Fission Neutron Spectrum Research Articles

Asymmetric fission of 235U nuclei induced by thermal neutrons and fast neutrons of pulsed nuclear reactors

The contributions that neutrons emitted from the neck of excited dumbbell-shaped fissioning 235U nuclei at the instant of their fission make to the prompt fission neutron spectrum, as well as the ratios of these contributions, have been measured for nuclear fission induced by thermal neutrons and fast neutrons of pulsed nuclear reactors.

Prompt fission neutron spectra from fission induced by 1 to 8 MeV neutrons onU235andPu239using the double time-of-flight technique

Prompt fission neutron spectra from $^{235}\mathrm{U}$ and $^{239}\mathrm{Pu}$ were measured for incident neutron energies from 1 to 200 MeV at the Weapons Neutron Research facility (WNR) of the Los Alamos Neutron Science Center, and the experimental data were analyzed with the Los Alamos model for the incident neutron energies of 1--8 MeV. A CEA multiple-foil fission chamber containing deposits of 100 mg $^{235}\mathrm{U}$ and 90 mg $^{239}\mathrm{Pu}$ detected fission events. Outgoing neutrons were detected by the Fast Neutron-Induced $\ensuremath{\gamma}$-Ray Observer array of 20 liquid organic scintillators. A double time-of-flight technique was used to deduce the neutron incident energies from the spallation target and the outgoing energies from the fission chamber. These data were used for testing the Los Alamos model, and the total kinetic energy parameters were optimized to obtain a best fit to the data. The prompt fission neutron spectra were also compared with the Evaluated Nuclear Data File (ENDF/B-VII.0). We calculate average energies from both experimental and calculated fission neutron spectra.

Prematurely Condensed Chromosome Rings after Neutron Irradiation of Human Lymphocytes

Calibration curves for fission spectrum neutrons and other high LET radiations are scarce in cytogenetic dosimetry and particularly for Prematurely Condensed Chromosome Rings (PCC-ring). Here we analyzed the behavior of the PCC-ring frequency and PCC index after neutron irradiation in a broad dose interval from 1 to 26 Gy. PCC-rings were induced in lymphocytes with Calyculin A. 6455 PCC cells in G1, G2/M and M/A stages were analyzed. The best fitting between the frequency of PCC ring (Y) and the Dose (D) was obtained with the equation Y = (0.059 ± 0.003) D. The saturation of the PCC-ring was observed after around 4 Gy, but it was still possible to analyze cells exposed up to 26 Gy. The distribution of rings by cell follows Poisson or Neyman type distribution for all doses. This PCC-ring dose effect curve can be used in case of accidental overexposure to neutron radiation, allowing a dose assessment in a reliable way. Additionally, the PCC index seems to be well correlated with radiation dose and decrease in a dose dependent manner from 13% in non exposed sample down to 0.2%. This observation allows the possibility to perform a quick classification of victims exposed to high doses of both gamma and neutron radiations. The PCC assay can then be used for both neutron dose estimation up to 4 Gy and for the rapid classification of victims exposed to higher doses. This assay could be included in the multiparametric approach developed to optimize the medical treatment of radiation victims.

Fission Product Yields of 233U, 235U, 238U and 239Pu in Fields of Thermal Neutrons, Fission Neutrons and 14.7-MeV Neutrons

The yields of more than fifteen fission products have been carefully measured using radiochemical techniques, for 235U(n,f), 239Pu(n,f) in a thermal spectrum, for 233U(n,f), 235U(n,f), and 239Pu(n,f) reactions in a fission neutron spectrum, and for 233U(n,f), 235U(n,f), 238U(n,f), and 239Pu(n,f) for 14.7 MeV monoenergetic neutrons. Irradiations were performed at the EL3 reactor, at the Caliban and Prospero critical assemblies, and at the Lancelot electrostatic accelerator in CEA-Valduc. Fissions were counted in thin deposits using fission ionization chambers. The number of fission products of each species were measured by gamma spectrometry of co-located thick deposits.

