Shielding Problems Research Articles

Built-in physics models and proton-induced nuclear data validation using MCNP, PHITS, and FLUKA – Impact on the shielding design for proton accelerator facilities

The MCNP, PHITS, and FLUKA are general-purpose Monte Carlo (MC) radiation transport codes that are widely used for many real-world shielding problems at accelerator facilities around the world. Nuclear interactions are described in these codes by either built-in physics models, or tables with evaluated cross sections and secondary energy-angular distributions, or a combination of both. Over the decades, many code validation efforts have been made, owing to the availability of shielding benchmarks to test the physics models and nuclear data and to verify the accuracy of simulation codes.For high beam energy and high beam current accelerator applications, neutron emission through the vacuum pipe along the reverse direction of incident proton beam is an important factor for a shielding design in order to correctly assess the dose rates for workers and the structural materials of the accelerator and handle with the waste activated by the backscattered neutron fluxes. In this work, neutron-production cross sections and thick-target yield predictions from MC codes relying on physics models and nuclear data libraries are benchmarked against the experimental data, in order to assess their accuracy in predicting neutron emission and furthermore to assess the corresponding impact on shielding design.The results of this study demonstrate that the nuclear data libraries and physics models, which are not expected to give good results at lower energies (< 150 MeV) but are used anyhow when there is no nuclear data available or above the energy range where the data tables end in the so-called “mix-and-match” strategy, need further improvements. Among the investigated proton induced nuclear data libraries, JENDL-4.0/HE produces the most satisfactory agreement to experimental data for all target materials, but may still benefit from refinement. Concerning the physics models of the codes, FLUKA V4-4.0 has the best performance in terms of reproducibility of the experimental values. It is also shown that all discrepancies between the calculations and the experiments for the energy range < 10 MeV (which is dominant on the dose rates through the shield thickness), are up to factor of two. This might be considered as an acceptable figure as it is equivalent to a normal safety margin (x2) considered in shielding calculations of accelerator facilities around the world.

A numerical investigation for the development of functionally graded Ti/HA tibial implant for total ankle replacement: Influence of material gradation law and volume fraction index.

Stress shielding is one of the major concerns for total ankle replacement implants nowadays, because it is responsible for implant-induced bone resorption. The bone resorption contributes to the aseptic loosening and failure of ankle implants in later stages. To reduce the stress shielding, improvements can be made in the implant material by decreasing the elastic mismatch between the implant and the tibia bone. This study proposes a new functionally graded material (FGM) based tibial implant for minimizing the problem of stress shielding. Three-dimensional finite element (FE) models of the intact tibia and the implanted tibiae were created to study the influence of material gradation law and volume fraction index on stress shielding and implant-bone micromotion. Different implant materials were considered that is, cobalt-chromium, titanium (Ti), and FGM with Ti at the bottom and hydroxyapatite (HA) at the top. The FE models of FGM implants were generated by using different volume fractions and the rule of mixtures. The rule of mixtures was used to calculate the FGM properties based on the local volume fraction. The volume fraction was defined by using exponential, power, and sigmoid laws. For the power and sigmoid law varying volume fraction indices (0.1, 0.2, 0.5, 1, 2, and 5) were considered. The geometry resembling STAR® ankle system tibial implant was considered for the present study. The results indicate that FGMs lower stress shielding but also marginally increase implant-bone micromotion; however, the values were within the acceptable limit for bone ingrowth. It is observed that the material gradation law and volume fraction index influence the performance of FGM tibial implants. The tibial implant composed of FGM using power law with a volume fraction index of 0.1 was the preferred option because it showed the least stress shielding.

The CADIS-DXTRAN method in NECP-MCX for Radiation Shielding Simulations

This paper discusses the implementation of the Consistent Adjoint-Driven Importance Sampling-Deterministic Transport (CADISDXTRAN) method in NECP-MCX to speed up Monte Carlo simulations for neutron radiation shielding applications. It provides a brief theoretical overview of the CADIS-DXTRAN method and describes the coding process in NECP-MCX. The study compares the performance of the CADIS-DXTRAN method with the CADIS method and the DXTRAN method in a labyrinth problem and a channel shielding problem. The results of the labyrinth problem reveal that the CADIS-DXTRAN method is highly efficient with a Figure of Merit (FOM) value 45 times that of DXTRAN and 17 times that of the CADIS method. While there is no significant improvement of the CADIS-DXTRAN method in the channel shielding problem. The particle behavior in the two problems is analyzed in detail to explain the performance of the CADIS-DXTRAN method.

