Prompt fission neutron multiplicity and spectrum evaluations for n+238U reaction

The prompt fission neutron multiplicity and spectra for n+²³⁸U reaction are calculated using an improved Los Alamos model which includes the linear relation between the average prompt gamma ray energy and the prompt neutron multiplicity and also the average fission fragment kinetic energy dependence on the incident neutron energy. The coefficients describing the quadratic variation of the fission fragment kinetic energy versus the incident energy are obtained by extrapolation of the data and procedure used for n+²³⁵U reaction. The inverse process compound nucleus cross-section of the fissioning nucleus is calculated using the coupled channel method. In the incident energy range where only the first fission chance is involved the comparison of present spectrum evaluation with spectrum calculation using multi-modal model is made too. The calculated prompt neutron multiplicity and spectra of ²³⁸U neutron induced fission are in good agreement with the experimental data for the entire incident energy range required in evaluations, proving the validity of the used procedure.     

<Superscript>239</Superscript>Pu prompt fission neutron spectra

Calculations of ²³⁹Pu(n, F) prompt fission neutron spectra have been performed for neutron energy up to 20 MeV. The exclusive spectra of pre-fission neutron reactions (n, xnf) were calculated on the basis of the Hauser-Feshbach model simultaneously with the cross sections of (n, F) and (n, 2n) reactions. The spectra of neutrons emitted by fission fragments were approximated by a sum of two Watt distributions. The components of the prompt fission neutron spectra due to pre-fission neutrons are manifested in the prompt fission neutron spectra and the average neutron energy. A correlation is established between this effect in the contribution of emissive fission (n, xnf) in the fission cross-section of ²³⁹Pu(n, F) and ²³⁵U(n, F). It is shown that the ²³⁹Pu(n, F) prompt fission neutron spectra used in applied calculations do not correspond to the experimental differential data and the systematic regularities in the spectra and their average energy found for the most carefully studied nuclei ^235,238U and ²³²Th. __________ Translated from Atomnaya énergiya, Vol. 103, No. 2, pp. 119–124, August, 2007.     

Improved Los Alamos model applied to the neutron induced fission of 239Pu and 240Pu and to the spontaneous fission of Pu isotopes

The Los Alamos model with multiple fission chances upgraded with (a) the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity and (b) the dependence of the average fission fragment kinetic energy on the incident neutron energy, is used for the n+²³⁹Pu and n+²⁴⁰Pu reactions, and also for the spontaneous fission of ^237–241Pu isotopes. In the case of ²⁴⁰Pu fissioning nucleus the variation of the average energy released versus the incident neutron energy is also taken into account. The calculated prompt fission neutron spectra and average prompt neutron multiplicity well represent the experimental data, proving a better predictive power of the improved Los Alamos model.     

Interpretation of delayed neutron phenomena

The Bohr-Wheeler mechanism of delayed neutron emission has been used, together with recent fission mass- and charge-distribution data, to interpret the observed characteristics of delayed neutrons from fission. This interpretation accounts satisfactorily for recently measured periods and relative abundances of the delayed neutrons from six fissionable nuclides: U²³³' U²³⁶, U²³⁸, Pu²³⁹, Pu²⁴⁰, and Th²³². A set of selection criteria are developed which lead to prediction of the “most probable” main precursors as well as possible contributors for each delayed neutron group. Assuming the most probable set of delayed neutron precursors, total delayed neutron yields have been calculated for each of the six fissionable nuclides studied. These calculated yields are in agreement with experiment, thus providing a reasonable explanation of the striking variation in observed total delayed neutron yields, which increase by an order of magnitude from Pu²³⁹ to Th²³² fission. Throughout the analysis, special attention is given to the dominant role of closed-shell effects in delayed neutron systematics.     

Prompt fission neutron spectrum evaluation for 252Cf(SF) in the frame of the multi-modal fission model

In the present work, an attempt to improve the evaluation of the prompt fission neutron spectrum of ²⁵²Cf(SF) is made. The multi-modal fission concept is included into the Los Alamos model. A more generalized form of the fission fragment residual nuclear temperature distribution and a possible anisotropy effect of the prompt neutron emission in the center-of-mass system are taken into account, too. The multi-modal fission parameters entering the prompt fission neutron spectrum model are determined on the basis of the experimental data concerning the fission fragment total kinetic energy TKE(A) and mass distribution Y(A) measured at IRMM. The calculated prompt neutron spectrum is obtained in better agreement with the standard point-wise evaluation of Mannhart and compared to other evaluations made with different models.     

