Independent Fission Research Articles

235U(nth,f)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{235}\ ext {U(n}_{\ ext {th}}\ ext {,f)}$$\\end{document} mass yield evaluation with associated covariance matrix

The study of fission yields has a major impact on the characterization and understanding of the fission process and its applications. Mass yield evaluation represents a key element in order to perform the best estimation of independent and cumulative fission product yields. Today, the lack of analysis-based correlation matrix between the different fission observables induces many inconsistencies in the evaluations. In particular, the mass yield uncertainties are drastically overestimated in comparison to chain yields while these two quantities only differ from the emission of delayed neutrons. In this work, a new consistent process of mass yield evaluation is proposed taking into account the description of the covariance matrix. For 235U(nth,f)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{235}\ ext {U(n}_{\ ext {th}}\ ext {,f)}$$\\end{document} mass yields, existing data cover a large range of produced masses which makes it possible to propose an evaluation of mass yields independently of any model with a precision closed to 1.5%\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$1.5\\%$$\\end{document} for the high yields. This new precision brings the possibility to discuss the origin of the structures in the post-neutron mass yields, in particular the consistency with the pre-neutron mass yields and the prompt neutron multiplicity per mass. The work is the first part of the 235U(nth,f)\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{235}\ ext {U(n}_{\ ext {th}}\ ext {,f)}$$\\end{document} thermal neutron induced fission yield evaluation which will be included in the new JEFF-4 library.

Probability Density Functions with Correlations of Experimental and Evaluated 235U(nth,f) Fission Yields and Their Impacts on Reactivity Loss with Burnup

The study of fission yields has a major impact on the characterization and understanding of the fission process and its applications. For the latter, it is crucial to provide fission yields with their associated correlation matrix in order to estimate precisely the uncertainties of crucial quantities, such as the reactivity loss. In the last decade, different works have been proposed to estimate the correlations of the independent fission yields satisfying the consistency of the cumulative yield evaluations. In these previous works, only model parameters and conservation laws have driven the correlations. The novelty of the present work consists of new complete and consistent evaluations of 235U(nth,f) independent and cumulative yields and their correlation matrices, starting from experimental data. The characterization of the probability density functions of fission yields validates the multivariate Gaussian assumption and constitutes a major issue in the validation of the uncertainty propagation tools of the applications. In addition, the new 235U(nth,f) fission yield covariance data have resulted in a decrease in nuclear data related to uncertainty in associated burnup calculations.

On the Assumptions Behind Statistical Sampling: A 235U Fission Yield Uncertainty Propagation Case Study

Monte Carlo sampling is frequently employed for uncertainty quantification in depletion calculations. Several assumptions are needed to perform this analysis. In this work, an assessment of these assumptions is proposed via sample convergence studies and perturbation of the sampling distribution. The Uncertainty Analysis in Best-Estimate Modeling (UAM) Pincell Hot Full Power and the Turkey Point reference cases were considered for this purpose. The 235U thermal independent fission yield uncertainties evaluated in JEFF-3.3 and JEFF-4.0 were propagated to the nuclide vector and to the system multiplication factor. Using JEFF-4.0 data, a 75% reduction in the uncertainty of selected nuclide concentrations and an 80% reduction in the multiplication factor uncertainty were observed, showcasing the effect of full covariance evaluations. The presented results also prove that the uncertainty in the considered observables shows marginal dependence on the sampling distribution.

Fission cross-section measurements for the 238U(n,f)97m+gNb, 238U(n,f)133gTe and 238U(n,f)130gSb reactions induced by D-T neutrons

In this study, we conducted measurements of the independent fission cross-sections of 238U(n, f)97m+gNb, 238U(n, f)133gTe reactions and the cumulative cross section of 238U(n, f)130gSb reactions induced by neutron at energies around 14 MeV, i.e., 14.1 ± 0.3, 14.5 ± 0.3 and 14.7 ± 0.3 MeV. The measurement results were obtained by the neutron activation method in combination with off-line γ-ray spectrometry techniques. The neutron flux was monitored on line by the accompanying α-particle from T(d, n)4He reaction, and the neutron energies were determined by the cross-section ratio of 90Zr(n, 2n)8+gZr to 93Nb(n, 2n)92mNb reactions. The independent fission cross-sections of the fission reactions were obtained by subtracting the influence of precursor nuclei or excited states. The obtained results are as follows: for 238U(n, f)97m+gNb, the independent cross sections are 1.0 ± 0.89, 0.98 ± 0.85 and 0.78 ± 0.70 mb at the specified neutron energy points. For 238U(n, f)133gTe, the independent fission cross-sections are 26.8 ± 2.8, 27.7 ± 2.9 and 20.5 ± 2.3 mb, respectively, at the same neutron energy points. As for 238U(n, f)130gSb, the obtained cumulative fission cross-sections are 5.35 ± 0.58, 5.05 ± 0.53 and 4.03 ± 0.44 mb, respectively, at the specified neutron energy points.

