Neutron Cross Section Library Research Articles

Thermal Neutron Filter Design for the Neutron Radiography Facility at the LVR-15 Reactor

In 2011 a decision was made to build a neutron radiography/ facility at one of the unused horizontal channels of the LVR-15 research reactor in Rez, Czech Republic. One of the key conditions for operating an effective radiography facility is the delivery of a high intensity, homogeneous and collimated thermal neutron beam at the sample location. Additionally the intensity of fast neutrons has to be kept as low as possible as the fast neutrons may damage the detectors used for neutron imaging. As the spectrum in the empty horizontal channel roughly copies the spectrum in the reactor core, which has a high ratio of fast neutrons, neutron filter components have to be installed inside the channel in order to achieve desired beam parameters. As the channel design does not allow the instalment of complex filters and collimators, an optimal solution represent neutron filters made of large single-crystal ingots of proper material composition. Single-crystal silicon was chosen as a favorable filter material for its wide availability in sufficient dimensions. Besides its ability to reasonably lower the ratio of fast neutrons while still keeping high intensities of thermal neutrons, due to its large dimensions, it suits as a shielding against gamma radiation from the reactor core. For designing the necessary filter dimensions the Monte-Carlo MCNP transport code was used. As the code does not provide neutron cross-section libraries for thermal neutron transport through single-crystalline silicon, these had to be created by approximating the theory of thermal neutron scattering and modifying the original cross-section data which are provided with the code. Carrying out a series of calculations the filter thickness of 1 m proved good for gaining a beam with desired parameters and a low gamma background. After mounting the filter inside the channel several measurements of the neutron field were realized at the beam exit. The results have justified the calculated values. After the successful filter installing and a series of measurements, first test neutron radiography attempts with test samples could been carried out.

The Impact of Updated Cross Section Libraries on ITER Neutronics Calculations

In ITER, determination of radiation loads such as nuclear heating due to neutrons and photons (gammas) is an important part of the design process. Monte Carlo transport codes need accurate neutron and photon cross section libraries to produce accurate results. Because photon heating dominates the contribution to total nuclear heating for common materials like stainless steel and copper in several key components of ITER, the photon cross section library is particularly important. In this work, two ITER realistic benchmark calculation models are used to determine the impact on nuclear heating by the cross section library used in the calculation. The results show that the nuclear heating can be as much as 5% lower to as much as 6% higher than the nuclear heating calculated using the standard fusion neutron and photon cross section library.

A tightly coupled scheme for neutronics and thermal–hydraulics using open-source software

Coupling neutronic and thermal–hydraulic analyses of a nuclear reactor core is important because it helps identify the most relevant safety issues without conservative assumptions. Currently coupled computations solve the same governing equations using different coupling methods, which can be sorted into two categories: loose coupling and tight coupling. This paper proposes and verifies a third coupling approach called “the Integrated Tight Coupling (ITC) method”. The mathematical equations in the nuclear fuel are rearranged to be integrated via a novel concept of the energy-group function. In addition, the data from the neutron cross section library can be used directly. The ITC method is implemented using two open-source codes: the DRAGON code and OpenFOAM. Additionally, a coupled computation using these two codes is new and has not been done. The ITC method is verified using a 1.5-D (1-D neutronics and 2-D thermal–hydraulics) symmetric unit cell example. The mesh of the tightly integrated computation is 25% coarser than the loosely coupled one. Starting from a similar initial guess, the number of iterations for the ITC method is 24% fewer than that for the loosely coupled computation to reach the same accuracy. In addition, the ITC method is tested with different initial guesses. For all cases tested, the scheme converged to the same solution. With further improvement and additional testing, the ITC method has the potential to be incorporated with other neutronics and thermal–hydraulics codes.