Estimation of weight window parameters based on recursive Monte Carlo approach for reactor shielding problems

Weight window, a variance reduction tool, is often used to improve the performance of radiation shielding calculations. One common issue with the weight window is determining the optimal set of weight window parameters for solving deep penetration shielding problems. To address this issue, the recursive Monte-Carlo methodology has been used with the in-house TRAWEI code. The program is responsible for generating the optimal weight parameters in a single run with minimum computational time. This paper presents the results of a numerical test conducted using a simple reactor model to evaluate the performance of the developed weight generator program. The findings reveal that MCNP simulations utilizing TRAWEI-generated weight values exhibit significantly higher calculation efficiency compared to both analog simulation and MCNP simulation using weights generated by the existing MCNP weight window generator. Overall, the utilization of the RMC methodology has shown its potential to significantly contribute to deep penetration shielding calculations.

Obtaining Boron Carbide and Nitride Matrix Nanocomposites for Neutron-Shielding and Therapy Applications

The very high capture cross-section of (epi)thermal neutrons by the boron isotope 10B makes elemental boron and its compounds and composites prospective for serving as materials intensively interacting with neutron irradiation. In their nanostructured form, boron-rich materials reveal properties that improve their radiation-performance characteristics. In this regard, new technologies have been proposed for the synthesis of nanocomposites with matrices of boron carbide B4C and hexagonal boron nitride h-BN. For the first time, boron carbide-tungsten and hexagonal boron nitride–(iron,magnetite) composites were obtained, respectively, in the form of layered/sandwich structures of components B4C and W and h-BN nanopowders coated/intercalated with magnetic nanoclusters of iron Fe or magnetite Fe3O4. Studying of their chemical/phase composition, structure/morphology, and some other properties leads to the conclusion that the developed B4C–W and h-BN–(Fe,Fe3O4) composites would be useful for solving important problems of boron-based neutron shielding and BNCT (Boron Neutron Capture Therapy), such as attenuating the gamma-radiation accompanying the absorption of neutrons by 10B nuclei and targeted delivery of 10B nuclei, as BNCT therapeutic agents, to tumor tissues using control by an external magnetic field, respectively.

Topology Optimization of a Femoral Stem in Titanium and Carbon to Reduce Stress Shielding with the FEM Method

Arthroplasty is commonly performed to treat advanced osteoarthritis or other degenerative joint conditions; however, it can also be considered for young patients with severe joint damage that significantly limits their functionality and quality of life. Young patients are still at risk of aseptic mobilization and bone resorption due to the phenomenon of stress shielding that causes an uneven distribution of tensions along the femoral contact surface prosthesis. This phenomenon can be limited by choosing the material of the prosthesis appropriately or by varying its stiffness, making sure that its mechanical behavior simulates that of the femur as much as possible. The aim of this study is to evaluate the mechanical strength of a prosthesis optimized both in shape and material and compare the results with a standard titanium prosthesis. Methods: Through three-dimensional modeling and the use of finite element method (FEM) software such as ANSYS, the mechanical behavior of traditional prosthesis and prosthesis optimized topologically respecting the ASTM F2996-13 standard. Results: With topological optimization, there is a stress reduction from 987 MPa to 810 MPa with a mass reduction of 30%. When carbon fiber is used, it is possible to further reduce stress to 509 MPa. Conclusions: The reduction in stress on the femoral stem allows an optimal distribution of the load on the cortical bone, thus decreasing the problem of stress shielding.

