therapeutic method, greatly depends on both high-quality neutron beam and highly target-selective boron drug. Tumor killing in cell-level accuracy can be achieved only when the two elements work together and collaborate closely. To realize such therapeutic effect, BNCT sets high requirement on both neutron source and boron drug. The research on boron drug is related with multi-disciplines, such as chemistry, biology, medicine, radiology, pharmacy and physics. This article focuses on the neutron source for BNCT. Commonly, there are two types of BNCT neutron sources: reactor-based neutron source and accelerator-based neutron source. Up to now, almost all clinic trails of BNCT therapy were performed with the former. Due to limited resources of reactor neutron source, only less than 2000 cases BNCT treatments have been carried out in the world since BNCT method was invented. Thanks for the intense beam proton accelerator development, accelerator based neutron source can provide better beam quality, and especially importantly it can be installed in a hospital environment, which is essential for wide-range application of BNCT. In accelerator-based BNCT field, China has a sound base owing to more than <sc>20 years</sc> research and development in high intensity neutron source for spallation neutron source and accelerator-driven subcritical system. It is meaningful work to transfer the related technology to accelerator-based BNCT (AB-BNCT). In this paper we will firstly introduce the demand on neutron beam specification recommended by IAEA. Then the difficulties in BNCT slow development in passing decades are reviewed. New era of BNCT is coming and one can expect a prosperous future, owing to accelerator-based BNCT. The core technology of AB-BNCT is explored and recent research achievements on BNCT research and development in China are reported. Various types of accelerators now can be used for BNCT facility, including electro-static high voltage, cyclotron and RF linac. Their pro and con are reviewed. Majorly two kinds of neutron generation targets are utilized for BNCT, and the characteristics comparison of these two are analyzed. On the newly constructed BNCT research platform in China, new boron drug research and development work are conducted in many institutes, universities and pharmacy companies. And in recent, a new AB-BNCT facility is under construction for clinic trails in a hospital. So we can have an optimistic viewpoint for BNCT future in China.

Design and beam dynamic studies of a 30-MW superconducting linac for an accelerator-driven subcritical system

The Japan Atomic Energy Agency (JAEA) is working on the research and development of a 30-MW continuous wave (CW) proton linear accelerator (linac) for the JAEA accelerator-driven subcritical system (ADS) proposal. The linac will accelerate a 20 mA proton beam to 1.5 GeV, using mainly superconducting cavities. The main challenge for an ADS accelerator is the high reliability required to prevent thermal stress in the subcritical reactor; thus, we pursue a robust lattice to achieve stable operation. To this end, the beam optics design reduces the emittance growth and the beam halo through the superconducting part of the linac. First, we simulated an ideal machine without any errors to establish the operation conditions of the beam. Second, we applied element errors and input beam errors to estimate the tolerance of the linac design. Finally, we implemented a correction scheme to increase the lattice tolerance by reducing the beam centroid offset on the transverse plane. Massive multiparticle simulations and a cumulative statistic of $1\ifmmode\times\else\texttimes\fi{}{10}^{8}$ macroparticles have shown that the JAEA-ADS linac can operate with less than $1\text{ }\text{ }\mathrm{W}/\mathrm{m}$ beam losses in error scenarios.

Development of code IMPC-MC1.0 by implementing the function for proton transport simulation in OpenMC

For the target-reactor coupling calculation in Accelerator Driven Sub-critical System (ADS). We developed the proton-neutron transport code IMPC-MC1.0by embedding the Intra-Nuclear Cascade model of Liège (INCL++5.1) and abrasion-ABLAtion model (ABLA) fission-evaporation de-excitation model into the OpenMC (v0.12.0). To verify the code, the Brookhaven National Laboratory - Alternating Gradient Synchrotron (BNL-AGS) experimental benchmark was investigated. And the IMPC-MC1.0 results for neutron yield of the Pb and W target are in perfect agreement with those of measured values. Moreover, the simulated neutron leakage spectra are consistent with MCNPX and FLUKA calculations. Then the ADS target models filled with different materials were simulated, the results of neutron yield, energy spectrum are reasonable when compared with LAHET calculation. Finally, the homogenized Japan Atomic Energy Agency (JAEA) 800 MW lead-bismuth cooled target-reactor model was verified for ADS application. The related results are reliable when compared with the those calculated by MCNPX.

