Fission Of 235U Research Articles

Nuclear Data Sensitivity Study for the EBR-II Fast Reactor Benchmark Using SCALE with ENDF/B-VII.1 and ENDF/B-VIII.0

The EBR-II benchmark, which was recently included in the International Handbook of Evaluated Reactor Physics Benchmark Experiments, served as a basis for assessing the performance of the SCALE code system for fast reactor analyses. A reference SCALE model was developed based on the benchmark specifications. Great agreement was observed between the eigenvalue calculated with this SCALE model and the benchmark eigenvalue. To identify potential gaps and uncertainties of nuclear data for the simulation of various quantities of interest in fast spectrum systems, sensitivity and uncertainty analyses were performed for the eigenvalue, reactivity effects, and the radial power profile of EBR-II using the two most recent ENDF/B nuclear data library releases. While the nominal results are consistent between the calculations with the different libraries, the uncertainties due to nuclear data vary significantly. The major driver of observed uncertainties is the uncertainty of the 235U (n,γ) reaction. Since the uncertainty of this reaction is significantly reduced in the ENDF/B-VIII.0 library compared to ENDF/B-VII.1, the obtained output uncertainties tend to be smaller in ENDF/B-VIII.0 calculations, although the decrease is partially compensated by increased uncertainties in 235U fission and ν¯.

ФИЛЬТР КАЛЬМАНА В ЗАДАЧЕ ОЦЕНКИ 8-ГРУППОВЫХ СПЕКТРОВ ЗАПАЗДЫВАЮЩИХ НЕЙТРОНОВ ПРИ ДЕЛЕНИИ 235U ТЕПЛОВЫМИ НЕЙТРОНАМИ

In the present work the estimation of the 8-group delayed neutron energy spectra emitted in the fission of 235U by thermal neutrons have been done. Aggregate delayed neutron spectra that were measured with high resolution in the different time intervals after the thermal neutron irradiation of the 235U sample presented in the IAEA reference database for beta-delayed neutron emission have been used for the estimation process. Due to the lack of the uncertainties for the spectra presented in this database and the necessity of this data in the process of estimation of delayed neutron group spectra at first the estimation of uncertainties of these spectra have been done. In the estimation process the following components of uncertainty have been considered: statistical; the component due to the neutron background; component due to the distinctive features of neutron spectra restoration process measured using spectrometer filled by helium-3 and also the component appearing due to introduction of the correction on the distortion of the neutron spectra as a result of passing through the lead sheilding. Estimation of spectra have been made using the technique based on the Kalman filter that allows to take into account the uncertainties of the input data. Estimation results are shown in the numerical and graphic form.

Compact nuclear-driven semiconductor reactor/laser system

A model for a solid state nuclear pumped laser (SSNPL) has been developed. The laser model includes the spatial inhomogeneities of electron-hole pairs generated by fission products. A MCNP model was produced which transported the 20 most common U-235 fission fragments to dictate the 1-dimensional spatial inhomogeneity. The MCNP model included a variety of thicknesses to the layers involved. Core properties include a minimally critical cubic bare nuclear core which has dimensions less than one cubic meter on a side and laser thresholds in the sub-megawatt range. Laser efficiencies ranged from 2 to 5%. The SSNPL has the potential to be compact, low mass and efficient making it a promising space-based laser system.

Validation of several sets of delayed-neutron group parameters for thermal fission of 235U by a Feynman-α analysis

ABSTRACT Time-sequence count data of a neutron counter were acquired at subcritical states of UTR-KINKI reactor and then a Feynman-α analysis considering delayed neutrons was carried out to determine a measured ratio of delayed to prompt neutron contribution to the correlation amplitude Y. The zero-power reactor was driven by a neutron source inherent in highly enriched uranium fuels to reduce any spatial effect. The delayed-to-prompt ratio was also calculated using several sets of delayed-neutron group parameters for thermal fission of 235U nuclide, to validate these sets by comparing with the measured ratio. The ratios calculated using Keepin’s 6-group and Spriggs’ 8-group sets agreed very well with the measured ratio. However, the ratios calculated using the other sets were almost the same as each other and in a very poor agreement with the measured ratio.