Calculation of Beta-Ray Spectra from Individual and Aggregate Fission Products

The β-ray spectra of individual fission products were calculated by using the β-decay data assuming every β-decay to be allowed transition. For the nuclides without measured decay data the β-feeding function was evaluated with the gross theory of β-decay and the β-ray spectrum was calculated from the function. The measured decay data were also supplemented with the data calculated by the gross theory for the excitation energy range above the highest measured excitation energy level. The β-ray spectra from aggregate fission products after a burst fission were calculated by using the β-ray spectrum and the atom number of each fission product nuclide and they were compared with the ones measured for thermal neutron induced fission of ²³⁵U, ²³⁹Pu and ²⁴¹Pu at Oak Ridge National Laboratory. The spectrum calculations showed excellent agreement with the measured data at shorter cooling times than 10s when many short-lived nuclides without measured decay data contributed considerably to the spectrum.     

Integral testing of Np cross-sections in GCFR spectra

Integral measurements of ²³⁷Np capture and fission rates have been carried out relative to the ²³⁹Pu fission rate in two GCFR lattices with different neutron spectra. The results have been compared with calculated values based, respectively, on ENDF/B-IV and FGL5 cross-sections. Both data sets predicted the measured fission ratios correctly within the experimental errors of ±2% but, in the case of the ratio of ²³⁷Np capture to ²³⁹Pu fission, the calculated ratios differed from the measured values by about 10% for both sets.     

Sources: A code for calculating (alpha,n), spontaneous fission,and delayed neutron sources and spectra

SOURCES is a computer code that determines neutron production rates and spectra from (alpha, n) reactions, spontaneous fission, and delayed neutron emission due to the decay of radionuclides in homogeneous media, interface problems, and three-region interface problems. The code is also capable of calculating the neutron production rates due to (alpha, n) reactions induced by a monoenergetic beam of alpha particles incident on a slab of target material. The (alpha, n) spectra are calculated using an assumed isotropic angular distribution in the center-of-mass system with a library of 107 nuclide decay alpha-particle spectra, 24 sets of measured and/or evaluated (alpha, n) cross sections and product nuclide level branching fractions, and functional alpha particle stopping cross sections for Z < 106. Spontaneous fission sources and spectra are calculated with evaluated half-life, spontaneous fission branching, and Watt spectrum parameters for 44 actinides. The delayed neutron spectra are taken from an evaluated library of 105 precursors. The code outputs the magnitude and spectra of the resultant neutron sources. It also provides an analysis of the contributions to that source by each nuclide in the problem.     

Research-reactor neutron fields for calibrating in-reactor detectors in nuclear power plants

The neutron spectra of research and power reactors are compared. The spectra were measured by the neutron-activation method and calculated using the KASKAD computer code. The a priori spectrum in the calculation was constructed as a superposition of physically validated spectra. A method of calibrating in-reactor detectors in nuclear power plants on the basis of the sensitivity to the ²³⁵U fission rate in 1 g of uranium using the neutron fields of research reactors is proposed. __________ Translated from Atomnaya énergiya, Vol. 100, No. 2, pp. 97–107, February, 2006.     

Representation of the fission neutron spectrum of235U,239Pu, and252Cf and the reactor spectrum as a superposition of five inelastic scattering functions

The results of investigations of the representation of the fission neutron spectra of²³⁵U,²³⁹Pu, and²⁵²Cf and neutron spectra at the core center of fast reactors as a superposition of five Weisskopf functions are presented. This representation best reflects the physics of nuclear fission and shows that the spectra at the core center of fast reactors are reconstructed with an error close to the error of the best studied spectra in the worldwide practice of neutron measurements. 3 tables, 13 references. All-Russia Scientific-Research Institute of Physical-Technical and Radio Engineering Measurements. Translated from Atomnaya énergiya, Vol. 88, No. 4, pp. 292–299, April, 2000.     

Prompt-Neutron Spectrum Formation Mechanism in 235U, 239Pu, and 252Cf Fission

The mechanism of the formation of the prompt-neutron spectrum in fissioning of ²³⁵U and ²³⁹Pu by thermal and fast neutrons and spontaneous fission of ²⁵²Cf is investigated. A method is proposed for the formation of the prompt-neutron spectrum in fissioning of nuclei as a superposition of three partial Weisskopf evaporation spectra with average neutron energy 0.4, 2.06, and 2.8 MeV.     

Improved Los Alamos model applied to the neutron induced fission of 235U and 237Np

The Los Alamos model, with multiple fission chances, was upgraded to include the linear relation between the average prompt gamma ray energy and the average prompt neutron multiplicity. A global, model-free parameterization of this relation versus the charge and mass number of fissioning nuclei is obtained. The average fission fragment kinetic energy dependence on the incident neutron energy is also taken into account. Employing this model for the n+²³⁵U and n+²³⁷Np reactions, the prompt fission neutron spectra, the average prompt neutron multiplicity and the average prompt gamma ray energy are obtained in very good agreement with the experimental data, proving a better predictive power.     