Comparative Safety Analysis of Accelerator Driven Subcritical Systems and Critical Nuclear Energy Systems

The accelerator driven subcritical system (ADS) has been chosen as one of the best candidates for Generation IV nuclear energy systems which could not only produce clean energy but also incinerate nuclear waste. The transient characteristics and operation principles of ADS are significantly different from those of the critical nuclear energy system (CNES). In this work, the safety characteristics of ADS are analyzed and compared with CNES by a developed neutronics and thermal-hydraulics coupled code named ARTAP. Three typical accidents are carried out in both ADS and CNES, including reactivity insertion, loss of flow, and loss of heat sink. The comparison results show that the power and the temperatures of fuel, cladding, and coolant of the CNES reactor are much higher than those of the ADS reactor during the reactivity insertion accident, which means ADS has a better safety advantage than CNES. However, due to the subcriticality of the ADS core and its low sensitivity to negative reactivity feedback, the simulation results indicate that the inherent safety characteristics of CNES are better than those of ADS under loss of flow accident, and the protection system of ADS would be quickly activated to achieve an emergency shutdown after the accident occurs. For the loss of heat sink, it is found that the peak temperatures of the cladding in the ADS and CNES reactors are lower than the safety limit, which imply these two reactors have good safety performance against loss of heat sink accidents.

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.

Detection and suppression of the trapped-electrons-transportation-type flashover in a linear accelerator

The polarization energy relaxation around trapped electrons type of flashover happened in the Superconducting Demo Linac of the China Initiative Accelerator Driven Subcritical System project, which greatly limited the acceleration capability of the accelerator. This problem seriously affects the stability of accelerator operation and becoming the most urgent issue to be solved. From the reticulated and radial traces through the inner and outer conductors on the electrical aging windows of the RF couplers, the surface discharge or flashover was inferred from the pick-up couplers. The simulation of field-emission electron trajectory in the superconducting cavity and the online monitoring of the flashover signal showed that the discharge occurred by the bombardment and accumulation of field-emission electrons from the cavity. Therefore, the antenna of the pick-up coupler was redesigned and applied to the Linac. The new design removed the abnormal signal caused by discharges during more than one year operation of the cavity, thereby improving the acceleration capability and stability performance of the accelerator.

Physics design of the CiADS MEBT

The superconducting linac for China initiative Accelerator Driven Subcritical System (CiADS) is the world-leading ADS-driver under construction with state-of-the-art technologies. This system is designed to accelerate a 5 mA proton beam to 500 MeV energy, and then delivering 2.5 MW of beam power to the neutron production target. The Middle Energy Beam Transport (MEBT) is designed with great emphasis on smoothing matching between the upstream and downstream acceleration sections, beam diagnostics layout and beam quality control. It is proposed and successfully implemented to immigrate the phase space distortion and reduce the increase of beam emittance. The acceptance of MEBT is optimized to make all the particles inside the effective acceptance of the superconducting section even with machine errors. This pertinent design of MEBT is suitable for the high-power superconducting accelerator of CiADS.

Structure Design and Analysis of Canted-Cosine-Theta (CCT) Superconducting Quadrupole Magnet

A superconducting quadrupole magnet based on the Canted-Cosine-Theta (CCT) type coil with a gradient field of 40 T/m and a bore diameter of 60 mm has been designed for the preresearch of two projects of high intensity accelerator facility (HIAF) and accelerator driven subcritical system (ADS). The magnet is comprised of two-layer coils embedded in the formers and the end plates for locating. The coil formers made of aluminum alloy are machined with grooves according to the drive equations of CCT for placing the wires. The existence of ribs between two adjacent wires can avoid accumulation of the electromagnetic force. It is important to take the mechanical design for the complex structure to avoid tensile stresses on the conductor and confine the stresses within a reasonable value. The stress analysis for the quadrupole magnet has been carried out considering the thermal shrinking due to cool down as well as the electromagnetic force on the coil. This paper reports the detailed stress analysis for the CCT quadrupole magnet structure, discusses the calculating results, and gives a reasonable mechanical design.

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.