Manufacturing LEU-foil annular target in Brazil

Molybdenum-99 is the most important isotope because its daughter isotope, technetium-99m, has been the most used medical radioisotope. The primary method used to produce Mo-99 derives from the fission of U-235 incorporated in so-called irradiation targets. Two routes are being developed to make Mo-99 by fissioning with low enriched uranium (LEU) fuel. The first adopts UAlx-Al dispersion plate targets. The second uses uranium metal foil annular targets. The significant advantage of uranium foil targets over UAlx-Al dispersion targets is the high density of uranium metal. This work presents the experience obtained in the development of the uranium metal annular target manufacturing steps. An innovative method to improve the procedure for assembling the uranium foil on the tubular target was presented. The experience attained will help the future production of Mo-99 in Brazil through the target irradiation in the Brazilian Multipurpose Reactor (RMB).

Increasing productivity in the manufacture of UAl2–Al dispersion-plate targets for Mo-99 production

Molybdenum-99 is the most important isotope because its daughter isotope, technetium-99 m, has been the most widely used medical radioisotope. The primary method employed to produce Mo-99 derives from the fission of U-235 incorporated in so-called irradiation targets. Pushed by the international Mo-99 crisis that occurred in 2009/2010, Brazil has decided to construct a new research reactor, the Brazilian Multipurpose Reactor (RMB), to produce this vital radioisotope to meet the Brazilian domestic demand. As part of this effort, it has been developed the process for manufacturing the target to be used in the production of Mo-99 by nuclear fission. The low enriched uranium (LEU) aluminide with the predominant phase UAl2 was the starting material. The picture-frame technique was used to clad UAl2–Al briquette with aluminum to obtain plate-type targets. It was developed an innovative method that allows increasing the productivity of this type of target based on multi-core rolling. A thermomechanical treatment was designed to get targets composed basically of a mixture of UAl3/UAl4 that are the required phases for a proper radiochemical dissolution after irradiation. The manufacturing process proved to be suitable for domestic production of targets, fulfilling the specification to produce Mo-99 in the Brazilian Multipurpose Reactor.

Generation of fast neutron removal cross sections using a multi-layered spherical shell model

A new method is explored for estimating the fast or fission neutron removal cross sections in single-element or multi-element media. A spherical Monte Carlo model conforming to the removal theory for fission neutrons was used. The MCNP5 code coupled with ENDF/B-VII.0 library was used to simulate the transport of U-235 fission neutrons within the model. Several removal cross sections were generated for abundant elements in construction and nuclear materials including H, Li, Be, B, C, O, F, Al, Cl, Fe, Ni, Cu, W, Pb, Bi, and U, and multi-element materials such as several compounds and composites. Validations were made using experimentally produced fast neutron removal cross section values, and those given by Phy-X/PSD and MRCsC software, and an empirical method. The comparison of the Monte Carlo generated removal cross sections showed good agreements with experimental values and those by theoretical methods. For multi-element media, the Monte Carlo generated removal cross sections were found to best fit the experimental values, in comparison with those produced by other available methods. Furthermore, a comparison of the thermal fluence trends obtained between the Monte Carlo simulations and several neutron removal experiments resulted in good agreement. The proposed method is highly useful in the generation of ENDF/B based removal cross sections, for point-kernel applications and fast neutron shielding materials comparisons.

Experimental Analyses of 243Am and 235U Fission Reaction Rates at Kyoto University Critical Assembly

Measurements of 243Am and 235U fission reaction rates are conducted with the use of two single fission chambers in the solid-moderated and solid-reflected core at the Kyoto University Critical Assembly (KUCA). Critical irradiation experiments of 243Am and 235U foils are carried out, and the measured result of 243Am/235U is 0.0424 ± 0.0019; also, calculation/experiment values between calculated (MCNP6.1 with JENDL-4.0, ENDF/B-VIII.0, and JEFF-3.3) and measured results of 243Am/235U range among 0.93 ± 0.04, 0.94 ± 0.04, and 0.93 ± 0.04, respectively. Through a comparison between the measured and calculated results, the 243Am fission cross-section data of the three major nuclear data libraries are successfully validated, demonstrating the same accuracy as that of previous minor actinide irradiation experiments at KUCA. Importantly, the comparison also provides the complemental data of integral experiments of 243Am fission reaction rates that confirm the accuracy of the 243Am fission cross-section data.