Use of correlations for neutron induced fission yields in DARWIN/PEPIN2 Uncertainty Quantification

Uncertainty quantification on Decay Heat is of utmost importance for the safety evaluations linked to nuclear waste management. International nuclear data libraries like JEFF, ENDF/B or JENDL provide evaluations on independent fission yields, but generally no correlations between those fission yields. With those data, independent fission yields can be considered as the most important contributor to the Decay Heat uncertainty. Nevertheless, other data are available, like correlation matrices for some fissile systems, or the use of fission yields by mass. Complete correlation matrices appear in nuclear data libraries, like JEFF-4, but are still very scares as only the most prominent fissile systems for nuclear reactor applications have been added. Another approach is to consider the use of the fission yields by mass, which uncertainty is smaller than those of independent fission yields, and adding a constraint when propagating uncertainties. In the work, we show the impact of considering the fission yields by mass, as it was implemented in DARWIN/PEPIN2.4.8. We compare DARWIN/PEPIN2.4.8 results, obtained on a first order direct perturbation approach, with a random sampling method through the use of an URANIE/MENDEL scheme.

The role of Fission in the search of the Super Heavy Land: Fission modes in heavy and superheavy nuclei. Case study of 180Hg

Many observations strongly support the hypothesis that nuclei may fission through several independent fission modes (multimodal fission) interpreted as different prescission shapes and fission paths in a multidimensional potential energy landscape where shell effects are dominant. Mass distributions of the fission fragments are sensitive to the potential energy landscape and appear to be single humped (symmetric) or double humped (asymmetric). In many cases a mixture of both modes is observed. We propose here our study on 180Hg. Binary fission fragments formed in the reaction 68Zn + 112Sn → 180Hg at different excitation energies around the Coulomb barrier were detected using the double-arm time-of-flight technique with the spectrometers CORSET. The experiment was performed at JYFL (Jyvaskyla, Finland). We will discuss an analysis of the mass distributions in terms of fission modes predicted by a five-dimensional fission model. We have found out that the mass distributions can be well reproduced by considering a symmetric fission mode and two asymmetric modes at (AL ≈ 80, AH ≈ 100) and (AL ≈ 70 and AH ≈ 110).

Connection of four-dimensional Langevin model and Hauser-Feshbach theory to describe statistical decay of fission fragments

ABSTRACT We have developed a novel theoretical method to obtain independent fission product yields and prompt neutron observables by connecting mass and total kinetic energy (TKE) distributions calculated by a four-dimensional Langevin dynamical model to a Hauser-Feshbach statistical decay model. In the Langevin calculations, mass distributions corresponding to the standard I and II modes were obtained separately and superposed to reproduce the fission fragment yield of spontaneous fission of 238 , 240 , 242 Pu and thermal neutron-induced fission of 239 Pu . This was achieved by using different neck parameters for these two modes in the two-center shell model shape parametrization, and a systematics of the superposing ratio was obtained as a function of N − Z / A of the fissioning nuclei. The Hauser-Feshbach calculations were performed using a nuclear reaction code TALYS for 239 Pu n , f reaction in the incident energy range from thermal up to 5 MeV , and the calculated prompt fission observables were compared with experimental and evaluated data. Although further improvements are needed for the most important nuclides, it turned out that the present methodology has the capability to prepare fission-related nuclear data for nuclides for which measurements are difficult.

Studying differential cross section for elastic proton scattering by a tensor model

Previous studies have shown the reliability of the Bayesian Gaussian CANDECOMP/PARAFAC tensor decomposition (BGCP) algorithm in predicting the independent fission yields and evaluating global uncertainty of the data. This work proposes to apply the BGCP algorithm for the purpose of predicting and evaluating the differential cross section for elastic proton scattering. The tensor model is developed by training with the experimental data of elastic proton scattering from the EXFOR database. The Root Mean Square Error is used to evaluate the experimental data and search for potential errors and omissions in EXFOR, in order to identify outlying data of 98Mo, 100Mo, 144Sm and 209Bi. After the revision for the outlying data in EXFOR, the reproduced values for 144Sm are compared with the remaining controversial entries. Besides, the predicted data for 9C are compared with the published data not available in EXFOR. The results demonstrate that the tensor model can make reasonable predictions and has a role to play in evaluating the quality of elastic proton scattering data. The tensor model may become a powerful tool for nuclear data evaluation in EXFOR.