ADS design concept for disposing of the U.S. spent nuclear fuel inventory

The United States inventory of the spent nuclear fuel (SNF) is ~80,000 metric tons of heavy metal (MTHM) at the end of 2018, which contains ~131 tons of minor actinides (MAs) and ~669 tons of plutonium. This paper describes a conceptual design of an accelerator-driven subcritical (ADS) system for disposing of this SNF inventory by utilizing the 131 tons of MAs inventory and a fraction of the plutonium inventory for energy production and transmuting some long-lived fission products.An ADS system with a homogeneous subcritical fission blanket was first examined to define the main system parameters. A spallation neutron source is used to drive the fission blanket and it is generated from the interaction of a 1-GeV proton beam with a lead–bismuth eutectic (LBE) target. The fission blanket has a liquid mobile fuel using LBE as the fuel carrier. The fuel materials are dissolved, mixed, or suspended in the liquid fuel carrier. Monte Carlo simulations were performed for three fission blankets to determine the main concept parameters. The loaded amount of actinide materials in the LBE carrier is either 5, 7, or 10% of the total volume of the fission blanket, respectively. The plutonium fraction in the MAs of each fission blanket was selected so that the neutron multiplication factors of the three fission blankets are ~0.980. In addition, Monte Carlo burnup simulations using the MCB5 computer program were performed to analyze the performance of the three conceptual ADS systems. During operation, fresh fuel is fed into the fission blankets to adjust its subcriticality and to control the system power. The burnup results show that the total MA materials consumed are about 30.6, 35.3, and 37.2 tons, after operating the three fission blankets for 35 full-power years with 25-MW 1-GeV proton beam. Thus, the corresponding numbers of ADS systems to utilize the 131 tons of MA materials of the SNF inventory are 4.3, 3.7, or 3.5, respectively.ADS systems with tube bundles inserted in the fission blanket were analyzed to overcome the disadvantages of the homogeneous fission blanket concept. The liquid lead is used as the target material, the mobile fuel carrier, and the primary coolant to avoid the polonium production from bismuth. Physics and thermal–hydraulic analyses were coupled to determine the parameters of the heterogeneous fission blanket. The engineering requirements for a satisfactory performance of the HT-9 ferritic steel structure material have been realized. Two heterogeneous fission blanket concepts with the liquid lead mobile fuel inside or outside the tube bundles were considered. The heterogeneous fission blanket with the mobile fuel inside the tubes shows better performance than the fission blanket with mobile fuel outside the bundle tubes and it is presented in this paper. MCB5 and Serpent, the Monte Carlo burnup computer programs were both used to simulate the fuel burnup in the ADS concept with the mobile fuels inside the tubes for 35 full power years. The results show that five ADS systems can dispose of the total United States inventory of the spent nuclear fuel.

Coupled multi-physics simulation for the evaluation of an accelerator-driven Aqueous Homogeneous Subcritical System for medical isotope production

The most common medical isotope (99mTc) is widely used for the diagnosis and therapy of complex diseases, with approximately 30–40 million nuclear medicine procedures worldwide every year. Although 99mTc can be directly produced in particle accelerators, it is generally obtained from the beta-decay of 99Mo. The utilization of an accelerator-driven Aqueous Homogeneous Subcritical System to produce 99Mo has several advantages and therefore, represents an attractive alternative to the conventional method of irradiating uranium solid targets in nuclear research reactors. This paper aims to perform a multi-physics evaluation of an Aqueous Homogeneous Subcritical System conceptual design with low enriched uranium (LEU), driven by a D-T neutron generator, to produce 99Mo in safe conditions. A coupled iterative calculation methodology was implemented to predict the steady state operation of the system at 20 kWth using the MCNP6 and ANSYS-CFX 18.1 codes for neutronic and thermal-hydraulic calculations, respectively. The main neutronic and thermal-hydraulic parameters were monitored to verify if they met the established convergence criterion of 1% to stop the calculation process. The methodology convergence was reached after four calculation steps when the most changing parameter, average fuel solution velocity, satisfied the criterion. However, others such as the average bubble velocity, the ion beam current, the fuel solution height and the effective multiplication factor varied less than 0.1% in the fourth step. A burnup calculation has shown that the system is capable of producing 114.3 six-days Ci of 99Mo in a one-week operation period and significant amounts 133Xe, 131I and 89Sr.

