Thermal Neutron-induced Fission Research Articles

Prompt-fission observable and fission yield calculations for actinides by TALYS

The nuclear reaction code TALYS adopts the Hauser-Feshbach statistical decay theory, to de-excite fission fragments. This involves for instance the evaporation of prompt fission neutrons and γ-rays. TALYS incorporates databases of primary fission fragment distribution which consists of primary fission fragment yield and data for excitation energy distribution of fission fragments. We conducted a sensitivity study on three parameters in TALYS and fitted them in order to reproduce experimental and evaluated data, in thermal neutron-induced fission of 235U. Moreover, we demonstrate a large-scale calculation of average prompt neutron and γ-ray multiplicities for 243 selected actinide isotopes.

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.

The description of dynamical fission process using improved quantum molecular dynamics model incorporated with microscopic potential energy surface

A new method, namely the improved quantum molecular dynamics model incorporated with the microscopic potential energy surface, is proposed to simulate the dynamical evolution from saddle to scission point and to calculate the isotope yields of fission fragments in thermal neutron-induced fission of actinide nuclei near 236U. With this approach, the shell and pairing effects are introduced by the microscopic potential energy, while the isospin effect, the dynamical effects of fluctuations and correlations in fission process are automatically included in the improved quantum molecular dynamics model. These two aspects eventually influence the productions of fission fragments. The calculated both charge and mass distributions of fission fragments for a series of actinide nuclei are in overall agreement with the evaluated data from JEFF-3.3 and ENDF/B-VIII.0. The isotope distributions of fission fragments in the fission of 236U reproduce the data reasonable well, from which we extract the relationship between peak mass number of the isotope distributions and the charge number of fission fragments. We find that this relationship deviates linear relation slightly at certain cases, which indicates the isospin nonequilibrium effect. By tracking back to the fission process on the two dimension plane of elongation and mass asymmetry of prefragments, we find that the necked-in shape and the separation of two parts happen much faster for the 132Sn than 128,136Sn in the time evolution of the distance R between the centers of two parts of the fissioning system. The events producing 128,132,136Sn and 136,140,144Xe present two kinds of average fission paths with bypassing and going through the minimum-energy valley of the microscopic potential energy surface, respectively.

Post-Neutron Mass Yield Distribution in the Thermal Neutron Induced Fission of 233U

In the thermal neutron–induced fission of 233U, the cumulative and independent yields of various fission products within the mass ranges of 77 to 109 and 123 to 155 have been measured by using an off-line gamma-ray spectrometric technique. The lower yields of 86Br and 136I than usual trend indicate the formation of delayed neutron emitters 87Br and 137I. From the cumulative yields, the post-neutron mass yield distribution was obtained after applying the charge distribution correction. The data from the present and earlier work of our laboratory in the 233U(nth,f) reaction were compared with similar data of 232,235U(nth,f) and 238U(n,f) reactions to examine the effect of mass difference of the fissioning systems on the fine structure of the mass yield distribution. The mass yield distribution in the 233U(nth,f) reaction was also compared with those of 229Th(nth,f), 241Pu(nth,f), and 245Cm(nth,f) reactions to examine the effect of charge and mass difference of the fissioning systems and also to examine the different behaviors of standard I and standard II asymmetric modes of fission.

Post-Neutron Mass Yield Distribution in the Thermal Neutron Induced Fission of 235U

The cumulative and independent yields of various fission products within the mass ranges of 78 to 108 and 123 to 155 have been measured in the thermal neutron–induced fission of 235U by using an off-line gamma-ray spectrometric technique. The post-neutron mass yield distribution was obtained from the cumulative yields after applying the charge distribution correction. The data from present and earlier work of our laboratory in the 235U(nth,f) reaction were compared with similar data of 229Th(nth,f), 245Cm(nth,f), and 252Cf(SF) reactions to examine the fine structure in the mass yield distribution for four different even-even fissioning systems with charge of 90 to 98. The comparison shows that the fine structure in the mass yield distribution depends on spherical and deformed neutron shell combinations. The shell combination favors the standard I asymmetric mode of fission in the 235U(nth,f) and 245Cm(nth,f) reactions, whereas it favors the standard II asymmetric mode of fission in the 229Th(nth,f) and 252Cf(SF) reactions.