TALYS calculations of prompt fission observables and independent fission product yields for the neutron-induced fission of ^{235}U

The TALYS nuclear reaction code’s Hauser-Feshbach statistical decay model has been adapted in order to calculate prompt fission neutron and gamma -ray observables by iterating over deexciting fission fragments. Several fission fragment generators such as GEF, HF^{3}D, and SPY were employed to provide TALYS with databases. These databases contain standardized tables with fission fragment yields, mean excitation energies and their widths, and average total kinetic energy, as a function of charge and mass number of primary fission fragments. The resulting calculations, including prompt particle multiplicities, spectra, average energies, and independent fission product yields, were compared with experimental and evaluated data. This work first outlines the new methodology implemented in TALYS and examines the effects of three important parameters on the final evaporation data. Furthermore, the neutron-induced fission of ^{235}U is investigated in detail as a function of incident energy. The results from TALYS, with input from GEF and HF^{3}D, were compared with available experimental data and the results of the stand-alone GEF code. The proposed methodology contributes to an improved capability to model the fission process.

Fission cross-section measurement for [formula omitted] and [formula omitted] in [formula omitted] reaction induced by 14MeV D-T neutrons

In this work, the independent fission cross-sections of U(n,f)238Xe135g and U(n,f)238Xe135m reactions induced by neutrons with the energies of 14.1 MeV, 14.5 MeV, and 14.7 MeV were measured by activation methods. The neutrons from T(d,n)He4 were used in the experiments and their energies were determined by the ratio of Zr(n,2n)90Zr89 and Nb(n,2n)93Nb92m reaction cross-sections. Aluminum films were chosen as reference samples to measure the neutron fluence relative to the cross section of the Al(n,α)27Na24 reaction. The effect of self-absorption, geometry, and cascade coincidence were also considered during the data analysis. Besides, the increase in the daughter nuclide yield due to the decay of the parent nuclides in the same decay chain was deducted. As a result, the independent fission cross-sections of the U(n,f)238Xe135g reaction are 2.54 ± 0.14 mb, 3.05 ± 0.19 mb and 2.94 ± 0.19 mb, while the cross-sections of U(n,f)238Xe135m reaction are 2.11 ± 0.16 mb, 2.47 ± 0.18 mb and 2.34 ± 0.21 mb for 14.1 MeV, 14.5 MeV and 14.7 MeV neutrons, respectively. This work provides experimental data for the database of nuclear fission reactions.

Consideration of memory of spin and parity in the fissioning compound nucleus by applying the Hauser-Feshbach fission fragment decay model to photonuclear reactions

Prompt and $\beta$-delayed fission observables, such as the average number of prompt and delayed neutrons, the independent and cumulative fission product yields, and the prompt $\gamma$-ray energy spectra for the photonuclear reactions on $^{235,238}$U and $^{239}$Pu are calculated with the Hauser-Feshbach Fission Fragment Decay (HF$^3$D) model and compared with available experimental data. In the analysis of neutron-induced fission reactions to the case of photo-induced fission, an excellent reproduction of the delayed neutron yields supports a traditional assumption that the photo-fission might be similar to the neutron-induced fission at the same excitation energies regardless of the spin and parity of the fissioning systems.

Independent isotopic fission yields of 252Cf spontaneous fission via mass measurements at the FRS Ion Catcher

We present first preliminary results of a novel method for measuring independent isotopic fission yields (IIFYs) of spontaneous fission (SF) via direct mass measurements, at the FRS Ion Catcher (FRS-IC) at GSI. Fission products were generated from a 252Cf source installed in a cryogenic stopping cell, and were identified and counted with the multiple-reflection time-of-flight mass spectrometer (MR-TOF-MS) of the FRS-IC, utilizing well-established measurement and data analysis methods. The MR-TOF-MS resolves isobars unambiguously, even with limited statistics, and its non-scanning nature ensures minimal relative systematic uncertainties amongst fission products. The analysis for extracting IIFYs includes isotope-dependent efficiency corrections for all components of the FRS-IC. In particular, we applied a self-consistent technique that takes into account the element-dependent survival efficiencies in the CSC, due to chemical reactions with the buffer gas. Our IIFY results, which cover several tens of fission products in the less-accessible high-mass peak (Z = 56 to 63) down to fission yields at the level of 10−5, are generally similar to those of the nuclear database ENDF/B-VII.0. Nevertheless, they reveal some structures that are not observed in the database smooth trends. These are the first results of a planned campaign to investigate IIFY distributions of spontaneous fission at the FRS-IC. Upcoming experiments will extend our results to wider Z and N ranges, lower fission yields, and other spontaneously-fissioning actinides.