Effect of burnup on the neutronic parameters of ITU TRIGA Mark II research reactor

This study aims to estimate burnup of the fuel elements for the Istanbul Technical University TRIGA Mark II Research and Training Reactor using a Monte Carlo-based burnup-depletion code. Effect of burnup on the core neutronic parameters, effective core multiplication factor, fast/epithermal/thermal neutron fluxes, and core-average neutron spectrum, and incoming neutron spectrum of the piercing beam port (PBP), is investigated at the Beginning of Life (BOL) and End of Life (EOL). Operational data peculiar to a selected operation sequence, which contains positions of CRs, power level of the reactor, material temperatures and latest core map, are used to determine the current fuel burnup of fuel elements at the time under consideration. A specific operation sequence is selected for the analysis. Furthermore, all control rods are considered fully withdrawn to assess the excess reactivity. Results are obtained using MONTEBURNS2 with ENDFB/V-II.1 neutron/photon library for a full power of 250 kW. Neutron cross-section libraries at the full-power operating temperatures are generated using NJOY. From the results, the calculated burnup values of the core at the sequence considered and EOL are found to be 420 MWh and 560 MWh, respectively. Remaining excess reactivity is calculated to be less than 0.3 $. It is observed that core average thermal neutron flux reduces by 1 % while the fast and epithermal neutron fluxes remain almost unchanged.

New Standard Evaluated Neutron Cross Section Libraries for the GEANT4 Code and First Verification

The Monte Carlo simulation of the interaction of neutrons with matter relies on evaluated nuclear data libraries and models. The evaluated libraries are compilations of measured physical parameters (such as cross sections) combined with predictions of nuclear model calculations which have been adjusted to reproduce the experimental data. The results obtained from the simulations depend largely on the accuracy of the underlying nuclear data used, and thus it is important to have access to the nuclear data libraries available, either of general use or compiled for specific applications, and to perform exhaustive validations which cover the wide scope of application of the simulation code. In this paper we describe the work performed in order to extend the capabilities of the GEANT4 toolkit for the simulation of the interaction of neutrons with matter at neutron energies up to 20 MeV and a first verification of the results obtained. Such a work is of relevance for applications as diverse as the simulation of a neutron detector, an entire particle physics experiment, a fission or fusion nuclear reactor, a neutron shielding or a hadron therapy cancer treatment facility.

Analysis of ITU TRIGA Mark II research reactor using Monte Carlo method

Research reactors include many complicated components with various shapes and sizes. Such complex parts also observed in TRIGA core are modelled by the researchers as simplified physical geometries when a particle transport computer code is used to analyse the reactors. These models are used to gain information on possible modifications in the reactors with no cost except a certain computational time demand. Besides, they can be used to understand the fabrication uncertainties of the core components and the methodologies used in the design process. The main objective of this study is to make a detailed three-dimensional full-core model of ITU (Istanbul Technical University) TRIGA Mark II research reactor for the use of Monte Carlo method and making a comparison of the simulation with the experimental observations. In case of lacking of experimental values reported, Final Safety Analysis Report values are used as reference. Furthermore, it is aimed to observe possible influences of using various neutron cross-section libraries (ENDF/Bs and JEFFs) onto the simulation results. The Monte Carlo simulations are carried out by using MCNP5 radiation transport code. All unsteady conditions are ignored, assuming the reactor operates at cold-zero power under the steady-state condition. For comparison, effective core multiplication factor (keff) and effective delayed neutron fraction (βeff) are computed. Reactivity worth ($) of control rods with rod position is presented. Pin power distribution within the fuel elements, axial power peaking distribution along the fuel length and normalized distribution of fast/thermal neutron flux throughout the core are analysed. The simulation results show that MCNP5 model of the reactor is properly established with sufficient detail in such a way that all simulation results are in an excellent agreement with the experimental data (or FSAR values). Results also show that the model yields more or less the same value even different neutron libraries are used.