Semi-empirical model to determine pre- and post-neutron fission product yields and neutron multiplicity

Post-neutron emission fission product mass distributions are calculated by using pre-neutron emission fission product yields (FPYs) and neutron multiplicity. A semi-empirical model is used to calculate the pre-neutron FPY, first. Then the neutron multiplicity for each fission fragment mass is used to convert the pre-neutron FPY to the post-neutron FPY. In doing so, assumptions are made for the probability for a pre-emission fission fragment with a mass number $A^*$ to decay to a post-emission fragment with a mass number $A$. The resulting post-neutron FPYs are compared with the data available. The systems where the experimental data of not only the pre- and post-neutron FPY but also neutron multiplicity are available are the thermal neutron-induced fission of $^{233}$U, $^{235}$U and $^{239}$Pu. Thus, we applied the model calculations to these systems and compared the calculation results with those from the GEF and the data from the ENDF and the EXFOR libraries. Both the pre- and post-neutron fission product mass distributions calculated by using the semi-empirical model and the neutron multiplicity reproduce the overall features of the experimental data.

Post-Neutron Mass Yield Distribution in the Thermal Neutron Induced–Fission of 239Pu

Mass chain yield distribution has been done in the thermal neutron–induced fission of 239Pu by measuring the cumulative yields of various fission products within the mass range of 78 to 159 and the independent yields of a few products. An off-line gamma-ray spectrometric technique was used to measure the gamma-ray activities of the fission products. From the measured values of the cumulative yields, the post-neutron mass chain yield distribution was obtained after applying the charge distribution correction. Data from the present and earlier work of our laboratory in the 239Pu(nth,f) reaction were compared with similar data of 238,241Pu(nth,f) and 240Pu(n,f) reactions, and it was found that the fine structures of the mass yield distributions are similar. The mass yield distribution in the 239Pu(nth,f) reaction was also compared with those of 229Th(nth,f) and 252Cf(SF) reactions to examine the effect of charge and mass difference of the fissioning systems on the mass yield distribution. It was found that the asymmetric standard I mode of fission is favorable in the 238,239,241Pu(nth,f) and 240Pu(n,f) reactions whereas the standard II mode is favorable in the 229Th(nth,f) and 252Cf(SF) reactions.

Fission Fragment Decay Heat by Using the Most Recent Evaluated Nuclear Data Library ENDF/B-VIII

In this paper, a home-made code was designed to calculate the decay heat emitted by fission fragments as a result of successive radioactive emissions after a fission burst. The nuclear data necessary for the calculations was extracted from the latest version of the Evaluated Nuclear Data Library ENDF/B-VIII.0. The code can calculate the decay heat of thermal and fast neutron-induced fission reactions on the isotopes of Thorium, Protactinium, Uranium, Neptunium, Plutonium, Americium, Curium, California, Einsteinium, and Fermium. A numerical method was used in this work to calculate the decay heat of all fission fragments due to the individual thermal or fast fissions of the isotopes of the previous ten actinides. The most influential nuclei in the decay heat were also identified at different times after the fission event. Moreover, the code showed high capability in calculating the fission fragments inventories and decay heats due to the decay of fission fragments of 31 fissionable nuclei.

Analysis of Fission Fragments Contributors on Total Decay Heat of Thermal Neutron-Induced Fission of U-235

Calculation of the decay heat from the decay/buildup of radionuclides generated after nuclear fission is one of the highest priorities in the nuclear industry. These calculations become more important if they are made together with the analysis of the most important isotopes affecting the decay heat. They are useful in designing the necessary nuclear safety for spent fuels, and their importance cannot be overlooked in the designs of transporting fuel storage containers as well as in the management of the radioactive waste generated. In this paper, by using MATLAB, the decay heat after the thermal fission of a U-235 nucleus was numerically calculated by solving linear differential equations for all the buildups/decays of the fission products. Also, the most contribution of radioactive isotopes to the decay heat was analyzed by using Microsoft Excel. The most influential isotopes were deduced in two ways; either by calculating the most influential isotopes at specific times, or by determining the largest influences in a cumulative manner. All required nuclear data such as decay constants their branching ratios, independent fission yield, and average α-, β-, and γ-energies released per disintegration of any nuclide, have been extracted from the latest version of the Evaluated Nuclear Data Files (ENDF) database ENDF/B-VIII.0. The two different methods used showed a difference in the contributing isotopes, which is logical for the difference in the method of calculation. The first method is suitable for instantaneous data while the second method is more suitable when there is a need to know the cumulative calculations. In sum, we can say that both methods complement each other, and neither of them can be dispensed with in the accurate calculations related to transportation and storage of spent fuel.