Fission Product Yields from Fission Spectrum n+ 239Pu for ENDF/B-VII.1

We describe a new cumulated fission product yield (FPY) evaluation for fission spectrum neutrons on plutonium that updates the ENDF/B-VI evaluation by England and Rider, for the forthcoming ENDF/B-VII.1 database release. 1 1 We intend to release the ENDF/B-VII.1 database in December 2011, and all released data are subject to CSEWG approval. It is possible that the released evaluated data will differ from those presented in this paper; the evaluated date presented here can be referred to as ENDF/B-VII.1 beta 0. We focus on FPs that are needed for high accuracy burnup assessments; that is, for inferring the number of fissions in a neutron environment. Los Alamos conducted an experiment in the 1970s in the Bigten fast critical assembly to determine fission product yields as part of the Interlaboratory Reaction Rate (ILRR) collaboration, and this has defined the Laboratory's fission standard to this day. Our evaluation includes use of the LANL-ILRR measurements (not previously available to evaluators) as well as other Laboratory FPY measurements published in the literature, especially the high-accuracy mass spectrometry data from Maeck and others. Because the measurement database for some of the FPs is small — especially for 99Mo — we use a meta-analysis that incorporates insights from other accurately-measured benchmark FP data, using R-value ratio measurements. The meta-analysis supports the FP measurements from the LANL-ILRR experiment. Differences between our new evaluations and ENDF/B-VI are small for some FPs (less than 1–2%-relative for 95Zr, 140Ba, 144Ce), but are larger for 99Mo (4%-relative) and 147Nd (5%-relative, at 1.5 MeV) respectively. We present evidence for an incident neutron energy dependence to the 147Nd fission product yield that accounts for observed differences in the FPY at a few-hundred keV average energy in fast reactors versus measurements made at higher average neutron energies in Los Alamos' fast critical assemblies. Accounting for such FPY neutron energy dependencies is important if one wants to reach a goal of determining the number of fissions to accuracies of 1–2%. An evaluation of the energy-dependence of fission product yields is given for all A values based on systematical trends in the measured data, with a focus on the energy dependence over the fast neutron energy range from 0.2–2 MeV. Based on these trends, we present an evaluation of the FPY data at 0.5 and 2.0 MeV average incident neutron energies. This new set of ENDF/B-VII data will enable users to linearly interpolate between the pooled FPY data at ∼0.5 MeV and our new data at 2 MeV to obtain FPYs at other energies.

Fission Product Data Measured at Los Alamos for Fission Spectrum and Thermal Neutrons on 239Pu, 235U, 238U

We describe measurements of fission product data at Los Alamos that are important for determining the number of fissions that have occurred when neutrons are incident on plutonium and uranium isotopes. The fission-spectrum measurements were made using a fission chamber designed by the National Institute for Standards and Technology (NIST) in the BIG TEN critical assembly, as part of the Inter-laboratory Liquid Metal Fast Breeder Reactor (LMFBR) Reaction Rate (ILRR) collaboration. The thermal measurements were made at Los Alamos' Omega West Reactor. A related set of measurements were made of fission-product ratios (so-called R-values) in neutron environments provided by a number of Los Alamos critical assemblies that range from having average energies causing fission of 400–600 keV (BIG TEN and the outer regions of the Flattop-25 assembly) to higher energies (1.4–1.9 MeV) in the Jezebel, and in the central regions of the Flattop-25 and Flattop-Pu, critical assemblies. From these data we determine ratios of fission product yields in different fuel and neutron environments (Q-values) and fission product yields in fission spectrum neutron environments for 99Mo, 95Zr, 137Cs, 140Ba, 141,143Ce, and 147Nd. Modest incident-energy dependence exists for the 147Nd fission product yield; this is discussed in the context of models for fission that include thermal and dynamical effects. The fission product data agree with measurements by Maeck and other authors using mass-spectrometry methods, and with the ILRR collaboration results that used gamma spectroscopy for quantifying fission products. We note that the measurements also contradict earlier 1950s historical Los Alamos estimates by ∼5–7%, most likely owing to self-shielding corrections not made in the early thermal measurements. Our experimental results provide a confirmation of the England-Rider ENDF/B-VI evaluated fission-spectrum fission product yields that were carried over to the ENDF/B-VII.0 library, except for 99Mo where the present results are about 4%-relative higher for neutrons incident on 239Pu and 235U. Additionally, our results illustrate the importance of representing the incident energy dependence of fission product yields over the fast neutron energy range for high-accuracy work, for example the 147Nd from neutron reactions on plutonium. An upgrade to the ENDF library, for ENDF/B-VII.1, based on these and other data, is described in a companion paper to this work.