First lifetime investigations of N>82 iodine isotopes: The quest for collectivity

We report on spectroscopic information and lifetime measurements in the neutron-rich I135,137,139 isotopes. This is the first lifetime data on iodine isotopes beyond N=82. Excited states were populated in fast neutron-induced fission of U238 at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν-ball spectrometer. The level schemes of the I135,137,139 isotopes are revised in terms of excited states with up to maximum spin-parity of (33/2+), populated for the first time in fast neutron-induced fission. We provide first results on the lifetimes of the (9/21+) and (13/21+) states in I137 and I139, and the (17/21+) state in I137. In addition, we give upper lifetime limits for the (11/21+) states in I135−139, the (15/21+) state in I137, the (17/21+) state in I139, and reexamine the (29/21+) state in I137. The isomeric data in I135 are reinvestigated, such as the previously known (15/21+) and (23/21−) isomers with T1/2 of 1.64(14) and 4.6(7) ns, respectively, as obtained in this work. The new spectroscopic information is compared to that from spontaneous or thermal-neutron induced fission and discussed in the context of large scale shell-model (LSSM) calculations for the region beyond Sn132, indicating the behavior of collectivity for the three valence-proton iodine chain with N=82,84,86.5 MoreReceived 9 March 2021Revised 31 May 2021Accepted 25 June 2021DOI:https://doi.org/10.1103/PhysRevC.104.014316©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsFissionLifetimes & widthsNuclear structure & decaysProperties90 ≤ A ≤ 149Nuclear Physics

First antineutrino energy spectrum from 235U fissions with the STEREO detector at ILL * * Stereo Collaboration, http://www.stereo-experiment.org.

This article reports the measurement of the 235U-induced antineutrino spectrum shape by the Stereo experiment. 43 000 antineutrinos have been detected at about 10 m from the highly enriched core of the ILL reactor during 118 full days equivalent at nominal power. The measured inverse beta decay spectrum is unfolded to provide a pure 235U spectrum in antineutrino energy. A careful study of the unfolding procedure, including a cross-validation by an independent framework, has shown that no major biases are introduced by the method. A significant local distortion is found with respect to predictions around E ν ≃ 5.3 MeV. A Gaussian fit of this local excess leads to an amplitude of A = 12.1 ± 3.4% (3.5σ).

I-129 DETERMINATION IN EVAPORATOR CONCENTRATE USING OXIDATIVE EXTRACTION AND CHROMATOGRAPHIC RESIN

Determination of long-lived radionuclides is very important for study of the radioactive waste final deposition. In this work will be studied 129I radionuclide which present 1.6 x 107 years half-life, β-particle-emitting (Emax = 194 keV) and X-ray-emitting (E = 29.78 keV). It’s produced primarily from fission of 235U and 239Pu and for fission induced by thermal neutrons. For this reason, radiochemical procedures for 129I determination in evaporator concentrate wastes from nuclear power plants were carried out. The first procedure was based on oxidative extraction and alkaline absorption and the second one, was based on selective extraction using a chromatographic resin in order to separate iodine from its interferents. After the separation steps, the iodine activity was measured by ultra low gamma spectrometry technique. To set up the yield recovery for 129I, a tracer solution of 129I was used in order to follow the behavior of iodine during the separation steps. The yield recovery for iodine was around 75-80% for the first procedure and 80-85% for the second. The two procedures used mutually, ensure a greater efficiency in the separation of iodine from their respective interferents.

A semimicroscopic algebraic cluster model for heavy nuclei

An extension of the Semimicroscopic Algebraic Cluster Model (SACM) is proposed, based on the pseudo-SU(3) model. The Hamiltonian and the spectroscopic factor operator of the model are presented and a procedure of constructing the model space. Because a huge number of SU(3) irreducible representations (irrep) appear, one has to be careful in designing a practical, consistent path to reduce the Hilbert space. The concept of forbiddenness, taking into account excitations of the clusters, is introduced and applied. The applications are to two systems with a low forbiddenness, namely to 236U -> 210Pb + 26Ne and 224Ra -> 210Pb + 14C, and to 236U -> 146Xe + 90Sr, which appears in the fission of 236U, which requires a large forbiddenness. Energies, electromagnetic transitions and spectroscopic factors are calculated.

Neutron emission and fast ion simulation for high performance long pulses at EAST.

Neutron emission spectroscopy and neutron yield measurements are important for high neutral beam injection (NBI) power heating at the Experimental Advanced Superconducting Tokamak (EAST). The neutron yields mainly depend on the deposition from NBI to the deuterium plasmas in the EAST. We have recently used TRANSP with time dependent diagnostic results to simulate the transport process of 30s long pulse deuterium plasma discharges in the EAST, obtaining the time dependent fast ion distribution, neutron emission spectrum, and total neutron emission rate. Combined with the time trace of the result measured by a standard 235U fission chamber, the effects of different configurations of NBI heating in EAST fusion plasmas have been evaluated.