New study of the neutron rich 136Te isotope through decay spectroscopy

136Sb is a neutron rich nucleus with one valence proton and three valence neutrons outside the doubly-magic nucleus 132Sn. It plays an important role in the rapid neutron capture process (r-process) as participates in its path further connected to the nucleosynthesis and the type II supernovae. In this work, a new β decay γ ray data on 136Te is obtained. The experiment is performed at the Institute Laue-Langevin using the Lohengrin spectrometer and the thermal neutron-induced fission reaction on $_{92}^{235}{{\rm{U}}_{143}}$. A specific β-β condition method is investigated for the γ-ray detection.

Monte Carlo simulation of the measurement by the 2E technique of the average prompt neutron multiplicity as a function of the mass of fragments from thermal neutron-induced fission of 239Pu

Using a Monte Carlo method, we simulate the measurement, by the 2E technique, of the average prompt neutron multiplicity as a function of the mass of fragments from the thermal neutron-induced fission of 239Pu. The input data for the simulation, associated with the primary fragment mass (A), consist of the yield (Y), the distribution of the total kinetic energy characterized by its average ((TKE) ̅) and its standard deviation (σ_TKE), the average prompt neutron multiplicity (ν ̅_s, a sawtooth approach of experimental data), and the slope of neutron multiplicity against total kinetic energy (dν_s/d<TKE>). The output data, associated with the simulated as the fragment mass measured by the 2E technique (µ), consist of the yield (y), the distribution of the total kinetic energy characterized by its average ((tke) ̅) and its standard deviation (σ_tke), and the average prompt neutron multiplicity (ν ̅_µ). In the mass regions A≈115 and A>150, ν ̅_µ is higher than ν ̅_s. This result suggests that, in those mass regions, the 2E experimental values associated with the average neutron multiplicity are overestimated, referred to the corresponding to the primary fragments.

Post-Neutron Mass Yield Distribution in the Thermal Neutron Induced Fission of 232U

In the thermal neutron–induced fission of 232U, cumulative and independent yields of various fission products within the mass ranges of 72 to 107 and 123 to 158 have been measured using an off-line gamma-ray spectrometric technique. The fission yields were determined relative to the yield of a monitor product 92Sr. Charge distribution correction was applied on the cumulative yields to obtain the post-neutron mass yield distribution. Mass yield distribution parameters such as full-width at tenth-maximum of light and heavy mass wings, average light mass number <AL> and heavy mass number <AH>, and average number of emitted neutrons <ν> were obtained. Data from the present and earlier work on the 232U(nth,f) reaction were compared with similar data of the 235U(nth,f) reaction. It was found that the mass chain yield distribution in the 232U(nth,f) reaction is asymmetric with two major humps as in the case of the 235U(nth,f) reaction. Besides this, in the 232U(nth,f) reaction, the mass yield distribution shows a small third hump for the symmetric fission products. It was also found that the standard II asymmetric mode of fission is favorable in the 232U(nth,f) reaction whereas the standard I asymmetric mode of fission is favorable in the 235U(nth,f) reaction.

Prompt fission neutron yields in thermal fission ofU235and spontaneous fission ofCf252