Uncertainty Quantification of Prompt Fission Neutron Spectrum for n(0.5 MeV) + 239Pu

Uncertainties associated with the prompt fission neutron spectrum (PFNS) of n(0.5 MeV) + 239Pu evaluated for the ENDF/B-VII.0 library are estimated using known experimental information and model parameter uncertainties in the framework of the Madland-Nix model. The model parameters used for the ENDF/B-VII.0 evaluation are also used in the present work. A covariance matrix is obtained, and its eigenvalues are estimated. Sampled spectra are then used in PARTISN transport simulations to infer the impact of PFNS uncertainties on the calculation of the multiplication factor keff in the Jezebel critical assembly. The present evaluated PFNS uncertainties lead to ˜0.24% uncertainty in the Jezebel keff. Finally, multigroup covariance matrices are produced in 33- and 590-group structures.

A Background-Free Direction-Sensitive Neutron Detector

The detection and measurements of properties of neutrons are of great importance in many fields of research, including neutron scattering and radiography, measurements of solar and cosmic ray neutron flux, measurements of neutron interaction cross sections, monitoring of neutrons at nuclear facilities, oil exploration, and searches for fissile weapons of mass destruction. Many neutron detectors are plagued by large backgrounds from x-rays and gamma rays, and most current neutron detectors lack single-event energy sensitivity or any information on neutron directionality. Even the best detectors are limited by cosmic ray neutron backgrounds. All applications would benefit from improved neutron detection sensitivity and improved measurements of neutron properties. Here we show data from a new type of detector that can be used to determine neutron flux, energy distribution, and direction of neutron motion. The detector is free of backgrounds from x-rays, gamma rays, beta particles, and relativistic singly charged particles. It is relatively insensitive to cosmic ray neutrons because of their distinctive angular and energy distributions. It is sensitive to thermal neutrons, fission spectrum neutrons, and high energy neutrons, with detection features distinctive for each energy range. It is capable of determining the location of a source of fission neutrons based on characteristics of elastic scattering of neutrons by helium nuclei. A portable detector could identify one gram of reactor grade plutonium, one meter away, with less than one minute of observation time.

The 235U(n, f) Prompt Fission Neutron Spectrum at 100 K Input Neutron Energy

A measurement of the 235U prompt fission neutron spectrum (PFNS) was performed at the Budapest Nuclear Research Reactor at 100 K incident neutron energy. The motivation for this investigation was to verify some literature data measured over the past 20 years that contradict the Los Alamos model, as well as integral data, benchmark (Keff) experiments, and recent spectral data taken at 0.5 MeV incident neutron energy. The measured spectra using three neutron detectors are in excellent agreement with each other. The average spectrum confirms literature data within the error bars in the neutron energy range of 0.7 to 10 MeV. However, the present PFNS shape cannot predict integral experimental data. It seems to be clear now that the disagreement between microscopic and macroscopic data is not connected with a systematic experimental error in the PFNS at low incident neutron energy.

Investigation of Some Structural Fusion Materials for (n, 2n) Reactions up to 40 MeV

The knowledge of (n, 2n) cross section is very important and necessary in the reactor technology since a significant portion of the fission neutron spectrum lies above the threshold of (n, 2n) reaction for the most of the reactor materials. The (n, 2n) cross section is neutron multiplier reaction in fusion reactor design. Therefore, (n, 2n) reactions of the selected blanket materials can play a key role for multiplying neutrons in reactor environment. In this study, for some structural fusion materials (n, 2n) reactions such as 27Al(n, 2n)26Al, 51V(n, 2n)50V, 52Cr(n, 2n)51Cr, 55Mn(n, 2n)54Mn, 56Fe(n, 2n)55Fe and 58Ni (n, 2n)57Ni have been carried out up to 40 MeV incident neutron energy. In these calculations, the pre-equilibrium and equilibrium effects have been investigated. Also in the present work, the (n, 2n) reaction cross-sections have calculated by using evaluated empirical formulas developed by Tel et al. at 14–15 MeV energy. The calculated results are discussed and compared with the experimental data taken from EXFOR database.