Study of the Effects of Moderators on ADS System Performance Based on UN-ThO2 Fuel

The neutron spectrum is an essential factor in making possible the increase of 233U isotope breeding from thorium fuel in an accelerator-driven subcritical (ADS) system; therefore, studying the effects of various moderators and coolants on 233U breeding is an important step in ADS performance. This study aims to evaluate the effect of using different moderators and coolants on the ADS system characteristics. Sodium, which was the most common coolant used in ADS reactors, was replaced by light water (LW) and graphite + CO2, separately. In this study, we used uranium nitride as the seed fuel associated with ThO2 as the blanket fuel for all cases. The Monte Carlo transport code MCNPX 2.7.0 was used to calculate neutronic parameters such as effective multiplication factor (Keff), power peaking factor (Pmax/Pav) in the radial direction of the ADS reactor core, actinide isotope evolution during fuel burnup, and power fraction from each fuel type for all cases. The results show that the utilization of graphite as the moderator with CO2 as the coolant allows more 233U production in thorium fuel compared with sodium and LW. On the other hand, LW showed great ability for plutonium and minor actinide transmutation and for energy generation.

A non-intercepting monitoring method for beam position on the target in an accelerator driven system

In an accelerator driven sub-critical system (ADS), an intense external neutron source provided by an accelerator coupled to a target is used to drive a sub-critical reactor. To ensure the stable operation of the reactor, it is of crucial importance to locate the beam spot on the target during the operation. In this paper, we have proposed a method for beam position measurement on the target’s beam entrance window. The proposed method is to measure the neutron flux outside the spallation target in the four directions of the east, south, west, and north. Since the neutron flux decreases with the distance between the detector and the beam center, the position of the beam center on the target’s entrance window can be extracted from the measured four neutron fluxes. The simulations based on FLUKA and OpenMC codes have indicated that the x and y positions of the beam center can be extracted from the measured four neutron fluxes. Since the four neutron detectors are fixed inside the reactor core, this method allows for continuous measurement of the beam position.

650 MHz elliptical superconducting RF cavities for CiADS Project

650 MHz multicell superconducting radio frequency (SRF) elliptical cavities are proposed for effective acceleration of proton beam in the Chinese initiative Accelerator Driven Subcritical System (CiADS). Two families of such cavities will be used in the driver superconducting linac to accelerate the H+ beam from 175 MeV to 500 MeV, with the possibility to upgrade the energy to 1 GeV and higher. Detail designs of a six-cell βopt = 0.62 elliptical cavity and a five-cell βopt = 0.82 elliptical cavity will be presented in this paper, including the multi-parameter electromagnetic design and optimization, high order modes analyses, multipacting simulations, mechanical and engineering analyses. Sub-systems of the cavity such as the fundamental power coupler antenna tip design and mechanical design of cavity-helium vessel-tuner system will also be discussed in detail. Three single cell prototype cavities were fabricated and vertical tested to verify the electromagnetic properties, the fabrication and post-processing technologies. Four multi-cell prototype cavities are under manufacturing and expected to be ready in 2021.

Analysis of integrated target in thorium‐based molten salt fast energy amplifier

Conventional neutron targets in accelerator driven subcritical system (ADS) are based on the design using heavy metal like lead and lead-bismuth eutectic (LBE), which generally needs a separated loop for target cooling. In the thorium-based molten salt fast energy amplifier, an integrated target is conceived that the proton beam is directly placed in the chloride/fluoride molten salt core without a separated cooling loop for the target in consideration of the neutron productivity of the liquid molten salt fuel bombarded with the proton. The physical analysis of the spallation target is presented, including the spallation neutron yield, neutron spectrum, spallation products, and energy deposition. Chloride molten salt is a reasonable choice among four kinds of simulated target based on considering about the neutron yield, neutron spectrum, spallation products and energy deposition. The simulation code is the Monte Carlo code MCNP6.1. Besides, evaluation of the target size is presented, and two kinds of the beam tube layout are compared by considering the neutron economy in the system.

Core power control of the ADS based on genetic algorithm tuning PID controller

The accelerator driven subcritical system (ADS) reactor, an accelerator driven transmutation research facility relies on control rod and external neutron source to realize core power regulation. This paper researches the dynamic and control characteristics of the ADS core under the external neutron source disturbance. The state space model of the ADS core with multi-input multi-output is established by linearizing the core nonlinear model with small disturbance. The response characteristics of the ADS core power control under external neutron source strength regulation are simulated by using genetic algorithm tuning proportional-integral-derivative (PID) controller and traditional PID controller. The results show that under different external neutron source relative strength, the effect of genetic algorithm tuning PID controller is better than that of traditional PID controller. The ADS core power can be well controlled by using genetic algorithm tuning PID controller.