Fission Modeling Research Articles

Shedding light on the 239Pu fission source term with new high-precision experiments and advanced fission modeling

In the last decade, there has been a renaissance of fission research resulting in new high-precision experiments and advanced fission modeling. For instance, the Chi-Nu and CEA teams supplied, for the first time, the 239Pu prompt fission neutron spectrum (PFNS) for broad ranges of incident and outgoing neutron energies. The CEA team also measured 239Pu average prompt neutron multiplicities, ν̄p, with lower statistical uncertainties and a technique significantly different than the one used in the past. The NIFFTE collaboration provided 239Pu(n,f)/235U(n,f) cross section shape ratios with uncertainties below 1% utilizing a novel detector type. Advanced fission event generators were developed, among them CGMF, FIFRELIN, FREYA, and GEF, which calculate post-scission fission observables in a correlated manner. These new experimental data and more consistent fission models change the evaluated PFNS, ν̄p, and (n,f) cross sections, some only modestly, compared to ENDF/B-VIII.0. In turn, the individual new nuclear data distinctly change simulated effective neutron multiplication factors of fast critical assemblies, but their combined impact is small, while affecting the prediction of LLNL pulsed sphere neutron leakage spectra and reaction rates only within experimental uncertainties. Also, the parameters obtained from fitting to ν̄p reproduce various post-scission fission observables within the uncertainties of experimental data. This indicates that new differential experiments and consistent fission modeling reduce compensating errors present in ENDF/B-VIII.0.

Propagation of radiation source uncertainties in spent fuel cask shielding calculations

The propagation of radiation source uncertainties in spent nuclear fuel (SNF) cask shielding calculations is presented in this paper. The uncertainty propagation employs the depletion and source term outputs of the deterministic code STREAM as input to the transport simulation of the Monte Carlo (MC) codes MCS and MCNP6. The uncertainties of dose rate coming from two sources: nuclear data and modeling parameters, are quantified. The nuclear data uncertainties are obtained from the stochastic sampling of the cross-section covariance and perturbed fission product yields. Uncertainties induced by perturbed modeling parameters consider the design parameters and operating conditions. Uncertainties coming from the two sources result in perturbed depleted nuclide inventories and radiation source terms which are then propagated to the dose rate on the cask surface. The uncertainty analysis results show that the neutron and secondary photon dose have uncertainties which are dominated by the cross section and modeling parameters, while the fission yields have relatively insignificant effect. Besides, the primary photon dose is mostly influenced by the fission yield and modeling parameters, while the cross-section data have a relatively negligible effect. Moreover, the neutron, secondary photon, and primary photon dose can have uncertainties up to about 13%, 14%, and 6%, respectively.

Correlated Production and Analog Transport of Fission Neutrons and Photons Using Fission Models FREYA, FIFRELIN, and the Monte Carlo Code TRIPOLI-4

Fission modeling in general-purpose Monte Carlo transport codes often relies on average nuclear data provided by the international evaluation libraries. As such, only average fission multiplicities are available and correlations between fission neutrons and photons are missing. Whereas uncorrelated fission physics is usually sufficient for standard reactor core and radiation shielding calculations, correlated fission secondaries are required for specialized nuclear instrumentation and detector modeling. For coincidence counting detector optimization for instance, the precise simulation of fission neutrons and photons that remain correlated in time from birth to detection is essential. New developments were recently integrated into the Monte Carlo transport code TRIPOLI-4 to model fission physics more precisely, the purpose being to access event-by-event fission events from two different fission models: Fission Reaction Event Yield Algorithm (FREYA) and Fission Fragment Evaporation Leading to an Investigation of Nuclear Data (FIFRELIN). TRIPOLI-4 simulations can now be performed, either by connecting via an application programming interface to the Lawrence Livermore National Laboratory fission library including FREYA, or by reading the external fission event data files produced by FIFRELIN beforehand. These new capabilities enable us to easily compare results from the Monte Carlo transport calculations using the two fission models in a nuclear instrumentation application. In the first part of this paper, the broad underlying principles of the two fission models are recalled. We then present the experimental measurements of neutron angular correlations for 252Cf(sf) and 240Pu(sf). The correlations were measured for several neutron kinetic energy thresholds. In the latter part of this paper, simulation results are compared with the experimental data. Spontaneous fissions in 252Cf and 240Pu are modeled by FREYA or FIFRELIN. Emitted neutrons and photons are subsequently transported to an array of scintillators by TRIPOLI-4 in the analog mode to preserve their correlations. Angular correlations between fission neutrons obtained independently from these TRIPOLI-4 simulations, using either FREYA or FIFRELIN, are compared with the experimental results. For 240Pu(sf), the measured correlations were used to tune one of the FREYA model parameters. Similarly, different sets of parameters have been tested in FIFRELIN to try to improve the agreement with the experimental data.

Correlated Production and Analog Transport of Fission Neutrons and Photons using Fission Models FREYA, FIFRELIN and the Monte Carlo Code TRIPOLI-4® .