Generation of Optimal Weight Values Based on the Recursive Monte Carlo Method for Use in Monte Carlo Deep Penetration Calculations

In nuclear engineering, Monte Carlo (MC) methods are commonly used for reactor analysis and radiation shielding problems. These methods are capable of dealing with both simple and complex system models with accuracy. The application of MC methods experiences challenges when dealing with the deep penetration problems that are typically encountered in radiation shielding cases. It is difficult to produce statistically reliable results due to poor particle sampling in the region of interest. Therefore, such calculations are performed by the Monte Carlo N-Particle Transport (MCNP) code in association with the weight window (WW) variance reduction technique, which increases the particle statistics in the desired tally region. However, for large problems, MCNP’s built-in weight window generator (WWG) produces zero WW parameters for tally regions located far from the source. To address this issue, the recursive Monte Carlo (RMC) method was proposed. This paper focuses on the RMC methodology and its implementation in the Helmholtz-Zentrum Dresden-Rossendorf’s (HZDR’s) in-house code TRAWEI, which is responsible for producing optimal zone weight parameters used for optimizing deep penetration MC calculations. In addition, this paper discusses the verification of the TRAWEI weight generator program to that of an existing MCNP WWG. The performance of TRAWEI-generated weight values is assessed using a handful of test cases involving two shield materials. Globally, the TRAWEI-generated weight values improved not only the statistical variance and computational efficiency of the MC run compared to the analog MCNP simulation but also those of the simulation with WW values generated by the standard MCNP WWG.

Unified multiconductor transmission-line model of multiple-shields multiconductor cables: evaluation of shield connections performances

This paper presents a transmission-line model of a multiple-shields multiconductor cable. This unified model includes at the same time the propagation and cross-coupling characteristics of the electrical wires and of the cable-shields. It also includes the electromagnetic characteristics of the shields (in terms of transfer impedance and transfer admittance). It is derived in compliance with the multiconductor-transmission-line theory and it is valid whatever the connection configurations at the shield ends are. Therefore, it makes it possible the modelling of realistic connection problems ranging from ideal 360° shield connections to simple bonding wires. In addition, it is suitable for both electromagnetic susceptibility and emission problems. The paper proposes a physical explanation of the derived per-unit-length matrices. This unified model is also used to define the required conditions for being able to calculate the response of a shielded-cable in a two-steps model in which the shield problem and the inner shield problem are solved in sequence. Finally, the paper illustrates an application of the model in order to evaluate performances of a shielded-cable-link on crosstalk configurations with respect to various electrical bonding techniques of the shield.

Novel Method for the Production of Titanium Foams to Reduce Stress Shielding in Implants.

Titanium foams have potential applications in orthopedic and dental implants because of their low elastic modulus and good bone in-growth properties. In the present study, a novel method for the preparation of three-dimensional interconnected microporous titanium foams has been developed. This method is based on the insertion of a filler metal into the titanium metal by arc melting, followed by its removal by an electrochemical dealloying process for the development of foams. Complete removal of the filler metal by the electrochemical dealloying process was confirmed by an X-ray diffractometry (XRD) analysis, whereas scanning electron microscopy (SEM) analysis of the developed foams showed the development of interconnected porosity. Ti foams with different levels of porosities were successfully developed by varying the amount of the filler metal. Mechanical and thermal characterizations of the developed foams were carried out using compression testing and laser flash apparatus, respectively. The yield strength and elastic modulus of the developed foams were found to decrease by increasing the volume fraction of pores. The elastic modulus of the developed titanium foams (15.5-36 GPa) was found to be closer to that of human bones, whereas their yield strength (147-170 MPa) remained higher than that of human bones. It is therefore believed that the developed Ti foams can help in reducing the problem of stress shielding observed in orthopedic implants. The thermal diffusivity of the developed foams (4.3-0.69 mm2/s) was found to be very close to that of human dentine.