Background: The sharing of excitation energy between the fission fragments is one of the key issues in studying nuclear fission. One way to address this is by studying prompt-fission neutron multiplicities as a function of other fission observables such as the mass, $\overline{\ensuremath{\nu}}\left(A\right)$. These are vital benchmark data for both fission and nuclear deexcitation models, putting constrains on the fragment excitation energy and hence on the competing prompt neutron/$\ensuremath{\gamma}$-ray emission. Despite numerous detailed studies, recent measurements done at JRC-Geel with the SCINTIA array in the epithermal region show surprisingly strong discrepancies to earlier thermal fission data and the Wahl systematics.Purpose: The purpose was to perform measurements of the prompt-fission neutron multiplicity, as a function of fragment mass and total kinetic energy (TKE), in $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$ and $^{252}\mathrm{Cf}\left(\text{sf}\right)$, to verify and extend the SCINTIA results. Another goal was to validate the analysis methods, and prepare for planned investigations at excitation energies up to 5.5 MeV.Methods: The experiments were conducted at the former 7 MV Van de Graaff facility in JRC-Geel, using a Twin Frisch-Grid Ionization Chamber and two liquid scintillation detectors. A neutron beam with an average energy of 0.5 MeV was produced via the $^{7}\mathrm{Li}$(p,n) reaction. The neutrons were thermalized by a 12 cm thick block of paraffin. Digital data acquisition systems were utilized. Comprehensive simulations were performed to verify the methodology and to investigate the role of the mass and energy resolution on measured $\overline{\ensuremath{\nu}}\left(A\right)$ and $\overline{\ensuremath{\nu}}\left(\text{TKE}\right)$ values. The simulation results also revealed that the $\ensuremath{\partial}\overline{\ensuremath{\nu}}(A)/\ensuremath{\partial}A$ and $\ensuremath{\partial}\overline{\text{TKE}}/\ensuremath{\partial}\overline{\ensuremath{\nu}}$ are affected by the mass and energy resolution. However, the effect is small for the estimated resolutions of this work. Detailed Fluka simulations were performed to calculate the fraction of thermal neutron-induced fission, which was estimated to be about $98%$.Results: The experimental results on $\overline{\ensuremath{\nu}}\left(A\right)$ are in good agreement with earlier data for $^{252}\mathrm{Cf}\left(\text{sf}\right)$. For $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$, the $\overline{\ensuremath{\nu}}\left(A\right)$ data is very similar to the data obtained with SCINTIA, and therefore we verify these disclosed discrepancies to earlier thermal data and to the Wahl evaluation. The experimental results on $\overline{\ensuremath{\nu}}\left(\text{TKE}\right)$ are also in agreement with the data at epithermal energies. For $^{252}\mathrm{Cf}\left(\text{sf}\right)$ a slope value of $\ensuremath{\partial}\overline{\text{TKE}}/\ensuremath{\partial}\overline{\ensuremath{\nu}}=\left(\ensuremath{-}12.9\ifmmode\pm\else\textpm\fi{}0.2\right)\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/\mathrm{n}$ was obtained. For $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$ the value is $\left(\ensuremath{-}12.0\ifmmode\pm\else\textpm\fi{}0.1\right)\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}/\mathrm{n}$. Finally, the neutron spectrum in the center-of-mass system was derived and plotted as a function of fragment mass.Conclusions: This work clearly proves the lack of accurate correlation between fission fragment and neutron data even in the best-studied reactions. The new results highlight the need of a new evaluation of the prompt-fission multiplicity for $^{235}\mathrm{U}\left({\text{n}}_{\text{th}},\text{f}\right)$.

Lognormal-Based Sampling for Fission Product Yields Uncertainty Propagation in Pebble-Bed HTGR

Uncertainty analyses of fission product yields are indispensable in evaluating reactor burnup and decay heat calculation credibility. Compared with neutron cross section, there are fewer uncertainty analyses conducted and it has been a controversial topic by lack of properly estimated covariance matrix as well as adequate sampling method. Specifically, the conventional normal-based sampling method in sampling large uncertainty independent fission yields could inevitably generate nonphysical negative samples. Cutting off these samples would introduce bias into uncertainty results. Here, we evaluate thermal neutron-induced U-235 independent fission yields covariance matrix by the Bayesian updating method, and then we use lognormal-based sampling method to overcome the negative fission yields samples issue. Fission yields uncertainty contribution to effective multiplication factor and several fission products’ atomic densities at equilibrium core of pebble-bed HTGR are quantified and investigated. The results show that the lognormal-based sampling method could prevent generating negative yields samples and maintain the skewness of fission yields distribution. Compared with the zero cut-off normal-based sampling method, the lognormal-based sampling method evaluates the uncertainty of effective multiplication factor and atomic densities are larger. This implies that zero cut-off effect in the normal-based sampling method would underestimate the fission yields uncertainty contribution. Therefore, adopting the lognormal-based sampling method is crucial for providing reliable uncertainty analysis results in fission product yields uncertainty analysis.

Oversize of the average prompt neutron multiplicity measured by the 1V1E method in the symmetric region of thermal neutron-induced fission of 239Pu

Using a Monte Carlo method, the measurement by the 1V1E technique of the average prompt neutron multiplicity as a function of the mass of fragments from the thermal neutron-induced fission of 239Pu is simulated. The input data, associated with the masses of complementary primary fragments (A1, A2), consist of the yield (Y), the average total kinetic energy (TKE-) and its standard deviation (σTKE), the average prompt neutron multiplicity (ν-s, a sawtooth approach of an experimental curve ν-), and the slope of neutron multiplicity against total kinetic energy, -dν/d<TKE>, plotted as a function of fragment mass. The output data, associated with the provisional masses of complementary fragments (m1∗, m2∗) calculated with the 1V1E method, consist of the simulated quantities Y, TKE-, σTKE, N-, comparable to the measured ones. In comparison with ν-s, N- and ν-are oversized in the region of light fragment masses near the symmetric fission. To interpret the results, the neighboring masses ratio RA =YA+1/Y(A) and the term FA=A-m∗, where m∗ is the average of provisional mass as a function of A, are defined. It is shown that N--ν-s and ν--ν-s are correlated with -F(R-1). It is concluded that the oversize of ν- relative to ν-s is due to the interplay of the prompt neutron emission and the slope of the mass yield curve.