A compact stilbene crystal neutron spectrometer for NBI-heated plasma neutron diagnostics at EAST.

Stilbene crystal detectors are widely used as fast neutron measurement tools based on recoil proton detection, such as liquid scintillators. A compact stilbene crystal neutron spectrometer (CSCNS) has been installed at the Experimental Advanced Superconducting Tokamak (EAST) to obtain information on fuel ions produced in the plasma core because of its merits of good n/γ discrimination capability, high detection efficiency, and fast response. For the first time, CSCNS has been used for neutron emission spectroscopy measurements in EAST plasmas with neutral beam injection (NBI) heating. The CSCNS has the same horizontal line of sight as the time-of-flight enhanced diagnostics neutron spectrometer. Under NBI heating scenarios, the time trace of the neutron yield monitored by the CSCNS is similar to the one monitored by a standard 235U fission chamber. The experimental pulse height spectra are also similar to the simulated ones generated by folding the simulated neutron energy spectrum with the detector response functions. These results demonstrate the capability of the CSCNS for neutron diagnostics and the study of fast-ion physics in EAST.

Spectroscopy and lifetime measurements in Te134,136,138 isotopes and implications for the nuclear structure beyond N=82

We report on spectroscopic information and lifetime measurements of even-even neutron-rich Te isotopes. Excited states were populated in fast-neutron induced fission of U238 at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν-ball spectrometer. We provide first results on lifetimes of the 61+ state in Te136 and the (61+), (41+), and (21+) states in Te138 and discuss the results in the context of large-scale shell-model calculations. The level schemes of Te136 and Te138 are revised in terms of lifetimes of their 21+,41+ states and updated information on the (42+) state in Te136 is presented. In addition, previously reported data on spectroscopy and lifetimes in Te134 are reexamined. This work provides new insights into the evolution of collectivity for Te isotopes with N=82,84,86.6 MoreReceived 30 September 2020Revised 4 February 2021Accepted 9 March 2021DOI:https://doi.org/10.1103/PhysRevC.103.034317©2021 American Physical SocietyPhysics Subject Headings (PhySH)Research AreasElectromagnetic transitionsLifetimes & widthsNuclear structure & decaysProperties90 ≤ A ≤ 149TechniquesShell modelNuclear Physics

Development of a Wide Dynamic Range Neutron Flux Measurement Instrument Having Fast Time Response for Fusion Experiments

A wide-range neutron flux measurement instrument is developed herein for monitoring the total neutron emission rate and yield of the Large Helical Device (LHD) during deuterium experiments implemented from March 2017 in the National Institute for Fusion Science (NIFS), Japan. The instrument is designed for and installed on the Neutron Flux Monitoring (NFM) system, which measures the counting rate using a 235U Fission Chamber. By combining the pulse counting and Campbell methods, the Digital Signal Processing Unit (DSPU) realized a wide dynamic range of over six orders of magnitude from 1 × 103 counts/s (cps) to 5 × 109 cps. This study explains and discusses how the instrument is developed, including topics from the predevelopment activities to the verification test at the Kyoto University Critical Assembly (KUCA). Experimental results in the LHD using the finished products suggest that the NFM system works well during deuterium experiments.

Thermal annealing of implanted <sup>252</sup>Cf fission tracks in monazite

Abstract. A series of isochronal heating experiments were performed to constrain monazite fission track thermal annealing properties. The 252Cf fission tracks were implanted into monazite crystals from the Devonian Harcourt granodiorite (Victoria, Australia) on polished surfaces oriented parallel to (100) pinacoidal faces and perpendicular to the crystallographic c axis. Tracks were annealed over 1, 10, 100 and 1000 h schedules at temperatures between 30 and 400 ∘C. Track lengths were measured on captured digital image stacks and then converted to calculated mean lengths of equivalent confined fission tracks that progressively decreased with increasing temperature and time. Annealing is anisotropic, with tracks on surfaces perpendicular to the crystallographic c axis consistently annealing faster than those parallel to the (100) face. To investigate how the mean track lengths decreased as a function of annealing time and temperature, one parallel and two fanning models were fitted to the empirical dataset. The temperature limits of the monazite partial annealing zone (MPAZ) were defined as length reductions to 0.95 (lowest) and 0.5 (highest) for this study. Extrapolation of the laboratory experiments to geological timescales indicates that for a heating duration of 107 years, estimated temperature ranges of the MPAZ are −44 to 101 ∘C for the parallel model and −71 to 143 ∘C (both ±6–21 ∘C, 2 standard errors) for the best-fitting linear fanning model (T0=∞). If a monazite fission track closure temperature is approximated as the midpoint of the MPAZ, these results, for tracks with similar mass and energy distributions to those involved in spontaneous fission of 238U, are consistent with previously estimated closure temperatures (calculated from substantially higher energy particles) of < 50 ∘C and perhaps not much higher than ambient surface temperatures. Based on our findings we estimate that this closure temperature (Tc) for fission tracks in monazite ranges between ∼ 45 and 25 ∘C over geological timescales of 106–107 years, making this system potentially useful as an ultra-low-temperature thermochronometer.