Comparison of frequency domain and time domain methods for the numerical simulation of contactless ultrasonic cavitation

The use of a top-mounted electromagnetic induction coil has been demonstrated as a contactless alternative to traditional ultrasonic treatment (UST) techniques that use an immersed mechanical sonotrode for the treatment of metals in the liquid state. This method offers similar benefits to existing UST approaches, including degassing, grain refinement, and dispersion of nanoparticles, while also preventing contact contamination due to erosion of the sonotrode. Contactless treatment potentially extends UST to high temperature or reactive melts. Generally, the method relies on acoustic resonance to reach pressure levels suitable for inertial cavitation and as a result the active cavitation volume tends to lie deep in the melt rather than in the small volume surrounding the immersed sonotrode probe. Consequently, (i) with suitable tuning of the coil supply frequency for resonance, the treatment volume can be made arbitrarily large, (ii) the problem of shielding and pressure wave attenuation suffered by the immersed sonotrode is avoided. However, relying on acoustic resonance presents problems: (i) the emergence of bubbles alters the speed of sound, resonance is momentarily lost, and cavitation becomes intermittent, (ii) as sound waves travel through and reflect on all the materials surrounding the melt, the sound characteristics of the crucible and supporting structures need to be carefully considered. The physics of cavitation coupled with this intermittent behaviour poses a challenge to sonotrode modelling orthodoxy, a problem we are trying to address in this publication. Two alternative approaches will be discussed, one of which is in the time domain and one in the frequency domain, which couple the solution of a bubble dynamics solver with that of an acoustics solver, to give an accurate prediction of the acoustic pressure generated by the induction coil. The time domain solver uses a novel algorithm to improve simulation time, by detecting an imminent bubble collapse and prescribing its subsequent behaviour, rather than directly solving a region that would normally require extremely small time steps. This way, it is shown to predict intermittent cavitation. The frequency domain solver for the first time couples the nonlinear Helmholtz model used for studying cavitation, with a background source term for the contribution of Lorentz forces. It predicts comparable RMS pressures to the time domain solver, but not the intermittent behaviour due to the underlying harmonic assumption. As further validation, the frequency domain method is also used to compare the generated acoustic pressure with that of traditional UST using a mechanical sonotrode.

Three-dimensional-printed titanium prostheses with bone trabeculae enable mechanical-biological reconstruction after resection of bone tumours.

ABSTRACTReconstruction after resection has always been an urgent problem in the treatment of bone tumours. There are many methods that can be used to reconstruct bone defects; however, there are also many complications, and it is difficult to develop a safe and effective reconstruction plan for the treatment of bone tumours. With the rapid development of digital orthopaedics, three-dimensional printing technology can solve this problem. The three-dimensional printing of personalised prostheses has many advantages. It can be used to print complex structures that are difficult to fabricate using traditional processes and overcome the problems of stress shielding and low biological activity of conventional prostheses. In this study, 12 patients with bone tumours were selected as research subjects, and based on individualised reverse-engineering design technology, a three-dimensional model of each prosthesis was designed and installed using medical image data. Ti6Al4V was used as the raw material to prepare the prostheses, which were used to repair bone defects after surgical resection. The operation time was 266.43 ± 21.08 minutes (range 180-390 minutes), and intraoperative blood loss was 857.26 ± 84.28 mL (range 800-2500 mL). One patient had delayed wound healing after surgery, but all patients survived without local tumour recurrence, and no tumour metastasis was found. No aseptic loosening or structural fracture of the prosthesis, and no non-mechanical prosthesis failure caused by infection, tumour recurrence, or progression was observed. The Musculo-Skeletal Tumour Society (MSTS) score of limb function was 22.53 ± 2.09 (range 16-26), and ten of the 12 patients scored ≥ 20 and were able to function normally. The results showed that three-dimensional printed prostheses with an individualised design can achieve satisfactory short-term clinical efficacy in the reconstruction of large bone defects after bone tumour resection.

Design and validation of a broad-group shielding library for fast reactors by using particle swarm optimization

To improve the efficiency of shielding calculation for fast reactors, the process of generating a broad-group shielding library for fast reactors was constructed by using advanced burner test reactors (ABTR) to construct the 1-D fast reactor model. To obtain energy broad-group structures appropriate for fast reactor shielding, particle swarm optimization (PSO) was used. Five structures were optimized, including 47-group, 38-group, 29-group, 25-group and 20-group. The corresponding broad-group shielding libraries using these optimized structures were generated and validated by four shielding problems for fast reactors. Numerical results show that the accuracy of NECL-EF47 with the 47-group structure was the best. The accuracy of NECL-EF25 is not as high as NECL-EF47, whereas NECL-EF25 with the 25-group structure is computationally more efficient than NECL-EF47 because of its smaller number of energy group. Conclusively, they are all suitable for shielding calculation for fast reactors and users can make the choice based on demand.