Nuclear Data Library Effects on Fast to Thermal Flux Shapes Around PWR Control Rod Tips

The development of a high-fidelity computational scheme to estimate the accumulated fluence at the tips of PWR control rods (CR) has been initiated at the Paul Scherrer Institut (PSI). Both the fluence from high-energy (E>1MeV) neutrons as well as for the thermal range (E<0.625eV) are required as these affect the CR integrity through stresses/strains induced by coupled clad embrittlement / absorber swelling phenomena. The concept of the PSI scheme under development is to provide from validated core analysis models, the volumetric neutron source to a full core MCNPX model that is then used to compute the neutron fluxes. A particular aspect that needs scrutiny is the ability of the MCNPX-based calculation methodology to accurately predict the flux shapes along the control rod surfaces, especially for fully withdrawn CRs. In that case, the tip is located a short distance above the core/reflector interface and since this situation corresponds to a large part of reactor operation, the accumulated fluence will highly depend on the achieved calculation accuracy and precision in this non-fueled zone. The objective of the work presented in this paper is to quantify the influence of nuclear data on the calculated fluxes at the CR tips by (1) conducting a systematic comparison of modern neutron cross-section libraries, including JENDL-4.0, JEFF-3.1.1 and ENDF/B-VII.0, and (2) by quantifying the uncertainties in the neutron flux calculations with the help of available neutron cross-section variances/covariances data. For completeness, the magnitude of these nuclear data-based uncertainties is also assessed in relation to the influence from other typical sources of modeling uncertainties/biases.

Reanalysis of the Gas-cooled fast reactor experiments at the zero power facility Proteus – Spectral indices

The gas-cooled fast reactor (GCFR) concept was investigated experimentally in the PROTEUS zero power facility at the Paul Scherrer Inst. during the 1970's. The experimental program was aimed at neutronics studies specific to the GCFR and at the validation of nuclear data in fast spectra. A significant part of the program used thorium oxide and thorium metal fuel either distributed quasi-homogeneously in the reference PuO{sub 2}/UO{sub 2} lattice or introduced in the form of radial and axial blanket zones. Experimental results obtained at the time are still of high relevance in view of the current consideration of the Gas-cooled Fast Reactor (GFR) as a Generation-IV nuclear system, as also of the renewed interest in the thorium cycle. In this context, some of the experiments have been modeled with modern Monte Carlo codes to better account for the complex PROTEUS whole-reactor geometry and to allow validating recent continuous neutron cross-section libraries. As a first step, the MCNPX model was used to test the JEFF-3.1, JEFF-3.1.1, ENDF/B-VII.0 and JENDL-3.3 libraries against spectral indices, notably involving fission and capture of {sup 232}Th and {sup 237}Np, measured in GFR-like lattices. (authors)

Automatic fuel lattice design in a boiling water reactor using a particle swarm optimization algorithm and local search

The axial section of fuel assembly in a boiling water reactor (BWR) consists of five or six different distributions; this requires a radial lattice design. In this study, an automatic procedure based on a particle swarm optimization (PSO) algorithm and local search was developed to design the radial enrichment and gadolinia (Gd) distribution of the fuel lattice. The design goals were to achieve the minimum local peaking factor (LPF), and to come as close as possible to the specified target average enrichment and target infinite multiplication factor (k∞), in which the number of fuel pins with Gd and Gd concentration are fixed. In this study, three axial sections are designed, and lattice performance is calculated using CASMO-4. Finally, the neutron cross section library of the designed lattice is established by CMSLINK; the core status during depletion, such as thermal limits, cold shutdown margin and cycle length, are then calculated using SIMULATE-3 in order to confirm that the lattice design satisfies the design requirements.

Characterization methods for an accelerator based fast-neutron facility

A fast neutron facility provides a number of complexities in both detection and shielding, the latter arising not only due to uncertainty in the behaviour of the scattered radiation (neutron and gamma-rays) from a fast neutron source, but also on shielding requirements that have to take into account internal and external factors, such as dose limitations, space availability for implementing bulky shielding and secondary interactions of the radiation with materials. This has possible influence on experimental measurements with a low signal to noise ratio. This paper reports on some of the investigations performed at a RFQ accelerator facility generating > 1011 neutrons per second with energies up to 14 MeV, which are used to perform fast neutron radiography studies. Areas highlighted are the neutron cross section libraries, where important data needs to be reviewed or updated.