Fission modeling in general-purpose Monte Carlo transport codes often relies on average nuclear data provided by international evaluation libraries. As such, only average fission multiplicities are available and correlations between fission neutrons and photons are missing. Whereas uncorrelated fission physics is usually sufficient for standard reactor core and radiation shielding calculations, correlated fission secondaries are required for specialized nuclear instrumentation and detector modeling. For coincidence counting detector optimization for instance, precise simulation of fission neutrons and photons that remain correlated in time from birth to detection is essential. New developments were recently integrated into the Monte Carlo transport code TRIPOLI-4 to model fission physics more precisely, the purpose being to access event-by-event fission events from two different fission models: FREYA and FIFRELIN. TRIPOLI-4 simulations can now be performed, either by connecting via an API to the LLNL fission library including FREYA, or by reading external fission event data files produced by FIFRELIN beforehand. These new capabilities enable us to easily compare results from Monte Carlo transport calculations using the two fission models in a nuclear instrumentation application. In the first part of this paper, broad underlying principles of the two fission models are recalled. We then present experimental measurements of neutron angular correlations for 252Cf(sf) and 240Pu(sf). The correlations were measured for several neutron kinetic energy thresholds. In the latter part of the paper, simulation results are compared to experimental data. Spontaneous fissions in 252Cf and 240Pu are modeled by FREYA or FIFRELIN. Emitted neutrons and photons are subsequently transported to an array of scintillators by TRIPOLI-4 in analog mode to preserve their correlations. Angular correlations between fission neutrons obtained independently from these TRIPOLI-4 simulations, using either FREYA or FIFRELIN, are compared to experimental results. For 240Pu(sf), the measured correlations were used to tune the model parameters.

Bruyères-le-Châtel Neutron Evaluations of Actinides with the TALYS Code: The Fission Channel

For several years, various neutron evaluations of plutonium and uranium isotopes have been performed at Bruyères-le-Châtel (BRC), from 1 keV up to 30 MeV. Since only nuclear reaction models have been used to produce these evaluations, our approach was named the “Full Model” approach.Total, shape elastic and direct inelastic cross sections were obtained from the coupled channels model using a dispersive optical potential developed for actinides, with a large enough coupling scheme including the lowest octupolar band. All other cross sections were calculated using the Hauser-Feshbach theory (TALYS code) with a pre-equilibrium component above 8–10 MeV.In this paper, we focus our attention on the fission channel. More precisely, we will present the BRC contribution to fission modeling and the philosophy adopted in our “Full Model” approach. Performing evaluations with the “Full Model” approach implies the optimization of a large number of model parameters. With increasing neutron incident energy, many residual nuclei produced by nucleon emission also lead to fission. All available experimental data assigned to various fission mechanisms of the same nucleus were used to determine fission barrier parameters. For uranium isotopes, triple-humped fission barriers were required in order to reproduce accurately variations of the experimental fission cross sections. Our BRC fission modeling has shown that the effects of the class II or class III states located in the wells of the fission barrier sometimes provide an anti-resonant transmission rather than a resonant one.Consistent evaluations were produced for a large series of U and Pu isotopes. Resulting files were tested against integral data.

Defect evolution in single crystalline tungsten following low temperature and low dose neutron irradiation

The tungsten plasma-facing components of fusion reactors will experience an extreme environment including high temperature, intense particle fluxes of gas atoms, high-energy neutron irradiation, and significant cyclic stress loading. Irradiation-induced defect accumulation resulting in severe thermo-mechanical property degradation is expected. For this reason, and because of the lack of relevant fusion neutron sources, the fundamentals of tungsten radiation damage must be understood through coordinated mixed-spectrum fission reactor irradiation experiments and modeling. In this study, high-purity (110) single-crystal tungsten was examined by positron annihilation spectroscopy and transmission electron microscopy following low-temperature (∼90 °C) and low-dose (0.006 and 0.03 dpa) mixed-spectrum neutron irradiation and subsequent isochronal annealing at 400, 500, 650, 800, 1000, 1150, and 1300 °C. The results provide insights into microstructural and defect evolution, thus identifying the mechanisms of different annealing behavior. Following 1 h annealing, ex situ characterization of vacancy defects using positron lifetime spectroscopy and coincidence Doppler broadening was performed. The vacancy cluster size distributions indicated intense vacancy clustering at 400 °C with significant damage recovery around 1000 °C. Coincidence Doppler broadening measurements confirm the trend of the vacancy defect evolution, and the S–W plots indicate that only a single type of vacancy cluster is present. Furthermore, transmission electron microscopy observations at selected annealing conditions provide supplemental information on dislocation loop populations and visible void formation. This microstructural information is consistent with the measured irradiation-induced hardening at each annealing stage, providing insight into tungsten hardening and embrittlement due to irradiation-induced matrix defects.