Validation of MCS code for shielding calculation using SINBAD

The MCS code is a computer code developed by the Ulsan National Institute of Science and Technology (UNIST) for simulation and calculation of nuclear reactor systems based on the Monte Carlo method. The code is currently used to solve two main types of reactor physics problems, namely, criticality problems and radiation shielding problems. In this paper, the radiation shielding capability of the MCS code is validated by simulating some selected SINBAD (Shielding Integral Benchmark Archive and Database) experiments. The whole validation was performed in two ways. Firstly, the functionality and computational rationality of the MCS code was verified by comparing the simulation results with those of MCNP code. Secondly, the validity and computational accuracy of the MCS code was confirmed by comparing the simulation results with the experimental results of SINBAD. The simulation results of the MCS code are highly consistent with the those of the MCNP code, and they are within the 2σ error bound of the experiment results. It shows that the calculation results of the MCS code are reliable when simulating the radiation shielding problems.

Hyperbolic Basis Functions for Time-Transient Analysis of Eddy Currents in Conductive and Magnetic Thin Sheets

This paper presents a new time-domain finite-element approach for modelling thin sheets with hyperbolic basis functions derived from the well-known steady-state solution of the linear flux diffusion equation. The combination of solutions at different operating frequencies permits the representation of the time-evolution of field quantities in the magnetic field formulation. This approach is here applied to solve a planar shielding problem in harmonic and time-dependent simulations for materials with either linear or nonlinear characteristics. Local and global quantities show good agreement with the reference solutions obtained by the standard finite element method on a complete and representative discretization of the region exposed to a time-varying magnetic field.

Internal field study of 21700 battery based on long-life embedded wireless temperature sensor

The safety of lithium-ion batteries is an essential concern where instant and accurate temperature sensing is critical. It is generally desired to put sensors inside batteries for instant sensing. However, the transmission of internal measurement outside batteries without interfering their normal state is a non-trivial task due to the harsh electrochemical environment, the particular packaging structures and the intrinsic electromagnetic shielding problems of batteries. In this work, a novel in-situ temperature sensing framework is proposed by incorporating temperature sensors with a novel signal transmission solution. The signal transmission solution uses a self-designed integrated-circuit which modulates the internal measurements outside battery via its positive pole without package breaking. Extensive experimental results validate the noninterference properties of the proposed framework. Our proposed in-situ temperature measurement by the self-designed signal modulation solution has a promising potential for in-situ battery health monitoring and thus promoting the development of smart batteries.Graphic abstract

Marvin: A parallel three-dimensional transport code based on the discrete ordinates method for reactor shielding calculations

Marvin is a parallel three-dimensional discrete ordinates (SN) code dedicated for reactor shielding calculations. It employs the multi-group and SN approximations to discretize the energy and angular variables, respectively. Multiple discretization methods, namely weighted diamond difference, theta-weighted diamond difference and step-characteristic, have been implemented to discretize the spatial variables. The within-group transport equation is solved by the source iteration or Krylov subspace solvers, both of which employ the Koch-Baker-Alcouffe (KBA) algorithm to parallelize the transport sweep process. The upscattering in the thermal energy range is treated with thermal upscattering iteration scheme. The code design of Marvin features the mixed programming model and a three-layer architecture design. Two benchmark problems devised from realistic reactor core configurations, namely the VENUS-3 and HBR-2 problems, were calculated to evaluate the correctness, accuracy and parallel computing performance of Marvin. The calculation results were compared against TORT solutions as well as measured data. The validation results indicate that: Marvin is properly implemented and can faithfully produce solutions to the reactor shielding problems; the computing efficiency of Marvin can be significantly enhanced via the KBA parallel transport sweep algorithm.