Alpha-decay induced shortening of fission tracks simulated by in situ ion irradiation

The diffusion of defects (e.g., vacancies and interstitials) and elements used for dating (e.g., He and Pb) in a mineral structure is a thermal process: This is the primary assumption used to determine the age and thermal history of minerals. For instance, thermal history reconstruction, through the number and length distribution of tracks produced by spontaneous fission of 238U, is obtained by assuming a thermal event to be the only energy source for shortening of fission tracks. Here, we report a new, non-thermal energy source that induces additional shortening of fission tracks by the irradiation of alpha-recoils from the alpha-decay of 238U and 232Th. We simulate alpha-decay induced track-shortening by combining ion accelerator irradiations with transmission electron microscopy. This allows for the first observation of track-shrinkage during in situ ion irradiation. We show that rather than alpha-particles, alpha-recoils induce a significant shortening of fission tracks by nuclear-collisions. The shortening of track-length can be quantified as a function of alpha-decay event dose. However, apatite is less sensitive than zircon to this non-thermal process. The findings exemplify the interactions among different types of self-irradiation from alpha-particles, alpha-recoils and fission-fragment nuclei in single mineral grains and have important implications for the use of zircon and apatite for radiometric dating and thermochronology.

Improved short-baseline neutrino oscillation search and energy spectrum measurement with the PROSPECT experiment at HFIR

We present a detailed report on sterile neutrino oscillation and U-235 antineutrino energy spectrum measurement results from the PROSPECT experiment at the highly enriched High Flux Isotope Reactor (HFIR) at Oak Ridge National Laboratory. In 96 calendar days of data taken at an average baseline distance of 7.9 m from the center of the 85 MW HFIR core, the PROSPECT detector has observed more than 50,000 interactions of antineutrinos produced in beta decays of U-235 fission products. New limits on the oscillation of antineutrinos to light sterile neutrinos have been set by comparing the detected energy spectra of ten reactor-detector baselines between 6.7 and 9.2 meters. Measured differences in energy spectra between baselines show no statistically significant indication of antineutrinos to sterile neutrino oscillation and disfavor the Reactor Antineutrino Anomaly best-fit point at the 2.5$\sigma$ confidence level. The reported U-235 antineutrino energy spectrum measurement shows excellent agreement with energy spectrum models generated via conversion of the measured U-235 beta spectrum, with a $\chi^2$/DOF of 31/31. PROSPECT is able to disfavor at 2.4$\sigma$ confidence level the hypothesis that U-235 antineutrinos are solely responsible for spectrum discrepancies between model and data obtained at commercial reactor cores. A data-model deviation in PROSPECT similar to that observed by commercial core experiments is preferred with respect to no observed deviation, at a 2.2$\sigma$ confidence level.

Gas phase synthesis of 4d transition metal carbonyl complexes with thermalized fission fragments in single-atom reactions

Abstract The formation of carbonyl complexes using atom-at-a-time quantities of short-lived transition metals from fusion and fission reactions was reported in 2012. Numerous studies focussing on this chemical system, which is also applicable for the superheavy elements followed. We report on a novel two-chamber approach for the synthesis of such complexes that allows spatial decoupling of thermalization and gas-phase carbonyl complex synthesis. Neutron induced fission on 235U and spontaneous fission of 248Cm were employed for the production of the fission products. These were stopped inside a gas volume behind the target and flushed with an inert-gas flow into a second chamber. This was flushed with carbon monoxide to allow the gas-phase synthesis of carbonyl complexes. Parameter studies of the transfer from the first into the second chamber as well as on the carbonyl complex formation and transport processes have been performed. High overall efficiencies of more than 50% were reached rendering this approach interesting for studies of superheavy elements. Our results show that carbonyl complex formation of thermalized fission products is a single-atom reaction, and not a hot-atom reaction.