Parameters of <Emphasis Type="Italic">s</Emphasis>-resonances in <Superscript>246–248</Superscript>Cm fission cross sections

The fission cross sections of ²⁴⁶Cm, ²⁴⁷Cm, and ²⁴⁸Cm isotopes measured with an SVZ-100 spectrometer, based on neutron moderation in lead, at the Institute of Nuclear Research of the Russian Academy of Sciences are analyzed. The area of the resolved resonances is calculated, and their neutron and fission widths are determined. The results are compared with existing data and recommended evaluations.     

Measurement of Double-Differential Neutron Emission Spectra from Uranium-238

We have performed the measurement of neutron emission spectra from ²³⁸U using a time-of-flight technique, and deduced the following data; (1) the prompt fission neutron spectra for 2 MeV incident neutrons at two emission angles of 90° and 135°, (2) the double-differential neutron emission cross sections at the incident energies of 1.2, 2.0, 4.2, 6.1 and 14.1 MeV. The emission spectra and the cross sections for scattering process were also deduced by subtracting the fission neutrons from the experimental spectra. The experimental results were compared with other experiments and the evaluations of JENDL-3 and ENDF/B-IV.

From the fission spectrum data ranging from 2 to 12 MeV, we have derived the best fit parameters for the Maxwellian and Watt type distribution functions. The experimental spectra are described with the Maxwellian spectrum with temperature of 1.24–1.26 MeV and are softer than both evaluations.

The spectra and cross sections for inelastic-scattering showed substantial disagreement with the evaluations concerning the discrete levels between 0.5 and 1.2 MeV, and continuum neutrons due to evaporation and pre-equilibrium processes. The secondary neutron angular distributions at 14 MeV incident energy were reproduced fairly well with the systematics.     

“Point by Point” model calculation of prompt neutron emission data for Cm(SF) and Cm(SF)

This work is devoted to the modeling of the prompt neutron emission in the spontaneous fission of ^248,244Cm. The Point by Point model calculations of relevant quantities (like: the multi-parametric matrix ν(A, TKE), the fission fragment multiplicity as a function of fragment mass (usually named sawtooth) ν(A), fission fragment pair multiplicity as a function of total excitation energy ν_pair(TXE), total average prompt neutron multiplicity 〈ν_p〉 and spectra N(E), total average multiplicity as a function of the total kinetic energy 〈ν_p〉 (TKE) describe very well all existing experimental data.     

Measurement of the 239Pu(n,f) cross section from thermal up to 30 keV neutron energy

At GELINA measurements of the ²³⁹Pu fission cross-section were performed covering the neutron energy region from thermal up to 30 keV. Fission fragment as well as fission neutron detection techniques were used. Also for the neutron flux determination different methods were applied. From the σ_f-data, several fission integrals were calculated and compared with other results.     

<Superscript>246–248</Superscript>Cm Neutron fission cross sections and resonance integrals

The fission cross sections of ^246–248Cm nuclei for neutron energies 0.1 eV–20 keV have been measured using an SVZ-100 lead moderation time neutron spectrometer at the Institute of Nuclear Research of the Russian Academy of Sciences. The fission resonance integrals have been calculated. The results are compared with the existing data and recommended evaluations.     

Statistical Estimation of Distributions of Physical Quantities in Nuclear Fission

Making use of a model based on the statistical theory, calculations were performed to obtain the mass yields, the most probable charges, the kinetic energies and the prompt neutron yields of fission fragments, and the mass yields of fission products from thermal-neutron-induced fission of ²³³U, ²³⁵U, ²³⁹Pu, ²⁴¹Pu and from the spontaneous fission of ²⁵²Cf. The calculations are further extended to fast-neutron-induced fission. The scission-point distance is treated as a parameter varying with the mass number of the heavy fragments of fission. This proved successful in approaching the calculated curves closer to the observed values.

It is possible to predict unknown physical quantities in nuclear fission with use made of the method developed in the present work.     

Fission products and secondary gamma-ray spectra in a plutonium-fueled fast reactor

The fission products' gamma-ray and gamma-ray energy source spectra for a gas-cooled fast breeder reactor (GCFR) are calculated for different times after shutdown by modifying the RIBD computer code. The secondary gamma-ray energy source spectrum in the core of a GCFR, from fission, inelastic scattering, and capture reactions, is calculated using a typical GCFR neutron spectrum. The computer code LAPHANO is used to generate the multigroup (n, xγ) neutron-coupled gamma-ray transfer matrix. The weak dependence of capture and inelastic gamma ray source spectrum on the neutron flux spectrum has been noted. The fission products and secondary gamma-ray source spectra obtained can be used to calculate heat generation and refueling shielding requirements, etc.