Multi-modal analysis for low energy neutron-induced fission of 235U

Low energy neutron induced fission of 235U is studied in the framework of the multi-modal fission model. The fission fragment properties, such as the yields, the average total kinetic energy distribution and the average neutron separation energy, are investigated for incident neutron energies from thermal to 6.0 MeV. The multi-modal fission approach is also used to evaluate the prompt fission neutron multiplicity and spectra for the neutron-induced fission of 235U with an improved version of the Los Alamos model for incident neutrons below the (n, nf) threshold. The three most dominant fission modes are taken into account. The model parameters are determined on the basis of experimental data. The calculated results are in good agreement with the experimental data.

Fission Research at IRMM

Fission Research at JRC-IRMM has a longstanding tradition. The present paper is discussing recent investigations of fission fragment properties of 238 U(n,f), 234 U(n,f), prompt neutron emission in fission of 252 Cf(SF) as well as the prompt fission neutron spectrum of 235 U(n,f) and is presenting the most important results.

Computational and Experimental Validation of a WGPu Neutron Leakage Source Using a Shielded PuBe (α,n) Neutron Source

We propose here a unique, patented shield design that transforms the complex neutron spectrum from a plutonium-beryllium (PuBe) neutron source to nearly the precise neutron signature leaking from a sphere of weapons-grade plutonium (WGPu) material. This will facilitate testing for detection of a significant quantity of weapons plutonium without the expense or risk of testing detector components with real materials. The Monte Carlo (MCNP5) and Deterministic (PENTRAN) computational codes have been used in developing the shield assembly. A nickel composite alloy shield for a PuBe capsule has been designed, built, and laboratory-tested to enable the neutron leakage spectrum from a standard 1-Ci PuBe source (mean energy of 4.6 MeV) to be transformed, through interactions in the shield, into a very close reproduction of the neutron spectrum leaking from a large, subcritical mass of WGPu metal (average neutron energy of 2.1 MeV). Nearly all current calibrations of neutron detectors use 252Cf for generation of a fission neutron spectrum, which decays with a half-life of ~2.7 yr and is very expensive to procure. By converting to this design, PuBe sources relying on 239Pu (T1/2 = 24110 yr) and lasting hundreds of years could then be used to precisely calibrate and test detectors for simulated WGPu neutrons. Alternative custom designs are also possible with further transport-based modeling.

Impact of Incident Energy Dependence of Prompt Fission Neutron Spectra on Uncertainty Analyses

This paper investigates the impact of the incident energy dependence of prompt fission neutron spectra (PFNS) on uncertainty propagation calculations. Uncertainty propagation from incident energy-dependent PFNS to criticality is formulated and its impact is evaluated numerically. It is found that the conventional procedure, in which representative PFNS covariance data for a specific incident energy are used, results in a larger PFNS-induced uncertainty than the straightforward procedure, in which different PFNS covariance data are used for each incident energy range given in the nuclear data libraries. The present study suggests that the correlation between different incident energies of PFNS has a large impact on uncertainty propagation calculation results for nuclear characteristics.

Evaluation and use of the prompt fission neutron spectrum and spectra covariance matrices in criticality and shielding

The prompt fission neutron spectra covariance matrices of 235U, 238U and 239Pu were evaluated using the Monte Carlo method for two empirical spectra formulations, Watt and Kornilov, and compared to the existing evaluations such as JENDL-3.3. The fission spectra, together with the covariance matrices and the methods for the calculation of the corresponding sensitivity profiles were validated on several applications, including shielding (VENUS-3 benchmark, pressure vessel dosimetry) as well as on fast and thermal reactor systems (KRITZ and SNEAK critical experiments). A new set of parameters for a Kornilov model is proposed that improve both benchmark performance and the agreement with recently measured differential spectra. A recommendation to use constrained sensitivity coefficients was confirmed for the case of the fission spectrum induced uncertainty in the effective multiplication factor ( k eff ) and in the detector responses, in order to assure the proper normalisation of the corresponding covariance matrices.