Applications of Event-by-Event Fission Modeling with FREYA

The recently developed codeFREYA (Fission Reaction Event Yield Algo- rithm) generates large samples of complete fission events, consisting of two receding product nuclei as well as a number of neutrons and photons, all with complete kinematic information. Thus it is possible to calculate arbitrary correlation observables whose behav- ior may provide unique insight into the fission process. We first discuss the present status ofFREYA, which has now been extended to include spontaneous fission. Concentrating on 239 Pu(n th,f), 240 Pu(sf) and 252 Cf(sf), we discuss the neutron multiplicity correlations, the dependence of the neutron energy spectrum on the neutron multiplicity, and the relation- ship between the fragment kinetic energy and the number of neutrons and their energies. We also suggest novel fission observables that could be measured with modern detectors.

Determination of Onset of Uplifting for the Mid‐Upper Yangtze Area Since Indosinian Event

AbstractThe onset of uplifting of the Mid‐upper Yangtze area is constrained by the combination of analysis of apatite fission track (AFT) age and modeling of fission track length distribution. The results show that the general trend of onset of uplifting is characterized by decreasing westward in geological time for the different tectonic units in the Mid‐upper Yangtze area. Two episodes of uplifting in Jianghan basin occurred within 157~97 Ma and 10~0 Ma; Xiang'exi‐Wuling area experienced an uplifting in 137~0 Ma; E'xi Yudong area, Chuandong folds, Qianzhong Hights and Dian‐Qian‐Gui areas in 97~0 Ma; Northern Sichuan basin and Chuanzhong areas in 56~0 Ma; and Chuanxi depression and Dianxi region in 23~0 Ma. The major faults of Qiyueshan, Huanyinshan and Longquanshan controlled the boundaries of the onsets of uplifting. The onsets of uplifting for the areas east of Qiyueshan fault, in between Qiyueshan and Huayinshan faults, in between Huayinshan and Longquanshan faults, west of Longquanshan fault are late Jurassic‐early Cretaceous (J3‐K1), late Cretaceous (K2), Paleogene (E) and Neogene (N), respectively.

Modeling and Neutron-Induced Fission Cross Sections for Americium

Neutron-induced fission cross sections for eight americium isotopes (A = 239 to 244) are predicted with the nuclear reaction model code EMPIRE-2.19. The code incorporates advanced fission modeling, presented by describing the implementation of barrier penetration, transmission mechanism, and decay probabilities. The fission cross-section calculations for americium isotopes are compared to experimental data and evaluations from the ENDF/B-VI.8, JEFF-3.1, JENDL-3.3, or preliminary ENDF/B-VII libraries when available. An analysis of the first and second fission barrier heights validates the existing systematic trend for odd and even neutron numbers.

Low temperature thermochronology of the southern East Greenland continental margin: Evidence from apatite (U–Th)/He and fission track analysis and implications for intermethod calibration

The southern coast of East Greenland is characterized by high topographic relief and deep fjords, but the evolution of the landscape and the low-temperature thermochronology of the region is not well understood. Here we present apatite fission-track and (U–Th)/He ages that suggest several important features of the long-term geomorphic history of this region, but which also illustrate an important discrepancy in the thermal histories derived from each technique. Apatite from bedrock of the southern coast of East Greenland between 62°N and 67°N has fission-track ages ranging from 60 to 840 Ma and (U–Th)/He ages ranging from 21 to 250 Ma. The ages generally increase with elevation and distance from the coast, and fission-track analyses show significant differences in thermal histories for the different regions. In the Kangertittivatsiaq area (c. 66–67°N) apatite fission track data and models suggest two separate periods of slow cooling: prior to c. 200 Ma and between c. 160 Ma and the late Cenozoic (more recently than c. 20 Ma), each of which was followed by a period of rapid cooling and inferred exhumation. Apatite He data in the Kangertittivatsiaq region, including crystal-size–age correlations in low-elevation samples, are most simply interpreted as recording an incision event of at least 1.5 km later than 20 Ma near the coast. This may have been caused by glacial erosion. The (U–Th)/He data also indicate an earlier phase of rapid exhumation at c. 250 Ma. In the Skjoldungen/Kap Møsting area (c. 62–64°N) approximately 200 km south of the Kangertittivatsiaq, apatite fission-track data suggest slow exhumation from c. 200 Ma into the Neogene followed by fast exhumation. The similarity of fission track ages (200 Ma) at sea level in the fjords in the Skjoldungen area (c. 62–64°N) do not suggest tilting in the hinterland related to the breakup of the East Greenland continental margin. Furthermore, the Cenozoic fission track ages and modeling fission track data suggest that pre-breakup basins may have covered the outer coast. Despite the broadly similar topographic implications of the fission-track and (U–Th)/He data, the thermal histories derived from these systems are inconsistent. Fission-track data require thermal histories that predict He ages younger than observed, and He data require thermal histories that predict fission-track ages older than observed. Similar discrepancies have also been observed in other settings characterized by long-term low-temperature thermal histories, and may reflect changes in annealing or diffusion behavior (or both) that either develop or become more apparent in such cases.