Test of isospin conservation in thermal neutron-induced fission of $$^{245}\mathrm{Cm}$$ 245 Cm

We have recently shown that the general trends of partition-wise fission fragment mass distribution in heavy-ion-induced compound nuclear (CN) fission of heavy nuclei can be reproduced reasonably well by using the concept of isospin conservation, hence providing a direct evidence of isospin conservation in neutron-rich systems [Jain et al, Nucl Data Sheets 120, 123 (2014); Garg and Jain, Phys. Scr. 92, 094001 (2017); Jain and Garg, EPJ Web of Conference 178, 05007 (2018); Garg et al, Phys. Scr. 93, 124008 (2018)]. In this paper, we test the concept of isospin conservation to reproduce the fission fragment mass distribution emerging from thermal neutron-induced CN fission reaction, $$^{245}\mathrm{Cm}(n_{\mathrm{th}}, \hbox {f})$$ . As earlier, we use Kelson’s conjectures [I Kelson, Proceedings of the Conference on Nuclear Isospin (Academic Press, New York, 1969)] to assign isospin to neutron-rich fragments emitted in fission, which suggest the formation of fission fragments in isobaric analogue states. We calculate the relative yields of neutron-rich fragments using the concept of isospin conservation and basic isospin algebra. The calculated results reproduce the experimentally known partition-wise mass distributions quite well. This highlights the usefulness of isospin as an approximately good quantum number in neutron-rich nuclei. This also allows us to predict the fragment distribution of the most symmetric Cd–Cd partition and the heavier mass fragment distributions, both not measured so far.

Estimation of the Yield of “Scission” Neutrons from Thermal Neutron-Induced Fission of 239Pu

Results of investigations of the angular and energy distributions of prompt neutrons from 239Pu fission observed on WWR-M type research reactor (Gatchina) are presented. Peculiarities observed in both angular and energy distributions of prompt neutrons can be explained by assuming the existence of neutrons emitted near the instant of fissioning nucleus rupture. Anisotropy of the angular distribution of prompt neutrons in the frame of the center of mass of fission fragments (ψ(0°)/ψ(90°) ≈ 1.07–1.09) should also be taken into consideration.

New measurements on isobaric fission product yields and mean kinetic energy for 241Pu thermal neutron-induced fission

Nuclear fission yields data measurements for thermal neutron induced fission of 241Pu have been carried out at the Institut Laue Langevin (ILL) in Grenoble, using the Lohengrin mass spectrometer. Mass, isotopic and isomeric yields have been extracted for the last measurements. A focus is given in this document to the mass yield results which are obtained for almost the entire heavy peak and most of the light high yields masses, along with the covariance matrix. The mean kinetic energy as a function of the fission product mass has also been extracted from the measurements. The total mean kinetic energy pre and post neutron emission have been assessed and compared to other works showing a rather good agreement.

Isotopic distribution and dependency to fission product kinetic energy for 241Pu thermal neutron-induced fission

Nuclear fission yields data measurements for thermal neutron induced fission of 241Pu have been carried out at the ILL in Grenoble, using the Lohengrin mass spectrometer. The relative isotopic yields for the masses 137 up to 141 have been derived with the associated experimental covariance matrices. Moreover, from preliminary results for the masses 92, 138 and 139, a clear evolution over fission product kinetic energy of the isotopic total count rate is observed.

Position sensitive twin ionization chamber for nuclear fission investigations

In this article, we report the recent achievements in the design of a back-to-back twin ionization chamber (TIC) for fission fragment (FF) mass, kinetic energy, and orientation. Correlated FF kinetic energies, their masses, and the angle of the fission axes in 3D Cartesian coordinates can be determined from analysis of the heights and shapes of the pulses induced by the FFs on the anodes of the TIC. These anodes were designed to consist of isolated [Formula: see text]-shaped strips connected to nodes of a chain filter made of serially-connected two-port networks. The double charge division method was implemented by digitizing four waveforms at the endpoints of the chain filters. It was shown that the fission fragments emission point on the target plane may be determined using the measured data. A position-sensitive neutron-induced fission detector for neutron imaging applications with both thermal and low energy neutrons was found as another possible implementation of the designed TIC. Preliminary measurements with thermal neutron-induced fission were done with RC chain filters and the results are reported here.