Analysis of prompt fission neutron spectrum and multiplicity for 237Np(n, f) in the frame of multi-modal Los Alamos model

The improved version of Los Alamos model with the multi-modal fission approach is used to analyse the prompt fission neutron spectrum and multiplicity for the neutron-induced fission of 237Np. The spectra of neutrons emitted from fragments for the three most dominant fission modes (standard I, standard II and superlong) are calculated separately and the total spectrum is synthesized. The multi-modal parameters contained in the spectrum model are determined on the basis of experimental data of fission fragment mass distributions. The calculated total prompt fission neutron spectrum and multiplicity are better agreement with the experimental data than those obtained from the conventional treatment of the Los Alamos model.

Evaluation of sensitivity coefficients of effective multiplication factor with respect to prompt fission neutron spectrum

Sensitivity coefficients with respect to the fission neutron spectrum can be formulated in two ways: in terms of the unconstrained and constrained sensitivity coefficients. Differences in results obtained using the two sensitivity methods were investigated for the case of the perturbation of the fission spectrum, and for the uncertainty of the effective multiplication factor ( k eff ) with respect to the fission spectrum. It is shown analytically that both sensitivity coefficient methods ideally result in the same uncertainty predictions. However, if the unconstrained sensitivity coefficients are used, the zero-sum constraint for the absolute covariance matrix must be satisfied with high numerical accuracy.

Covariances of Prompt Fission Neutron Spectra

Following the outcomes and the recommendations of the Working Party on Evaluation Cooperation (WPEC) SG-26 the impact of the uncertainties and energy correlations in the prompt neutron fission spectra (PFNS) was studied in several critical benchmarks, both thermal and fast. Two different approaches to calculate the sensitivity of keff with respect to the fission neutron spectra, including the new so-called normalized sensitivity method, were used as a means to check and assure the mathematical correctness of the derived fission spectrum covariance matrices. A new Monte Carlo (MC) method has been proposed and applied to produce the covariance matrices of the PFNS for the neutron induced fission on 235U, 238U and 239Pu nuclei. The MC method was validated by comparison with the matrix derived analytically using the so-called File-30 formalism in ENDF terminology. Several sets of covariance matrices of the PFNS of 235U, 238U and 239Pu nuclei were derived from the uncertainties assigned to the model parameters of the Watt and the recently proposed Kornilov PFNS parameterizations.

Current Inconsistencies in 238Pu, 241,243Am and 242Cm Evaluations and their Impact on Uncertainties

Improvements in the nuclear reaction modeling and nuclear parameter systematics, consistent with the description of neutron data on major actinides 232 Th, 233 U, 235 U, 238 U and 239 Pu will be shown to provide a sound basis for critical assessment of the fission, capture, inelastic scattering, and (n,xn) reaction cross sections on minor actinides. This will largely help to avoid substituting possible model deficiencies by enlarging parameter uncertainties in the conventional nuclear models. In a number of minor actinides, the uncertainty estimation of cross sections and prompt fission neutron spectra would require complete re-evaluation of basic data. Otherwise, extremely large cross section uncertainty estimates will have to be adopted, especially in the case of poorly investigated Np, Pu, Am, Cm targets.

Uncertainties and Covariances of the Fission Cross Sections and the Fission Neutron Multiplicities for Actinides

Experimental data on the fission cross sections and the fission-neutron multiplicities of actinides are analyzed on the basis of the unrecognized error-estimation method. Such an approach allows us to estimate reasonable systematic uncertainties of available data, which are underestimated by authors of most measurements as a rule. The corresponding uncertainties and covariances of evaluated data are obtained for the most important actinides in the 15-energy group representation. Differences between the present evaluations and the recent BOLNA results are discussed for the main actinides. It is shown that the fission cross section uncertainties are overestimated essentially in the BOLNA analysis for minor actinides. Uncertainties of the cross sections and the neutron multiplicities averaged over the Cf-252 fission-neutron spectrum are considered.