The Extension of the Hauser-Feshbach Fission Fragment Decay Model to Multi-chance Fission and its Application to 239Pu

The Hauser-Feshbach fission fragment decay model, HF3D, calculates the statistical decay of fission fragments through both prompt and delayed neutron and γ-ray emissions in a deterministic manner. While previously limited to the calculation of only first-chance fission, the model has recently been extended to include multi-chance fission, up to neutron incident energies of 20 MeV. The deterministic decay takes as input prescission quantities–fission probabilities, pre-fission neutron energies, and the average energy causing fission– and post-scission quantities–yields in mass, charge, total kinetic energy, spin, and parity. From those fission fragment initial conditions, the full decay is followed through both prompt and delayed particle emissions. The evaporation of the prompt neutrons and γ rays is calculated through the Hauser-Feshbach statistical theory, taking into account the competition between neutron and γ-ray emission, conserving energy, spin, and parity. The delayed emission is taken into account using time-independent calculation using decay data. This whole formulation allows for the calculation of prompt neutron and γ-ray properties, such as multiplicities and energy distributions, both independent and cumulative fission yields, and delayed neutron observables, in a consistent framework. Here, we describe the implementation of multi-chance fission into the HF3D model, and show an example of prompt and delayed quantities beyond first-chance fission, using the example of neutron-induced fission on 239Pu. This expansion represents significant progress in consistently modeling the emission of prompt and delayed particles from fissile systems.

Generation and analysis of independent fission yield covariances based on GEF model code

The fission yield data provided by the evaluated nuclear data files do not contain covariance information, which is not conducive to uncertainty analysis. To generate covariance information, the model parameters of the code GEF which describes the fission process are sampled and the independent fission yield samples are calculated. The covariances of independent fission yields of 235U, 239Pu, and 241Pu thermal neutron-induced fissioning systems are generated individually based on the above samples. This method is verified by comparing the uncertainties of burnup-related responses based on fission yield samples calculated by GEF and based on fission yield samples generated with the covariances. The influence of correlations among fissioning systems is also quantified and the joint covariances among different fissioning systems calculated with GEF are demonstrated correct. In addition, the Bayesian Monte Carlo method is adopted to adjust the model parameters of GEF, and the numerical results prove the effectiveness of the adjustment.

From fission yield measurements to evaluation: Propositions of 235U(nth,f) Fission Yields for JEFF-4

The study of fission yields has a major impact on the characterization and understanding of the fission process and its applications. The fission products have a direct influence on the amount of neutron poisons that limit the fuel burnup and on the evaluation of the decay heat of the reactor after shutdown. Fission yield evaluation represents the synthesis of experimental and theoretical knowledge in order to perform the best estimation of mass and independent fission product yields. Today, the lack of correlations between the different fission observables induces some inconsistencies in the evaluations. For instance, the mass yield uncertainties are drastically overestimated while this observable is the best known. In this last decade, different covariance matrices have been proposed but the experimental part of those are neglected. A consistent covariance matrix depends on the global process of evaluation from data up to the models used. The collaboration develops a new methodology in the field of fission products for the future version of the JEFF-library coupling the analyses of the CEA and NNL institutes.

New design and simulation of the ion guide for neutron-induced fission products at the IGISOL facility

Measurements of independent fission yield distributions in neutron-induced fission at high neutron energies are important for our fundamental understanding of the fission process, and are also relevant for reactor physics applications. So far, measurements of independent fission yields in proton-induced fission have been performed at the IGISOL facility at the University of Jyväskylä, using the Penning trap as a high resolving-power mass-filter. In order to also facilitate measurements of neutron-induced fission, a dedicated ion guide and a proton-to-neutron converter was developed. However, the first measurement indicates that fewer fission products than expected reach the Penning trap. To explore potential reasons and possible improvements, a simulation model was also developed and benchmarked. The benchmark showed that the model is able to reproduce the performance of the ion guide remarkably well and that the main reason for the low yield of fission products is the low collection efficiency of the ion guide. Based on the benchmark, a new ion guide is being designed. In the new design, the positions of the uranium targets and volume of the ion guide have been changed to increase the collection of fission products. This results in a five-fold increase of the yield. However, the collection efficiency of the new ion guide still needs to be improved in order to achieve intensities of the extracted fission products that are large enough to allow for reasonable measurement times. Because the volume of the ion guide is increased significantly, the extraction time of the ions is expected to be longer than that from the previous ion guide. Therefore, an electric field guidance system that consists of a combination of a stationary electric field and an RF-carpet is considered to be deployed. The stationary field, produced from a set of DC-ring electrodes, accelerates the ions towards the RF-carpet at end plate of the ion guide. The RF-carpet consists of a time-dependent field, produced from a radio-frequent structure of concentric rings, with a DC-component that guides the ions towards the exit hole in the center of the end plate. In this paper we present the current status of the simulations and design of the new ion guide.