Validation of an ENDF/B-VII.0-Based Neutron and Photon Shielding Library in MATXS-Format

A MATXS-format neutron and photon cross section library for various shielding applications was generated based on the ENDF/B-VII.0, which has been disseminated through the NEA Data Bank (NEA-1818). The library contains 204 nuclide data including 8 thermal scattering law data processed by the NJOY99 code, while it adopts the VITAMIN-B6 energy group structure, i.e., 199 groups for the neutron and 42 groups for the photon. In this study, the library has been validated through some shielding benchmark experiments taken from SINBAD. The calculations were carried out with the TRANSX/DANTSYS code system, and the results were compared with those of the KASHIL-E6 library, which was the preceding library based on the ENDF/B-VI.5. In addition, the applicability of the library to the reactor criticality calculation was examined for some fast and thermal benchmark problems taken from the ICSBEP benchmark specifications.

Neutronics modeling of the High Flux Isotope Reactor using COMSOL

The High Flux Isotope Reactor located at the Oak Ridge National Laboratory is a versatile 85 MWth research reactor with cold and thermal neutron scattering, materials irradiation, isotope production, and neutron activation analysis capabilities. HFIR staff members are currently in the process of updating the thermal hydraulic and reactor transient modeling methodologies. COMSOL Multiphysics has been adopted for the thermal hydraulic analyses and has proven to be a powerful finite-element-based simulation tool for solving multiple physics-based systems of partial and ordinary differential equations. Modeling reactor transients is a challenging task because of the coupling of neutronics, heat transfer, and hydrodynamics. This paper presents a preliminary COMSOL-based neutronics study performed by creating a two-dimensional, two-group, diffusion neutronics model of HFIR to study the spatially-dependent, beginning-of-cycle fast and thermal neutron fluxes. The 238-group ENDF/B-VII neutron cross section library and NEWT, a two-dimensional, discrete-ordinates neutron transport code within the SCALE 6 code package, were used to calculate the two-group neutron cross sections required to solve the diffusion equations. The two-group diffusion equations were implemented in the COMSOL coefficient form PDE application mode and were solved via eigenvalue analysis using a direct (PARDISO) linear system solver. A COMSOL-provided adaptive mesh refinement algorithm was used to increase the number of elements in areas of largest numerical error to increase the accuracy of the solution. The flux distributions calculated by means of COMSOL/SCALE compare well with those calculated with benchmarked three-dimensional MCNP and KENO models, a necessary first step along the path to implementing two- and three-dimensional models of HFIR in COMSOL for the purpose of studying the spatial dependence of transient-induced behavior in the reactor core.

Comparison of calculated and measured reaction rates obtained through foil activation in the subcritical dual spectrum facility YALINA-Booster

Reaction rates were measured by the foil activation technique to obtain neutron spectrum information in a subcritical core driven by an external neutron source. The experimental results are compared with Monte Carlo calculations in order to examine the capability of the Monte Carlo code MCNP together with ENDFB-6.8, JEFF-3.1.1 and CENDL-3.1 neutron cross section libraries to predict the neutron spectrum dependent reaction rates correctly in a subcritical core. The focus lies on fast neutrons. A discrepancy is found in the calculated-to-experimental values of the reaction rates and an inaccurate cross section is identified in CENDL-3.1.

Production and testing of HENDL-2.1/CG coarse-group cross-section library based on ENDF/B-VII.0

A coarse-group coupled neutron and photon (27 n + 21γ) cross-section library HENDL-2.1/CG, based on ENDF/B-VII.0 evaluate data source, has been produced by FDS Team in Institute of Plasma Physics, Chinese Academy of Sciences (ASIPP). HENDL-2.1/CG containing 350 nuclide cross-section files (from 1H to 252Cf) was generated in MATXS format with the NJOY processing system and then by compiling coarse-group problem-dependent format using the TRANSX code. In order to verify the availability and reliability of the HENDL-2.1/CG data library, requisite benchmark calculations were performed and compared with HENDL-2.0/MG fine-group coupled neutron and photon (175 n + 42γ) cross-section library. In general, results using the coarse-group library showed similarly believable as fine-group library.