Design Method of Porous Titanium Alloy Based on Meta-Structure

To meet the personalized demands of different bone tissues in regard of the mechanical properties of titanium alloy implants and solve the problem of stress shielding, this paper studies TC4 and thereby proposes a design method of porous titanium alloy based on meta-structure for additive manufacturing. In this paper, the construction method of the meta-structure is illustrated, and the relationship model between the meta-structure and the key mechanical properties of metal implants is established. This paper determines certain relevant parameters in the model through simulation method, verifies the correctness of the model through additive manufacturing experiment, and proposes two construction methods of porous titanium alloy based on meta-structure, thus providing a systematic and quantitative solution for the design of porous structure of titanium alloy for implants.

WITHDRAWN: Shielding performance analysis of flexible multi-layered textile sheets based on polymer substrate

Abstract The need for effective Electro Magnetic Interference (EMI) Shielding became inevitable due to the rise of terahertz range technologies based electronic devices in civil, military and healthcare domains. In this paper we analyzed shielding performances of multilayered textile sheets with flexible polymeric substrate. The analysis considered factors like sensitivity of shielding effectiveness, conductivity of polymer inter-layers, thickness of textiles and frequency range. Many researchers contributed to the problem of EM shielding. The literature revealed that carbon based shielding is not mechanically flexible. Metal based shielding is heavy weight and not easy for tuning performance. Synthetic material with laminated sheets containing textile fabrics and conductive polymers was found a promising solution. Recently Aloia et al. explored shielding performances of flexible multilayer screens based on chemically doped graphene on polymer substrate. However, their research was not considering textiles. Textiles with polymer substrate will have impact in making EM shielding. Therefore further investigation is required to focus on textiles with conductive polymers in making EM shielding. This is the focus of the paper which analyzes various combinations and permutations of textile sheets and polymer substrate. Our mathematical and analytical results revealed that the proposed methodology can help improve decision making on EM shielding effectiveness significantly.

Low-frequency Magnetic Shielding of Planar Shields: A Unified Wave Impedance Formula for the Transmission Line Analogy

This article focuses on wave impedance calculations employed in the transmission line analogy for the low-frequency magnetic shielding problem: a planar shield against a loop current source placed parallel to the shield. We present a unified formula for wave impedance calculations that is distinct from traditional definitions of the transverse E -field to H -field ratio, which is related to the ratio of the H -field components to its longitudinal derivative. The formula is relatively simple in terms of mathematical complexity when applied to derive a concrete wave impedance expression. For a circular loop, the formula produces the same wave impedance expression as the traditional definition for field points on the central axis of the loop. For field points off the axis, the formula more accurately predicts the shielding effectiveness relative to traditional definitions. Based on the proposed formula, we also obtain wave impedance expressions for rectangular, elliptical, and regular polygon loops under quasi-static assumptions. The effectiveness of these expressions is verified through comparisons with finite element simulations.

High-efficiency simulation of VENUS-3 neutron-shielding problem with an automatic and enhanced hybrid Monte-Carlo-Deterministic method

To solve the deep-penetration problem in the Monte-Carlo simulation, the classic consistent adjoint driven importance sampling (CADIS) and forward-weighted CADIS (FW-CADIS) hybrid MC-deterministic method is adopted and enhanced in this paper. An automatic hybrid MC-deterministic code, NECP-MCX, is newly developed. NECP-MCX embeds the discrete-ordinate code NECP-Hydra to automatically perform the forward and adjoint neutron-transport calculations and set up the importance parameters for the MC simulation. In massive parallel calculation for large shielding problems, huge amounts of memory are required to save detailed weight-window information. To mitigate the memory limitation, a mesh-coarsening algorithm is developed based on the contributon theory. It can be applied in the traditional Cartesian-mesh and the nested-mesh structure. NECP-MCX was applied to the VENUS-3 benchmark. The results show that NECP-MCX with the enhanced hybrid method is effective in variance reduction and memory saving for the simulation of large and complicated shielding problems on parallel computer resources.