Covariance analysis of 235U(nth,f) independent and cumulative fission yields: Propositions for JEFF4

The study of fission yields has a major impact on the characterization and understanding of the fission process and its applications. Fission yield evaluations represent the synthesis of experimental and theoretical knowledge in order to perform the best estimation of independent fission yields. Today, the lack of correlations between the different fission observables induces several inconsistencies in the evaluations. Different works proposed to estimate the correlations of the independent fission yields satisfying the consistency to the chain yield evaluations. Nevertheless, none of them introduces a prior correlations of the independent and chain evaluations in the evaluation process. Covariance matrix of fission yields depends on the evaluation method used according to the kinds of existing measurements. The consistency is deeply entangled to the statistical agreement between each dataset considering the covariance of measurements. Moreover, covariance of model parameters does not represent the only contribution to the evaluation covariance matrix. Thus, a new evaluation process is crucial to provide a complete and coherent evaluation file. The LEPh Laboratory of CEA Cadarache is developing this program for the future version of the JEFF-library.

Measurement of the fission yield of 136Cs in the 239Pu(nth,f) reaction

A recent experimental campaign performed at the LOHENGRIN spectrometer at ILL aimed at measuring the independent fission yield of 136Cs in the 239Pu(nth,f) reaction. This nuclide can have an important contribution to the total dose rate coming from spent light water reactor UOX and MOX fuels. Moreover, its impact is of first order on the uncertainty of the total dose rate calculated in specific areas of Nuclear Power Plants within accidental conditions. One of the most important sources of uncertainty is its independent fission yield. Therefore, a new measurement of its independent yield along with a rigorous uncertainty analysis was performed. Due to its low independent yield, a new measurement technique has been applied. Ions recoiling from neutron-irradiated 239Pu target were collected by implantation into an Al foil placed inside a vacuum chamber. This foil was then transferred to a low γ-ray background setup located at nearby laboratory (LPSC). The procedure was then repeated for different LOHENGRIN settings. Compared to the JEFF-3.1.1 library, a decrease of the fission yield of 136Cs with a reduced uncertainty is obtained.

Stopping Force Analysis of 235U Elemental Fission Product Yields for En = 0.11–92.4 MeV

Most evaluated elemental fission product yield distributions are not experimentally measured. Instead, the majority of evaluated distributions are based on analytic expressions of the Zp-model for relevant cumulative yields. Here we report independent elemental fission product yield distributions of a 235U target for incident neutron energies ranging from 0.11 MeV through 92.4 MeV. Atomic numbers are calculated by an approach that combines a 2E analysis with a stopping force analysis method, developed within this paper. These analyses are applied to more than 6.1 × 106 fission fragment ionization tracks captured within the NIFFTE (Neutron Induced Fission Fragment Tracking Experiment) collaboration fission time projection chamber (fissionTPC). A 3-Z resolution was obtained with the fissionTPC spatial and energy resolutions. Tabulated results are presented for the atomic yield and experimentally derived Zp values as a function of pre-neutron-emission fragment masses for the complete range of incident neutron energies. The stopping-force-derived Zp values tend to support the unchanged charge distribution theory within uncertainty.

Bayesian approach to heterogeneous data fusion of imperfect fission yields for augmented evaluations

We demonstrate that Bayesian machine learning can be used to treat the vast amount of experimental fission data which are noisy, incomplete, discrepant, and correlated. To supply the application needs, the two-dimensional cumulative fission yields (CFY) of neutron-induced fission of $^{238}\mathrm{U}$ are evaluated for energy dependencies and uncertainty qualifications by cross-experiment data fusion. For independent fission yields (IFY) with very few experimental data, the heterogeneous data fusion of CFY and IFY is employed to interpolate the energy dependence. This work shows that Bayesian data fusion can facilitate the maximum utilization of imperfect raw nuclear data.