A stylized three dimensional PWR whole-core benchmark problem with Gadolinium

Recent advancements in computer technology have led to an increased ability to solve complicated reactor problems, and as a result, there has been an interest in codes and methods that utilize whole-core transport in three-dimensional, large-scale, highly-heterogeneous configurations. Most current reactor benchmark problems are either small-scale or involve spatial homogenization (pin cell or full-assembly) for diffusion codes; therefore, new whole-core benchmark problems must be designed in order to validate whole-core transport methods. This study focuses on an PWR case, with a low leakage loading pattern and Gadolinium as a burnable absorber. In this paper, a detailed description of a new 3D whole-core numerical benchmark problem based on a 4-loop layout is presented with geometric, material and cross section specifications. A set of 2-group and 4-group region-wise neutron cross section libraries is generated using the lattice depletion code HELIOS, and an MCNP model is described for three configurations (all rods in, all rods out, power-shaping rods inserted). Eigenvalues are presented for the 2-group calculations of all three configurations, and detailed pin fission density results are presented for the power-shaping configuration.

Performance of a Monte-Carlo solution for the H.B. Robinson-2 Pressure Vessel Dosimetry Benchmark

At the Paul Scherrer Institut, a methodology for PWR fast neutron fluence estimations, based on the Monte-Carlo particle transport code MCNPX with general-purpose neutron data libraries and using neutron source data from deterministic 3-D core-follow calculations, has been developed. The methodology has been validated on the basis of experimental data related to the fluence at the inner surface of a Swiss PWR Reactor Pressure Vessel. In this technical note, a first objective is to enlarge the validation basis of the methodology as well as to extend it for applications to RPV outer-surfaces. To that aim, a preliminary analysis with the MCNPX-2.4.0 code, along with the JEFF-3.1 continuous-energy neutron data library, of the “H.B. Robinson-2 Pressure Vessel Benchmark”, providing in-vessel and out-vessel experimental dosimetry data, is presented. In addition, considering that the available original solutions of the benchmark employed deterministic transport methods with associated libraries, a second objective of this technical note is to assess the progress achieved for this type of problems when applying modern Monte-Carlo based methods. The results show that for the 237Np and 238U fission dosimeters, which were of primary interest in the given study, a non-negligible improvement is seen in the MCNPX solution, indicating thus a rather good performance of the employed Monte-Carlo method and providing thereby, additional confidence for the overall PSI fluence methodology. For other high-energy dosimeters, the presented new results do not show yet any significant accuracy improvement versus previously reported results. This can however not be confirmed before additional studies, e.g., with focus on improvements of the involved modelling approximations and the statistical precision of the MCNPX calculations, be carried out. Similarly, investigations of neutron cross-section and dosimetry libraries effects remain to be addressed. These further studies are however not included here since at this stage, the principal aim was mainly to model and analyse this benchmark at most consistent manner with the previous solutions using a continuous-energy Monte-Carlo based method.

Neutronics experiments on HCPB and HCLL TBM mock-ups in preparation of nuclear measurements in ITER

In support of the breeder blanket development program, the EU is conducting a dedicated neutronics R&D effort to provide the basis for the design of nuclear tests to be performed in ITER on the Test Blanket Modules (TBMs). It includes the development of computational tools comprising both Monte-Carlo and deterministic transport, sensitivity and uncertainty codes, the generation of high quality neutron cross-section and covariance data libraries. These are validated experimentally in view of their application in the ITER TBM and the DEMO design. To this purpose, two neutronics experiments have been carried out on mock-ups of both the Helium Cooled Pebble Bed (HCPB) and the Helium Cooled Lithium Lead (HCLL) variants of ITER TBMs, at 14-MeV neutron sources. Redundant experimental techniques have been used to measure the resulting tritium production rate and the neutron and gamma ray spectra which are needed to predict the blanket shielding performance, nuclear power production and all nuclear loads. The comparison of experiment and corresponding calculation is obtained with the associated uncertainty margin based on experimental as well as calculational uncertainties. At the same time, suitable nuclear measuring techniques for TBMs in ITER, in particular for the tritium production, are being developed, optimised, and tested in the mock-up experiments. In particular, the present paper summarizes the preliminary results of the latest of such experiments, i.e. the one conducted on a mock-up of the HCLL blanket. It describes also the dedicated neutronics R&D activities, including the efforts to specify the neutronics tests and objectives in TBMs in ITER, considering the various design concepts.

The effect of modern thermal neutron scattering sublibraries on criticality safety evaluations of wet storage pools

Within the framework of a recent criticality safety evaluation (CSE) performed for the licensing of a new commercial wet storage pool using MCNPX-2.5.0 along with the ENDF/B-VII.0 and JEFF-3.1 continuous energy cross-section libraries, the maximum permissible initial fuel-enrichment limit for water reflected configurations was found to be dependant upon the applied neutron cross section library. More detailed investigations showed that the difference is mainly caused by different sublibraries for thermal neutron scattering based on parameterizations of the S ( α , β ) scattering matrix. This paper addresses the main findings of these investigations and emphasizes which configurations are particularly sensitive to the parameterization of S ( α , β ) applied in the CSE analysis. The results are presented in an attempt, on the one hand, to assess more precisely the sensitivity upon thermal scattering modeling data in CSE of wet storage pools, and on the other hand, to assess whether the data employed in the most recent neutron data libraries can be considered as an improvement for this type of analyses.

The EAF-2007 Neutron, Deuteron, and Proton Activation Libraries

The European Activation System, EASY, includes as the source of nuclear data the European Activation File (EAF). A new version of EAF, EAF-2007, has been developed that contains cross-section data for deuteron- and proton-induced reactions in addition to the traditional neutron-induced data. The main reason for the addition of these data to EAF is to enable activation calculations to be performed for the International Fusion Materials Irradiation Facility, which is a planned materials test facility, and for other accelerator-driven devices with an incident upper energy limit of 60 MeV. EAF-2007 has benefited from the generation and maintenance of comprehensive activation files and the development of the processing code SAFEPAQ-II. Cross-section validation exercises against both experimental data and systematics, which were started on the EAF-4 file (1995), enable a comprehensive assessment of the data. Although EAF-2007 is the best-validated activation neutron cross-section library in the world, currently <3% of all the reactions can be compared with experimental information and sometimes then only for a very limited, and not always relevant, energy range. As with EAF-2001, EAF-2003, and EAF-2005, results of integral experiments have been used to correct, adjust, and validate data. This can be done using SAFEPAQ-II by inputting the measured effective cross sections. Validation using integral data has been performed by means of direct comparison with measurements of various materials under relevant neutron spectra. A tool has been recently developed that is of importance now that libraries contain so much calculated data. Statistical Analysis of Cross Sections is used to look for trends in the library data for a particular reaction type, and this has proved efficient in identifying reactions with data that need correction or improvement. This method has been used with EAF-2005 and EAF-2007 and is a valuable additional validation method.

Design of an energy-independent spherical-type neutron dose monitor

The thickness of a spherical-type neutron dose monitor has been optimized in order to obtain improved dose response compared to that of existing instruments. The monitor structure includes four layers of the spherical shell. The outermost layer is composed of a poly-methyl methacrylate (PMMA) shell. A boron nitride shell is used as the second layer while two polyethylene shells are used as the third and the core layers. Twelve radial directional thermo-luminescent detectors (TLD) detectors are arranged between the layers at two different depths and one in the center. Considering the reaction rate of 6Li(n,α) T for the TLD detectors, a series of calculations were performed using MCNP5 by varying the thickness of the layers. Neutron cross-section libraries based on JENDL-3.3 were applied for the calculations. To evaluate the dose response of the monitor, the spectrum of the D 2O-moderated 252Cf source was used as mentioned in ISO 8529. The monitor is sensitive to measurements of three different energy groups at three depths of the moderator. The neutron dose was calculated from the responses of a linear combination of TLD group detectors. The dose response was improved and well agree with expected dose, H*(10), compared to that of existing instruments, especially for intermediate energies of neutrons. The total ambient dose equivalent was varied only 3% for two different incident directions, hence the direction distribution was found small for dose measurement.